Method and apparatus for moving carriage assembly from initial position to target position and optical disc system including same

[0002] This application is a divisional of U.S. patent application Ser. No. 08/420,899, filed Apr. 11, 1995, now U.S. Pat. No. 5,677,899, which is a continuation-in-part of U.S. patent application Ser. No. 08/376,882, filed Jan. 25, 1995, now U.S. Pat. No. 5,729,511.

[0003] 1. Field of the Invention

[0004] The present invention relates to data storage systems of the type that include a housing having an opening for receipt of a removable disc cartridge in which an information recording medium is mounted for protection. More particularly, it relates to a system for rapidly encoding and writing information onto optical disks in a high density format, and for reading and decoding the information written thereon.

[0005] 2. Description of the Related Art: Overview

[0006] The demand for mass data storage continues to increase with expanding use of data processing systems and personal computers. Optical data storage systems are becoming an increasingly popular means for meeting this expanding demand. These optical data systems provide large volumes of relatively low-cost storage that may be quickly accessed.

[0007] In optical disc systems, coded video signals, audio signals, or other information signals are recorded on a disc in the form of information tracks on one or both planar surfaces of the disc. At the heart of an optical storage system is at least one laser (or other light source). In a first operating mode, the laser generates a high-intensity laser beam that is focused on a small spot on an information track of a rotating storage disc. This high-intensity laser beam raises the temperature of the recording surface of the material above its Curie Point—the point at which the material loses its magnetization and accepts the magnetization of the magnetic field in which the disc is placed. Thus, by controlling or biasing this surrounding magnetic field, and allowing the disc to cool below its Curie Point in a controlled magnetic environment, information may be recorded on the disc in the form of magnetic domains referred to as “pits” on the recording medium.

[0008] Subsequently, when the operator desired to reproduce or read the previously recorded information, the laser enters a second operating mode. In this mode, the laser generates a low-intensity laser beam that is again focused on the tracks of the rotating disc. This lower intensity laser beam does not heat the disc above its Curie Point. The laser beam is, however, reflected from the disc surface in a manner indicative of the previously recorded information due to the presence of the previously formed pits, and the previously recorded information may thereby be reproduced. Since the laser may be tightly focused, an information processing system of this type has advantages of high recording density and accurate reproduction of the recorded information.

[0009] The components of a typical optical system include a housing with an insertion port through which the user inserts the recording media into the drive. This housing accommodates, among other items, the mechanical and electrical subsystems for loading, reading from, writing to, and unloading an optical disc. The operation of these mechanical and electrical subsystems is typically within the exclusive control of the data processing system to which the drive is connected.

[0010] Within the housing of a conventional system that uses disc cartridges, a turntable for rotating a disc thereon is typically mounted on the system baseplate. The turntable may comprise a spindle having a magnet upon which a disc hub is mounted for use. The magnet attracts the disc hub, thereby holding the disc in a desired position for rotation.

[0011] In optical disc systems, as discussed above, it is necessary to magnetically bias the disc during a writing operation by applying a desired magnetic field to at least the portion of the disc being heated by the laser during the writing (recording or erasing) operation. Thus, it is necessary to mount a magnetic field biasing device where it may be conveniently placed in close proximity to the disc surface when the disc is held in position by the magnet associated with the spindle.

[0012] A variety of media or disc types are used in optical data storage systems for storing digital information. For example, standard optical disc systems may use 5¼ inch disks, and these optical disks may or may not be mounted in a protective case or cartridge. If the optical disc is not fixedly mounted in a protective cartridge, an operator manually removes the disc from the protective case. The operator would then manually load the disc onto a loading mechanism, using care to prevent damage to the recording surface.

[0013] Alternatively, for purposes of convenience and protection, a disc may be mounted within an enclosure or a cartridge that is itself inserted into the insertion port of the drive and is then conveyed to a predetermined position. These disc cartridges are well known in the computer arts. The disc cartridge comprises a cartridge housing containing a disc upon which data may be recorded.

[0014] Cartridge Loading

[0015] To protect the disc when the cartridge is external from the drive, the disc cartridge typically includes at least one door or shutter that is normally closed. The cartridge shutter may have one or more locking tabs associated with it. The corresponding disc drive includes a mechanism for opening the door or shutter on the cartridge as the cartridge is pushed into the system. Such a mechanism may comprise a door link that makes contact with a locking tab, thereby unlocking the shutter. As the cartridge is inserted further into the drive, the shutter is opened to partially expose the information recording medium contained therein. This permits a disc hub to be loaded onto a spindle of a motor or other drive mechanism, and permits entry of a read-write head and a bias magnetic into the protective cartridge. The disc, when rotated by the drive mechanism, permits the read-write head to access all portions of the disc media.

[0016] To conserve space in optical storage systems, it is desirable to minimize the size required by the apparatus that loads a disc onto and unloads the disc from a spindle. Conventional loading and unloading devices vary depending upon the type of disc being used. A conventional disc loading and unloading system that uses disc cartridges is typically capable of automatically transporting a disc cartridge from a receiving port onto the spindle. When the disc is no longer required, a conventional disc loading and unloading system automatically unloads the disc from the spindle. A loading device for performing this loading and unloading of the disc is generally constructed so that during disc loading (i.e., when the disc is moved from an ejected position into the player and onto the spindle), the disc is moved horizontally, parallel to the baseplate and turntable, towards the turntable. When the disc has been positioned above the turntable, the disc is lowered vertically, perpendicular to the face of the turntable, onto the spindle. Once on the turntable, a spindle magnet attracts the disc hub fixed to the center of the media, thereby clamping the disc in a rotatable condition for read-write operations.

[0017] When an operator is finished using the disc, the operator initiates an eject operation. The most common solution for ejecting a cartridge and disc from a spindle is the technique used in most Japanese drives. In this type of disc unloading apparatus, a cartridge “box” has four pins at its sides, and the pins ride in tracks in an adjacent sheet metal guide. During disc ejection, the cartridge box lifts the disc straight up and off the spindle. The apparatus then moves the disc horizontally, parallel to the baseplate and turntable, towards the disc receiving port in the front of the player. When the disc is thus lifted from the spindle during the unloading operation, it is necessary to generate sufficient upward force on the cartridge to overcome the magnetic clamping force holding the disc hub on the spindle magnet. The peak upward force required to overcome the magnetic clamping force may be produced by the mechanical operation of an ejection lever or by the activation of an electric ejection system.

[0018] In conventional electric ejection systems, wherein the disc cartridge unloading apparatus vertically lifts the disc cartridge to break the magnetic force between the spindle magnet and the disc hub, the electric ejection motor must generate a large load to effect removal of the disc cartridge. Consequently, when an operator opts to use the electric ejection system, a large motor having a large torque is required to generate sufficient vertical lifting force. Space must be reserved in the system housing to accommodate this large motor, thereby increasing the overall size of the housing for the cartridge-loading apparatus. In addition, the large motor consumes a considerable amount of power.

[0019] It is thus desirable to reduce the complexity of the disc player, while reducing the overall size of the player to facilitate the drive's convenient use in computer applications. In order to be able to receive a 5¼ inch disc cartridge and yet be small enough to be conveniently used in conjunction with a personal computer, optical disc drives must use compact and carefully located mechanical and electrical subsystems. With this in mind, it is desirable to reduce the size of the required ejection motor. One way to effect this result is to reduce the amount of force required to break the magnetic clamping force holding the disc hub on the spindle magnet. By reducing this required force, it is possible to use a smaller ejection motor in the player. It is thus desirable to design a disc loading apparatus wherein the disc is not vertically lifted off of the spindle magnet, but is, rather, “peeled” from the magnet.

[0020] A conventional method that attempts to achieve this peeling action has the turntable and spindle swing down away from the disc. This method is discussed in U.S. Pat. No. 4,791,511 granted to Marvin Davis and assigned to Laser Magnetic Storage International. It remains desirable, however, to design a drive wherein the disc is peeled from the spindle magnet.

[0021] Focus and Tracking Actuation

[0022] In order to attain a precise reading of the information stored on the disc, it is necessary to be able to move the objective lens in both a focusing (i.e., perpendicular to the plane of the disc) or Z direction in order to focus the laser beam to a small point of light on a precise location of the disc to write or retrieve information, and in a tracking (i.e., radial from the center of the disc) or Y direction to position the beam over the exact center of the desired information track on the disc. Focus and tracking corrections may be effected by moving the objective lens in either the direction of the optical axis of the lens for focusing, or in a direction perpendicular to the optical axis for tracking.

[0023] In these systems, the position of the objective lens in the focus and tracking directions is commonly adjusted by control systems. Actuators support the objective lens and convert position correction signals from the feedback control systems into movement of the objective lens. Most commonly, these actuators comprise moving coils, stationary magnets, and a stationary yoke, wherein a magnetic field is produced in an air gap between the yoke and magnets. U.S. Pat. No. 4,568,142 issued to Iguma and entitled “Objective Lens Driving Apparatus” illustrates an actuator of this type wherein the actuator includes rectangular magnets positioned within U-shaped yokes. The yokes are spaced from one another with their north poles opposing, in close enough proximity to one another to form a magnetic circuit. A square-shaped focusing coil is bonded to the outsides of a square-shaped lens frame. Four tracking coils are bonded on the corners of the focusing coil. The ends of the focusing coil are then positioned within the air gaps formed by each of the U-shaped yokes so that the focusing coil straddles the yokes. Because the focusing coil must extend around these “center” or “inner” yoke plates, the coil cannot be wound as tightly as desired and the rigidity of the coil construction is compromised. Further, in this type of closed magnetic circuit design, the majority of coil wire is positioned outside the air gaps, significantly reducing the efficiency of the actuator.

[0024] In most optical systems, the stiffness of the coil in the air gap has to be very high and the coil decoupling resonance frequency should be above 10 kHz, and is most desirably above 25 kHz. In many types of prior actuator designs, large amounts of coil wire in the magnetic air gap are often required to achieve maximum motor performance. To place such a large amount of coil within the air gap and still conform to the limited space constraints of the actuator design, the coil must be wholly or partially “freestanding”, or must be wound on the thinnest bobbin possible. These types of coil configurations have low stiffness and typically decouple at lower frequencies. The dynamic resonance behavior of many actuator designs can also cause the coil to unwind during operation.

[0025] Other actuator designs have used the same magnetic air gap to develop focus and tracking motor forces such that the tracking coil(s) is glued onto the focus(s) coil or vice versa, in an attempt to save parts, space, and weight. In these types of designs, the decoupling frequency of the tracking coil(s) glued onto a freestanding focus coil is typically around 15 kHz, significantly below the preferred decoupling frequency.

[0026] Focus Sensing

[0027] Optical recording and playback systems, such as those utilizing optical memory disks, compact disks, or video disks, require precise focusing of an illuminating optical beam through an objective lens onto the surface of an optical disc. The incident illuminating beam is generally reflected back through the objective lens, and is then used to read information stored on the disc. Subsequent to passing back through the objective lens, a portion of the reflected beam is typically directed to an apparatus designed to gauge the focus of the illuminating beam on the disc. Information extracted from the reflected beam by this apparatus may then be used to adjust the focus of the illuminating beam by altering the position of a movable objective lens relative to the disc.

[0028] A number of techniques for detecting the focus of an illuminating optical beam are known. For example, U.S. Pat. Nos. 4,423,495; 4,425,636; and 4,453,239 employ what has been termed the “critical angle prism” method of determining beam focus. In this method an illuminating beam reflected from a storage disc is made incident upon a detection prism surface which is set very close to a critical angle with respect to the reflected illuminating beam. When the focus of the illuminating beam on the surface of the disc deviates from a desired state, the variation in the amount of optical energy reflected by the detection prism surface may be used to derive a focus error signal used to adjust the focus of the illuminating beam.

[0029] The critical angle prism method generally requires that the orientation of the detection prism surface relative to the reflected illuminating beam be precisely adjusted. This requirement arises as a result of reflectivity characteristic of the detection prism in the neighborhood of the critical angle and makes focus error detection systems based on this method extremely sensitive. The critical angle technique, however, has several disadvantages. First, the focus error signal it produces depends on the light reflection at the interface between the detection prism surface and air. Thus, changes in altitude, which change the index of refraction of the air, can cause false focus readings (offsets) to occur. Also, the critical angle technique is inherently unsuitable for use in differential focus sensing systems.

[0030] Differential systems are increasingly important because they allow cancellation of certain types of noise that can occur in optical disc drives. The critical angle method is unsuited to differential operation for two reasons. First, the transmitted beam produced by the sensing prism is compressed along one axis, making it unsymmetrical with the reflected beam. Symmetry of the two beams is preferred in a differential system to optimize the noise-cancellation properties in varied environments. Second, at the point on the reflectivity curve of a critical angle prism where the intensities of the two beams are balanced, the slope is far too low to produce a useful differential focus error signal.

[0031] A focus detecting apparatus which requires somewhat less precise adjustment of the optical surface on which the reflected illuminating beam is incident, when compared to the critical angle technique is disclosed in U.S. Pat. No. 4,862,442. In particular, the optical surface described therein comprises a dielectric multilayer coating having a reflectivity which varies continuously with respect to the angle of incidence of the reflected illuminating beam. It follows that rotational maladjustment of the surface comprising the multilayer coating will have smaller effect on the value of the focus error signal, but that also the technique will have reduced angular sensitivity. Also, inaccuracies in the focus error signal produced by multilayer dielectric systems may develop in response to relatively slight changes in the wavelength of the reflected illuminating beam. Such sensitivity to wavelength changes is undesirable since the focus error signal is designed to relate solely to the focus of the illuminating beam.

[0032] In addition, certain systems using a dielectric multilayer reflecting surface provide focus error signals having only a limited degree of sensitivity. For example, FIG. 37 of U.S. Pat. No. 4,862,442 shows a particular reflectivity characteristic for a layered dielectric reflecting surface, with the slope of the reflectivity characteristic being proportional to the sensitivity of the focus error signal. The disclosed reflected intensity ranges in value from approximately 0.75 to 0.05 over angles of incidence extending from 42 to 48 degrees. This reflectivity change of approximately 10% per degree produces a focus error signal of relatively low sensitivity.

[0033] Accordingly, a need in the art exists for an optical arrangement characterized by a reflectivity profile which allows generation of a highly sensitive focus error signal relatively immune to changes in altitude and to chromatic aberration, and which is capable of use in differential systems.

[0034] Seek Actuation

[0035] Optical data storage systems that utilize a focused laser beam to record and instantaneously playback information are very attractive in the computer mass storage industry. Such optical data storage systems offer very high data rates with very high storage density and rapid random access to the data stored on the information medium, most commonly an optical disc. In these types of optical disc memory systems, reading and writing data is often accomplished using a single laser source functioning at two respective intensities. During either operation, light from the laser source passes through an objective lens which converges the light beam to a specific focal point on the optical disc. During data retrieval, the laser light is focused on the recording medium and is altered by the information of the data storage medium. This light is then reflected off the disc, back through the objective lens, to a photodetector. It is this reflected signal that transmits the recorded information. It is thus especially important that, when information is being written to or read from the memory, the objective lens, and the exiting focused beam, be precisely focused at the center of the correct track so that the information may be accurately written and retrieved.

[0036] In order to attain a precise reading of the information stored on the disc, it is necessary to be able to move the objective lens in both a focussing (i.e., perpendicular to the plane of the disc) or Z direction in order to focus the laser beam to a small point of light on a precise location of the disc to write or retrieve information, and in a tracking (i.e., radial) or Y direction to position the beam over the exact center of the desired information track on the disc. Focus and tracking corrections may be effected by moving the objective lens in either the direction of the optical axis of the lens for focusing, or in a direction perpendicular to the optical axis for tracking.

[0037] In these systems, the position of the objective lens in the focus and tracking directions is commonly adjusted by control systems. Actuators support the objective lens and convert position correction signals from the feedback control systems into movement of the objective lens. As will be appreciated, failure to focus the light on a small enough area of the medium will result in too large a portion of the disc being used to store a given amount of information, or in too broad an area of the disc being read. Likewise, the failure to precisely control the tracking of the laser light will result in the information being stored in the wrong location, or in information from the wrong location being read.

[0038] In addition to translation along the Z axis to effect focusing, and translation along the Y axis to effect tracking, there are at least four additional motion modes for the actuator, each of which reduces the accuracy of the reading and writing operations and is thus undesirable during normal operation of the system. These undesirable modes of motion are rotation about the X axis (an axis orthogonal to both the X direction and the Z direction), or pitch; rotation about the Z axis, referred to as yaw; rotation about the Y axis, called roll; and linear motion along the X axis, or tangential translation. Motion in these directions is often caused by motor and reaction forces acting on the carriage and/or actuator. These modes typically produce undesired movement during tracking or focussing operations which subsequently affects the alignment of the objective lens relative to the optical disc.

[0039] Anamorphic, Achromatic Prism System

[0040] Optical disc systems often employ an anamorphic prism for adjustment of laser beam ellipticity, for the removal of laser beam astigmatism, and/or for beam steering. References such as U.S. Pat. No. 4,333,173 issued to Yonezawa, et al., U.S. Pat. No. 4,542,492 issued to Leterme, et al. and U.S. Pat. No. 4,607,356 issued to Bricot, et al. describe using simple anamorphic prisms for beam shaping in optical disc applications.

[0041] Frequently, the anamorphic prism systems have an embedded thin film to reflect some or all of a returning beam (reflected from optical media) to a detection system. U.S. Pat. No. 4,573,149 to Deguchi, et al. describes the use of thin films to reflect a return beam to detection systems. Furthermore, the entrance face of the anamorphic prism is often used to reflect the returning beam to a detection system as described in U.S. Pat. Nos. 4,542,492 and 4,607,356. Often, it is advantageous to have multiple detection channels. For instance, in optical disks, one detector may provide data signals and another detector may provide control signals such as tracking and/or focus servo signals.

[0042] A typical problem with conventional prisms is that the anamorphic prism suffers from chromatic dispersion which can result in lateral chromatic aberration. In other words, when the wavelength of the light source changes, the resulting angles of refraction through the anamorphic prism also change. These changes result in a lateral beam shift when the beam is focused onto optical media such as an optical disc. In optical disc systems, a small shift in the beam may cause erroneous data signals. For instance, if the shift is sudden and in the data direction, the beam may skip data recorded on the optical disc.

[0043] If the light source (e.g., a laser) were truly monochromatic, the chromatic aberration in the prism would not cause a problem. Several factors, however, often cause the laser spectrum to change. For instance, most laser diodes respond with a change in wavelength when the power increases. In magneto-optic disc systems, an increase of power occurs when pulsing the laser from low to high power to write to the optical disc, as is well understood in the art. This increase in laser power often causes a wavelength shift of around 1.5 to 3 nanometers (nm) in conventional systems. Most laser diodes also respond to a change in temperature with a change in the wavelength. Additionally, random “mode-hopping” can cause unpredictable wavelength changes commonly ranging from 1-2 nanometers. RF modulation is often applied to laser diodes operating at read power in order to minimize the effect that “mode-hopping” has on the system. The RF modulation, however, increases the spectral bandwidth and can change the center frequency. Moreover, RF modulation is not generally used when the laser is operating at write power. In a non-achromatic system, a sudden change in the wavelength of the incident light typically results in a lateral beam shift in the focused spot of up to several hundred nanometers. A lateral beam shift of this magnitude could cause significant errors in the data signal.

[0044] Using multi-element prism systems to correct chromatic dispersion is known in the art of optical design. Textbooks such as Warren J. Smith, Modern Optical Engineering, McGraw-Hill, 1966, pp. 75-77 discuss this idea. Furthermore, some optical disc systems use multi-element anamorphic prism systems which are achromatic. Typical existing multi-element prism systems, however, require the multiple prism elements to be separately mounted. Mounting the multiple elements increases the expense and difficulty of manufacturing because each element must be carefully aligned with respect to the other elements in the system. Small deviations in alignment can cause significant variations in function. This also complicates quality control. Other existing mult-element prism systems have attached elements to form a unitary prism, but these prism systems require that the prism material of each prism be different in order for the system to be achromatic. Finally, existing systems which are achromatic do not provide return beam reflections to multiple detection systems.

[0045] Data Retrieval—Transition Detection

[0046] For many years, various types of recordable and/or erasable media have been used for data storage purposes. Such media may include, for example, magnetic tapes or disks in systems having a variety of configurations.

[0047] Magneto-optical (“MO”) systems exist for recording data on and retrieving data from a magnetic disc. The process of recording in a magneto-optical system typically involves use of a magnetic field to orient the polarity of a generalized area on the disc while a laser pulse heats a localized area, thereby fixing the polarity of the localized area. The localized area with fixed polarity is commonly called a pit. Some encoding systems use the existence or absence of a pit on the disc to define the recorded data as a “1” or “0”, respectively.

[0048] When recording data, a binary input data sequence may be converted by digital modulation to a different binary sequence having more desirable properties. A modulator may, for example, convert m data bits to a code word with n modulation code bits (or “binits”). In most cases, there are more code bits than data bits, that is m<n.

[0049] The density ratio of a given recording system is often expressed according to the equation (m/n)×(d+1), where m and n have the definitions provided above, and d is defined as the minimum number of zeroes occurring between ones. Thus, the RLL 2/7/1/2 code has, according to the above equation, a density ratio of 1.5, while the GCR 0/3/8/9 code has a density ratio of 0.89.

[0050] For reading data in an MO system, a focused laser beam or other optical device is typically directed at the recording surface of a rotating optical disc such that the laser beam can selectively access one of a plurality of tracks on the recorded surface. The rotation of the laser beam reflected from the recorded surface may be detected by means of Kerr rotation. A change in Kerr rotation of a first type, for example, represents a first binary value. A change in Kerr rotation of a second type represents a second binary value. An output signal is generating from the first and second binary values occurring at specified clock intervals.

[0051] Although there has been a continual demand for disc systems capable of storing increasingly higher data densities, the ability to achieve high data storage densities has met with several limitations. As a general matter, the reasonable upper limit for data density is determined in part by reliability requirements, the optical wavelength of laser diode, the quality of the optical module, hardware cost, and operating speed. Maximum data densities are also affected by the ability to reject various forms of noise, interference, and distortion. For example, the more densely data is packed, the more intersymbol interference will prevent accurate recovery of data. Moreover, because the technology for many intermediate and high performance optical disc drives has been limited by downward compatibility constraints to older models, signal processing techniques have not advanced as rapidly as they might otherwise have.

[0052] When attempting to recover stored data, existing read channels of magneto-optical and other types of disc drives commonly suffer from a number of problems due to the unintended buildup of DC components in the read signal. One cause of DC buildup results from the recording of unsymmetrical data patterns over a number of bytes or data segments. A symmetrical data pattern may be considered as one having an average DC component of zero over a region of interest. Because sequences of recorded bits may be essentially random in many modulation codes, however, localized regions of recorded data having particular patterns of 1's and 0's will produce an unsymmetrical read signal having unwanted DC components. Because the data patterns vary over time, the level of DC buildup will also vary, causing wander of the DC baseline, reduction of threshold detection margins, and greater susceptibility to noise and other interference.

[0053] Undesired DC buildup is also caused by variance in pit size due to thermal effects on the writing laser or the storage medium. As the writing laser heats up, for example, the spot size may increase leading to wider pits. When the recorded pits are read, variations in pit size will cause an unsymmetrical input signal having DC components. Variation in pit size not only causes undesired DC buildup but also causes the relative locations of the data to appear shifted in time, reducing the timing margin and leading to possible reading errors.

[0054] Various attempts have been made to overcome the described problems. For example, various tape drive systems commonly use a DC-free code such as a 0/3/8/10 code, otherwise referred to simply as an 8/10 code. Because an 8/10 code requires 10 stored bits to yield 8 data bits, however, it is only 80% efficient which is a drawback when attempting to record high data densities.

[0055] Another method for handling DC buildup involves the use of double differentiation. This method typically involves detection of the peaks of a first derivative of the input signal by detecting zero-crossings of the second derivative of the input signal. Thus, the DC components are effectively filtered out. One drawback of this method is that differentiation or double differentiation can cause undesirable noise effects. A second drawback is that the method may decrease the timing margin to unacceptably low levels (e.g., by as much as 50 percent).

[0056] In another method for addressing DC buildup, the data to be stored is randomized prior to recording such that none of the data patterns repeat over a data sector. This method, however, may not be acceptable by ISO standards and may lack downward compatibility with previous disc drive systems. As a further drawback to this method, de-randomizing the data may be complex.

[0057] Yet another method for controlling DC buildup involves the use of so-called resync bytes between data segments. This method generally involves the examination and manipulation of data before it is recorded in order to minimize DC buildup upon readback. Before recording, two consecutive data segments are examined to determine if the patterns of 1's and 0's are such as to cause positive DC, negative DC, or no DC components when read back. If for example, two consecutive data segments have the same DC polarity, one of the data segments is inverted prior to being recorded on the medium. In order to stay within the constraints of the particular encoding system, however, a resync byte between the segments may need to be written so that the pattern of contiguous bits and of flux reversals is proper. A drawback of such a method is that it will not necessarily reduce all DC buildup, and time constants must be determined such that the predictable DC buildup will not affect performance. Further, the method requires additional overhead including the examination of data segments to determine their relative polarity.

[0058] It would therefore be advantageous to have a method and device for reading stored data from a medium without suffering the undesirable effects of DC buildup, without creating unacceptable levels of noise or significantly reducing timing margins, without the requirement of large amounts of overhead or de-randomizing algorithms, and while providing high data storage efficiency.

[0059] Data Storage and Other Aspects of Data Retrieval

[0060] Recordable/erasable optical disks are currently available for use as data storage media. Magneto-optical recording is the technique commonly used to store the data on and/or retrieve the data from the disc. During recording, a magnetic field orients the polarity of a generalized area on the disc, while a laser pulse heats a localized area thereby fixing the polarity of the smaller area. The localized area with fixed polarity is commonly called a pit. Some encoding systems use the existence or absence of a pit on the disc to define the recorded data as a “1” or “0”, respectively. The most commonly used encoding system for this pit-type recording is the run length limited (RLL) 2,7 code because it gives the highest data-to-pit ratio. This type of recording, however, does not lead to higher density because amplitude and timing margins deteriorate very rapidly as frequency is increased.

[0061] There is disclosed a method for moving a carriage assembly from an initial position to a target position relative to a storage medium rotating at a circumferential velocity. The method comprises the steps of determining a first radial distance between the initial position and a center of the storage medium, determining a second radial distance between the target position and the center of the storage medium, determining a circumferential distance between the initial position and the target position, determining an initial circumferential velocity of the storage medium, calculating-a velocity trajectory relative to the first radial distance, the second radial distance, the circumferential distance, and the initial circumferential velocity, and moving the carriage assembly from the initial position to the target position substantially at the velocity trajectory. The velocity trajectory is calculated such that the carriage assembly will arrive radially and circumferentially at the target position at substantially the same time.

[0062] Additionally, a target circumferential velocity may be determined. The rotation of the storage medium may be changed from the initial circumferential velocity to the target circumferential velocity, and the velocity trajectory is further related to the target circumferential velocity.

[0063] Further objects of the present invention together with additional features contributing thereto and advantages accruing therefrom will be apparent from the following description of certain preferred embodiments of the invention which is shown in the accompanying drawing, wherein:

[0064] FIG. 1 is an isometric view of an optical disk drive embodying the present invention;

[0065] FIG. 2 is a top view of the disk drive of FIG. 1, with the housing of the drive removed;

[0066] FIG. 3 is a cross-sectional view of the disk drive of FIG. 1, taken in the direction of arrows 3—3 in FIG. 1;

[0067] FIG. 4A is a top view of an optics module of the disk drive of FIG. 1;

[0068] FIG. 4B is a diagram of the optical path of the disk drive of FIG. 1;

[0069] FIG. 5 is a system block diagram of the electronics of the disk drive of FIG. 1;

[0070] FIG. 6 is another isometric view of a disc drive with a disc cartridge about to be inserted therein;

[0071] FIG. 7 is an exploded isometric view of the disc drive of FIG. 6, depicting the major subassemblies thereof;

[0072] FIGS. 8A-8B are isometric views of the baseplate depicted in FIG. 7;

[0073] FIG. 9 is a top elevational view of the drive of FIG. 6 with some features removed to better show the tiller, the tiller-driving gears, the motor that drives these gears, and the operative relationship between these features;

[0074] FIGS. 10A-10F are elevational and isometric views of a tiller;

[0075] FIGS. 11A-11C comprise elevational and isometric views of a left slider;

[0076] FIGS. 12A-12E are elevational and isometric views of a right slider;

[0077] FIG. 13 is a top plan view of the parking arm in two positions, one drawn in phantom, showing its action of parking the carriage at the back of the drive while the drive is at rest;

[0078] FIG. 13A is a perspective view of the disk drive of FIG. 1, illustrating in particular the fine actuator assembly carriage which supports the optics used to focus the laser beam on the data track of the optical disk;

[0079] FIGS. 14A-14C comprise elevational and isometric views of a parking arm;

[0080] FIGS. 15A and 15B are isometric views of a cartridge receiver;

[0081] FIGS. 16A and 16B are elevational views, during insertion of a disc cartridge, of the drive of FIG. 6 with some features removed to better show the trip lug on the right door link, the latch, and the operative relationship between these features;

[0082] FIGS. 17A and 17B are isometric views of a latch that holds the cartridge receiver in the up position;

[0083] FIG. 18 is an isometric view of a bias coil assembly clamp;

[0084] FIG. 19 is an isometric view of a bias coil assembly;

[0085] FIG. 20 is an exploded isometric view of the major components comprising the bias coil assembly;

[0086] FIG. 21 is an isometric view of a pivot bar or rail that rotatably supports the bias coil assembly;

[0087] FIG. 22 is an isometric view of the bias coil assembly flexure to which the bias coil assembly is mounted and which is, in turn, mounted to the pivot bar depicted in FIG. 21;

[0088] FIG. 23 is an elevational view of the right side of the cartridge receiver and the cartridge just before initiation of an cartridge-eject cycle, depicting the disc mounted in operating position on the spindle;

[0089] FIG. 24 is an elevational view of the right side of the cartridge receiver and the cartridge during the cartridge-eject cycle, depicting the cartridge being tipped and the disc being peeled off the spindle;

[0090] FIG. 25 is an elevational view of the right side of the cartridge receiver and the cartridge during the cartridge-eject cycle, depicting the cartridge loading system in the up position and the disc starting to be ejected from the disc drive;

[0091] FIG. 26 is a schematic perspective view of an actuator in accordance with the present invention;

[0092] FIG. 27 is a perspective view of the lens holder for the actuator of FIG. 26;

[0093] FIG. 28 is a perspective view of the actuator of FIG. 26 within a magnetic field housing as employed in conjunction with a recording system;

[0094] FIG. 29 is a top plan view of the recording system of FIG. 28;

[0095] FIG. 30 is a right side elevational view of the recording system of FIG. 28;

[0096] FIG. 31 is a front elevational view of the recording system of FIG. 28;

[0097] FIG. 32 is a schematic perspective view illustrating the magnetic fields produced by the magnet pairs of the actuator of FIG. 26;

[0098] FIG. 33 is a perspective view of the focus coils and permanent magnets of the actuator of FIG. 26;

[0099] FIG. 34 is a schematic cross-sectional view of the focus coils and permanent magnets of the actuator of FIG. 26 taken along section lines 34—34 of FIG. 33 illustrating the focus forces acting on the actuator;

[0100] FIG. 35 is a schematic cross-sectional view of the tracking coil and permanent magnets of the actuator of FIG. 26 illustrating the tracking forces acting on the actuator;

[0101] FIG. 36 is a block diagrammatic presentation of a preferred embodiment of the beam focus sensing apparatus of the present invention;

[0102] FIG. 37 is a magnified top cross-sectional view of a differential version of the inventive beam separation module (FTR prism);

[0103] FIG. 38 is an illustrative front view of the first and second quad detectors included within the inventive focus sensing apparatus;

[0104] FIG. 39 is a graph showing the reflectivity of the FTR prism as a function of the angle of incidence of the servo beam;

[0105] FIG. 40 is a graph of the value of a differential focus error signal generated by a preferred embodiment of the apparatus of the present invention as a function of the position of the objective lens relative to an optical disc;

[0106] FIG. 41 schematically illustrates an exemplary optical read/write system in which the carriage and actuator assembly of the present invention may be used;

[0107] FIG. 42 is a perspective view of the carriage and actuator assembly;

[0108] FIG. 43 is an exploded view of the carriage and actuator assembly;

[0109] FIG. 44 is an exploded view of the actuator;

[0110] FIG. 45 is a schematic top view illustrating the coarse tracking forces acting on the assembly;

[0111] FIG. 46 is a side schematic view further illustrating the coarse tracking forces;

[0112] FIG. 47 is an exploded view which illustrates the focus forces acting on the actuator;

[0113] FIG. 48 is an exploded view which illustrates the fine tracking forces acting on the actuator;

[0114] FIG. 49A is a schematic top view illustrating the symmetry of coarse tracking forces in the horizontal plane;

[0115] FIG. 49B is a schematic side view illustrating the symmetry of coarse tracking forces in the vertical plane;

[0116] FIG. 50A is a schematic top view illustrating the symmetry of fine tracking forces in the horizontal plane;

[0117] FIG. 50B is a schematic end view illustrating the alignment of the net fine tracking force with the center of mass of the fine tracking motor;

[0118] FIG. 51A is a schematic top view illustrating the symmetry of fine tracking reaction forces in the horizontal plane;

[0119] FIG. 51B is a schematic end view illustrating the alignment of the net fine tracking reaction force with the center of mass of the fine tracking motor;

[0120] FIG. 52A is a schematic side view illustrating the symmetry of focus forces in the horizontal plane;

[0121] FIG. 52B is a schematic end view illustrating the alignment of the net focus force with the optical axis of the objective lens;

[0122] FIG. 53A is a schematic side view which illustrates the symmetry of focus reaction forces in the horizontal plane;

[0123] FIG. 53B is a schematic end view which illustrates the alignment of the net focus reaction force with the optical axis of the objective lens;

[0124] FIG. 54 is a schematic top view illustrating the flexure forces and fine motor reaction forces generated in response to the flexure forces;

[0125] FIG. 55A is a schematic side view which illustrates the symmetry of carriage suspension forces in the horizontal plane;

[0126] FIG. 55B is a schematic end view illustrating the alignment of the net carriage suspension force with the optical axis of the objective lens;

[0127] FIG. 56A is a schematic top view which illustrates the symmetry of friction forces in the horizontal plane;

[0128] FIG. 56B is a schematic side view illustrating the alignment of the friction forces with the center of mass of the carriage;

[0129] FIG. 57 is a schematic end view which illustrates the net inertial forces acting at the center of mass of the fine motor and center of mass of the carriage in response to a vertical acceleration;

[0130] FIG. 58A is a schematic side view illustrating the alignment of the net inertial force of the fine motor with the optical axis of the objective lens;

[0131] FIG. 58B is a schematic side view illustrating the alignment of the net inertial force of the carriage with the optical axis of the objective lens;

[0132] FIG. 59A is a schematic top view which illustrates the inertial forces acting on components of the carriage and actuator assembly for horizontal accelerations;

[0133] FIG. 59B is a schematic top view illustrating the net inertial forces for horizontal accelerations;

[0134] FIG. 60A is a schematic end view which illustrates the fine motor and carriage inertial forces for accelerations above the flexure arm resonance frequency;

[0135] FIG. 60B is a schematic end view which illustrates the fine motor and carriage inertial forces for accelerations below the flexure arm resonance frequency;

[0136] FIGS. 61A-61D are diagrams illustrating the relationship between the fine tracking position versus fine motor current;

[0137] FIGS. 62A-62C illustrate the effects of asymmetrical focus forces acting on the assembly;

[0138] FIG. 63 illustrates an alternative embodiment of a carriage and actuator assembly;

[0139] FIG. 64 illustrates the operation of the actuator to move the lens holder in a focusing direction;

[0140] FIG. 65 illustrates the operation of the actuator to move the lens holder in a tracking direction;

[0141] FIG. 66 depicts a simple anamorphic prism and illustrates the effect of chromatic aberration in the prism;

[0142] FIG. 67 depicts an existing multi-element anamorphic prism system;

[0143] FIG. 68 depicts an exemplary air-spaced prism system according to the present invention;

[0144] FIGS. 69 and 69A depict one embodiment of an air-spaced, multi-element prism system of the present invention;

[0145] FIGS. 70, 70A, and 70B depict side, bottom, and top plan views, respectively, of the plate prism of the prism system embodiment depicted in FIG. 69;

[0146] FIGS. 71, 71A, and 71B depict side, top, and bottom plan views, respectively, of the trapezoidal prism of the embodiment of the prism system shown in FIG. 69;

[0147] FIGS. 72 and 72A depict a side view and a plan view of one optical surface, respectively, of an embodiment of the chromatic correcting prism of the prism system embodiment shown in FIG. 69;

[0148] FIG. 73 depicts an alternative embodiment of an air-spaced, multi-element prism system of the present invention;

[0149] FIGS. 74, 74A, and 74B depict side, top and bottom plan views, respectively, of the quadrilateral prism of the alternative embodiment illustrated in FIG. 73;

[0150] FIG. 75 is a block diagram showing an optical data storage and retrieval system;

[0151] FIG. 76 is a series of sample waveforms;

[0152] FIGS. 77A and 77B are waveform diagrams of a symmetrical and unsymmetrical input signal, respectively;

[0153] FIG. 78 is a block diagram of a read channel;

[0154] FIG. 79A is a more detailed block diagram of various stages of a read channel;

[0155] FIG. 79B is a detailed circuit diagram of a partial integrator stage;

[0156] FIGS. 80A-80E are frequency response diagrams of various stages of a read channel;

[0157] FIG. 80F is a plot of group delay for a combination of stages in a read channel;

[0158] FIGS. 80G(1)-80G(4) are waveform diagrams showing signal waveforms at various stages in the read channel;

[0159] FIG. 81 is a block diagram of a peak detection and tracking circuit;

[0160] FIG. 82 is a schematic diagram of the peak detection and tracking circuit of FIG. 81;

[0161] FIG. 83 is a waveform diagram showing tracking by a threshold signal of the DC envelope of an input signal;

[0162] FIGS. 84A-84D are diagrams showing exemplary waveforms at various points in a read channel;

[0163] FIG. 85 is a block diagram showing the optical data storage and retrieval system;

[0164] FIG. 86 is a series of waveforms showing uniform laser pulsing under a pulsed GCR format and nonuniform laser pulsing under an RLL 2,7 format;

[0165] FIG. 87 is a series of waveforms showing laser pulsing for various data patterns adjusted by the write compensation circuit;

[0166] FIG. 88 is a schematic diagram showing the write compensation circuit;

[0167] FIG. 89 is a series of waveforms showing laser pulsing for amplitude asymmetry correction;

[0168] FIG. 90 is a schematic diagram showing the amplitude asymmetry correction circuit;

[0169] FIG. 91 is a block diagram showing the basic relationship of elements of the pulse slimming means;

[0170] FIG. 92 is a series of waveforms showing threshold adjustments by the dynamic threshold circuit;

[0171] FIG. 93 is a schematic diagram for the dynamic threshold circuit;

[0172] FIG. 94 is a schematic block diagram of an optical data storage and retrieval system incorporating downward compatibility;

[0173] FIG. 95 is a diagram of the track layout of the high-density optical disks;

[0174] FIG. 96 is a diagram of the sector format of the high-density optical disks;

[0175] FIG. 97 is a block diagram in more detail showing the read/write circuitry of FIG. 94;

[0176] FIG. 98 is a table depicting, for each of the 21 zones in the preferred format of the high-density optical disc, the tracks within the zone, the number of sectors per track within the zone, the total number of sectors in the zone, and the write frequency of the data recorded in the zone;

[0177] FIG. 99 provides a table of the equations used to compute the CRC bits of the ID field;

[0178] FIG. 100A is the first half of a table (Hex 00 to 7F) showing how the 8-bit bytes in the three address fields and in the data field, except for the resync bytes, are converted to channel bits on the disc;

[0179] FIG. 100B is the second half of a table (Hex 80 to FF) showing how the 8-bit bytes in the three address fields and in the data field, except for the resync bytes, are converted to channel bits on the disc;

[0180] FIGS. 101A-119C are schematic diagrams of the electronic circuitry in a preferred embodiment of the invention;

[0181] FIG. 120 is an isometric view of a mechanical isolator and a pole piece in accordance with a first preferred embodiment;

[0182] FIG. 121 is an isometric view of the mechanical isolator in a second preferred embodiment;

[0183] FIG. 122 is a state diagram of the read mode firmware modeule employed in conjunction with the present invention;

[0184] FIG. 123 is a state diagram of the write mode firmware modeule utilized in conjunction with this invention;

[0185] FIG. 124 shows a Nyquist diagram of the focus loop transfer function for selected amounts of closed loop peaking;

[0186] FIG. 125 is a graphical representation of magnitude responses of the focus loop transfer function for open and closed conditions;

[0187] FIG. 126 is a graphical representation of phase responses of the focus loop transfer function for open and closed conditions;

[0188] FIG. 127 illustrates the magnitude response curves for focus compensation transfer functions; and

[0189] FIG. 128 shows the phase response curves for focus compensation transfer functions.

[0190] System Overview: Main Optical, Electrical, and Mechanical Components

[0191] Referring initially to FIG. 1, there is shown an optical disk drive 10 having a housing 14. Disk drive 10 plays and/or records on a disk (not shown here) that is housed in removable disk cartridge 12. Alternatively, the disk could be contained within the housing 14 of disk drive 10.

[0192] Referring now to FIGS. 2 and 3, wherein FIG. 2 shows a top view of the drive 10 with the housing 14 removed to reveal certain important mechanical, electrical, and optical components of the drive 10. FIG. 3 is a cross-sectional view of the drive 10, taken in the direction of section lines 3—3 of FIG. 1. In FIG. 2 there is shown a base plate 16, a spindle 17, a linear actuator assembly 20, an objective lens carriage assembly 22, an optics module 24, a drive circuit board 26, and a flexible circuit connector 28. FIG. 3 shows a main circuit board 30, a spindle motor 18, an optics module circuit board 27, and the drive circuit board 26.

[0193] In brief, the base plate 16 acts as a base for the other components of the drive 10, positioning and aligning the components with respect to each other. Preferably the base plate 16 is made of cast steel for low cost.

[0194] As shown in FIG. 2, the linear actuator assembly 20 includes a pair of linear voice coil actuators 23. Each voice coil actuator 23 consists of a rail 34 that is rigidly attached to the base plate 16. The rails 34 are substantially parallel to each other. Adjacent each rail 34 is a pole piece 32. Surrounding a portion of each pole piece 32 is one of the actuator coils 23. Each actuator coil 23 is attached to an opposite portion of lens carriage assembly 22, so that when the coils 23 are selectively energized, the lens carriage assembly 22 moves along the rails 34. The actuator coils 23 are driven by signals from the drive circuit board 26, which result in linear motion of the lens carriage assembly 22 relative to the optics module 24, and relative to a respective disk (not shown here) inserted in the drive 10. In this manner, the lens carriage assembly 22 enables coarse tracking of the disk.

[0195] The optics module 24 and lens carriage assembly 22 together contain the principle optics of the drive 10. Optics module 24 is rigidly attached to the base plate 16, and contains a laser, various sensors, and optics (not shown here). In operation, the laser directs a beam (shown later) from the optics module 24 towards the lens carriage assembly 22, and optics module 24 in turn receives a return beam (shown later) from the lens carriage assembly 22. The lens carriage assembly 22 is attached to the linear actuator assembly 20, as described above. The lens carriage assembly 22 contains a pentaprism (shown later), an objective lens (shown later), servomotors (not shown) for focusing the objective lens, and servomotors (shown later) for fine adjustments of the objective lens position relative to the position of the linear actuator assembly 20 and to the inserted disk, to enable fine tracking of the disk. Electrical information and control signals are transferred between the lens carriage assembly 22 and the main circuit board 30 on the one hand, and the drive circuit board 26 on the other hand by means of the flexible circuit connector 28.

[0196] The optics module circuit board 27 contains a laser driver and preamplifiers (shown later). The drive circuit board 26 controls the spindle motor 18, the linear coil actuators 23 of the linear actuator assembly 20, and the servomotors of the lens carriage assembly 22. The drive circuit board 26 is controlled by the main circuit board 30. The main circuit board 30 includes most of the electronic components that various design considerations (e.g., noise reduction, EMI and power loss) do not require to be located on the optics module circuit board 27, or the drive circuit board 26.

[0197] The spindle motor 18 is rigidly attached to the base plate 16. Motor 18 directly drives the spindle 17, which in turn spins the disk.

[0198] Optics: Optics module and Objective Lens Assembly

[0199] With reference now to FIG. 4A, there is shown a top cross-sectional view of the optics module 24. Optics module 24 includes a housing 40, a semiconductor laser diode 42, a collimating lenses 44, an achromatizing prism 46, an anamorphic expansion prism 48, a leaky beamsplitter 49, a DFTR prism 50, cylinder lenses 51, a read lens 52, a microprism 54, servodetector sensors 56 and 58, a forward sensor 60, and a data detector sensor 62. These elements are also shown in FIG. 4B, which presents a diagram of the optical path followed by a laser beam 64. FIG. 4B shows the optical elements of the optics module 24 in conjunction with a pentaprism 66 and an objective lens 68 of the lens carriage assembly 22. For ease of illustration in FIG. 4B, a portion 70 of the laser beam 64 between the pentaprism 66 and the objective lens 68 is shown to lie in the same plane as the portions of the laser beam 64 that pass through the optics module 24. Actually, the pentaprism 66 is positioned to direct the laser beam portion 70 perpendicular relative to the portions of the laser beam 64 that pass through the optics module 24.

[0200] With continuing reference to FIG. 4B, it is to be understood that in operation the laser beam 64 is a collimated beam produced by the lenses 44 from the diverging beam emitted by the laser diode 42. The beam 64 transmits through the prisms 46 and 48, transmits through the beamsplitter 49 and exits the optics module 24 toward the lens carriage assembly 22. There it passes through the pentaprism 66 and is focused onto the disk surface by the objective lens 68.

[0201] Upon reflection from the disk, a reflected portion of the laser beam 64 returns through the objective lens 68 and the pentaprism 66 to re-enter the optics module 24. A first portion of the beam 64 reflects on the beamsplitter interface between the prism 48 and the beamsplitter 49, transmits through and is focused by the read lens 52, and enters the microprism 54. There the beam is split into two parts according to polarization, and each part is detected by a separate element of data detector sensor 62.

[0202] A second portion of the beam 64 transmits through the beamsplitter 49 and is internally reflected in the anamorphic prism 48. This second portion of the beam 64 exits the anamorphic prism 48 and enters the DFTR prism 50. There this second portion of the beam 64 is divided into two parts, which are each focused by the cylinder lenses 51 onto the respective surfaces of corresponding servo sensors 56 and 58. In response, the sensors 56 and 58 generate signals that are directed to the optics module circuit board 27, where the signals are used to generate tracking and focus error signals.

[0203] Electronic Systems: Main Circuit Board, Drive Circuit Board, and Optics Module Circuits

[0204] Referring now to FIGS. 1, 2, 4A, and 5, there is shown in FIG. 5 a system block diagram of the electronic subsystems of the drive 10 in which a block 80 encompasses a read sensor preamplifier 82, a laser driver 84, and servo sensor preamplifiers 86. As represented by FIGS. 4A and 5, the read sensor preamplifier 82 is connected to the data detector sensor 62, and amplifies the signal generated by data detector 62. Similarly, the servo sensor preamplifiers 86 are connected to the servo detectors 56 and 58, and amplifies the signal generated by servo detectors 56 and 58. The laser diode 42 is connected to the laser driver 84, which provides signals that drive the laser 42. The subsystems 82, 84, and 86 of the block 80 are grouped together on the optics module circuit board 27 that is positioned in close proximity to the optics module 24. This minimizes the distance that signals must travel from the sensors 62 to the preamplifier 82, and from the sensors 56 and 58 to the preamplifiers 86, to reduce the adverse effect of noise on these signals. Since the signal that the laser driver 84 generates to drive laser diode 42 is of a relatively high frequency, good design practice requires the laser driver 84 to be positioned close to laser diode 42.

[0205] Block 88 of FIG. 5 encompasses a spindle motor Interface 90, a mechanical subassembly (MSA) interface 92, a position sensor interface 94, and an assembly of switches and displays 96. The components 90, 92, 94, and 96 of block 88 all reside on the drive circuit board 26. The spindle motor interface 90 controls the spindle motor 18. The MSA interface 92 interfaces with the various displays and switches 96, including the front panel displays, the eject circuit, and switches related to the disk cartridge 12. Position sensor interface 94 connects to the coil actuators 23 of actuator assembly 20, which are powered by power amplifiers 102.

[0206] The remaining subsystems of the system block diagram of FIG. 5 reside on the main circuit board 30 illustrated in FIG. 3. These subsystems include an analog read channel 100, a encoder/decoder 104, an SCSI chip set 106, a buffer dram 108, and a GLIC interface 110 and an associated EEPROM 112. The main circuit board 30 also includes an analog interface circuit 114, a Digital Signal Processor (DSP) 116, an embedded controller 118 and its associated RAM/EPROM 120. Note that for optical drives 10 that are MO recordable drives, power amps 102 also drive a bias coil 122.

[0207] Cartridge Loading Apparatus

[0208] Referring first to FIG. 6, there is shown a magnetic disc storage system, generally designated 1-10. FIG. 6 depicts a replaceable disc cartridge 1-13 positioned for insertion into the disc drive 1-10 incorporating the cartridge loading and unloading apparatus of the instant invention. The disc drive 1-10 includes a bottom housing 1-16 and a face plate 1-19. The face plate 1-19 includes a disc receiving port 1-22, a drive activity indicator light 1-25, and an ejection button 1-28.

[0209] The outer housing of the disc cartridge 1-13, which is of a conventional type, includes an upper planar surface 1-31 and a lower planar surface 1-32 which is shown in FIG. 25. The disc cartridge 1-13 also has a forward-facing label end 1-34. In the preferred embodiment, the forward-facing label end 1-34 of the disc cartridge 1-13 remains visible to a user while the disc cartridge 1-13 is inserted in the disc drive 1-10. Side walls, for example side wall 1-37, extend between the upper planar surface 1-31 and the lower planar surface 1-32, and the cartridge further comprises a rear wall 1-38 extending between the upper planar surface 1-31 and the lower planar surface 1-32 parallel to the forward-facing label end 1-34. Near the label end 1-34 of the side walls 1-37 are channels 1-40 to accommodate cartridge locating pins 1-43 (FIGS. 8A-8B) located on a base plate 1-46.

[0210] The disc cartridge 1-13 also includes a cartridge door or shutter 1-49. The shutter 1-49 is spring-loaded in a closed position (FIGS. 6, 7, and 16). When the shutter 1-49 is open, it rests in a recessed portion 1-52 of the upper planar surface 1-31. Since the disc drive 1-10 of the preferred embodiment reads two-sided disc cartridges 1-13, a similar shutter and recessed portion exists on the lower planar surface 1-32, but these features are not shown in the figures. The shutter typically has a shutter latch 1-55 (not shown) on the rear wall 1-38 of the disc cartridge 1-13.

[0211] Protected within the disc cartridge 1-13 is a disc 1-14 (FIGS. 23-25), having a metallic disc hub 1-15. As known in the relevant arts, the disc 1-14 may be formed as a rigid substrate having a magnetic material coating thereon. Embedded in the magnetic material coating are tracks in the form of concentric or spiraling rings. The magnetic coating may be on either one or both surfaces of the rigid substrate, and the coating enables data to be magnetically recorded on the disc 1-14 by magnetic transducers, typically referred to as heads. At the center of the rigid substrate is the metallic disc hub 1-15.

[0212] Referring now to FIG. 7, the primary component groups within the disc drive 1-10 of the instant invention include the following. There is the bottom housing 1-16 in which the base plate 1-46 rests. In FIG. 7, a spindle motor 1-61 is shown mounted on the base plate 1-46. The spindle motor 1-61 includes a spindle magnet 1-63 which attracts the metallic disc hub 1-15 of the disc 1-14 (FIGS. 23-25) when the disc cartridge 1-13 is installed in the disc drive 1-10. An ejection mechanism according to the present invention is generally referenced 1-67. The ejection mechanism 1-67 includes a left slider 1-70, a right slider 1-73, and a tiller 1-76. The ejection mechanism 1-67 is described more fully below. A parking arm 1-79 is also depicted in FIG. 7 in its position above the left slider 1-70. A cartridge receiver is shown generally at 1-82. Also shown in FIG. 7 are a left door link 1-85, a right door link 1-88, and a receiver door 1-91, each of which is pivotally attached to the cartridge receiver 1-82. The drive face plate 1-19 is depicted in front of the cartridge receiver 1-82. Finally, a rotatable, magnetic bias coil assembly 1-94 is depicted attached to a bias coil arm 1-97, with bias coil clamps 1-100 depicted above the bias coil arm 1-97. Further details about each of these primary component assemblies will next be provided.

[0213] With continuing reference to FIG. 7, it is illustrated that the bottom housing 1-16 includes side walls 1-103 and a back wall 1-106. On the inside base of the bottom housing 1-16 are four mounting stations 1-109 to which the base plate 1-46 is secured. The bottom housing 1-16 would also encase the control electronics, which are not depicted in the figures.

[0214] In reference to FIGS. 8A and 8B, further details of the construction of the base plate 1-46 will now be provided. The base plate 1-46 is mounted on the four mounting stations 1-109 (FIG. 7) of the bottom housing 1-16. The base plate 1-46 has many components either molded into, embedded into, attached to, or associated with it. Base plate 1-46 is the“glue” that brings the many components of this invention together and permits them to interact. Around the periphery of the base plate 1-46 there is a forward wall 1-112, a left outer side wall 1-115, a left inner side wall 1-118, a right outer side wall 1-121, a right inner side wall 1-124, and a rear vertical wall 1-127. The left and right outer side walls 1-115, 1-121, respectively, each include a vertical slot 1-130, 1-133, respectively. The left vertical slot 1-130 accommodates a left lift pin 1-136 (FIG. 15A) on the cartridge receiver 1-82 when the cartridge receiver 1-82 is in place around the base plate 1-46. The right vertical slot 1-133 similarly accommodates a right lift pin 1-139 (FIG. 15B) of the cartridge receiver 1-82.

[0215] The two cartridge locating pins 1-43, FIG. 8B, are positioned near the forward ends of the left and right outer side walls 1-115, 1-121, respectively. These locating pins 1-43 are adapted to engage the cartridge channels 1-40 (FIG. 6). When the pins 1-43 are located in the channels 1-40, the pins 1-43 hold the disc cartridge 1-13 and prevent it from moving both laterally (i.e., side-to-side) and longitudinally (i.e., forward and backward).

[0216] A spindle motor mount 1-142 is molded into the bottom of the base plate 1-46. The spindle motor 1-61 (FIG. 7) may be held on the spindle motor mount 1-142 by, for example, spring clips (not shown) attached to an intermediate rib 1-145.

[0217] The base plate 1-46 has various axes and mounting pins associated therewith. For example, a tiller pivot axis 1-148 is mounted on the base plate 1-46 adjacent to the spindle motor mount 1-142. A tiller-spring pin 1-151 is fixed to the bottom of the base plate 1-46 near the forward wall 1-112 (FIG. 8A). The other pins attached to the bottom of the base plate 1-46 near the forward wall 1-112 act as pivot shafts for the gears in the ejection gear train. The base plate 1-46 also includes a left slider channel 1-154 and a right slider channel 1-157. The slider channels 1-154, 1-157 extend along the sides of the base plate 1-46. The left slider channel 1-154 is formed between the left outer side wall 1-115 and the left inner side wall 1-118. When in position, the left slider 1-70 is sandwiched between the left inner side wall 1-118 and the left outer side wall 1-115, and rides in the left slider channel 1-154 (see FIGS. 9,13, and 16A). Similarly, the right slider channel 1-157 is formed between the right outer side wall 1-121 and the right inner side wall 1-124. When in position, the right slider 1-73 is sandwiched between the right inner side wall 1-124 and the right outer side wall 1-121, and rides in the right slider channel 1-157. The left and right sliders 1-70, 1-73, respectively, may be held in their respective channels 1-154, 1-157 by, for example, “ears” on the spring clips (not shown) that hold the spindle motor 1-61 in position on the spindle motor mount 1-142.

[0218] At the end of the right slider channel 1-157, adjacent to the rear vertical wall 1-127, a socket 1-160 is formed in the base plate 1-46 where the rear of the right inner side wall 1-124 merges with the rear of the right outer side wall 1-121. This socket 1-160 accommodates a pivot pin 1-163 (FIGS. 17B and 17A) of a receiver latch 1-166. The receiver latch 1-166 has a vertical surface 1-169 (FIG. 17B) upon which a latch-release trip lug 1-172 (FIGS. 7 and 16A), which is fixed to the right door link 1-88, impacts to release the receiver latch 1-166.

[0219] The base plate 1-46 has a port 1-175 in the rear vertical wall 1-127. The laser diode 42 (not shown), which would be located behind the rear vertical wall between a left corner pillar 1-178 and a right corner pillar 1-181, shines through the port 1-175 and into a carriage 1-184 (best shown in FIGS. 9, 13, 13A, 16A and 16A), which contains the optics that focus the laser beam on an information track on the disc 1-14. The carriage 1-184 is discussed further below.

[0220] The base plate 1-46 also has a hole 1-187 molded therein to accommodate a pivot shaft 1-190 (FIG. 14B) of the parking arm 1-79. This hole 1-187 is molded as an integral part of the left inner side wall 1-118. FIG. 9, for example, shows the parking arm 1-79 in place with its pivot shaft 1-190 in the hole 1-187. The disc drive 1-10 includes an optics module 1-189 which performs similarly to the optics module 24 discussed above.

[0221] Referring now to FIGS. 14A through 14C, further features of the parking arm 1-79 will be described. In addition to the pivot shaft 1-190, the parking arm 1-79 includes a pressing end 1-193. The parking arm 1-79 has a jaw 1-196 formed on the end remote from the pressing end 1-193. The jaw 1-196 has a long side 1-199 and a short side 1-202. When the parking arm 1-79 is in position, the jaw 1-196 straddles a lug 1-205 (FIG. 11C) on the left slider 1-70. The parking arm 1-79 in position, with its jaw 1-196 straddling the lug 1-205 of the right slider 1-70, may be seen to best advantage in FIGS. 9, 13, 16A and 16B. The position of the parking arm 1-79 is thereby dictated by the location of the left slider 1-70 in the left slider channel 1-154.

[0222] As seen to best advantage in FIG. 13, the parking arm 1-79 parks the carriage 1-184. The carriage 1-184 focuses the laser beam coming through the port 1-175 (FIGS. 8A and 8B) in the rear vertical wall 1-127 of the base plate 1-46. In particular, the carriage positions the laser beam over the center of a data track containing data to be read. The carriage 1-184 rides on support rails 1-208, FIG. 9. A conventional magnetic arrangement drives the carriage 1-184 along the rails 1-208. When the cartridge receiver 1-82 is in the up condition, the parking arm 1-79, which is powered by the left slider 1-70, holds the carriage 1-184 toward the rear of the drive. This condition is illustrated in FIGS. 9 and 16A, and is illustrated in FIG. 13 by the parking arm 1-79 shown in solid lines. When the left slider 1-70 is driven forward by the tiller 1-76 during ejection of the disc cartridge 1-13, the parking arm 1-79 is rotated by the lug 1-205 pressing against the short side 1-202 of the jaw 1-196 until the pressing end 1-193 of the parking arm 1-79 holds the carriage 1-184 toward the back of the disc drive 1-10. When the cartridge receiver 1-82 is in its down position, the left slider 1-70 has been driven toward the rear of the disc drive 1-10 by the tiller 1-76. Under this scenario, the lug 1-205, which was driven rearward with the left slider 1-70, has rotated the parking arm 1-79 toward the front of the disc drive 1-10. With the left slider 1-70 and parking arm 1-79 in these positions, the carriage 1-184 is not influenced by the pressing end 1-193 of the parking arm 1-79 and may move freely below the disc 1-13 in the disc drive 1-10.

[0223] The ejection mechanism 1-67, which may be seen to best advantage in FIGS. 7 and 9, includes the following key features. An ejection motor 1-209 powers the ejection mechanism. In particular, the ejection motor 1-209 powers a gear train that powers the output cam which, in turn, forces the tiller 1-76, FIG. 9, to rotate in a first direction (counterclockwise in FIG. 9), thereby ejecting a disc cartridge 1-13 from the disc drive 1-10. When the ejection process is initiated, the motor 1-209 drives a corresponding worm gear 1-211. The worm gear 1-211 is fixed to the central shaft of the ejection motor 1-209. This worm gear 1-211 drives a first large gear 1-214 about a first axis 1-217. This rotation of the first large gear 1-214 rotates a first small gear 1-220, which is fixed to the bottom of the first large gear 1-214 for rotation therewith about the first gear axis 1-217. The first small gear 1-220 drives a second large gear 1-223 about a second gear axis 1-226. A second small gear 1-229 is fixed to the top of the second large gear 1-223 for rotation therewith about the second gear axis 1-226. The second small gear 1-229, in turn, drives a third large gear 1-232 about a third gear axis 1-235. The third large gear 1-232 drives a cam 1-238 that forces the tiller 1-76 to rotate about the tiller axis 1-148.

[0224] The tiller 1-76 will now be described with reference to FIGS. 10A-10F and FIG. 9. The tiller 1-76 is pivotally attached to the base plate 1-46 by the tiller axis 1-148. A tiller-spring hook 1-239 is molded on the slender portion of the tiller 1-76. A tiller spring 1-241 (FIG. 9) is attached between the tiller-spring hook 1-239 and the tiller-spring pin 1-151. The tiller-spring 1-241 biases the tiller 1-76 in a second direction (clockwise in FIG. 9) about the tiller axis 1-148. This is the cartridge-loading direction, which drives the right slider 1-73 forward and the left slider 1-70 rearward, to seat the disc cartridge 1-13 on the spindle motor 1-61. The tiller further includes a tiller skirt or webbed portion 1-244 that rides on top of the tiller gear train and thereby helps to contain the ejection gears in position on their respective gear axes. The end of the tiller near the tiller skirt 1-244 comprises a U-shaped jaw 1-247, and the tiller end remote from the skirt 1-244 comprises a similar U-shaped jaw 1-250. The U-shaped jaw 1-247 fits rotatably around a cylindrical connection post 1-253 of the left slider 1-70 (FIG. 11 C). Similarly, the U-shaped jaw 1-250 of the tiller 1-76 fits rotatably around the cylindrical connection post 1-256 (FIG. 12E) of the right slider 1-73. The tiller 1-76 is thereby pivotally connected to the forward ends of the left and right sliders 1-70, 1-73, respectively. In addition, since the left and right sliders 1-70, 1-73 are held in their respective slider channels 1-154, 1-157 by the spring clips (not shown) which also hold the spindle motor 1-61 in position, the tiller 1-76 is held on the tiller axis 1-148 by the interaction between the U-shaped jaws 1-247, 1-250 and the cylindrical connecting posts 1-253, 1-256.

[0225] When the tiller 1-76 rotates in a first direction (counterclockwise in FIG. 9), the left slider 1-70 is driven forward in the left slider channel 1-154, while the right slider 1-73 is simultaneously driven rearward in the right slider channel 1-157. Thus, rotation of the tiller 1-76 in the first direction (counterclockwise in FIG. 9) raises the cartridge receiver 1-82 so that a disc cartridge 1-13 may be ejected from or loaded into the disc drive 1-10. On the other hand, when the tiller 1-76 rotates in a second direction (clockwise in FIG. 9), the left slider 1-70 is driven rearward in the left slider channel 1-154, while the right slider 1-73 is simultaneously driven forward in the right slider channel 1-157. Rotation of the tiller 1-76 in this direction lowers the cartridge receiver 1-82, placing the disc on the spindle motor. The raising and lowering of the cartridge receiver 1-82 by the rotation of the tiller 1-76 is discussed further below.

[0226] As discussed above, the left slider 1-70 rides in the left slider channel 1-154, and the right slider 1-73 rides in the right slider channel 1-157 under the influence of the tiller 1-76. Further details concerning the sliders 1-70, 1-73 is provided next.

[0227] Referring now to FIGS. 11A-11C, the features of the left slider 1-70 are as follows. The left slider includes the cylindrical connecting post 1-253 on its forward end. The parking arm lug 1-205 exists on a first recessed portion 1-259. The parking arm 1-79 slides along the first recessed portion 1-259 of the left slider 1-70 under the influence of the lug 1-205. An S-shaped slot 1-262 is formed into the left slider 1-70. When the left slider 1-70 is in position in the left slider channel 1-154, the S-shaped slot 1-162 opens toward the left outer side wall 1-115, adjacent to and behind the left vertical slot 1-130. When the cartridge receiver 1-82 is in position around the base plate 1-46, the left lift pin 1-136 (FIG. 15A) of the cartridge receiver 1-82 rides in the left vertical slot 1-130 of the base plate 1-46. The left lift pin is longer than the left outer side wall 1-115 is thick. Therefore, the left lift pin 1-136 projects through the left vertical slot 1-130 and rides in the S-shaped slot 1-262 in the left slider 1-70. When the cartridge receiver 1-82 is thus positioned about the base plate 1-46, with the left lift pin 1-136 riding in the vertical slot 1-130 and the S-shaped slot 1-262, the cartridge receiver 1-82 is restricted from traveling forward or backward and may only travel up and down vertically. The vertical slot 1-130 restricts the forward-to-backward movement of the cartridge receiver 1-82, while the S-shaped slot 1-262 in the left slider 1-70 defines the vertical height of the cartridge receiver. In other words, depending upon which portion of the S-shaped slot 1-262 is behind the vertical slot 1-130 at any particular moment, the cartridge receiver 1-82 may be in its highest position, its lowest position, or at some position between its highest and lowest positions.

[0228] A second recessed portion 1-265 is present on the top of the left slider 1-70. A horizontal pin (not shown) may be attached to the base plate 1-46 so as to slip along the second recessed portion 1-265. This horizontal pin (not shown) would limit the most forward and most rearward positions of the left slider 1-70 because the pin would impact the edges of the second recessed portion 1-265 upon reaching one of the extreme positions of the left slider.

[0229] The rear-most end of the left slider 1-70 includes a notch 1-268, which is best illustrated in FIGS. 11B and FIG. 7. The notch 1-268 is located on a displaced end portion 1-272 of the left slider 1-70. The notch 1-268 receives a lever arm 1-275 of the bias coil arm 1-97, FIG. 7. This lever arm 1-275 rotates the bias coil arm 1-97 depending upon the position of the left slider 1-70, and in particular, the position of the notch 1-268. The displaced end portion 1-272 of the left slider 1-70 rides in a recess 1-278 (FIG. 8B) in the left outer side wall 1-115 of the base plate 1-46.

[0230] Referring now to FIGS. 12A-12E, the features of the right slider 1-73 will be presented. As stated above, the tiller 1-76 is connected to the right slider 1-73 via the cylindrical connection post 1-256. The right slider 1-73 has an S-shaped slot 1-281 formed therein. This S-shaped slot 1-281 is a flipped version of the S-shaped slot 1-262 in the left slider 1-70. This is best shown in FIG. 7. Upon close consideration of FIG. 7, it becomes apparent that, when the sliders 1-70, 1-73 are connected to the tiller 1-76, the S-shaped slots 1-262, 1-281 are flipped mirror images of each other. This arrangement is necessary since the sliders 1-70, 1-73 move in opposite directions under the influence of the tiller 1-76. The S-shaped slot 1-281 in the right slider 1-73 also opens toward the right outer side wall 1-121 when the right slider 1-73 is in its operating position in the right slider channel 1-157. Similar to what was described above with reference to the left slider 1-70, when the cartridge receiver 1-82 is in position around the base plate 1-46, the right lift pin 1-139 (FIG. 158) rides in the right vertical slot 1-133 (FIG. 8B). Since the right lift pin 1-139 is longer than the right outer side wall 1-121 is thick, the right lift pin 1-139 projects through the right outer side wall 1-121 at the right vertical slot 1-133 and rides in the S-shaped slot 1-281 in the right slider 1-73. The right vertical slot 1-133 restricts the right lifting pin 1-139 from traveling parallel to the longitudinal axis of the base plate 1-46 (i.e., parallel to a line passing perpendicularly through the forward wall 1-112 and the rear vertical wall 1-127). Since the right lift pin 1-139 rides in the S-shaped slot 1-281, the vertical height of the cartridge receiver 1-82 is defined by the location of the right lift pin 1-139 in the S-shaped slot 1-281. The S-shaped slot 1-281 in the right slider 1-73 travels behind the right vertical slot 1-133 at the same rate that the S-shaped slot 1-262 in the left slider 1-70 passes behind the left vertical slot 1-130, but in an opposite direction. The flipped mirror image design of the S-shaped slots 1-262, 1-281, however, ensures that the left and right lift pins 1-136, 1-139, respectively, are held at substantially the same vertical height above the bottom of the base plate 1-46 at any particular time.

[0231] Still referring primarily to FIGS. 12A-12E, the right slider 1-73 includes the following additional features. A recessed portion 1-284 is provided on the top surface of the right slider 1-73. A pin (not shown) may be mounted horizontally across the right slider channel 1-157 so as to slide along the recessed surface 1-284. The horizontal pin sliding along the recessed surface 1-284 would limit the maximum forward and rearward travel of the right slider 1-73 since the horizontal pin would hit the edges of the recess 1-284 at the extremes of travel of the right slider 1-73. The right slider 1-73 also includes a notched region 1-287 to accommodate a paw 1-290 (FIGS. 17A and 17B) of the receiver latch 1-166. A raised portion 1-293 is provided on the rear end of the right slider 1-73. When the tiller 1-76 rotates in the first direction (counterclockwise in, for example, FIG. 13), driving the right slider 1-73 rearward in the right slider channel 1-157, a latching action takes place between the paw 1-290 of the receiver latch 1-166 and the raised portion 1-293 of the right slider 1-73. In particular, a first slipping surface 1-296 (FIG. 17A), which is located on the paw 1-290, slides past a second slipping surface 1-299 (FIGS. 12C and 12E), which is on the raised portion 1-293 of the right slider 1-73. When the surfaces 1-296 and 1-299 slip past each other, the paw 1-290, which is spring-loaded in the direction indicated by arrow 1-302 in FIG. 17A, enters the notched region 1-287 of the right slider 1-73, which holds the right slider 1-73 in the rearward position and, consequently, holds the cartridge receiver 1-82 in its uppermost position. When the cartridge receiver is in this position, any disc cartridge 1-13 in the drive 1-10 would be ejected, or, alternatively, a disc cartridge 1-13 could be loaded into the disc drive 1-10.

[0232] The S-shaped slots 1-262 and 1-281 in the left and right sliders 1-70, 1-73, respectively, play a significant role in generating the peeling action accomplished by the instant invention when loading a disc cartridge onto and unloading a disc cartridge from the spindle motor. This role of the S-shaped slots 1-262, 1-281 in facilitating the peeling action generated by the instant invention is discussed further below.

[0233] Referring now to FIGS. 15A and 15B, the cartridge receiver 1-82 and the components attached thereto will be described. The cartridge receiver 1-82 is a one-piece, injection molded piece of plastic to which the left door link 1-85 (FIG. 7) and right door link 1-88 are added. When the disc drive 1-10 is fully assembled, the cartridge receiver 1-82 rides on the outside of the left and right outer side walls 1-115, 1-121 of the base plate 1-46. The cartridge receiver 1-82 travels vertically up and down as the lift pins 1-136, 1-139 move up and down as they follow their respective S-shaped slots 1-262, 1-281. The cartridge receiver 1-82 also pitches slightly up and down about an imaginary lateral axis passing through the left and right lift pins 1-136, 1-139. It is this slight pitching motion in conjunction with the up and down motion that generates the beneficial peeling action achieved by the instant invention. The cartridge receiver 1-82 may be snapped or lifted off of the remainder of the mechanism if the cover of the disc drive 1-10 is removed.

[0234] The cartridge receiver 1-82 has a left cartridge receiving channel 1-305 and a right cartridge receiving channel 1-308 formed therein. A stop bumper 1-311 is positioned in the rear of the right cartridge-receiving channel 1-308 to prevent improper insertion of a disc cartridge 1-13. As may be seen in FIGS. 6 and 7, the disc cartridge 1-13 has a pair of slots 1-314 molded into the side walls 1-37. If the disc cartridge 1-13 is inserted correctly, with its rear wall 1-38 entering the disc receiving port 1-22 first, one of the slots 1-314 in the disc cartridge 1-13 will accommodate the stop bumper 1-311 and permit the cartridge 1-13 to be fully inserted into the drive 1-10. If, on the other hand, the user inserts the disc cartridge 1-13 with the forward-facing label end 1-34 entering the disc receiving port 1-22 first, the stop bumper 1-311 will impact the label end 1-34 of the disc cartridge 1-13, thereby preventing full insertion of the disc cartridge 1-13 into the disc drive 1-10. A rear wall 1-317 of the cartridge receiver 1-82 has a notched region 1-320 formed therein. This notched region 1-320 permits the latch-release trip lug 1-172 (FIG. 16) fixed to the right door link 1-88 to impact the vertical surface 1-169 (FIG. 17B) of the receiver latch 1-166. Since the left and right door links 1-85 and 1-88, respectively, are rotated toward the rear of the disc drive 1-10 as the disc cartridge 1-13 is inserted in the cartridge receiver 1-82, as the disc cartridge 1-13 approaches full insertion, the trip lug 1-172 trips the receiver latch 1-166 by pressing against the vertical surface 1-169 to rotate the receiver latch 1-166. This rotation of the receiver latch 1-166 frees the paw 1-290 from its latched position around the raised portion 1-293 of the right slider 1-73. When the receiver latch 1-166 is tripped in this manner, the cartridge receiver 1-82 can be lowered, placing the disc cartridge 1-13 in operating position on the spindle motor 1-61.

[0235] Referring to FIGS. 7, 15A, 15B, 16A and 16B, the attachment of the left door link 1-85 and the right door link 1-88 to the receiver cartridge 1-82 will now be described. The left and right door links 1-85 and 1-88, respectively, are attached to the rear corners of the cartridge receiver 1-82, near the rear wall 1-317. Specifically, the left door link 1-85 is rotatably mounted to the cartridge receiver 1-82 at a first pivot point 1-323, and the right door link 1-88 is rotatably mounted to the cartridge receiver 1-82 at a second pivot point 1-326. The door links 1-85 and 1-88 are biased by a spring (not shown) toward the face plate 1-19 of the disc drive 1-10. In operation, one or the other of the door links 1-85, 1-88 unlatches the cartridge shutter lock and opens the cartridge shutter 1-49 as the disc cartridge 1-13 is inserted into the drive 1-10. Whether the left door link 1-85 or the right door link 1-88 opens the cartridge shutter 1-49 is determined by which side of the disc cartridge 1-13 is facing up when the cartridge 1-13 is inserted into the drive 1-10. If the disc cartridge 1-13 is inserted with a first side up, the right door link 1-88 operates the shutter latch and opens the shutter 1-49. If the disc cartridge 1-13 is inserted with its other side up, the left door link 1-85 operates the shutter latch and opens the shutter 1-49. When no disc cartridge 1-13 is in the drive 1-10, the door links 1-85 and 1-88 rest against door link stops 1-329, which are integrally formed as part of the cartridge receiver 1-82. These door link stops 1-329 ensure that free ends 1-332 of the door links 1-85 and 1-88 are properly positioned to release the shutter latch and open the shutter 1-49 as the disc cartridge 1-13 is inserted into the drive 1-10.

[0236] With reference now to FIGS. 18-22, the rotatable, magnetic bias coil assembly 1-94 will be more fully described. The bias coil assembly 1-94 is used during writing and erasing operations of the disc drive 1-10. The bias coil assembly 1-94 includes a steel bar 1-335 wrapped in a coil of wire 1-338. When the bias coil assembly 1-94 is positioned over a disc 1-14, as best shown in FIG. 23, it extends radially across the disc 1-14 and is thus capable of generating a strong magnetic field over a radial strip of the disc 1-14, extending from near the spindle 1-62 (FIGS. 23-25) to the edge of the disc 1-14. When the disc 1-14 is rotated under the bias coil assembly 1-94 by the spindle motor 1-61, it is possible to generate a magnetic field over the entire surface of the disc 1-14, thus enabling the user to write information to all portions of the disc 1-14 from its innermost to its outermost tracks. The coil 1-338 and bar 1-335 are covered by a bias coil housing top 1-341, which is mounted to a bias coil housing bottom 1-344.

[0237] The bias coil assembly 1-94 is mounted to a bias coil flexure 1-347, FIG. 22, which is, in turn, mounted on the bias coil arm 1-97, FIG. 21. The bias coil arm 1-97 straddles the width of the base plate 1-46 and is rotatably held by a pair of the bias coil clamps 1-100, FIG. 18, to the corner pillars 1-178 and 1-181, FIGS. 8A and 8B, of the base plate 1-46. The bias coil clamps 1-100 thus act as bearing blocks under which the bias coil arm 1-97 can rotate. The bias coil clamps 1-100 include a stop ledge 1-350, FIG. 18, which terminates the upward travel of the cartridge receiver 1-82 during an ejection operation, as discussed more fully below with reference to FIGS. 23-25. As previously discussed, the bias coil arm 1-97 includes the lever arm 1-275 in operative association with the notch 1-268 on the rearward end of the left slider 1-70 to lift and lower the bias coil assembly 1-94. Since the lever arm 1-275 engages the notch 1-268 in the left slider 1-70, the left slider 1-70 controls when the bias coil assembly 1-97 is rotated onto or off of the disc cartridge 1-13.

[0238] The bias coil assembly 1-94 may tilt or rotate about a point 1-353 near its center, and it is spring-loaded downward. In this manner, the bias coil assembly 1-94 can remain parallel to the disc cartridge 1-13 when in the down condition (i.e., the position depicted in FIG. 23, wherein the disc cartridge 1-13 is fully loaded), and when in the up condition (i.e., the position depicted in FIG. 25, wherein the disc cartridge 1-13 is unloaded). The ability of the bias coil assembly 1-94 to remain parallel to the disc cartridge 1-13 when in the up condition provides the clearance needed for the drive 1-10 to be able to complete a disk-ejection operation, as discussed below. When in the down condition and loaded in the disc cartridge 1-13, the bias coil assembly 1-94 rests on the disc cartridge 1-13 in three places.

[0239] With further reference now to FIGS. 23-25, the ejection of a disc cartridge 1-13 from the disc drive 1-10 will be described. FIG. 23 depicts a disc cartridge 1-13 with the disc hub 1-15 fully loaded onto the spindle 1-62 of the spindle motor 1-61. In this configuration, the bias coil assembly 1-94 is loaded into the disc cartridge 1-13 through the open shutter 1-49. When the disc cartridge 1-13 is fully loaded in this manner, the left slider 1-70 has been slid to its most rearward position by the tiller 1-76. The lever arm 1-275 of the bias coil arm 1-97 has been rotated toward the rear of the disc drive 1-10. It is this rotation of the lever arm 1-275 which has installed the bias coil assembly 1-94 into the disc cartridge 1-13. Since the lift pins 1-136 and 1-139 of the cartridge receiver 1-82 are restrained to only vertical movement by the vertical slots 1-130 and 1-133 (FIGS. 8A and 8B), when the left slider 1-70 has been driven toward the rear of the disc drive 1-10 by the tiller 1-76, as depicted in FIG. 23, the cartridge receiver 1-82, via its lift pins 1-133 and 1-136, has been driven to the lowest point in the S-shaped slots 1-262 and 1-281.

[0240] An intermediate stage of the ejection cycle will now be described with reference to FIG. 24. After a user initiates the ejection of the disc cartridge 1-13 from the disc drive 1-10, the ejection motor 1-208, FIG. 9, rotates the tiller 1-76 in a first direction (counterclockwise in FIG. 9). This rotation of the tiller pulls the left slider 1-70 toward the front of the drive 1-10, as illustrated in FIG. 24. As the left slider 1-70 slides forward, the notch 1-268 rotates the lever arm 1-275 forward, thereby lifting the bias coil assembly 1-94 out of the disc cartridge 1-13. As may also be seen in FIG. 24, the lift pins 1-136 and 1-139, which are fixed to the cartridge receiver 1-82, are being forced up the S-shaped slots 1-262 and 1-281 by the motion of the tiller 1-76. Since the lift pins 1-136 and 1-139 are positioned on the cartridge receiver at a point—where a lateral axis passing through both lift pins 1-136 and 1-139 would not also pass through the spindle 1-62, a“peeling” action for removal of the disc hub 1-15 from the spindle magnet 1-64 is achieved as the cartridge receiver 1-82 is raised. In other words, as shown in FIG. 24, the disc 1-14 is not lifted vertically from the spindle 1-62 during the ejection cycle. Rather, due to the location of the lift pins 1-136, 1-139 on the cartridge receiver 1-82, the rear portion of the disc cartridge 1-13 is lifted before the forward end of the disc cartridge 1-13 as the lift pins 1-136 and 1-139 follow their respective S-shaped slots 1-262 and 1-281. This peeling action lowers the peak force required to remove the disc hub 1-15 from the magnetic clamp 1-64 of the spindle motor 1-61.

[0241] Referring still to FIG. 24, it is apparent that after the cartridge receiver 1-82 has been lifted a predetermined amount by the motion of the sliders 1-70 and 1-73, a lip 1-356, FIG. 15A, on the rear wall 1-317 of the cartridge receiver 1-82 impacts the lower surface of the stop ledge 1-350, FIG. 18, on the bias coil clamps 1-100. This contact between the bottom surface of the stop ledge 1-350 and the top surface of the lip 1-356, in conjunction with the continued rotation of the tiller 1-76 and the resulting longitudinal motion of the sliders 1-70 and 1-73, causes the cartridge receiver 1-82 to pitch slightly upward in FIG. 24. This occures substantially about the point of contact between the stop ledge 1-350 and the lip 1-356, as the lift pins 1-136, 1-139 continue to pick up the receiver. This slight pitching motion of the cartridge receiver 1-82 effects the “peeling” action referred to above.

[0242] FIG. 25 depicts the configuration of the disc drive 1-10 after the slight upward pitching of the cartridge receiver 1-82 is complete and the cartridge receiver 1-82 has impacted the stops adjacent to the disc receiving port 1-22. At this point, the left slider 1-70 has reached its furthest forward position and has pulled the lever arm 1-275 to its furthest forward position, thereby rotating the bias coil assembly 1-94 out of the disc cartridge 1-13. The bias coil assembly is thus parked parallel to and above the disc cartridge 1-13, substantially against the inside of the top surface of the disc drive 1-10 or substantially against a printed circuit board located against the inside of the top surface of the disc drive 1-10. The bias coil assembly 1-94 travels vertically preferrably about 9 mm from its loaded position in the disc cartridge 1-13 to its just-described raised position.

[0243] As the cartridge receiver 1-82 is raised to its highest position (about 5 mm above its lowest position), the right slider 1-73 of FIGS. 12A-12E is latched in its rear-most position by the receiver latch 1-166, FIGS. 17A and 17B, as fully described above. When the cartridge receiver 1-82 is in the up position depicted in FIG. 25, the cartridge receiver 1-82 is positioned parallel to the base plate 1-46, ready for the cartridge 1-13 to be ejected. The spring force of the door links 1-85 and 1-88, which are biased toward the forward end of the disc drive 1-10 as described above, and the spring force of the cartridge shutter 1-49, which is biased toward a closed position, cause the disc cartridge 1-13 to be ejected from the disc drive 1-10, as shown in FIG. 25.

[0244] The disc loading process is essentially the reverse of the above described ejection process. Therefore, a detailed description of the disc insertion process will not be provided.

[0245] In the present invention, where the disc hub 1-15 is peeled from the spindle magnet 1-64, the required ejection force is effectively reduced by the manner in which the disc 1-14 is moved from the loaded position to the unloaded position. Through the use of the “peeling” motion employed in accordance with this invention, a smaller force is required to remove the disc hub 1-15 than is required in conventional, vertical-lifting systems. In addition, the design conserves overall drive height. The above-described design accomplishes the peeling of the disc hub 1-15 from the spindle magnet 1-64 with a mechanism that uses available space at the sides of the drive 1-10, rather than requiring parts that straddle the width of the base plate 1-46 to tie the motion of both sides of a cartridge receiver 1-82 together and using additional height to do so. Another advantageous feature of the design is the noncritical nature of most of the dimensions required. Further, the bias coil actuating mechanism that loads the bias coil assembly into the cartridge 1-13 is simple and has a minimum number of wear points. The entire design is easy to assemble and for the most part, can be manufactured using simple and easy to fabricate parts.

[0246] While what has been described above is a preferred embodiment of this invention, it will be obvious to those skilled in the art that numerous changes may be made without departing from the spirit or scope of the invention. For example, the present invention may be used for media systems which do not require the bias coil assembly 1-94 (i.e., phase change or write once systems), by eliminating the parts used to operate the bias coil arm 1-97. In addition, although in the preferred embodiment the storage media is a 5¼ inch magneto-optic disc cartridge, the present invention is applicable to all types of media and all sizes of drives.

[0247] Two-Axis Moving Coil Actuator

[0248] FIG. 26 schematically illustrates a two-axis electromagnetic actuator 2-10 constructed in accordance with the present invention. The actuator 2-10 includes an objective lens 2-12 positioned within a lens holder 2-14. A radial or tracking coil 2-16 is wound around and affixed to the lens holder 2-14 so as to be generally positioned perpendicular to the Z axis. First and second focus coils 2-18 and 2-20 are positioned at the sides of the lens holder 2-14 and are affixed to the tracking coil 2-16 so as to be generally positioned perpendicular to the Y axis. A first pair of permanent magnets 2-22 is positioned adjacent the first focus coil 2-18 and a second pair of permanent magnets 2-24 is positioned adjacent the second focus coil 2-20.

[0249] As shown in FIG. 27, the lens holder 2-14 includes a generally rectangular collar 2-30 having a circular aperture 2-32 centered therein. The objective lens 2-12 is glued into position on top of the circular aperture 2-32 in the collar 2-30. The collar 2-30 is supported by a generally I-shaped platform 2-34 having a pair of grooves 2-44 formed at the edges thereof to align and secure the tracking coil 2-16 therein when it is wound around the platform. A base 2-36 supporting the platform 2-34 includes first and second T-shaped sections 2-46 and 2-48 having a slot 2-50 formed therebetween. As will be explained in more detail below, this base 2-36 acts as a mass balance for the lens holder 2-14. The collar 2-30, platform 2-34, and base 2-36 are aligned on two sides to form first and second opposing faces 2-52 and 2-54 of the lens holder.

[0250] The focus coils 2-18 and 2-20 are affixed to the tracking coil 2-16 such that the central axes of the focus coils are coincident, intersect, and preferably perpendicular to the central axis of the tracking coil. The focus coils 2-18 and 2-20 are preferably formed from thermally bonded wire having a bond material layer thereon and are preferably wound on a suitable tool or support. The coils 2-18 and 2-20 are preferably wound around the support as tight as possible without deforming the wire. As those skilled in the art will appreciate, this tightness will vary with the type of wire. During the winding process, the focus coils 2-18 and 2-20 are preferably heated to melt the bond material layer on the wire, advantageously increasing the solidity and rigidity of the wound coils. The temperature is advantageously selected so as to be high enough to melt the bond material, but not so high as to melt the insulation. After cooling, the coils 2-18 and 2-20 are removed from the support and these freestanding coils are then affixed to the tracking coil 2-16 in a well-known manner using a suitable adhesive.

[0251] Each of the freestanding focus coils 2-18 and 2-20 is oval in shape and has two elongate sides 2-56 joined by a pair of shorter ends 2-58. The sides 2-56 and ends 2-58 of the coils 2-18 and 2-20 surround an open or hollow annular center 2-60. The tracking coil 2-16 is wound around the I-shaped platform 2-34 of the lens holder 2-14 such that the coil is received by and secured within the grooves 2-44 and positioned against the opposed faces 2-52 and 2-54 of the lens holder. Referring to both FIG. 26 and FIG. 27, the two focus coils 2-18 and 2-20 are affixed to the tracking coil 2-16 such that the tracking coil is positioned within the center 2-60 of each focus coil. The focus coils 2-18 and 2-20 are further positioned such that each coil abuts the opposed faces 2-52 and 2-54 of the lens holder 2-14. In this manner, the tracking coil 2-16 and focus coils 2-18 and 2-20 are rigidly secured to the lens holder 2-14, thereby creating a more rigid driven unit that behaves as a single lumped mass.

[0252] Referring to FIGS. 28, 29, 30, and 31, in operation, a light source element (not shown here), typically a laser diode, emits a laser light beam 2-70, FIG. 31. The beam 2-70 is incident upon a prism 2-72 which orthogonally reflects the light beam upward toward the objective lens 2-12. The lens 2-12 converges the beam 2-70 to a precise focal point or optical spot 2-74 on the surface of a recording medium, such as an optical disc 2-76. Upon striking the disc 2-76, the light beam 2-70 is altered by the information stored on the disc 2-76 and is reflected as a divergent light beam carrying information identical to that encoded on the disc 2-76. This reflected beam re-enters the objective lens 2-12 where it is collimated and is again reflected by the prism 2-72 to a photodetector (not shown here) which detects the data stored on the disc 2-76. In addition, if the light beam falling on the photodetector is out of focus or misaligned, the amount of misalignment or defocusing is measured electronically and used as feedback for a servo system (not shown) well-known in the art which properly realigns the objective lens 2-12 relative to the disc 2-76.

[0253] It is these feedback signals which determine the amount and direction of movement of the actuator 2-10 and objective lens 2-12 carried thereon needed to bring the light beam into the desired focus condition with respect to the disc 2-76. When radial or tracking movement is required to position the objective lens 2-12 beneath the center of a selected track on the optical disc 2-76, current is applied to the tracking coil 2-16. The current interacts with the magnetic field produced by the permanent magnet pairs 2-22 and 2-24 to produce forces which move the actuator 2-10 in the tracking direction. The forces are generated according to the Lorentz law F=B·X·I·1, wherein F represents the force acting on the tracking coil 2-16, B represents the magnetic flux density of the magnetic field between the permanent magnet pairs 2-22 and 2-24, I represents the current through the tracking coil 2-16, and 1 represents the length of the coil 2-16. When the current I applied to the tracking coil 2-16 travels through the coil in a counterclockwise direction, relative to the orientation of FIG. 29, a force is produced which moves the actuator 2-10 to the right. This rightward movement is indicated in FIG. 31 by arrow 2-15. When the current applied to the coil 2-16 travels through the coil in the opposite, or clockwise direction, a force is produced which moves the actuator 2-10 to the left as indicated in FIG. 31 by arrow 2-17. In this manner, the actuator 2-10 is moved radially to position the objective lens 2-12 beneath the center of a desired information track on the surface of the optical disc 2-76.

[0254] Movement of the actuator 2-10 to effect focusing is produced when current is generated in the two focus coils 2-18 and 2-20 affixed to the tracking coil 2-16 at the sides of the lens holder 2-14. When the current through these coils 2-18 and 2-20 is applied so that the current travels in a counterclockwise in the plane of FIG. 30, a force is produced which acts to move the lens holder 2-14 and objective lens 2-12 upward, as shown by arrow 2-19 in FIG. 31, towards the surface of the optical disc 2-76. Conversely, when current is applied such that current travels through the coils 2-18, 2-20 in a direction clockwise in the plane of FIG. 30, a force is produced which moves the lens holder 2-14 downward, as shown in FIG. 31 by the arrow 2-21, or farther away from the surface of the disc 2-76.

[0255] Because the tracking coil 2-16 is coupled to the lens holder 2-14, and, in turn, the focus coils 2-18 and 2-20 are coupled directly to the tracking coil 2-16, the coils and lens holder behave as a “lumped mass” and the frequencies at which the coils decouple with respect to the lens holder are significantly increased. Decoupling frequencies of up to 30 kHz have been measured with the actuator design of the present invention.

[0256] With reference now to FIGS. 28 and 29, the magnet pairs 2-22 and 2-24, remain stationary during movement of the lens holder 2-14 and are affixed within a generally rectangular housing or base 2-80. Two pairs of suspension wires 2-82 and 2-84 are provided to suspend the objective lens holder 2-14 between the magnet pairs 2-22 and 2-24. The wire pairs 2-82 and 2-84 are attached to a stationary printed circuit board 2-85 which is positioned vertically with respect to the lens holder 2-14 and acts as a support for the wire pairs 2-82 and 2-84. The wire pairs 2-82 and 2-84 are further attached to electrical contacts on a moving circuit board 2-87 which is attached to the lens holder 2-14, again in a vertical orientation. In particular, a free end of each focus coil 2-18 and 2-20 is soldered to electrical contacts 2-86 such that current is supplied to the focus coils 2-16 and 2-18, through the second or bottom wire pair 2-84 which is also soldered to the contacts 2-86. The other free end of each focus coil 2-18 and 2-20 is soldered to the circuit board 2-87 and joined along an electrical contact 2-88. The free ends of the tracking coil 2-16 and the first or top suspension wire pair 2-82 are soldered to electrical contacts 2-89 on the moving circuit board 2-87 such that current is supplied to the coil through the top pair of wires. The base 2-36 of the lens holder 2-14 acts as a mass balance by offsetting the weight of the objective lens 2-12 and the circuit board 2-87 to which the lens holder 2-14 is attached.

[0257] Alternatively, four flexures could be used to suspend the lens holder 2-14. The flexures would desirably act as parallel leaf springs which permit the objective lens holder 2-14 to move up-and-down for focusing while prohibiting changes in the orientation of the optical axis of the lens 2-12. In this manner, the objective lens 2-12 will not be canted with respect to the surface of the optical disc 2-76 as the lens holder 2-14 is moved in the focusing direction. Each flexure further includes narrow portions which operate as a hinge so as to allow some movement of the lens holder 2-14 in a side-to-side direction for tracking adjustments.

[0258] In addition to accomplishing fine focusing and tracking movements of the lens holder 2-14, it is often desirable to detect the position of the lens holder 2-14 with respect to the base 2-80. To ascertain the position of the objective lens 2-12 in both a tracking and/or a focusing direction, the actuator 2-10 is equipped with a position sensor 2-90. Preferably, a light emitting diode (LED) 2-92 is positioned on one side of the actuator 2-10, opposite the sensor 2-90, such that when the objective lens holder 2-14 is centered within the base 2-80, light emitted by the LED 2-92 will shine through the slot 2-50 in the lens holder 2-14 to illuminate a portion of the sensor 2-90. A position sensitive detector is advantageously implemented as the sensor 2-90 and the sensor is positioned such that when the lens holder 2-14 is centered within the base 2-80, light emitted by the LED 2-92 will pass through the slit 2-50 and will be distributed on the detector. Thus, as the lens holder 2-14 moves in a side-to-side direction, i.e., the tracking direction, various portions of the sensor 2-90 will be illuminated, indicative of the position of the lens holder 2-14 in the tracking direction. Consequently, when the lens holder 2-14 is not centered with respect to the base 2-80, a portion of the light emitted from the LED 2-92 will be blocked by the lens holder 2-14, causing an unequal distribution of light on the sensor 2-90. This unequal distribution may then be analyzed to determine the position of the lens holder 2-14 with respect to the base 2-80 by well-known circuitry and methods.

[0259] When a control signal is generated by the servo system, a given current is applied to the tracking coil 2-16 and/or the focus coils 2-18 and 2-20 depending on the direction in which the displacement of the lens holder 2-14 and objective lens 2-12 attached thereto is required. Such servo systems and feedback circuits which control the amount of current are well known in the art. As.discussed above, this current interacts with the electromagnetic field produced by the permanent magnet pairs 2-22 and 2-24 to create a force which displaces the lens holder 2-14 and objective lens 2-12 attached thereto in the appropriate focusing or tracking direction.

[0260] The operation and structure of the focus and tracking mechanism will now be described in greater detail. As illustrated in FIGS. 32 and 33, the permanent magnet pairs 2-22 and 2-24, are oriented with opposite poles opposing each other. More specifically, the first pair of magnets 2-22 includes a first or top magnet 2-100 and a second or bottom magnet 2-102 in a stacked relationship joined along a planar interface, such that the north pole of the top magnet 2-100 and the south pole of the bottom magnet 2-102, as represented in FIG. 33, are positioned adjacent the lens holder 2-14. The second pair of magnets 2-24 includes a third or top magnet 2-104 and a fourth or bottom magnet 2-106 in a stacked relationship joined along a planar interface having the opposite orientation, such that the south pole of the top magnet 2-104 and the north pole of the bottom magnet 2-106, as represented in FIG. 33, are positioned adjacent the lens holder 2-14. As shown in FIG. 32, the field lines produced by this orientation originate at the north pole of each magnet pair 2-22 and 2-24, and terminate at the south pole of each magnet pair. Iron plates 2-110 (shown in phantom for clarity) may be attached to each magnet pair 2-22 and 2-24 on the sides of the permanent magnets opposite the lens holder 2-14. The iron plates 2-110 effectively “shunt” the magnetic flux emanating from the sides of the magnets 2-100, 2-102, 2-104, and 2-106 opposite the lens holder 2-14, thereby increasing the magnetic flux adjacent the lens holder and producing a corresponding increase in actuator power.

[0261] The focus forces acting on the actuator 2-10 are illustrated in more detail in FIG. 34. When a current I is applied to the focus coils 2-18 and 2-20 in the direction indicated, i.e., out of the plane of the drawing sheet adjacent the top magnets 2-100, 2-104 and into the plane of the drawing sheet adjacent the bottom magnets 2-102 and 2-106, forces F_FOCUS1, and F_FOCUS2are generated which are translated to the lens holder 2-14 to accelerate or decelerate the moving mass (lens holder) and to the suspension wire pairs 2-82 and 2-84, bending the suspension wires to move the lens holder 2-14 and associated objective lens 2-12 closer to the optical disc 2-76. Because the lines of magnetic flux curve as described above, the direction of the magnetic field varies vertically in the focus coils 2-18, 2-20. For example, for the focus coil 2-18 positioned adjacent the first magnet pair 2-22, in the plane of FIG. 34 which vertically bisects the coil adjacent the top magnet 2-100, the magnetic field has a first direction at the top of the coil 2-18 given by B₁, and a second direction in the bisecting plane adjacent the bottom magnet 2-102 at the bottom of the coil 2-18 given by B₂. In accordance with the Lorentz law F=B·X·I·1, the current interacts with the magnetic field B1 to produce a first force component F1 acting on the portion of the focus coil 2-18 adjacent the top magnet 2-100, and interacts with the magnetic field B2 to produce a second force component F2 acting on the portion of the focus coil adjacent the bottom magnet 2-102. As the magnitude of the horizontal portions of the force components F1 and F2 are equal in magnitude but opposite in direction, these horizontal force components cancel one another in accordance with the rules of vector addition to produce the resultant force F_FOCUS1which is vertically upward in the plane of FIG. 34. Similarly, the horizontal force components throughout the rest of the coil 2-18 are canceled, giving a vertical resultant force which is strictly vertically upward (i.e., is vertically upward and has effectively no horizontal component) and therefore moves the lens holder 2-14 closer to the surface of the optical disc 2-76.

[0262] As the lines of flux generated by the second magnet pair 2-24 curve oppositely of those generated by the first magnet pair 2-22, the direction of the magnetic field at any point in the focus coil 2-20 is different than the direction of the field at the corresponding point in the focus coil 2-18. Again, because the flux lines curve, the direction of the field acting on the coil 2-20 varies vertically along the coil. In the plane of FIG. 34 which vertically bisects the coil adjacent the top magnet 2-104 of the second magnet pair 2-24, the magnetic field direction is given by B₃at the top of the coil 2-20 and a force is generated in accordance with Lorentz law in the direction F₃, while in the bisecting plane adjacent the bottom magnet 2-106, the magnetic field direction is given by B₄at the bottom of the coil 2-20 and a force F₄is generated. The forces add to produce a resultant force F_FOCUS2, which, as shown, is strictly vertically upward.

[0263] Thus, it can be seen that the forces F_FOCUS1and F_FOCUS2, act on the focus coils 2-18 and 2-20, respectively, to move the lens holder 2-14 upward. Conversely, if the current was applied to the focus coils 2-18 and 2-20, in the opposite direction, forces would be generated to move the lens holder 2-14 downward, or farther away from the surface of the optical disc 2-76. By moving the objective lens 2-12 closer to or farther away form the surface of the optical disc 2-76, the focus coils 2-18 and 2-20 act to precisely focus the laser beam exiting the objective lens 2-12 on the disc 2-76.

[0264] As illustrated in FIG. 35, movement of the actuator 2-10 to effect fine tracking is produced when current is generated in the tracking coil 2-16 affixed to the lens holder 2-14. In the plane of FIG. 35 which horizontally bisects the tracking coil 2-16, a magnetic field having direction B1 acts on the cross-section of the coil 2-16 located closest to the first magnet pair 2-22 and a magnetic field having the direction B2 acts on the cross-section of the coil located closest to the second magnet pair 2-24. If, for example, a current I is applied in a counterclockwise direction around the tracking coil 2-16, a force F1 acts on the portion of the tracking coil adjacent the first magnet pair 2-22 and a force F2 acts on the portion of the tracking coil adjacent the second magnet pair 2-24. These forces add under the laws of vector addition to produce a resultant force F_TRACKwhich acts to move the lens holder 2-14 to the right in the plane of FIG. 35. When the forces act on the tracking coil 2-16 in this manner, they are translated through the lens holder 2-14 to accelerate or decelerate the moving mass (lens holder), and into the suspension wire pairs 2-82 and 2-84 which bend in the corresponding direction to move the objective lens 2-12 and precisely center the laser beam exiting therefrom within the center of a selected data track on the surface of the optical disc 2-76. Conversely, if a current I is applied in a clockwise direction around the coil 2-16, a resultant force is produced which moves the lens holder 2-14 to the left in the plane of the FIG. 35.

[0265] Thus, it can be seen that the coupling arrangement of the present invention further reduces the distance between the resultant forces acting on the coils 2-16, 2-18, and 2-20 and the optical axis of the objective lens 2-12, decreasing adverse modes of motion such as pitch, roll, and yaw during focusing and tracking operations.

[0266] With the actuator design of the present invention, only two pair of permanent magnets, i.e., four total magnets, and three coils are required to effect movement in both the tracking and focusing directions, thereby reducing both the size and weight of actuator and yielding higher decoupling frequencies. As the component count for the actuator is low, the actuator is easy to manufacture and assemble as compared to prior actuator designs having many more coils, magnets, and pole pieces. In addition, because the tracking and focus coils 2-16, 2-18, and 2-20 are coupled directly to the lens holder 2-14 and are not wound around yokes or poles, coil rigidity and resonance frequency response is significantly improved. Furthermore, direct coupling of the coils 2-16, 2-18, and 2-20, reduces the distance between the point where the effective tracking and focus forces are generated and the optical axis of the objective lens, thereby decreasing adverse motions such as pitch, roll, and yaw.

[0267] The present invention improves motor performance. Values of merit as high as 130 m/s²/sq. rt. (W) for the focus direction and 70 m/s²/sq. rt. (W) for the radial direction have been measured for actuators constructed in accordance with the present invention. These values are significantly higher than previously realized. As those skilled in the art will recognize, the design of the present invention also ensures that approximately 40% of the coil wire is utilized, thereby increasing the efficiency of the actuator over prior designs.

[0268] The preferred embodiment of the two-axis electromagnetic actuator 2-10 has been described with reference to the coordinate system illustrated in FIG. 26 wherein the optical disc 2-76 is positioned above the objective lens 2-12 such that focusing is effected by moving the actuator 2-10 up and down along the Z-axis and tracking movement is effected by moving the actuator in a side-to-side motion along the Y-axis. Those skilled in the art will recognize, however, that the actuator 2-10 of the present invention could also be incorporated in optical systems having different orientations than those illustrated.

[0269] Focus Sensing Apparatus

[0270] FIG. 36 is a block diagrammatic representation of a preferred embodiment of the beam focus sensing apparatus 3-10 of the present invention. The apparatus 3-10 includes an optical arrangement 3-12 for providing a servo beam S indicative of the focus of an illuminating beam I upon an optical disc 3-14. The servo beam S comprises a portion of the illuminating beam I reflected by the disc 3-14. Techniques for generating such a servo beam are well known to those skilled in the art. For example, an optical system such as the optical arrangement 3-12 for generating the servo beam S is described in U.S. Pat. No. 4,862,442, which is herein incorporated by reference. A brief summary of the operation of the optical arrangement 3-12 is set forth below.

[0271] As shown in FIG. 36, the optical arrangement 3-12 includes a laser source 3-16 which generates a linearly polarized beam B. The beam B is collimated by a collimating lens 3-18, and the collimated beam is directed by an optical beamsplitting arrangement 3-20 to an objective lens 3-24. The collimated beam is then converged by the objective lens 3-24 onto the surface of the optical disc 3-14. The optical disc may, for example, comprise a compact disc, video disc, or optical memory disc. The disc 3-14 reflects the illuminating beam focused thereon back through the objective lens 3-24 to the beamsplitting arrangement 3-20. Those skilled in the art will appreciate that the beamsplitting arrangement 3-20 may include a first beamsplitter (not shown) to redirect a first portion of the reflected illuminating beam in order to form the servo beam S. The beamsplitting arrangement 3-20 will also generally include a second beamsplitter (not shown) to redirect a second portion of the reflected illuminating beam to create a data beam. Such a data beam carries information stored on the optical disc 3-14. The servo beam S is intercepted by an FTR prism 3-30, the design and construction of which is discussed more fully hereinafter.

[0272] As is also described more fully below, the servo beam S is divided into a transmitted beam T and a reflected beam R by the FTR prism 3-30. In the embodiment of FIG. 36, the transmitted and reflected beams T and R are of substantially equal cross section and intensity. The transmitted beam T is incident on a first quad detector 3-32, while the reflected beam R is incident on a second quad detector 3-34. Electrical signals produced by the quad detectors 3-32 and 3-34 in response to the intensity distributions of the transmitted and reflected beams T and R, are utilized by a control unit 3-37 to generate a differential focus error signal (DFES) indicative of the focus of the illuminating beam I on the disc 3-14. One preferred embodiment of the control unit 3-37 and associated method for generating the DFES is discussed hereinafter. The focus error signal may, for example, be used to control a mechanical arrangement (not shown here) disposed to adjust the focus of the illuminating beam I by altering the displacement of the objective lens 3-24 relative to the disc 3-14.

[0273] FIG. 37 shows a magnified top cross-sectional view of the FTR prism 3-30. The prism 3-30 includes first and second optical members 3-35 and 3-36 which sandwich a separation layer 3-38. The optical members 3-35 and 3-36 may be formed from glass having an index of refraction larger than that of the separation layer 3-38. For example, in one preferred embodiment, the optical members 3-35 and 3-36 may be manufactured from glass having an index of refraction of 1.55, while the separation layer 3-38 is composed of a solid such as either magnesium fluoride (MgF₂) or fused silica (SiO₂) having indices of refraction of 1.38 and 1.48, respectively. The separation layer 3-38 need not consist of a solid, and may be formed from a liquid or air provided that the optical members 3-35 and 3-36 are of a larger index of refraction.

[0274] A brief description of the physics of the interaction of the light in beam S with layer 3-38 is as follows. If layer 3-38 and optical member 3-35 are not present, the well-known phenomenon of total internal reflection takes place at the hypotenuse face of optical member 3-36, sending all of beam S in the direction of beam R. However, some light energy exists behind the hypotenuse face of optical member 3-36 in the form of “evanescent waves”, which do not propagate. When optical member 3-35 is brought close enough to optical member 3-36, this energy is coupled without loss into member 3-35 and propagates in the direction of beam T. This phenomenon is known as frustrated total reflection (FTR). In this condition, if the FTR prism is disposed with respect to beam S such that the incidence angle A of beam S at separation layer 3-38 is close to the region of frustrated total reflection, the transmission and reflection curves will have very steep slopes (angular sensitivities). This allows the fabrication of a very sensitive focus sensing system. Furthermore, the transmission and reflection curves for such a system based on the FTR principle will be relatively insensitive to the wavelength of the light in beam S, as compared to the curves of a multilayer structure.

[0275] The prism 3-30 may be fabricated by first depositing the separation layer on either of the optical members via conventional thin film techniques. The complementary optical member may then be affixed to the exposed surface of the separation layer with an optical glue. Although the indices of refraction of the first and second optical members 3-35 and 3-36 will generally be chosen to be identical, differing indices of refraction may also be selected. In the preferred embodiment, the first and second optical members have identical indices of refraction in such a geometry that the transmitted and reflected beams T and R are of substantially equal cross-section.

[0276] As shown in the illustrative front view of FIG. 38, the first quad detector 3-32 includes first, second, third, and fourth photodetective elements 3-40, 3-42, 3-44, and 3-46, respectively, which produce electrical signals hereinafter referred to as T1, T2, T3, and T4 in response to the intensity of the transmitted beam T impinging thereon. Similarly, the second quad detector 3-34 includes fifth, sixth, seventh, and eighth photodetective elements 3-50, 3-52, 3-54, and 3-56, respectively, which provide electrical signals hereinafter referred to as R1, R2, R3, and R4 in response to incidence of the reflected beam R. The photodetective elements may be PIN diodes, wherein the level of the electrical output from each diode is proportional to the optical energy received thereby.

[0277] When the objective lens 3-24 of FIG. 36 is situated relative to the disc 3-14 such that the illuminating beam I is properly focused, the rays included within the servo beam S are well collimated (i.e. substantially parallel) and are therefore incident on the separation layer 3-38 at a substantially identical angle A shown in FIG. 37. Contrary to this, when the objective lens 3-24 does not focus the illuminating beam in the plane occupied by the surface of the disc 3-14, the rays comprising the servo beam S will be either mutually convergent or divergent. It follows that all rays within the servo beam S will impinge on the separation layer 3-38 at the substantially same angle when the illuminating beam I is suitably focused, while rays of a different range of angles of incidence will address the separation layer 3-38 when the beam I is out of focus. The prism 3-30 is designed such that the reflectivity and transmissivity of the separation layer 3-38 is extremely sensitive to the angle at which optical energy is incident on the separation layer 3-38. Thus, the spatial distribution in the intensity of the transmitted and reflected beams T and R will vary as the focus position of the illuminating beam I varies relative to the surface of the disc 3-14. That is, an illuminating beam I which is properly focused gives rise to a well collimated servo beam S such that all the rays thereof experience the same degree of reflection by the separation layer 3-38. Accordingly, the transmitted and reflected beams T and R will be of substantially uniform intensity when the illuminating beam I is appropriately focused. Conversely, a convergent or divergent servo beam S will engender transmitted and reflected beams T and R of nonuniform spatial intensity distributions since the rays within the servo beam S will be subject to a variety of degrees of reflection by the separation layer 3-38. By detecting these spatial variations in the intensity of the transmitted and reflected beams, the photodetectors 3-32 and 3-34 produce electrical signals which may be utilized to produce a DFES indicative of the focus position of the illuminating beam I.

[0278] The manner in which a DFES may be synthesized in response to the degree of collimation of the servo beam S may be further understood with reference to FIG. 39. FIG. 39 is a graph showing the reflectivity (intensity of beam R÷intensity of beam S) of the FTR prism 3-30 as a function of the angle of incidence of rays within the servo beam S relative to the separation layer 3-38. Specifically, the graph of FIG. 39 depicts the reflectivities Rs and Rp of the prism 3-30 in response to illumination by both s-polarized and p-polarized optical energy of wavelength 0.78 microns. The reflectivity profiles of FIG. 39 pertain to a FTR prism 3-30 having a separation layer 3-38 with a thickness of 4.5 microns and an index of refraction of 1.38, with the separation layer being sandwiched by glass members having an index of refraction of 1.55. As represented in FIG. 39, the prism 3-30 is preferably positioned relative to the servo beam S at an angle of incidence A₁such that the prism 3-30 is operative about a working point P. That is, at the working point P, the prism 3-30 is positioned such that an illuminating beam I properly focused on the disc 3-14 engenders a well collimated servo beam S having rays which impinge on the separation layer 3-38 at the angle A₁. Since the reflectivity of the prism 3-30 is approximately 0.5 at the operating point P, the transmitted and reflected beams produced by the optical arrangement 3-12 including the prism 3-30 are of substantially identical average intensity.

[0279] When the separation between the objective lens 3-24 and the disc 3-14 varies such that the servo beam S decollimates in either a convergent or divergent manner, a first portion thereof will impinge on the separation layer 3-38 at an angle of incidence larger than the angle A₁. For example, at an angle of incidence of A2, FIG. 39, a corresponding portion of the servo beam will experience a reflectivity of approximately 0.7. Since the first servo beam portion is subject to a reflectivity of only 0.5 when the servo beam S is well collimated, the regions of the detectors 3-32 and 3-34 which receive the parts of the reflected and transmitted beams R and T derived from the first servo beam portion will collect more and less optical energy, respectively, than when the illumination beam I is properly focused. Similarly, the areas of the detectors 3-32 and 3-34 in optical alignment with parts of the transmitted and reflected beams T and R arising from a second portion of the servo beam S incident on the separation layer 3-38 at an angle of incidence A₃, which is smaller than the angle A₁, will be illuminated by more and less optical energy, respectively, than in a condition of proper focus. The DFES is produced in response to electrical signals engendered by the photodetectors 3-32 and 3-34 indicative of this spatial nonuniformity in the intensity distribution of the transmitted and reflected beams T and R. Moreover, since in the preferred embodiments described herein, the prism 3-30 is optically nonabsorbing, variation in the intensity of the transmitted beam T arising from a change in the angle of incidence of a portion of the servo beam S is mirrored by an equal, oppositely directed variation in the magnitude of the part of the reflected beam R engendered by the identical servo beam portion. Non-differential error signals may be generated independently from either the transmitted or reflected beam, using the equations:

[0280] FES(transmitted)=(T1+T2)−(T3+T4)  (1)

[0281] FES(reflected)=(R1+R2)−(R3+R4)  (2)

[0282] In the differential system, the differential focus error signal (DFES) is generated by the control unit 3-37 in accordance with the following expression:

[0283] DFES=(R1+R2+T3+T4)−(T1+T2+R3+R4)  (3)

[0284] The control unit 3-37 includes circuitry suitable for carrying out the arithmetic operations of equation (3) and for generating a DFES based on these operations. Preamplifiers (not shown here) are included to amplify the electrical signals from the photodetectors 3-32 and 3-34 prior to processing by the control unit 3-37.

[0285] Utilizing the dual quad photodetector arrangement described herein leads to the synthesis of differential focus error signals having a reduced sensitivity to certain beam imperfections not induced by inaccuracies in the focus position of the illuminating beam relative to the disc 3-14. Since a localized decrease in the intensity of the servo beam S unrelated to the focus position of the illuminating beam affects the detectors 3-32 and 3-34 in a substantially similar manner, such a decrease does not affect the value of the DFES due to the corresponding cancellation which occurs in equation (3).

[0286] As mentioned above in the Background of the Invention, prior focusing systems were generally ill-equipped to implement the differential focus sensing scheme described by equation (3). In particular, a feature of the present invention lies in the ability of the FTR prism 3-30 to provide transmitted and reflected beams of substantially similar cross section and intensity such that both may effectively contribute to the synthesis of a DFES.

[0287] In addition to providing a DFES for maintaining the focus of the illuminating beam I in the direction normal to the surface of the disc 3-14, the electrical outputs from the photodetectors 3-32 and 3-34 may also be used by the control unit 3-37 to generate a tracking error signal (TES). The TES is indicative of the radial position of the illuminating beam I relative to the conventional spiral or concentric guiding tracks (not shown) imprinted on the surface of the disc 3-14. The TES enables the beam I to follow the guiding tracks despite eccentricities therein by controlling a mechanical arrangement (not shown here) operative to adjust the radial position of the objective lens 3-24 relative to the disc 3-14. The TES is calculated by the control unit 3-37 on the basis of electrical outputs from the photodetectors 3-32 and 3-34 in accordance with the following equation:

[0288] TES=(T1+T3+R3+R1)−(T2+T4+R2+R4)  (4)

[0289] Again, the manner in which a tracking error signal may be derived from the relationship existing between spatial intensity changes of the servo beam and the tracking position of the illuminating beam is disclosed in, for example, U.S. Pat. No. 4,707,648.

[0290] In perhaps the majority of systems operative to control the focus of an illuminating beam relative to an optical disc, it will be desired to generate both tracking and focus error signals in response to the electrical outputs of the photodetective elements. Since generation of both the focus and tracking error signals is known to generally require at least one quad photodetector, the embodiments of the present invention disclosed herein have been described with reference to quad photodetectors. It is also known, however, that a focus error signal may be derived on the basis of electrical signals produced by photodetectors having only two independent photosensitive regions (bicell detectors). Accordingly, in applications requiring only the generation of a focus error signal, a single photodetective element could be substituted for the first and second elements 3-40 and 3-42 of the photodetector 3-32, and a single photodetective element could replace the third and fourth elements 3-44 and 3-46. Similarly, a single photodetective element could be used in lieu of the fifth and sixth elements 3-50 and 3-52 of the photodetector 3-34, and a single element could be substituted for the seventh and eighth elements 3-54 and 3-56.

[0291] The slope of the reflectivity profile of FIG. 39 about the working point P is proportional to the sensitivity of the DFES generated by the apparatus 3-10. Specifically, the sensitivity of the apparatus 3-10 to changes in the focus of the illuminating beam I is augmented by increases in the slope of the reflectivity profile. Accordingly, it is an object of the present invention to provide a prism 3-30 characterized by a reflectivity profile which is as steep as practically possible.

[0292] The shape of the reflectivity profile of FIG. 39 about the working point P may be altered by adjusting the thickness of the separation layer 3-38. For example, increasing the thickness of the separation layer 3-38 translates the angle of minimum reflectivity A_mtowards the critical angle A_c, see FIG. 39, without affecting the value of the latter. It follows that increasing the separation layer thickness serves to increase the slope of the reflectivity profile in the vicinity of the working point P. Similarly, reducing the thickness of the separation layer 3-38 enlarges the angular displacement between the critical angle A_cand the angle of minimum reflectivity A_m. The shape of the reflectivity profile of the prism 3-30 may be varied in order to adjust the sensitivity of the DFES. A reasonable slope can be obtained, for example, by use of a separation layer having a thickness that is greater than one half the wavelength of the illuminating beam I.

[0293] The value of the critical angle A_Cmay be adjusted by varying the index of refraction of the separation layer 3-38 relative to that of the glass members 3-35 and 3-36. Thus, adjustment of the separation layer thickness in conjunction with manipulation of the indices of refraction of the separation layer and surrounding glass members allows the prism 3-30 to be fabricated in accordance with a desired reflectivity profile.

[0294] FIG. 40 is a graph of the value of a normalized DFES (NDFES) generated by the apparatus 3-10 as a function of the deviation from the desired displacement of the objective lens 3-24 relative to the disc 3-14.

[0295] Again, the data in FIG. 40 was obtained by utilizing a prism 3-30 having a separation layer of index of refraction 1.38 and thickness 4.5 microns sandwiched between glass members of index of refraction 1.55, with the prism 3-30 being illuminated by a servo beam of wavelength 0.78 microns. As is shown in FIG. 40, the value of the DFES is preferably zero when the desired displacement exists between the objective lens 3-24 and the disc 3-14. The sign (+ or −) of the DFES is thus indicative of whether the displacement between the objective lens and disc surface exceeds or is less than that required for proper focusing. As mentioned above, the DFES may be used to control a mechanical arrangement (not shown here) disposed to adjust the separation between the objective lens 3-24 and the disc 3-14. It may be appreciated that the slope of the NDFES is approximately 0.16 micron⁻¹at the working point defined by 0 (zero) disc displacement.

[0296] Although the servo beam S has been represented herein to be substantially collimated when incident on the separation layer 3-38, the present invention is not limited to configurations giving rise to collimated servo beams. When a convergent or divergent servo beam is utilized, inaccuracies in the focus position of the illuminating beam will alter the degree of convergence or divergence thereof. Those skilled in the art will appreciate that the focus sensing apparatus of the present invention may be utilized to generate a DFES in response to such changes in convergence or divergence.

[0297] The inventive focus sensing apparatus has thus been shown to overcome the disadvantages inherent in other focus detection systems by providing reflected and transmitted beams of substantially similar shape and intensity from which a high precision, altitude insensitive focus error signal may be differentially derived. The focus sensing technique disclosed herein nonetheless retains features present in certain related focus detection systems such as low sensitivity to mechanical vibration, decreased sensitivity to disc tilt, and increased thermal stability.

[0298] Seek Actuator

[0299] FIG. 41 schematically illustrates the operation of an exemplary optical read/write system 4-50 in reading data from a precise location 4-52 on an information storage medium, such as an optical disc 4-54. While the system 4-50 illustrated is a write-once or WORM system, those skilled in the art will recognize that the carriage and actuator assembly of the present invention could also be used in magneto-optical erasable system. Information is transmitted to and read from the disc 4-54 utilizing a light beam 4-56 produced by a light source 4-58 which passes through a plurality of components including a cube-shaped beamsplitter 4-60 which separates the light beam 4-56 according to its polarization, a quarter wave plate 4-62 which changes the polarization of the light beam 4-56, a collimator lens 4-64, and an objective lens 4-66, which, in combination, direct the light beam 4-56 toward the desired location 4-52 on the disc 4-54.

[0300] In operation, the light source 4-58, typically a laser diode, emits the light beam 4-56 toward the convex collimator lens 4-64. The collimator lens 4-64 converts this source beam 4-56 into a parallel, linearly S polarized light beam 4-70 and conducts the beam 4-70 toward the beamsplitter 4-60. This cube-shaped beamsplitter 4-60 is formed by attaching two right angle prisms 4-72 and 4-74 along their respective hypotenuses and includes a polarization sensitive coating forming a beamsplitting interface 4-76 between the two hypotenuses. The beamsplitter 4-60 separates and/or combines light beams of differing polarization states, namely linear S polarization and linear P polarization. Separation is accomplished in conjunction with the polarization sensitive coating which transmits linearly P polarized light beams and reflects linearly polarized S light beams. Light exiting the beamsplitter 4-60 passes through the quarter wave plate 4-62 which converts the linearly polarized light beam 4-70 to a circularly polarized light beam 4-78. Upon exiting the quarter wave plate 4-62, the circularly polarized beam 4-78 enters an actuator 4-80.

[0301] The actuator 4-80 includes a mirror 4-82 which orthogonally reflects the light beam 4-78 upward toward the objective lens 4-66. This objective lens 4-66 converges the circularly polarized beam 4-78 to the precise focal point 4-52 on the surface of the optical disc 4-54. Upon striking the disc 4-54, the circularly polarized light beam 4-78 is altered by the information stored on the disc 4-54 and is reflected as a divergent circularly polarized light beam 4-84 carrying information identical to that encoded on the disc 4-54. This reflected circularly polarized light beam 4-84 re-enters the objective lens 4-66 where it is collimated. The light beam 4-84 is again reflected from the mirror 4-82 and re-enters the quarter wave plate 4-62. Upon exiting the quarter wave plate 4-62, the circularly polarized beam 4-84 is converted to a linearly P polarized light beam 4-86. As linearly P polarized light beams are transmitted through the beamsplitter 4-60 without reflection at the splitting interface, this light beam 4-86 continues to a photodetector 4-88, which detects the data stored on the disc 4-54. In addition, if the light beam 4-86 falling on the photodetector 4-88 is out of focus or misaligned, the amount of misalignment or defocusing is measured electronically and used as feedback for a servo system (not shown here) which properly realigns the objective lens 4-66.

[0302] FIG. 42 illustrates an electromagnetic carriage and actuator assembly 4-100 constructed in accordance with the present invention. The assembly can be used with an optics module 4-102 to read and write data onto the surface of an optical disc as described above in connection with FIG. 41, wherein the light source 4-58, detector 4-88, collimating lens 4-64, quarter wave plate 4-62, and beamsplitter 4-60 are all incorporated within the module 4-102. A spindle motor 4-104 is located adjacent the assembly 4-100 and rotates an optical disc (not shown here) about an axis of rotation A above the assembly 4-100. The assembly 4-100 includes a carriage 4-106 having first and second bearing surfaces 4-108 and 4-110 slidably mounted on first and second guide rails 4-112 and 4-114, respectively, and an actuator 4-116 which is mounted on the carriage 4-106. As will be appreciated, the rails 4-112 and 4-114 provide a frame along which the carriage moves. A beam of light 4-120 emitted from the light source 4-58 in the optics module 4-102 enters the actuator 4-116 through a circular aperture 4-118 and is reflected by a mirror contained inside the actuator through an objective lens 4-122 defining an optical axis O to the surface of the disc. As readily understood, the axis of rotation A of the disc is parallel to the optical axis O of the objective lens 4-122.

[0303] The carriage 4-106 and actuator 4-116 carried thereon are moved horizontally along the rails 4-112 and 4-114 in a tracking direction by a coarse tracking motor to access various information tracks on the surface of the disc. The tracking motor includes two permanent magnets 4-130 and 4-132 wherein each magnet is attached to a C-shaped outer pole piece 4-134 and 4-136, respectively. Two inner pole pieces 4-138 and 4-140 are positioned across the ends of the outer pole pieces 4-134 and 4-136 so as to form a rectangular box around the permanent magnets 4-130 and 4-132. Two coarse coils 4-142 and 4-144 of equal length are affixed to vertical plates 4-174 and 4-176, FIG. 43, and surround the inner pole pieces 4-138 and 4-140 with sufficient clearance to move over the pole pieces 4-138 and 4-140 when the carriage 4-106 is moved in the tracking direction. In this embodiment, these coarse coils 4-142 and 4-144 are the only portion of the course tracking motor that are movable. As will be described in more detail below, the actuator 4-116 can also move the objective lens 4-122 closer to or farther away from the disc, thereby focusing the exiting light beam 4-120 upon the desired location on the surface of the disc.

[0304] FIG. 43 is an exploded view illustrating the carriage 4-106 and actuator 4-116 in greater detail. The carriage 4-106 includes a generally rectangular base 4-150 to which the actuator 4-116 is attached. The base 4-150 has a substantially flat top surface 4-152 having a generally rectangular chamber 4-154 formed therein. The first bearing surface 4-108 is cylindrical in shape, while the second bearing surface 4-110 consists of two elliptical bearing sections 4-160 and 4-162 of approximately equal length which meet inside the base 4-150. The spacing of the rails 4-112 and 4-114 relative to the optical axis O is selected such that each bearing surface 4-108 and 4-110 is subjected to the same amount of preload. The bearing surfaces 4-108 and 4-110 are further designed such that both surfaces have substantially the same amount of surface area contacting the rails 4-112 and 4-114. The length of the bearing sections comprising the second bearing surface is approximately equal to the length of the first bearing surface, although minor variations in length may be necessary to account for wear.

[0305] Two vertical walls 4-156 and 4-158 extend upwardly from the top surface 4-152 of the base 4-150 adjacent the ends of the chamber 4-154. The base 4-150 further includes two platform regions 4-164 and 4-166 formed at the ends of the base 4-150 above the bearing surfaces 4-108 and 4-110. A step 4-168 joins the top surface 4-152 of the base 4-150 with the second platform region 4-166. A first U-shaped notch 4-170 is formed in the first platform region 4-164, and a second U-shaped notch 4-172 is formed in the second platform region 4-166 and step 4-168.

[0306] The coarse coils 4-142 and 4-144 are attached to the two vertical plates 4-174 and 4-176 respectively. The plates 4-174 and 4-176 are respectively positioned in notches 4-180 and 4-182 formed in the ends of the base 4-150. The base 4-150 further includes a mass balancing plate 4-184 which is attached to a bottom surface 4-186 of the base 4-150 via a screw 4-188, and a mass balancing projection 4-190 which extends outwardly from the base 4-150 adjacent the first coarse coil 4-142. A circular aperture 4-192 is formed in a front side 4-194 of the base 4-150 and receives the light beam 4-120 emitted from the optics module 4-102 of FIG. 42. A bracket 4-196 having a circular aperture 4-198 therein is positioned between the second vertical wall 4-158 and the first platform region 4-164 along the front side 4-194 of the base 4-150. The bracket 4-196 additionally includes a notch 4-200 which receives a photodetector 4-202 such that the photodetector 4-202 is positioned between the bracket 4-196 and the first platform region 4-164.

[0307] The actuator 4-116, often referred to as a “2-D” actuator for 2 degrees of motion, i.e. focusing and tracking, is mounted on the base 4-150 between the vertical walls 4-156 and 4-158 and the platform regions 4-164 and 4-166. A prism (not shown) is positioned within the chamber 4-154 in the base 4-150 to deflect the light beam 4-120 emitted from the optics module 4-102 such that the beam 4-120 exits the actuator 4-116 through the objective lens 4-122. The objective lens 4-122 is positioned within a lens holder 4-210 attached to a focus and fine tracking motor which moves the lens 4-122 so as to precisely align and focus the exiting beam 4-120 upon a desired location on the surface of the optical disc. The objective lens 4-122 defines the optical axis O which extends vertically through the center of the lens.

[0308] The components of the actuator 4-116 are best seen in FIG. 44. The lens holder 4-210 is generally rectangular in shape and includes a generally rectangular opening 4-212 formed therethrough. A top surface 4-214 of the lens holder 4-210 includes a circular collar 4-216 positioned between two shoulders 4-218 and 4-220. A circular aperture 4-222 having a diameter substantially equal to that of the collar 4-216 is formed in a bottom surface 4-224 of the lens holder. A rectangular focus coil 4-230 is positioned within the rectangular opening 4-212 in the lens holder 4-210. Two oval-shaped, fine tracking coils 4-232 and 4-234 are positioned at the corners of a first end 4-240 of the focus coil 4-230, and two more identical tracking coils 4-236 and 4-238 are positioned at the corners of a second end 4-242 of the focus coil 4-230. A first pair of U-shaped pole pieces 4-244 is positioned to surround the first end 4-240 of the focus coil 4-230 and tracking coils 4-232 and 4-234 attached thereto, while a second pair of U-shaped pole pieces 4-246 surrounds the second end 4-242 of the focus coil 4-230 and tracking coils 4-236 and 4-238 attached thereto. In addition, two permanent magnets 4-250 and 4-252 are positioned between the respective pole piece pairs 4-244 and 4-246, adjacent the respective tracking coils 4-232, 4-234, and 4-236, 4-238.

[0309] Two top flexure arms 4-260 and 4-262 are attached to the top surface 4-214 of the lens holder 4-210 while two additional bottom flexure arms 4-264 and 4-266 are attached to a bottom surface of the lens holder 4-210. Each flexure arm preferably consists of a thin sheet of etched or stamped metal (typically steel or beryllium copper) with a thickness in the order of 25 micrometers to 75 micrometers. For simplicity, only the flexure arm 4-260 will be described. It should be noted, however, that the remaining flexure arms 4-262, 4-264, and 4-266 are of identical structure. The flexure arm 4-260 includes a first vertical section 4-270 attached to first, second, and third horizontal sections 4-272, 4-274, and 4-276. The third horizontal section 4-276 is further attached to a perpendicular crossbar 4-280. The first horizontal section 4-272 includes a shoulder 4-278 which attaches to the corresponding shoulder 4-218 on the lens holder 4-210. In a similar manner, the shoulder of the second top flexure arm 4-262 attaches to the corresponding shoulder 4-220, while the shoulders of the bottom flexure arms 4-264 and 4-266 attach to the corresponding structures on the bottom surface of the lens holder 4-210.

[0310] The flexures 4-260, 4-262, 4-264, and 4-266 are further attached to a support member 4-290. The support member 4-290 includes a central notch 4-292 which receives the second pair of pole pieces 4-246. A ledge 4-294 is formed on each side of the notch 4-292 on the top and bottom surfaces of the support member 4-290. The crossbar sections 4-280 of the flexure arms 4-260 and 4-262 are attached to these ledges 4-294, while flexure arms 4-264 and 4-266 are connected to corresponding ledges on the bottom of the support member 4-290 so as to cooperatively suspend the lens holder 4-210 from the support member 4-290. The support member 4-290 further includes an aperture 4-296 for receiving a light emitting diode 4-300. The diode 4-300 is in alignment with the aperture 4-198 in the bracket 4-196, FIG. 43, and photodetector 4-202 positioned within the notch 4-200 in the bracket, such that when the light emitting diode 4-300 is energized, substantially collimated light is emitted through the aperture 4-198 in the bracket 4-196 and is incident upon the photodetector 4-202. Depending on the position of the lens holder 4-210 with respect to the support member 4-290, light emitted by the diode 4-300 will fall onto various portions of the detector 4-202. By analyzing the amount of light incident upon the detector 4-202, a position correction signal can be generated to determine the amount of displacement required for precise focusing and tracking at the desired location on the surface of the disc.

[0311] In the illustrated embodiment, the fine motor mass consists of the lens holder 4-210, the objective lens 4-122, the focus coil 4-230, and the fine tracking coils 4-232, 4-234, 4-236, and 4-238. The carriage mass consists of the base 4-150, course tracking coils 4-142 and 4-144, the bracket 4-196, and photodetector 4-202, the support member 4-290, the vertical plates 4-174 and 4-176, the mass balancing plate 4-184 and screw 4-188, the permanent magnets 4-250 and 4-252, the pole pieces 4-244 and 4-246, and the bearing surfaces 4-108 and 4-110.

[0312] With reference to the above description in connection with FIGS. 43 and 44, the coarse tracking coils 4-142 and 4-144 have equal dimensions and are symmetric about optical axis O of the objective lens. Further, the tracking coil pairs 4-232, 4-234 and 4-236, 4-238 have equal dimensions and are symmetric about optical axis O of the lens 4-122. The dimensions of the mass balance plate 4-184 and mass balance projection 4-190 are advantageously selected to compensate for the mass of the support member 4-290, flexures 4-260, 4-262, 4-264, 4-266, bearing surfaces 4-108, 4-110, bracket 4-196 and photodetector 4-202, such that the center of mass of the carriage and the center of mass of the fine and focus motors (consisting of the pole pieces 4-244, 4-246, the permanent magnets 4-250, 4-252, the focus coil 4-230, and tracking coils 4-232, 4-234, 4-236, 4-238) are generally intersected by the optical axis O of the lens 4-122. As will be described in more detail below, alignment of these centers of gravity with the optical axis O of the lens 4-122, and the symmetry of the motor forces and reaction forces acting on the carriage 4-106 and actuator 4-116 ensure that undesirable modes of motion which adversely affect position of the objective lens 4-122 are minimized.

[0313] Referring to FIG. 45, the permanent magnets 4-130, 4-132 adjacent the coarse tracking coils 4-142, 4-144 generate a magnetic field B whose lines of flux extend inwardly toward the coarse coils 4-142 and 4-144. When coarse tracking movement is required to position the objective lens 4-122 beneath a selected track on the optical disc, current is applied to the coarse tracking coils 4-142, 4-144. The current interacts with the magnetic field B to produce forces which move the carriage 4-106 in the tracking direction. The forces are generated according to the Lorentz law F=B·X·I·1, wherein, as stated above, F represents the force acting on the focus coil, B represents the magnetic flux density of the magnetic field between the two permanent magnets, I represents the current through the focus coil, and 1 represents the length of the coil. For example, when the current I applied to the first coarse tracking coil 4-142 travels through the portion of the coil positioned within the magnetic field B in the direction into the plane of FIG. 45, a force F_Coarse1in the direction of the arrow 4-320 is produced. Similarly, when current I travels through the portions of the second tracking coil 4-144 positioned within the magnetic field B in the direction out of the plane of FIG. 45, a force F_Coarse2in the direction of the arrow 4-322 is produced. The forces F_Coarse1and F_Coarse2act to move the carriage 4-106 horizontally to the left.

[0314] Conversely, FIG. 46 shows that if the direction of the current I within the portions of the tracking coils 4-142, 4-144 within the magnetic field B is reversed, forces F_Coarse′, and F_Coarse2′, are produced which act to move the carriage into the plane of the drawing sheet of FIG. 46 (to the right in FIG. 45). The amount of movement in the tracking direction depends on the amount of current applied to the coarse coils 4-142 and 4-144. In this manner, the carriage 4-106 is moved to position the objective lens 4-122 such that the laser beam 4-120 exiting the lens 4-122 is focused within a desired information track on the surface of the optical disc.

[0315] When a control signal is generated by the optics module 4-102, a given current is applied to either the fine tracking coils 4-232, 4-234, 4-236, and 4-238, or the focus coil 4-230 depending on the direction in which the displacement of the lens holder 4-210 and objective lens 4-122 attached thereto is required. Such servo system and feedback circuits which control the amount of current are well known in the art. This current interacts with the electromagnetic field produced by the permanent magnets 4-250 and 4-252 to create a force which displaces the lens holder 4-210 and the objective lens 4-122 attached thereto in the appropriate tracking or focusing direction. For example, if repositioning is desired in the focus direction, according to a focus error signal, this signal is transmitted to a servo amplifier (not shown here), which generates a current through the focus coil 4-230. As described above, a force is generated according to the Lorentz law F=B·X·I·1.

[0316] With reference now to FIG. 47, the permanent magnets 4-250 and 4-252 of the 2-D actuator 4-116 are oriented such that the south poles of each magnet 4-250, 4-252 face the lens holder 4-210. In this configuration, a magnetic field B is formed whose lines of flux originate at the magnets 4-250, 4-252 and are directed inwardly toward the lens holder 4-210 as shown. When a current I is applied to the focus coil 4-230 and travels through the portions of the coil 4-230 positioned within the magnetic field B in the direction shown, an upward force F_Focusis generated at each section of the focus coil 4-230 which is translated to the flexure arms 4-260, 4-262, 4-264, and 4-266, bending the flexure arms to move the lens holder 4-210 and associated objective lens 4-122 closer to the optical disc. Conversely, when the current I is run through the coil sections in the opposite directions as those illustrated, a downward force is generated which acts on the flexures to move the lens holder 4-210 and objective lens 4-122 farther away from the surface of the optical disc. The magnitude of the displacement is dependent upon the amount of current applied to the focus coil 4-230. By moving the objective lens 4-122 closer to or farther away from the surface of the optical disc, the focus coil 4-230 acts to precisely focus the laser beam 4-120 exiting the objective lens 4-122 within the desired information track on the disc.

[0317] As shown in FIG. 48, movement of the actuator 4-116 to effect fine tracking is produced when current is generated in the four fine tracking coils 4-232, 4-234, 4-236, and 4-238 affixed to the focus coil 4-230. When current is applied to the tracking coils in the directions shown through the portions of the tracking coils positioned within the magnetic field B, forces F_Trackare produced which move the lens holder 4-210 to the right. When the forces FTrack act on the tracking coils 4-232, 4-234, 4-236, and 4-238, they are translated through the focus coil 4-230 and lens holder 4-210 to the flexures 4-260, 4-262, 4-264, and 4-268 which bend in the corresponding direction, and the objective lens 4-122 is moved in the direction of the forces, to the right in FIG. 48. When current travels through the tracking coils 4-232, 4-234,4-236, and 4-238 in the opposite direction, a force is generated which acts to move the lens holder 4-210 to the left. The amount of current applied to the fine tracking coils 4-232, 4-234, 4-236, and 4-238 is relatively small in comparison with the amount applied to the coarse tracking coils 4-242, 4-244, and the dimensions of the fine tracking coils are much smaller than the coarse coils to increase resonance frequencies and thus enable higher servo bandwidths which can then control to tighter track errors.

[0318] FIGS. 49A-56B are schematic diagrams of the actuator and carriage assembly 4-100 which illustrate the symmetry and balancing of forces achieved with the design of the present invention.

[0319] FIG. 49A is a schematic diagram illustrating the symmetry of coarse or carriage motor forces acting on the actuator 4-116 in the horizonal plane. When current is applied to the coarse tracking coils 4-142 and 4-144 as described above, forces F_Coarse1and F_Coarse2are produced which are centered within the portion of the coarse coils 4-142, 4-144 located adjacent the permanent magnets 4-130 and 4-132, respectively. The dimensions of the first coarse coil 4-142 are selected to equal the dimensions of the second coarse coil 4-144, and the current applied to each coil is the same, such that the forces F_Coarse1and F_Coarse2acting on the coils are equal. Further, the coarse coils 4-142 and 4-144 are positioned at equal distances L_C1and L_C2from the objective lens 4-122 such that the resulting moments about the optical axis O of the objective lens 4-122 are equal, and the carriage yaw is minimized. In FIG. 49B, the centers of the coarse motor forces F_Coarse1and F_Coarse2are illustrated in the vertical plane. Because the forces F_Coarse1and F_Coarse2are vertically aligned with the center of mass of the carriage CM_C(i.e., are generally intersected by a line orthogonal to the radial direction and the optical axis O containing the center of mass of the carriage CM_C), the moments about the horizontal axis are equal, and carriage pitch which can cause the prism to deflect the beam angle, thereby introducing track offset, is reduced.

[0320] The fine tracking motor forces in the horizontal and vertical planes are illustrated in FIGS. 50A and 50B. The forces F_Track1and F_Track2produced by the energization of the fine tracking coils 4-232, 4-234, 4-236, and 4-238 within the magnetic field induced by the permanent magnets 4-250 and 4-252 are centered between the pairs of fine tracking coils 4-232, 4-234 and 4-236, 4-238, and extend horizontally in the tracking direction. The dimensions of the coils are equal and the amount of current applied to the coils is equal as well, such that the magnitude of the resulting forces F_Track1and F_Track2is equal. Additionally, the fine tracking coils 4-232, 4-234, 4-236, and 4-238 are positioned at equal distances L_Tfrom the optical axis O of the objective lens 4-122, and thus, the moments produced about the optical axis O are equal, such that yaw of the lens holder 4-210 and lens 4-122 carried thereon about the vertical axis is decreased. As illustrated in FIG. 50B, the resultant fine tracking force F_Trackacts on the center of mass of the fine motor mass CM_F, such that lens holder pitch is minimized.

[0321] FIG. 51A illustrates the reaction forces F_React1and F_React2resulting from the fine tracking motor which act upon the carriage 4-106 in opposition to the fine tracking motor forces F_Track1and F_Track2illustrated in FIG. 50A. These reaction forces F_React1and F_React2act on the pole pieces 4-244 and 4-246 positioned over the tracking coils 4-232, 4-234, 4-236 and 4-238 on each side of the lens holder 4-210. As described above, the magnitude of the tracking forces F_Track1and F_Track2is equal. Further, the dimensions of the pole pieces 4-244 and 4-246 are equal, such that the reaction forces F_React1and F_React2produced are equal. Because the pole pieces 4-244 and 4-246 are positioned at equal distances L_Rfrom the optical axis O of the lens 4-122, the moments about the optical axis O are equal in magnitude, reducing rotation about the vertical axis, or yaw. FIG. 51B illustrates the resultant reaction force F_Reactin the vertical plane. As shown, the reaction force F_Reactacts at the center of mass of the fine motor mass CM_F, at a distance L_RMabove the center of mass of the carriage mass CM_C, and thus a moment will act on the carriage 4-106. Because the distance L_RMand the reaction forces F_React1and F_React2are fairly small, however, this moment is relatively small and does not significantly affect carriage performance.

[0322] The resultant focus forces F_Focus1and F_Focus2acting on the actuator 4-116 are illustrated in FIG. 52A. The focus forces F_Focus1and F_Focus2are centered in the portions of the focus coil 4-230 located between the tracking coils 4-232, 4-234, 4-236 and 4-238 and pole pieces 4-244, 4-246, adjacent the permanent magnets 4-250 and 4-252. The focus coil 4-230 is wound within the opening 4-212 in the lens holder 4-210, FIG. 44, such that the same amount of current flows through each side of the coil 4-230 adjacent the magnets, thus producing equal forces F_Focus1and F_Focus2at the sides of the lens holder 4-210 which act to move the lens holder and objective lens 4-122 carried thereon in a vertical direction. The coil is positioned symmetrically within the opening 4-212 in the lens holder 4-210 such that the centers of the forces F_Focus1and F_Focus2produced are positioned equidistantly at distances L_Ffrom the optical axis O of the objective lens 4-122: In this configuration, the moments produced about the optical axis O of the lens 4-122 are equal, reducing roll of the lens holder 4-210. Additionally, as illustrated in FIG. 52B, when viewed from the end of the carriage, the focus forces F_Focus1and F_Focus2(F_Focusin the drawing) are aligned with the center of mass CM_Cof the carriage mass, thereby reducing pitch of the carriage 4-106.

[0323] The reaction forces F_FR1and F_FR2produced in response to the focus forces F_Focus1and F_Focus2shown in FIG. 52A are illustrated in the horizontal plane in FIG. 53A. The reaction forces F_FR1and F_FR2are equal in magnitude and opposite in direction to the focus forces F_Focus1and F_Focus2and are centered adjacent the fine motor permanent magnets 4-250 and 4-252 intermediate the pole pieces 4-244 and 4-246. As described above, the focus forces F_Focus1and F_Focus2are equal, and thus, the reaction forces F_FR1and F_FR2are equal as well. Further, the reactions forces F_FR1and F_FR2act at equal distances L_FRfrom the optical axis O of the objective lens 4-122 to further reduce pitch. Additionally, as illustrated in FIG. 53B, when viewed from the end of the carriage 4-106, the reaction forces F_FR1and F_FR2(F_FRin the drawing) are aligned with the center of mass CM_Cof the carriage mass, thereby reducing pitch of the carriage.

[0324] Forces F_Flex1and F_Flex2generated by the flexure arms 4-260, 4-262, 4-264, and 4-266 on the lens holder 4-210 are illustrated in FIG. 54. The forces F_Flex1and F_Flex2illustrated are those acting on the upper flexure arms 4-260, 4-262. It should be understood by those skilled in the art that identical forces act on the lower flexure arms 4-264 and 4-266, as well. The forces F_Flex1and F_Flex2acting on the upper flexure arms 4-260 and 4-262, respectively, are centered at the crossbar sections 4-280 of the flexure arms 4-260 and 4-262 where the flexure arms are attached to the support member 4-290. As previously described, when these forces F_Flex1and F_Flex2act on the flexure arms 4-260 and 4-262, the flexure arms bend in the appropriate direction to achieve fine tracking. To maintain the flexure arms 4-260 and 4-262 in their bent condition, the fine motor generates reaction forces F_RAand F_RBwhich are centered at the pole pieces 4-244 and 4-246 on either side of the lens holder 4-210. As shown, the flexure forces F_Flex1and F_Flex2act a distance L_Flexfrom the optical axis O of the focus lens 4-122, while the reaction forces F_RAand F_RBact distances L_RAand L_RBfrom the optical axis O, respectively. It will be apparent to those skilled in the art that the moments produced about the optical axis O of the lens 4-122 by the pairs of forces are not equal, since (F_Flex1+F_Flex2) does not equal (F_RAL_RA+F_RBL_RB). Since, however, these forces are effectively decoupled from the carriage except at very low frequencies (typically below around 40 hz), these forces do not affect actuator performance at most normal operating conditions.

[0325] As described above, the carriage 4-106 includes two bearing surfaces 4-108 and 4-110 which are slidably mounted on the guide rails 4-112 and 4-114 in order to position the carriage 4-106 beneath various data tracks on the optical disc. In essence, the bearings 4-108 and 4-110 act as “springs” which hold the carriage 4-106 above the rails 4-112 and 4-114. Bearing “spring” stiffness forces F_Bearing1and F_Bearing2are illustrated in FIG. 55A. The forces F_Bearing1and F_Bearing2are centered at the point of contact between the bearing surfaces 4-108 and 4-110 and the rails 4-112 and 4-114 and extend downwardly through the center of the rails. As described above, the surface contact area between the bearing surface 4-108 and rail 4-112 is approximately equal to the surface contact area between the bearing surface 4-110 and rail 4-114, and thus these stiffness forces F_Bearing1and F_Bearing2are substantially equal. The bearing surfaces 4-108 and 4-110 are positioned at equal distances L_Bearingfrom the optical axis O of the lens 4-122 so that the moments about the optical axis O produced by these forces F_Bearing1and F_Bearing2are equal, minimizing carriage yaw. Referring to FIG. 55B, in the vertical plane, the net carriage suspension force F_Bearingacts at a point directly between the two bearings and aligned with the optical axis O.

[0326] Friction forces F_Friction1A, F_Friction1B, and F_Friction2acting on the bearing surfaces 4-108, 4-110 and rails 4-112 and 4-114 are illustrated in FIG. 56A. As the first bearing surface 4-108 includes two sections 4-160 and 4-162, the two friction forces F_Friction1Aand F_Friction1Bare present, one associated with each bearing section 4-160 and 4-162, respectively, which are centered at the middle of the bearings along the area of contact with the rail 4-114. The second friction force F_Friction2acts on the second bearing surface 4-108 and is centered in the middle of the bearing along its contact with the rail 4-112 as shown. Because the area of contact of the bearing sections 4-160 and 4-162 forming the first bearing surface 4-110 substantially equals the area of contact of the second bearing surface 4-108, and the amount of pre-loading and coefficient of friction is the same for both bearing surfaces, the sum of the friction forces F_Friction1Aand F_Friction1Bequals the friction force F_Friction2. The bearing surfaces 4-112 and 4-114 are located at equal distances L_Ffrom the optical axis O of the focus lens 4-122, and the resulting moments about the optical axis of the lens are then equal as well. In the vertical plane, the forces F_Friction1A, F_Friction1B, and F_Friction2, act at the areas of contact between the rails 4-112, 4-114 and the bearing surfaces 4-108, 4-110, FIG. 56B which are advantageously designed to be horizontally aligned with the center of mass of the carriage mass CM_C, such that moments about the center of mass which can produce carriage pitch are reduced.

[0327] FIGS. 57-60 illustrate the inertial forces acting on the carriage 4-106 and actuator 4-116 for both vertical and horizontal accelerations. The inertial forces acting on the fine motor and carriage in response to a vertical acceleration of the assembly are shown in FIG. 57. A first downward inertial force F_IF, FIGS. 57 and 58A, equal to the mass of the fine motor multiplied by the acceleration acts at the center of mass of the fine motor mass CM_F. A second downward inertial force F_IC, FIG. 57 and 58B, acts at the center of mass of the carriage mass CM_Cand is equal to the mass of the carriage multiplied by the acceleration. FIGS. 58A and 58B further illustrate that the inertial forces F_IFand F_ICare horizontally aligned with the optical axis O of the objective lens 4-122.

[0328] FIG. 59A illustrates the inertial forces acting on the coarse coils 4-142, 4-144 and fine motor pole pieces 4-244, 4-246 for horizontal accelerations of the carriage and fine motor, respectively. An inertial force F_IC1acts at the center of upper portion of the first coarse coil 4-142 and an inertial force F_IC2acts at the center of the upper portion of the second coarse coil 4-144. As described above, the coils 4-142 and 4-144 are of identical dimensions, such that the mass of the first coil 4-142 equals the mass of the second coil 4-144. The magnitude of each force F_IC1and F_IC2is equal to mass of the respective coil multiplied by the acceleration, and thus, the inertial forces acting on the coils 4-142 and 4-144 are equal. Because the coils 4-142 and 4-144 are positioned at equal distances L_Cfrom the optical axis O of the objective lens 4-122, the resulting moments about the optical axis of the lens produced by the inertial forces F_IC1and F_IC2are equal. Similarly, because the fine motor pole pieces 4-244 and 4-246 are of equal dimensions and are located at equal distances L_Pfrom the optical axis O, the inertial forces F_IP1and F_IP2acting on the pole pieces are equivalent, and the resulting moments about the optical axis O of the objective lens 4-122 are equal. Applying this same analysis to all other components or “subparts” of the carriage and actuator assembly, and as will be explained in more detail below, the inertial forces produced by horizontal and vertical accelerations above the resonance frequency of the flexure arms are balanced and symmetric with respect to the optical axis O. The net inertial forces of the fine motor and carriage F_IFand F_ICfor acting on the assembly for horizontal accelerations thus act along a line through the center of the carriage which intersects the optical axis O as shown in FIG. 59B. The net inertial force due to the coarse motor F_ICis equal to the mass of the coarse motor multiplied by the acceleration, while the net inertial force due to the fine motor F_IFis equal to the mass of the fine motor multiplied by the acceleration.

[0329] At high frequencies, being accelerations in the tracking direction above the lens holder-flexure arm resonance frequency, approximately 40 Hz, components of the assembly 4-100 decouple and do not affect the position of the objective lens 4-122. Consequently, the inertial forces differ for accelerations above and below the flexure arm resonance frequency. The inertial forces for horizontal accelerations at these high frequencies are illustrated in FIG. 60A. At these high frequencies, the actuator 4-116 is decoupled from the carriage 4-106, such that a first inertial force F_I1equal to the mass of the fine motor multiplied by the acceleration acts at the center of mass of the fine motor mass CM_F, and a second inertial force F_I2equal to the mass of the coarse motor multiplied by the acceleration is centered at the center of mass of the carriage mass CM_C.

[0330] FIG. 60B illustrates the inertial forces at horizontal accelerations below the flexure arm resonance frequency. At these lower frequencies, the fine motor mass and carriage mass move as a unit which has a net center of mass at CM_C′. As illustrated, this net center of mass CM_C′ is located at a distance x vertically above the center of mass of the carriage mass CM_C, and thus the coarse motor forces F_Coarse1and F_Coarse2, and the friction forces F_Friction1and F_Friction2, are no longer aligned with the carriage mass center of mass, now shifted to CM_C′. Although this vertical shift in the carriage center of mass occurs, the symmetrical design of the assembly 4-100 ensures that the center of mass of the carriage mass CM_Cdoes not shift in the horizontal plane, and the forces acting on the carriage remain symmetrical about the center of mass and optical axis O in spite of the vertical shift in the center of mass from CM_Cto CM_C′.

[0331] Further, the symmetry of the design ensures that horizontal shifting of the center of mass CM_Cdoes not occur when subparts or components of the carriage decouple at high frequencies. For example, at frequencies in the KHz range, the fine motor poles pieces 4-244, 4-246 and magnets 4-250, 4-252 will decouple. Due to the symmetry of the design, however, the center of mass will not shift in the horizontal plane. Because there is no shift of the center of mass CM_Cin the horizontal plane, reaction forces of the focus motor will not pitch or roll the carriage at frequencies above those where subparts have come “loose”. Thus, by horizontally aligning the center of mass with the optical axis O of the objective lens 4-122, the lens sits “in the eye of the storm”, where its position is minimally affected by resonance, motor, and reaction forces acting on the assembly 4-100.

[0332] FIGS. 61A and 61B illustrate the Bode transfer diagram of fine tracking position versus fine motor current of the actuator 4-116 of the present invention for an objective lens of 0.24 grams suspended in a fine motor having a mass of 1.9 grams. As illustrated in FIG. 61A, the actuator exhibits an almost ideal dB curve 4-310 having an approximate 40 dB/decade slope and an ideal phase shift curve 4-312, FIG. 61B. The two dB and phase shift curves are identified trace lines 4-310 and 4-312, respectively. FIGS. 61C and 61D illustrate the same transfer function when the lens is off centered in the horizontal or tracking direction by 0.15 mm. Both the dB and phase shift curves, trace lines 4-410′ and 4-412′, respectively, reveal a disturbance, or glitch, which occurs at approximately 3.2 kHz. The phase margin dips approximately 25 degrees, reducing loop damping and increasing settling time and overshoot. In terms of lens positioning, the horizontal shift in lens position disturbs the symmetry or balance of the fine tracking forces acting on the lens and results in a moment about the optical axis of the lens, resulting in yaw. Thus, it can be seen that the balancing of forces in the assembly 4-100 about the optical axis O of the objective lens 4-122 markedly improves tracking position.

[0333] FIGS. 62A-62C illustrate the effects of asymmetrical focus forces acting on the assembly 4-100. FIG. 62A illustrates the tracking signal, illustrated as trace line 4-320, while crossing tracks for a track pitch of 1.5 μm, wherein each sine wave corresponds to an information track on the surface of the optical disc. In FIG. 62B, the focus force is centered with the center of mass of the fine motor CM_Fand the optical axis O. The top trace 4-322 shows the current applied to the focus coil during the step, while the bottom trace 4-324 shows the tracking error signal while following a particular track, for a focus current of 0.1 Amp, and a focus acceleration of 0.75 m/s². As illustrated, the tracking error signal remains virtually unaffected by the focus current level. FIG. 62C shows the effect on the current and tracking error signals as in FIG. 62B when the focus force is shifted out of alignment with the optical axis O and center of mass CM_Fby approximately 0.2 mm. The corresponding curves are identified as trace lines 4-422′ and 4-424′, respectively. The tracking signal is now visibly affected by the focus current. With the same focus current and acceleration, a tracking offset of 0.022 μm results. Typically, the total allowable track offset in an optical drive is in the range of 0.05 μm to 0.1 μm, and thus, by aligning the forces as in FIG. 62B, the tracking offset is significantly reduced.

[0334] An alternative embodiment of a carriage and actuator assembly 4-400 in which the center of mass of a 2-D actuator coincides with the center of mass of the carriage mass is illustrated in FIG. 63. In addition to being substantially symmetrical about the optical axis of an objective lens, the center of mass of the fine motor mass coincides with the center of mass of the carriage mass and is aligned with the optical axis. The carriage and actuator assembly 4-100 of the first embodiment is adequate for most frequency ranges. The assembly 4-400 of the present alternative embodiment, however, may be used in applications where it is desirable to avoid the shift in the center of mass of the carriage mass at frequencies below the flexure arm resonance frequency.

[0335] The assembly 4-400 includes a carriage 4-406 having first and second bearing surfaces 4-408 and 4-410 substantially identical to those in assembly 4-100 which can be slidably mounted on guide rails (not shown), and a 2-D actuator 4-416 which is mounted within the carriage 4-406. The carriage 4-406 includes a pair of coarse tracking coils 4-412 and 4-414 positioned within respective notches 4-417 and 4-418 formed in the carriage 4-406, adjacent the bearing surfaces 4-408 and 4-410, which act to move the carriage 4-406 horizontally in a tracking direction, FIG. 65, to access various information tracks on the surface of an optical disc.

[0336] The actuator 4-416 includes a lens holder 4-420 having an objective lens 4-422 mounted thereon. A pair of ledges 4-424 formed on the top surface of the carriage 4-406 support a pair of top flexure arms 4-426 which are further attached to the top surfaces of a pair of projections 4-428 formed on the lens holder 4-420. A pair of bottom flexure arms 4-429 which are identical in structure to the top flexure arms 4-426 are supported by corresponding ledges in the bottom of the carriage (not shown), and attach to corresponding bottom surfaces of the projections 4-428 on the lens holder 4-420. A beam of light 4-430 enters the actuator 4-416 through a oval aperture 4-432 and is reflected by a mirror (not shown) contained inside the actuator 4-416 through the objective lens 4-422 along an optical axis O′. The actuator 4-416 is further attached to a focus and fine tracking motor which moves the lens 4-422 so as to precisely align and focus the exiting beam upon a desired location on the surface of the optical disc. The focus and fine tracking motor includes two permanent 4-440 and 4-442 magnets affixed to opposing ends of the lens holder 4-420. An oval-shaped fine tracking coil 4-444 is affixed to each permanent magnet 4-440 and 4-442, adjacent the carriage bearing surfaces 4-408 and 4-4-410. A focus coil 4-448 is attached to the top and bottom surfaces of the carriage 4-406 and supported by ledges formed within the interior of the carriage such that the lens holder 4-420 is positioned between the focus coils 4-448.

[0337] Coarse tracking movement of the carriage 4-406 and actuator 4-416 is effected in a manner identical to that of the assembly 4-100 illustrated in FIGS. 46 and 47. When a current is applied to the coarse tracking coils 4-412 and 4-414 in the presence of a magnetic field, a force is generated according to Lorentz law which acts to move the carriage 4-406 and actuator 4-416 in a tracking directions, FIG. 65, so as to position the objective lens 4-422 beneath various information tracks on the optical disc.

[0338] FIG. 64 illustrates the operation of the actuator 4-416 to move the lens holder 4-420 and objective lens 4-422 carried thereon in a focusing direction. When a current is generated in the focus coils 4-448, an electromagnetic field 4-450 is induced in each of the coils. The electromagnetic field 4-450 differs in direction for the respective focusing coils as shown. In the example shown, both permanent magnets 4-440 and 4-442 will be attracted by the bottom focus coil 4-448 (not shown here) and repelled by the top focus coil 4-448, thus moving the objective lens holder 4-420 toward the bottom focus coil 4-448 and away from the top focus coil 4-448 to position the objective lens 4-422 further away from the surface of the optical disc, wherein the magnitude of the displacement depends on the strength of the induced electromagnetic field.

[0339] In a similar manner, FIG. 65 illustrates the permanent magnets 4-440 and 4-442 interacting with the fine tracking coils 4-444. Energization of the tracking coils 4-444 moves the lens holder 4-420 horizontally in the tracking direction to the right or to the left depending upon the direction of current through the coils. For example, in the presence of the magnetic field 4-460 illustrated, the lens holder 4-420 and objective lens 4-422 are moved towards the left. In this manner, the fine tracking coils 4-444 act to more precisely position the light beam exiting the objective lens 4-422 within the center of a desired information track on the optical disc.

[0340] In the following discussion, the identified forces and lengths correspond to those discussed above in conjunction with the assembly 4-100. For convenience of illustration, the prime symbol A′ A will be used to discuss corresponding values while reference will be made to FIGS. 46, 49B, 50A, 51A-53A, 55A, 56A, 58A, and 58B as employed in discussing the indicated forces and lengths associated with the assembly 4-100.

[0341] As described above, the coarse tracking motor operates in a manner identical to that of the coarse tracking motor in the assembly 4-100. The coarse tracking coils 4-412 and 4-414 are of identical dimensions and are positioned at equal distances from the optical axis O′ of the objective lens 4-422. Equal currents are applied to the coils such that corresponding forces F_Coarse1′ and F_Coarse2′, see FIG. 46 , acting on the carriage 4-406 act at equal corresponding distances L_C1′ and L_C2′, FIG. 49B, from the optical axis O′. In the vertical plane, in the radial direction, these forces F_Coarse1′ and F_Coarse2′ are aligned with the coincident centers of gravity of the corresponding fine motor mass CM_F′, FIG. 58A, and carriage mass CM_C′, FIG. 58B, thereby minimizing carriage and actuator pitch. In a similar manner, the bearing surfaces 4-408 and 4-410 are positioned at equal distances from the optical axis O′ such that the carriage suspension forces are also symmetric about the optical axis O′. Each force F_Bearing1′ and F_Bearing2′, see FIG. 55A for comparison, acts an equal distance L_Bearing1′ from the optical axis O′ such that the moments produced about the optical axis are equal and carriage and actuator pitch is further reduced. The surface area of the bearings which contacts the rails is designed to be substantially equal such that the friction forces acting on the carriage 4-406 are substantially equal. Since the bearing surfaces 4-408 and 4-410 are positioned equidistantly from the optical axis O′, the moments acting about the optical axis are equal and carriage and actuator is minimized. The assembly is further designed such that the friction forces are vertically aligned with the center of mass of the carriage 4-406 and actuator 4-416.

[0342] The fine tracking coils 4-444 are of equal dimensions and the current applied to the coils is equal such that the fine tracking forces acting on the actuator are equal. Further, the fine tracking coils 4-444 are positioned at equal distances L_T′, FIG. 50A, from the optical axis O′ such that the moments produced about this axis are equal. In the vertical plane, these forces F_Track1′ and F_Track2′, FIG. 50A, are also aligned with the centers of gravity of the actuator 4-416 and carriage 4-406, such that pitch of the actuator 4-416 is reduced. Since the fine tracking forces acting on the assembly are equal, it follows that the reaction forces F_React1′ and F_React2′, FIG. 51A, produced in response to the tracking forces F_Track1′ and F_Track2′ are equal as well. These reaction forces act at equal distances L_R′ from the optical axis and are vertically aligned with the centers of gravity, such that moments about the optical axis O′ are equal and yaw is reduced.

[0343] In a similar manner, the focus coils 4-448 have substantially equal dimensions and current applied to them such that the focus coils 4-448 produce equal forces F_Focus1′ and F_Focus2′ acting on the actuator. In this embodiment, however, the focus coils 4-448 are located at equal distances L_F′. FIG. 56A, from the coincident centers of gravity of the fine motor mass and carriage mass such that the moments about the optical axis O′ are equal. Further, because the focus forces F_Focus1′ and F_Focus2′, FIG. 52A, are equal, the focus reaction forces F_FR1′ and F_FR2′, FIG. 53A, acting on the fine motor mass are equal and act at equal distances L_FR′, FIG. 53A, from the coincident centers of gravity of the carriage mass CM_C′ and fine motor mass CM_F′. Thus, moments produced by the reaction forces about the optical axis O′ are equal and actuator pitch is further minimized.

[0344] The flexure forces F_Flex1′, F_Flex2′, acting on the actuator and fine motor reaction forces F_RA′, F_RB′, produced in response to the flexure forces are effectively the same as those illustrated in FIG. 54 for the assembly 4-100. Because the flexure and reactions forces are not symmetrical about the optical axis O′, the moments produced by these pairs of forces about the axis O′ are not equal. These forces, however, are effectively decoupled from the carriage 4-406 except at low frequencies (typically below around 40 Hz), such that these moments do not affect actuator performance under most operating conditions.

[0345] Thus, the motor and reaction forces acting on the assembly 4-400 are symmetric about the optical axis O′ and are vertically in alignment with the centers of gravity of the fine motor mass CM_F′ and carriage mass CM_C′. Because the centers of gravity of the fine motor mass and carriage mass coincide, decoupling of the actuator 4-416 or any of the subparts of the assembly 4-400 will not shift the center of mass, and the forces and moments acting on the assembly 4-400 will remain balanced for virtually all horizontal and vertical accelerations.

[0346] Anamorphic, Achromatic Prism System

[0347] FIG. 66 depicts a prior art optical system 5-100 having a light source 5-102, which provides an incident light beam 5-106 depicted in dashed lines, a simple anamorphic prism 5-108, a focusing lens 5-110, and an optical medium 5-112. The light beam 5-106 enters the prism 5-108 at an incidence angle 5-114 with respect to the normal to an entrance face 5-116 of the prism. Laser light sources usually generate an elliptical beam with some astigmatism, as is well understood in the art. The anamorphic prism 5-108 provides expansion along the minor axis of the ellipse to correct for beam ellipticity. The angle of incidence 5-114 is selected to provide the desired expansion along the minor axis. The anamorphic prism 5-108 can also correct astigmatism in the incident light beam 5-106. The lens 5-110 focuses a resulting corrected beam 5-118 to form a spot 5-120 on the optical medium 5-112.

[0348] The simple prism 5-108 is adequate as long as the wavelength of the incident light beam 5-106 remains constant. In practice, however, light sources typically change wavelength due to temperature changes, power shifts, random “mode hopping” and other conditions, as is well known in the art. In magneto-optic disc systems, the laser power continually shifts between the power level required for write operations and the power level required for read operations.

[0349] The angle of refraction of light at the interface of materials is calculated with Snell's law, as is well known in the art:

[0350] n₁sin θ₁=n₂sin θ₂

[0351] where:

[0352] n₁=index of refraction of material 1;

[0353] θ₁=angle of incidence with respect to normal;

[0354] n₂=index of refraction of material 2; and

[0355] θ₂=angle of refraction with respect to normal.

[0356] This relationship governs the refraction of the light beam 5-106 when it enters the prism 5-108. As seen in FIG. 66, when an incident beam 5-106 of one wavelength enters the anamorphic prism 5-108, the beam is refracted at a given angle dictated by the index of refraction of the prism 5-108 and the angle of incidence 5-114 of the light beam 5-106. The resulting light beam 5-118, corrected for ellipticity, and possibly, astigmatism of the incident beam 5-106, enters the focusing lens 5-110 and results in the focused light spot 5-120 on the optical medium 5-112. The index of refraction, however, changes with wavelength. This is referred to as chromatic dispersion. Accordingly, when the wavelength of the incident light beam 5-106 changes, the angle of refraction resulting from the interface between air and the prism 5-108 is different than the angle of refraction for the previous wavelength. FIG. 66 depicts with dotted lines, the effect of a shift in the wavelength of the incident beam 5-106. The incident light beam 5-106 refracts at a different angle and results in a light beam 5-122 which enters the focusing lens 5-110 at a different angle to result in a focused light spot 5-124 on the optical medium 5-112. As illustrated in FIG. 66, the light spot 5-124 is displaced from the light spot 5-120. This displacement, resulting from a change in wavelength in the incident light beam, is referred to herein as lateral beam shift.

[0357] The lateral beam shift may be avoided by not employing the anamorphic prism 5-108. For instance, a system may employ a circular lens to provide a circular spot on the optical medium. To form the circular spot with a lens, however, the lens only focuses a circular aperture within the elliptical light beam. This results in an inefficient use of the laser power because portions of the light beam outside the circular aperture are discarded. Accordingly, a system which does not employ the anamorphic prism for beam shaping does not benefit from the prismatic correction of ellipticity and astigmatism in the incident light beam. The beam shaping capabilities of the anamorphic prism provide efficient use of the laser power by expanding the elliptical beam into a circular beam. The efficient use of power is advantageous, particularly in optical disc systems when increased power is necessary in order to write to the disc.

[0358] FIG. 67 shows a conventional configuration for a multi-element prism system 5-130, as is well known in the art. The system depicted consists of three prism elements, prism 5-132, prism 5-134 and prism 5-136, a focusing lens 5-138, and a reflective-type optical medium 5-140. The prism system 5-130 could be designed to be achromatic by proper selection of the individual prism geometries, indexes of refraction, and dispersions for prism 5-132, prism 5-134 and, prism 5-136.

[0359] The prism system 5-130 illustrated in FIG. 67 also allows reflection of a return beam from the optical medium 5-140 to a detection system 5-144 by including a beam-splitting thin film 5-146 between the prism 5-134 and the prism 5-136.

[0360] As seen in FIG. 67, an entering light beam 5-148 passes through the prisms 5-132, 5-134, and 5-136, and is then focused by the lens 5-138 to form a spot 5-137 on the optical medium 5-140. The light beam 5-148 reflects from the optical media 5-140 back through the focusing lens 5-138 into the prism 5-136, and reflects from the thin film 5-146 as a light beam 5-150. The light beam 5-150 then enters the detection system 5-144.

[0361] If designed to be achromatic, changes in the input light beam 5-148 wavelength should not result in a lateral shift in the focused light spot 5-137 on the optical medium 5-140.

[0362] As previously explained, optical systems often benefit from more than one detector. A prism system with an air space in the light path could provide significant advantages, particularly in providing a compact, achromatic prism system capable of reflecting portions of the incident and return beams to multiple detectors. Furthermore, by using an air space, a symmetrical correcting prism can be added to an existing anamorphic prism system. Finally, a unitary prism system with an air space would be advantageous in order to provide a stable, compact, easy to manufacture and install, prism assembly.

[0363] In order to more fully explain the design of an achromatic prism system with an air space between prisms, reference is made to FIG. 68, which depicts a two-element prism system 5-152 having a chromatic correcting prism 5-154 added to a simple anamorphic prism 5-156. The correcting prism 5-154 has an index of refraction of n₁and the simple anamorphic prism 5-156 has an index of refraction of n₂, at a selected wavelength. The angles in the system are represented as shown in FIG. 68 as φ, a₁, a₂, a₃, a₄, a₅, a₆, a₇, β₁, β₂, and β_air. The deviation angle from the incident beam to the exit beam is referenced as α, where

[0364] α=β₁+β_air−(a₇+φ+β₂)

[0365] and a₇can be calculated through repeated applications of Snell's law and the geometry of triangles.

[0366] The design conditions are chosen to achieve a desired result (e.g., total deviation through the system). For instance, to design an achromatic system, the condition is that a be constant over some range of wavelengths.

[0367] For a total desired deviation angle, α=A, from the entrance beam to the exit beam, the condition is met as follows:

[0368] A=β₁+β_air−(a₇+φ+β₂)

[0369] Furthermore, the condition for making the correcting prism 5-154 a symmetrical prism with no net expansion of the incident light beam so that it can be added to the simple anamorphic prism 5-156, as shown in FIG. 68, is as follows:

[0370] φ=sin⁻¹[n₁*sin(β₁/2)]

[0371] By selecting this condition, the correcting prism 5-154 does not expand the incident light beam. The correcting prism, therefore, can be added to an existing anamorphic prism system selected to provide the appropriate expansion.

[0372] Finally, the prism assembly 5-152 can meet all of the desired design restraints by proper selection of φ, β₁, β₂, β_air, and of the glass dispersions.

[0373] In some cases it may be desirable for the exit beam to have a significant deviation angle from the entrance beam. For instance, a deviation of 90 degree(s) may be advantageous. This can be accomplished by providing a total internal reflection in the prism 5-156 before the beam exits the prism. This changes the above calculations, but the design goals can still be met by proper selection of the parameters.

[0374] Applying the above principles for adding a symmetrical correcting prism to an existing anamorphic prism, a prism system was designed which has multiple surfaces to partially reflect the return beam to different detectors. Embodiments of unitary, air-spaced, achromatic prism systems with significant deviation angles between the entrance beam and the exit beam, along with multiple reflections to various detection systems are described below.

[0375] FIG. 69 illustrates an embodiment of an air-spaced, anamorphic, achromatic prism system 5-170 according to the present invention. Preferably, the prism system 5-170, as depicted in FIG. 69, has three prisms bonded as a single unit. As previously explained, this provides the advantage that the prism assembly 5-170 is mounted as a single unit. Because the prisms are bonded together, they need not be separately mounted in the optical system. This reduces mounting time, increases stability of the system, decreases mounting costs, and minimizes functional deviations between different optical systems. The three prism elements are a plate prism 5-172, a trapezoidal prism 5-174, and a correcting prism 5-176. FIG. 69 also shows the light beam path as a light beam 5-178 from the light source 5-102, an air gap light beam 5-180, an exit/reflected light beam 5-182, a first detector channel light beam 5-184 to a first detector 5-185, a second detector channel light beam 5-186 to a second detector 5-187, and a third detector light beam 5-188 to a third detector 5-189. By including an air gap between the correcting prism 5-176 and the plate prism 5-172 through which the air gap light beam 5-180 passes, the correcting prism 5-176 can be designed as a symmetrical corrector with no net expansion to the incident beam 5-178. Therefore, the correcting prism 5-176 can be added to the plate prism 5-172 and the trapezoidal prism 5-174 combination in order to achromatize the prism system 5-170 shown in FIG. 69.

[0376] FIG. 69 also depicts a lens 5-190 positioned to focus the exit light beam 5-182 onto an optical medium 5-191. The specifics of the design shown in FIG. 69 are described and designed to be substantially achromatic for a design wavelength of 785±22 nm. At this wavelength, the system will have the properties described below.

[0377] The plate prism 5-172 is depicted in more detail in FIGS. 70, 70A and 70B. FIG. 70 is a side view of the plate prism 5-172, FIG. 70A is a bottom plan view illustrating a surface S15-200, and FIG. 70B is a top plan view illustrating a surface S25-202. The plate prism has the optical surface S15-200, the optical surface S25-202, an optical surface S35-204, a surface S45-206, and a surface S55-208. In one embodiment, the surfaces S15-200 and S25-202 are substantially parallel and spaced apart at a distance designated in FIG. 70 as 5-210. In the present embodiment, the distance 5-210 is advantageously 6.27 mm. The surface S55-208 and the surface S35-204 are also substantially parallel in the present embodiment. The surface S15-200 and the surface S35-204 intersect and terminate at an edge 5-211 (i.e., the S1/S2 edge) in FIG. 70, at an angle 5-212 (i.e., the S1/S2 angle), which is advantageously 50 degree(s) 21′±10′ in the present embodiment. The surface S35-204 and the surface S25-202 intersect and terminate at an edge 5-214; the surface S25-202 and the surface S45-206 intersect and terminate at an edge 5-216; the surface S45-206 and the surface S55-208 intersect and terminate at an edge 5-218; and the surface S55-208 and the surface S15-200 intersect and terminate at an edge 5-220, as designated in FIG. 70. The surface S25-202 has a length referenced as 5-222 in FIG. 70 and a width referenced as 5-224 FIG. 70A. In the present embodiment, the length 5-222 is 13.34 mm and the width 5-224 is 8.0 mm. The overall length of the prism, referenced as 5-225 in FIG. 70, from the edge 5-218 to the edge 5-211 measured parallel to the surface S15-200 is advantageously 23.61 mm in the present embodiment. The distance from the edge 5-218 and the edge 5-220, referenced as 5-227, measured along a reference plane 5-226 defined perpendicular to the surface S15-200 and the surface S25-202 is advantageously 2.14 mm. The plan view in FIG. 70A illustrates a clear aperture 5-230 and a clear aperture 5-232 defined on the surface S15-200. A clear aperture is simply an area of the surface of the prism over which the surface is specified to meet a selected quality. In the present embodiment, the clear apertures 5-230 and 5-232 are 8.5 mm by 6.5 mm ovals. Advantageously, the aperture 5-230 is centered with its minor axis a distance 5-233 from the edge 5-211 and with its major axis centered in the middle of the surface S15-200 as shown in FIG. 70A. In the present embodiment, the clear aperture 5-232 is centered with its minor axis a distance 5-234 from the edge 5-220, and with its major axis centered along the middle of the surface S15-200. Advantageously, in the present embodiment, the distance 5-233 is 6.15 mm and the distance 5-234 is 5.30 mm.

[0378] The plan view depicted in FIG. 70B illustrates a clear aperture 5-235 defined on the surface S25-202. The present embodiment defines this clear aperture as an 8.5 mm by 6.5 mm oval with its minor axis centered a distance 5-236 from the edge 5-214 and its major axis centered in the middle of the surface S25-202 as depicted in FIG. 70B. In the present embodiment the distance 5-236 is 5.2 mm. The clear apertures 5-230, 5-232, and 5-235 define portions of the surfaces over which the surface quality is preferably at least 40/20, as is well known in the art. In the illustrated embodiment, BK7 grade A fine annealed glass, well known in the art, is an appropriate optical material for the prism 5-172.

[0379] FIG. 71 shows additional detail of the trapezoidal prism 5-174 of the embodiment depicted in FIG. 69. The trapezoidal prism 5-174 has an optical surface S65-240, an optical surface S75-242, an optical surface S85-244, and an optical surface S95-246. The surface S65-240 and the surface S75-242 terminate and intersect at an edge 5-248. The surface S75-242 and the surface S85-244 intersect and terminate at an edge 5-250 at an angle referenced as 5-251. Advantageously, the angle 5-251 is substantially 135 degrees. The surface S85-244 and the surface S95-246 intersect. and terminate at an edge 5-252 at an angle 5-254 which is advantageously 50 degrees 21′n in the present embodiment. The surface S95-246 and the surface S65-240 intersect and terminate at an edge 5-256. The surface S65-240 has a length 5-258 shown in FIG. 71. Advantageously, the length 5-258 is 9.5 mm in the present embodiment. The surface S65-240 and the surface S85-244 are substantially parallel and spaced at a distance 5-260, FIG. 71. In the present embodiment, the distance 5-260 is 8.0 mm measured in a direction perpendicular to the surface S65-240 and the surface S85-244. The edges 5-250 and 5-248 are spaced at a distance 5-261 along a plane 5-262 defined parallel with the surface S85-244. Advantageously, the distance 5-261 is 8.0 mm in the present embodiment. FIG. 71A is a top plan view of the trapezoidal prism 5-174 illustrating the surface S65-240 and the surface S95-246. As depicted in FIG. 71A the trapezoid prism 5-174 has a thickness 5-263. Preferably, the thickness 5-263 is approximately 8 mm in the present embodiment. As shown in FIG. 71A, the surface S65-240 has a clear aperture 5-264 defined in the present embodiment as a 6.5 mm minimum diameter circular aperture centered across the width of the surface and centered at a distance 5-265 from the edge 5-248. Preferably, the distance 5-265 is 4.0 mm in the present embodiment. The surface S95-246 has a clear aperture 5-266 centered on the surface. In the present embodiment, the clear aperture 5-266 is defined as a 6.5 mm by 8.5 mm minimum oval.

[0380] FIG. 71B depicts a bottom plan view of the trapezoidal prism 5-174 illustrating the surface S75-242 and the surface S85-244 with clear apertures 5-268 and 5-270, respectively. As shown in FIG. 71B, the trapezoid prism 5-174 has a length 5-272 from the edge 5-252 to the edge 5-248 measured along the reference plane 5-262. Preferably, the length 5-272 is 16.13 mm in the present embodiment. In one embodiment, the clear aperture 5-268 for the surface S75-242 is defined as a 6.5 mm by 9.2 mm oval centered on the surface S75-242 with its minor axis parallel to and centered between the edge 5-248 and the edge 5-250. Advantageously, the clear aperture 5-270 is a 6.5 mm by 6.7 mm oval centered on the surface S85-244 with its major axis centered parallel between the edge 5-250 and the edge 5-252. In the present embodiment, the surface quality of the clear apertures 5-264, 5-266, 5-268, and 5-270 is advantageously 40/20, well known in the art.

[0381] Many of the surfaces in the prisms have coatings to facilitate the function of the prism. In the present embodiment, the surface S65-240 has an anti-reflection coating with transmission ≧99.8% at 90°±0.5 degrees angle of incidence. The surface S85-244 has a coating with transmission ≧98.5% at 10.7°±0.5 angle of incidence for internally incident light. The surface S95-246 has a low extinction thin film coating with reflection of the s polarization state (R_s) (i.e., normal to the plane of incidence)>90%, and with reflection of the p polarization state (R_p)=12.5%±2.5% at 39°39′±0.5° angle of incidence. The material for the trapezoidal prism 5-174 of the embodiment illustrated in FIGS. 69 and 71-71B is BK7 grade A fine annealed optical glass, as is well known in the art.

[0382] The chromatic correcting prism 5-176 of the embodiment of the prism system 5-170 depicted in FIG. 69 is shown in more detail in FIGS. 72 and 72A. As illustrated, the chromatic correcting prism 5-176 has an optical surface S105-290, an optical surface S115-292, and a surface S125-294 configured to form a triangular prism. The surface S115-292 and the surface S125-294 intersect and terminate at an edge 5-296. The surface S105-290 and the surface S125-294 intersect and terminate at an edge 5-298. Preferably, the surfaces S105-290 and S115-292 are symmetrical. The surface S125-294 has a length 5-300, which is 7.78 mm in the present embodiment. Thus, the edge 5-296 and the edge 5-298 are separated by the distance 5-300. The surface S105-290 and the surface S115-292 approach each other at an angle referenced as 5-302. In the present embodiment, the angle 5-302 is advantageously 38° 20′. The surface S115-292 and the surface S105-290 are terminated a distance 5-303 from the surface S125-294, measured perpendicular to the surface S125-294. The distance 5-303 is 10.5 mm in the present embodiment.

[0383] FIG. 72A depicts a view of the surface S105-290. In this embodiment, the prism 5-176 has a thickness referenced 5-304 in FIG. 72A. In the present embodiment, the thickness 5-304 is advantageously 8.0 mm. Desirably, the surface S105-290 has an oval clear aperture 5-306. In the present embodiment, the clear aperture 5-306 is an oval centered with the major axis parallel to, and a distance 5-308 from, the intersection at 5-298. The minor axis is centered on the surface S105-290 as illustrated. Preferably, the clear aperture 5-306 is defined as a 6.5 mm by 2.8 mm oval in the present embodiment, and the surface quality across the clear aperture 5-306 is advantageously 40/20, as known in the art. In the present embodiment, the surface S115-292 also has a similar clear aperture defined on its surface.

[0384] As with the trapezoidal prism 5-174, the chromatic correcting prism 5-176 has coatings on some of its surfaces to facilitate performance. In one embodiment, each of the surfaces S105-290 and S115-292 has an anti-reflective coating (e.g. reflectance ≦3% at 35.5°±1.0° angle of incidence, as is well known in the art). In the present embodiment, SFII grade A fine annealed glass is the material for the correcting prism 5-176.

[0385] When the prisms as described above are assembled as the unitary prism system 5-170 of the embodiment shown in FIG. 69, the light beams reflect as illustrated and explained below for a wavelength of 785±22 nm. For discussion purposes, a reference plane 5-237 is defined along one side of the prism system 5-170 as illustrated in FIG. 69A. The incident beam 5-178 from the light source 5-102 enters the surface S105-290 at an incidence angle 5-326 and parallel with the reference plane 5-237. The light beam 5-178 exits the prism 5-176 into the air-gap as the light beam 5-180 and enters the prism 5-172 through surface S25-202. A portion of the light beam reflects at the thin film on the surface S95-246 and exits the surface S35-204 as the light beam 5-188. In one embodiment, the beam 5-188 may be directed to the detection system 5-189. Because this reflected beam is a portion of the input beam, the detection system 5-189 receiving the light beam 5-188 may monitor the intensity of the incident light. The remainder of the light beam which does not reflect at the thin film on the surface S95-246, passes into the trapezoidal prism 5-174, reflects internally at the surface S75-242 and exits as the light beam 5-182 through the surface S65-240.

[0386] In the embodiment described, if the angle of incidence 5-326 of the light beam 5-178 is 35° 26′, the light beam exits the prism 5-174 with a total deviation from the entrance beam 5-178 to the exit beam 5-182 of 87° 37′±5′, parallel to the reference plane 5-237 within 5′, and the light beam 5-182 exits normal to the surface S65-240 within 5′.

[0387] The lens 5-190 focuses the light beam 5-182 onto the optical medium 5-191. The light beam reflects back through the lens and enters normal to the surface S65-240, reflects internally at the surface S75-242, and then reflects at the thin film between the trapezoidal prism 5-174 and the plate prism 5-172. The resulting beam exits the trapezoidal prism 5-174 through the surface S85-244 as the light beam 5-184 at a deviation angle 5-328. The light beam 5-184 enters the first detector 5-185.

[0388] Part of the light beam returned from the optical medium 5-190 also passes through the thin film, reflects at the surface S25-202 and exits the plate prism 5-172 as the light beam 5-186. This reflection is available because of the air gap in the prism system. In one embodiment, the light beam 5-184 and the light beam 5-186 can both be directed to separate detection systems 5-185 and 5-187, respectively. For instance, the detection system 5-185 may collect data signals, and the detection system 5-187 may collect control signals (e.g., focus and tracking servo information).

[0389] As explained above, the embodiment described is substantially achromatic within a typical range of wavelength changes from a conventional laser light source. Accordingly, shifts in the wavelength of the incident light do not significantly affect the resulting lateral position of the focused beam on the optical medium 5-190.

[0390] Calculations simulating the performance of the prism system 5-170 for variations in wavelength from 780 nm to 785 nm are shown in the table below. Phi is the incidence angle on the correcting prism (i.e., 35° 26′ in the present embodiment) and its variation is estimated as ±0.5°. The wavelength shift is indicated in one column and the corresponding shift in the focused spot from the prism system is indicated in the columns for incidence angles of Phi±0.5°. For instance, as seen in the first line of the table, for a wavelength shift of the incident light beam of 780 nm-781.5 nm, the focused spot shifts by −0.2 nm at the incident angle of Phi, by 2.6 nm for an incidence angle of Phi −0.5°, and by −2.9 nm for a incidence angle of Phi+0.5°.

[0391] As can be seen from the above table, the lateral displacement at the incidence angle, Phi, varies by less than 1 nm for a wavelength shift from 780 to 783 nm, with an incidence angle of Phi. This is contrasted with a lateral displacement of approximately 200 nm for a wavelength shift of 3 nm in an embodiment similar to that described above but without the chromatic correction. This indicates a substantially achromatic system.

[0392] FIG. 73 illustrates a prism system 5-339 as an alternative embodiment of the present invention. This embodiment has the correcting prism 5-340, a plate prism 5-342, and a.quadrilateral prism 5-344. The correcting prism 5-340 and the plate prism 5-342 are both substantially the same as the correcting prism 5-176 and the plate prism 5-172, respectively, of the prism system 5-170 shown in FIG. 69. The quadrilateral prism 5-344, however, differs from the trapezoidal prism 5-174.

[0393] The quadrilateral prism 5-344 of FIG. 73 is depicted in more detail in FIGS. 74, 74A and 74B. The quadrilateral prism 5-344 has a surface S135-346, a surface S145-348, a surface S155-350, and a surface S165-352. The surfaces S135-346, S145-348, S155-350, and S165-352 are configured similarly but not identical to the surfaces S65-240, S75-242, S85-244, and S95-246 of the trapezoidal prism 5-174. The surfaces S135-346 and S145-348 intersect at an edge 5-353 at an angle 5-354; the surfaces S145-348 and S155-350 intersect at an edge 5-355 at an angle referenced 5-356; and the surfaces S155-350 and S165-352 intersect at an edge 5-357 at an angle 5-358, as shown in FIG. 74. Finally, the surfaces S165-352 and S135-346 intersect at an edge 5-359. In one embodiment, the angle 5-354 is 49° 40′, the angle 5-356 is 135°, and the angle 5-358 is 50° 21°. The distance between the edge 5-353 and the edge 5-355, measured perpendicular to the surface S155-350 is referenced as 5-360 in FIG. 74. In one embodiment, the distance 5-360 is 8.0 mm. Additionally, the distance from the edge 5-353 to the edge 5-359 is referenced 5-362. In one embodiment, the distance 5-362 is 8.9 mm measured parallel to the surface S155-350. Finally, the distance between the edge 5-353 and the edge 5-355, measured along a plane parallel with the surface S155-350, is referenced as 5-364. In one embodiment, the distance 5-364 is preferably 8.0 mm.

[0394] FIG. 74A is a plan view of the surface S135-346 and also depicts the surface S165-352. FIG. 74A illustrates the thickness of the prism 5-344 referenced as 5-368. In one embodiment, the thickness 5-368 is 8.0 mm. Advantageously, the prism 5-344 has a clear aperture 5-370 defined along the surface S135-346, and a clear aperture 5-372 defined along the surface S165-352, as shown in FIG. 74A. In the present embodiment, the clear aperture 5-370 is a circular aperture centered across the surface and a distance 5-374 from the edge 5-353. In one embodiment, the clear aperture 5-370 is a circular aperture with a minimum diameter of 6.5 mm and the distance 5-374 is 4.0 mm. Advantageously, the surface S165-352 also has the clear aperture 5-372 centered on the surface. In one embodiment, the clear aperture 5-372 is a 6.5 mm by 8.5 mm oval aperture centered on the surface S165-352 as represented in FIG. 74A.

[0395] FIG. 74B is a plan view of the surface S145-348 and also illustrates the surface S155-350. The overall length of the prism 5-344 from the edge 5-353 to the edge 5-357 measured along a plane parallel to the surface S155-350 is referenced as 5-380 in FIG. 74B. In one embodiment, the length 5-380 is 16.13 mm. As seen in FIG. 74B, the surface S145-348 has a clear aperture 5-382 centered on the surface, and the surface S155-350 also has a clear aperture 5-384 centered on the surface. In one embodiment, the clear aperture 5-382 is a 6.5 mm by 9.2 mm oval, and the clear aperture 5-384 is a 6.5 mm by 6.7 mm oval.