Method and Apparatus for Effecting Data Transfer With High Precision Reference Data on a Rotatable Storage Media

This invention relates to systems and methods for writing and reading positioning patterns on data storage systems, and particularly to systems and methods for controlling the writing or reading of precisely positioned reference data and other reference data in sealed disk drive units.

In the present state of the art pertaining to digital storage on movable media, extremely high track densities (of the order of 1000-2000 tracks per inch) have become feasible. Workers in the art have constantly sought to increase the number of usable tracks per disk, i.e. the track density, since it is intuitively recognizable that doubling the number of tracks per inch effectively doubles the amount of stored data. As track densities were increased, it was soon found that open loop systems, such as incremental positioning systems without actual head position feedback, could not meet the accuracy requirements. Such previously minor factors as thermal errors and manufacturing tolerances could not be accepted because of the very small track spacings. Consequently, developers went to embedded servo track systems, and sophisticated dedicated servo surface systems which may also be combined with some embedded servo track information. Embedded servo track systems have prewritten reference patterns among the data tracks which enable precise location of magnetic heads for writing and reading during operation. Dedicated servo surface systems have a single disk surface on which there is only recorded head position information. This information is continuously read in order to position the remaining read/write heads mounted on the common actuator. The constant quest for ever higher track densities has led to modern systems which have 2,000 to 4,000 or more tracks per inch, for magnetic media.

Some of the techniques heretofore used have written the required reference patterns by accurately positioning the head assembly by a separate apparatus which is typically not part of the final drive assembly. For example, an external rotary or linear actuator may be attached to the head assembly drive actuator. With the position of the external actuator being accurately controlled, the head assembly actuator correspondingly follows. This, however, can result in positioning inaccuracies due to various factors such as the friction in the coupling between the two actuators, the relative position of the external actuator and the servo head, and errors between the external position and actual internal head position.

A serious disadvantage is the fact that the disk drive unit must be partially disassembled in order to obtain access to the head assembly actuator, and thereafter maintained in a clean room for the servo track writing operation. A clean room is required to provide the equivalent environment of a clean sealed disk drive assembly. A purified atmosphere without particulate contaminants is needed to "fly" the magnetic head assembly at the necessary close spacing (of the order of a few microinches) to the surface of the disk. Otherwise, particulates, such as ordinary dust, appear as relatively massive objects that affect both the aerodynamics and recording.

A substantially more costly technique, but one which may be adequately precise, mounts a retroreflector assembly on the head assembly actuator, and directs reflected light to a fixed laser interferometer, which can give an extremely precise position signal for the head actuator in the form of a distance measurement to the retroreflector. Then, an external actuator can be used to position the head drive, or the head drive can be placed in a servo loop which uses the interferometer for position feedback This expedient is, however, expensive and as usually practiced involves partial disassembly and clean room operation.

It is evident, therefore, that none of the available expedients provides a suitably low cost, high precision, method for writing position reference information on a disk storage medium, or particularly for doing so with a sealed type of drive and under conditions and with procedures suitable for mass production operations.

There are also other circumstances in which it is desired to sense or control a part of the dynamic interior mechanisms of an open or a sealed disk drive, in terms of determining where the head mechanism is, or the angular position of the disk itself, or both. In some instances in early testing of a unit, for example, it is desired to check data transfer operations at a number of specific track positions. Without a full operating system and servo information this cannot readily be accomplished, if at all, by present techniques. As a different example, the angular position of the rotating disk may be needed, again prior to or independently of, other position information without disassembly of the unit.

Systems and methods in accordance with the invention for writing and reading position reference data on a storage media employ a precision mechanical translator and a light beam reflected off a member associated with or on the media, such as part of a head drive assembly for writing on the media. The reflected beam is directed as a small area beam spot onto a precision detector associated with the translator. The geometry is arranged such that increments of movement of the translator can be substantially greater than increments of movement of the head drive assembly. By precisely locating the translator at different positions, and using an error signal derived from the detector to control the head drive assembly responsively, position information may be precisely written by the writing head at closely spaced relationships on the media.

The ability to control internal position in response to an external positioning and sensing system permits other data transfer operations to be carried out at precise locations. For example, reading and writing can be done at precise positions without use of the usual position information.

Important aspects of the invention derive from the multiplication factor obtained through use of the translator that moves through the same angle but substantially larger displacements due to the geometry than does the head drive actuator, together with minute beam cross section size, so that the position can be precisely sensed, as by a bicell detector. Very small changes in position at the translator thus generate precise signals for exact control of head position in a closed loop fashion. In a preferred arrangement, the beam is focused at an image region, and the centroid of the beam, whose area is less than the area of the detector, generates signals used in a differential manner to derive the error signal. The error signal is zero when the centroid is on the index line between the halves of the bicell and varies in a positive or negative sense dependent on the deviation from the index.

This arrangement is particularly advantageously used with a sealed magnetic disk drive having a rotary actuator for a magnetic head structure. The disk drive incorporates, at minimal additional cost, a window and a small planar mirror positioned on the actuator The light beam, such as a laser beam, is directed through the window to be reflected off the actuator onto the detector, which is movable on the translator mechanism along an arc centered on the axis of the rotary actuator. A sealed disk drive unit may therefore be mounted in position relative to the translator, and electrically coupled so that the error signal drives the rotary actuator Then position reference tracks may be written on the disk, starting with a known or a "home" position for the rotary actuator and proceeding through separate increments as defined by the translator.

It is particularly advantageous to use the light beam in an autocollimator configuration, such that the light source is disposed on the translator to form a beam along a parallel path with reflected beam to the detector, with beam splitters being utilized to guide the returned beam back to the detector. In this system, a pair of transimpedance amplifiers are coupled to the different parts of the bicell, and their signals are combined to generate the error signal which drives the rotary actuator. For convenience of control, the error signal that is generated is normalized to compensate for laser beam intensity variations. In addition a part of the laser beam is split off and directed toward a fixed reference detector to be used in generating a reference signal in compensating for beam pointing variations.

Another aspect of the invention is a sealed disk drive having a rotary actuator and characterized by a planar reflector mounted on the actuator to be pivoted in response to movement of the translator, and a light transmissive window spanning an arc that encompasses all angles of a line perpendicular to the mirror as it changes angle with the actuator between its limits of motion.

Further in accordance with the invention, the translator and detector system may be used to write position information with a data storage system that has an external rotary actuator shaft. In this instance the mirror may be removably or permanently mounted on the accessible portion of the shaft, and no window is needed.

A further feature of the invention lies in the combination of internal reflective indicia, one or more transmissive windows in the sealed housing, and the external beam generation and reflected beam sensing system to sense the operating position of internal dynamic mechanisms. The angular position of a disk can thus be sensed by providing peripheral index marks, the passage of which are remotely detected during rotation to generate control information for positioning, clocking or sector identifying purposes.

A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a combined perspective and block diagram view of a system in accordance with the invention;

FIG. 2 is a plan view of a more specific example of mechanism, optical path and error signal generating circuits in the arrangement of FIG. 1;

FIG. 3 is a side view of the system of FIG. 2;

FIG. 4 depicts more specific details of a beam focusing path, slightly modified for clarity, that may be utilized in the arrangement of FIGS. 2 and 3;

FIG. 5 is a different example of a different form of reference pattern writing system in accordance with the invention;

FIG. 6 is a perspective and generalized view of essential features of a system for controlling the writing of position information in a disk drive that has an externally accessible shaft for actuator control; and

FIG. 7(a) and FIG. 7(b) and a plan and side views respectively view of a different arrangement, showing how angular position of a rotating member within a sealed housing may be remotely detected.

An example of a reference pattern writing system in accordance with the invention is shown in generalized form in FIG. 1. Further details of a specific preferred version are shown, without duplication of some of the associated circuits and devices, in FIGS. 2 to 4. The examples given relate to a high density magnetic disk drive 10 of the design having a sealed housing 12 with air filtration means (not shown) for ensuring that particulates cannot intrude between the disk 14 and the closely spaced, aerodynamically supported, magnetic head 16. The head 16 is mounted on an actuator arm 18 that is pivoted about an actuator axis 20 by a drive actuator 22, such as a DC motor, a voice coil motor, a torque motor, or some other precision angular positioner. These elements and relationships are not shown in detail because they, as well as the drive spindles and motors, are well known in the art. Variants such as multiple disk assemblies should also be understood to be adaptable to use within the scope of the invention. The present specific example is directed to a 51/4" diameter disk system, but adjustment for smaller (e.g. 31/2", 21/2" and sub-2") and larger (e.g. 8", 91/2" and 14") disk systems will be understood to require no more than routine engineering effort.

The principles and embodiments of the invention also apply to optoelectronic disk memories, where these are to be used without prewritten or pre-embedded reference patterns. Most current optoelectronic memories incorporate physical reference patterns, but as new electrooptic media come into use this approach may not always be feasible or desirable.

The actuator arm 18 serves as the base for receiving a small planar mirror 28, mounted on the side of the base portion of the arm 18 so as to be approximately tangential to a circumference about the actuator axis 20, at a short radius from that axis. Also, the disk drive 10 is further modified by incorporation, in one wall 30 of the housing 12, of a small transparent window 32. The window 32 spans the arc which a line perpendicular to the plane of the mirror 28 traverses between the limits of movement of the actuator arm 18 as it moves between the innermost and outermost (or vice versa) tracks on the disk 14. The window 32 is joined to the sealed housing 12 so as to maintain the internally clean environment, and sealants or gaskets may be used to ensure against leakage if desired. Although the window 32 may be curved about the actuator axis 20, it is shown below that a planar window 32 does not introduce an unacceptable level of error, and thus a flat element is preferred for cost reasons. In any event the mirror 28 and window 32 add only a minimal increment of cost to the disk drive 10, and constitute the only modifications to the drive mechanical configuration. The mirror 28 is attached to the actuator arm 18 so as to bisect the axis 20 (if on the axis) or, more usually, on the side of the arm 18 tangential to a circumference about the axis 20. The window 32 spans an arc that encompasses a beam path perpendicular to the plane of the mirror 28 as the actuator 18 moves through its range of travel.

To the disk drive 10 can be added an actuator drive circuit 36 which operates in response to external commands and to internal servo information to move the actuator. The resident drive circuits of the disk drive 10 can be used but this is not preferred if they are not designed to accommodate the external error signal requirements. The drive 10 also incorporates means for defining a known track position, such as a "home" track position at one limit of the recording area. Read/write circuits 38 may incorporate internal means for writing and reading timing information and/or servo pattern signals that are to be applied from an external signal generation source (not shown). Further details are not given because timing signals are not used with all reference patterns, because a dedicated clock head can be used, and because various other expedients are available for writing timing data and servo patterns. These include the use of peripheral index marks, independently sensed through the window, on the disk in the fashion shown in FIGS. 7(a) and 7(b) and described below. They also include a system which does not require the use of indicia, such as that described by Martyn Lewis in a U.S. patent application filed concurrently herewith.

The reference pattern writing system in accordance with the invention also includes control circuits 40 which provide a "home" command to the actuator drive circuits 36 for initializing operations, and increment commands for thereafter selecting successively spaced apart reference positions for a mechanical translator 42 external to the disk drive 10. The mechanical translator 42 is mounted to pivot on a translator shaft 44 approximately collinear with the actuator axis 20, so that light beam devices on the translator 42 move in a plane normal to the actuator axis at constant radii from the axis 20.

The arm length of the translator 42 (sometimes L.sub.translator hereafter) is at a distance from the axis 20 that is substantially greater than the arm length (L.sub.actuator) between the axis 20 and the recording head gap on the magnetic head 16. Referring briefly to FIG. 4, this relationship is shown, although not to scale.

Referring again to FIG. 1, the circumferential arc of movement of any point on the translator 42 about the axis 20 corresponds to the arc described by a line perpendicular to the planar mirror 28. The translator 42 is shifted in position by an electromechanical positioner 46, operating in response to control circuit 40 commands. With the rotary arm 18 in the "home" position, however, the positioner 46 receives error signals so that the translator 42 may be driven to a corresponding start position from which track writing can begin. This is to say that the translator 42 is driven to "find" the beam reflected by the arm 18 in its home position. This initialization of the translator 42 is effected by the control circuits 40 in response to error signals generated as described below. The control circuits 40 may comprise a microprocessor or firmware for providing the two basic commands needed for positioner 46 operation.

Either one or both of two light beam devices, namely a light beam source 50 and a beam position detector 52, are mounted on the translator 42. When both are movable with the translator 42 the unit functions as a single plane autocollimator 54, with a projected light beam from the source 50 being reflected off the mirror on the arm 18 inside the housing 12, and the reflected beam being parallel to the projected beam when the system is adjusted to a stable position. However, in alternative configurations either the light beam source 50 or the beam position detector 52 may be fixed, with the other element 50 or 52 being mounted on and moving with the translator 42. The autocollimator version is generally preferred because it allows certain corrections to be made for higher resolution, because the length of arc is smaller by a factor of two, and the window 32 can therefore also be smaller.

The light beam source 50 can be any light source suitable for directing a small beam spot on the detector 52, but for cost and performance reasons a semiconductor laser is generally used. An optics system 53 is used to separate the direct and reflected beams and establish the desired small beam spot size at the object plane. The pattern of a laser beam spot has a generally Gaussian distribution and is elliptical in outline, but the beam centroid does not change (unless the beam direction shifts) because these factors are constant. The beam position detector 52 has an area larger than the beam spot size, and a configuration to provide a signal that varies with beam position on the detector. Thus the detector may comprise a linear array of elements, a two-dimensional array of elements, or a single device with variable amplitude response, such as a lateral cell. Preferably, however, the detector 52 comprises a bicell unit in which two adjacent detector cells abut at an intermediate index line or area which defines the desired position for the beam centroid when the rotary actuator is in stable operation at a selected position.

The signals generated by the detector 52 are converted into an error signal by error signal generating circuits 56, including typical preamplifier and signal compensation means, and coupled to the actuator drive circuits 36.

This system in accordance with the invention thus employs the translator 42 as a remote position reference for placing the magnetic head 16 at chosen radial positions, very precisely determined, relative to the disk 14. The rotary actuator arm 18 real time position is represented by the reflected light beam direction off the mirror 28, because the light beam from the source 50 in any event impinges off the mirror 28 through the window 32, and then reflects back through the window 32 onto the detector 52. Because the arm length at which translator 42 adjustments are made is substantially greater than the actuator arm length to the magnetic head 16, there is a corresponding advantage in positional accuracy. This is augmented by high gain resulting from very small beam size and high detector sensitivity. With a beam spot size of 54 .mu.m and a mechanical arm advantage of about 7 (L.sub.translator) to 1 (L.sub.actuator), for example, the position of the magnetic head 16 can be easily controlled to within .+-.5 microinches. This is a sufficiently precise control to enable approximately 4,000 reference patterns per inch to be written on the disk 14. Greater density in writing reference patterns can be provided by further increasing the mechanical advantage and adjustments in the optical path.

This capability is achieved without any more than the two minor structural additions to the sealed disk drive 10, and by a mechanical and optical mechanism that is entirely external to the sealed disk drive 10. Thus there is no need for disassembly of the unit, hence clean room conditions are not a prerequisite to writing the patterns. The disk drive 10, manufactured by mass production methods, need only be placed on a suitable fixture so that the actuator axis 20 is properly aligned. Then with the actuator arm 18 commanded to the home position, the translator 42 is correspondingly placed at a start position. Thereafter, incrementing the translator 42 causes the rotary actuator to be located at the chosen track positions on the disk. At each position, once the actuator arm 18 has stabilized with the magnetic head 16 at the radius defined by the translator 42, the reference patterns can be written using read/write circuits 38.

To fully realize the potential of this concept, however, advantage must be taken of it to correct for certain practical problems concomitant to low cost light beam sources and optical elements. Semiconductor lasers do not typically generate beams of constant uniformity and directionality. The attenuation of beam power as it traverses the optical path may also vary. These factors can adversely affect magnetic head positioning accuracy. Also consideration must be given to possible beam distortion introduced by the window and manufacturing tolerances as to placement of mechanical elements.

The system of FIGS. 2 and 3 is a practical implementation of the configuration of FIG. 1, with extension of the autocollimator technique. The translator 42 comprises an arm 66 mounted to pivot approximately about the actuator axis 20 as in FIG. 1. The structure to which the arm 66 is attached includes reference points or, not shown, surfaces against which the disk drive 10 may be indexed. These references may comprise registration holes or pins built into the bottom or sides of the housing 12 or reference surface, or a combination of such features. These may be used to locate the disk drive relative to the structure to which the translator 42 is attached. The positioner 46 comprises a transverse lead screw drive 68 turned by a microstepper 69 and mounted to pivot on a shaft 70 about a positioner reference axis 72 that is parallel to the actuator axis 20. Variations in the length of increments at the translator 42 may be desirable to assure equal increments of movement at the head 16. However, this variation is dependent on angle in wholly predictable fashion and the increments may thus be adjusted in predetermined fashion by the controller circuits 40. An additional pivot 76 is provided between the lead screw nut 74 driven by the lead screw 68 and the translator arm 66 which accommodates changes in the angular relationship between the translator 42 and the positioner 46 as the translator arm 66 moves arcuately through its range.

In this example the light beam source comprises a semiconductor diode laser 80 pointed toward the mirror 28 on the actuator 18, through the window 32. On each side of the laser beam path, facing in a parallel direction, are separate detectors, consisting of a position feedback detector bicell 82 (hereinafter the feedback bicell) and a beam direction detector bicell 84 (hereinafter the direction bicell). The beam from the laser 80 is passed through a collimating lens 86 and a focusing lens 87, to a beam splitter 90 angled at a 45.degree. angle to the beam path. A portion of the incident beam, a reference beam, is deflected off to a corner reflector 92 into a reverse but parallel path to the direction bicell 84. The remaining portion of the incident beam proceeds through the beam splitter 90 and the window 32 and after reflection off the mirror 28 on the actuator 18, returns to be reflected off the beam splitter 90 to a second corner reflector 93, and then along a second parallel but reverse path to the feedback bicell 82. If there is beam directional instability due to the laser 80 it is present both in the reference beam and the feedback beam and correspondingly varying signals are derived at the feedback bicell 82 and direction bicell 84, to be used as hereafter described.

The two currents from the feedback bicell 82 are applied to an arm position detector circuit 96 within the error signal generating circuits 56. The two currents are separately amplified in first and second transimpedance amplifiers 98, 99 respectively, and their difference is derived at a first difference junction 102, while their sum is derived at a summing junction 104. The difference signal, which represents the basic error signal but which may drift with variations in beam intensity, is thus adjusted to a suitable higher or lower normalized level by a conventional normalizer circuit 106 by using the summed amplitude of the two signals as a reference. Like circuits may be utilized in the beam direction circuit 110, which derives the two outputs from the direction bicell 84, and which then generates a normalized error signal output which is differentially combined at a second difference circuit 112 with the error signal output from the detector circuit 96, and the resultant signal is applied to an amplifier 58 via a compensation circuit 59 performing accepted control system functions.

A number of expedients aid in optimizing the performance of this system. For example, recognizing the elliptical character of the beam from the laser 80, the long dimension of the focused beam spot is parallel to the index line dividing the bicell, by proper orientation of the laser. The divergent light beam from the laser is, after formation into a collimated beam by the first lens 86, focused by the second lens 87 in a region close to the window 32. The shorter the focal length of the focusing lens 87, the smaller the beam size at the feedback bicell 82. In a practical example the laser 80 beam diverges, and the first lens 86 provides a collimated beam of 4.6 mm which is focused by the second lens 87 into the feedback bicell 82 to have a 54 .mu.m diameter in the long dimension. For a 51/4" disk drive unit, a practical example of the length relationships is 3" (76 mm) for the rotary arm distance from pivot axis to magnetic head recording gap, and 8" (203 mm) beam length to the feedback bicell 82 was selected. Under these conditions, a 1 microinch (0.0254 micron) displacement at the head gap is equivalent to a spot motion of 0.12% of the beam diameter, which with amplifier gain provides a significant error signal, hence the system has adequate static performance to achieve accurate positioning. The ideal situation during the pattern writing process is for the actuator to be "locked" in position so as not to have any error contributions from the actuator positioning system present in the written patterns. While it is not practical to "lock" the actuator, an actuator control with a high gain bandwidth product will approximate this condition. The present example provides a position control system with an open loop crossover frequency of over 500 Hz which provides adequate dynamic performance to achieve the very high desired accuracies.

This may be better appreciated by reference to FIG. 4, in which the incoming light path from the collimating lens 86 is shown in simplified form, and directed through the focusing lens 87 in the window 32 onto the planar mirror 28. The beam reflected back onto the feedback bicell 82 is controlled at an arm length, L.sub.translator, which is relatively long with respect to the distance between the actuator axis and the recording head gap, L.sub.actuator. In this Figure, the incoming beam is shown at an angle relative to the outgoing beam, rather than parallel, solely for purposes of clarity. The focal length and position of the lens 87 are chosen such that the focal plane coincides with the surface of the feedback bicell 82. Because the diffraction limited spot size is smaller with a smaller focal length, the feedback bicell 82 can be placed relatively close to the window 32, but practical consideration limit this spacing. The mechanical advantage is greater with a higher ratio between L.sub.translator /L.sub.actuator, and while 6:1 to 7:1 ratios are used in the present example, different ratios can be used if desired. Analysis of noise factors and system sensitivity, however, establishes that, as desired, a minute variation at the recording gap results in a significant signal derived at the error signal generating circuits to enable precise actuator control.

Two other factors are possible sources of error, but upon analysis, are seen to be inconsequential. The spherical aberration effect introduced by the window 32 is minimal, but an astigmatism that varies with the angle of incidence of the beam on the window is also introduced. However, this effect introduces defocusing which is an order of magnitude smaller than the beam spot size (54 microns) so that these aberrations are insignificant. Similarly, consideration has been given to the effects of possible misalignment of the translator axis relative to the actuator axis, and differences in placement of the mirror on the rotary actuator While such errors can have a small influence on beam spot size, and upon the optical path length, there is no effect on the relationship between the optical platform angle and the actuator angle. Consequently, when the optics are initially aligned relative to the home position of the rotary actuator, such errors are automatically cancelled. The maximum allowable pivot plus mirror displacement along the optical axis, and the perpendicular misalignment, are in the low millimeter range, which is well within manufacturing tolerances.

The capability for external control of the internal rotary actuator afforded by the system of FIGS. 1-4 enables other types of data transfer to take place, with or without position information. After assembling a disk drive and before or after the drive is sealed, for example, the system can be used to position the head assembly at different track locations for reading or writing purposes, or both.

FIG. 5 depicts a system that has the basic advantages of the concept, but involves different design tradeoffs. The mirror 28' and window 32' are incorporated in the drive unit 10,, as previously described. However, the light source comprises a fixed laser 80', which, together with collimating and focusing lenses (not shown) is disposed to direct a light beam onto the mirror 28', for deflection out to a feedback bicell 82'. The feedback bicell 82, is on a translator 42' moved by a positioner 46', either linearly (with corrections) or in an arc about the actuator axis. Error signal generating circuits, including a compensation circuit 59', and a power amplifier 58', are utilized to energize the actuator drive and to control magnetic head position. The advantages include a simpler optical system and a double angle beam deviation with translator 42' position The result is that the system has double the beam sensitivity at the cost of a larger window.

As seen in FIG. 6, systems and methods in accordance with the invention may also be utilized in a combination that does not require an internal mirror or adjacent window. The rotary actuator internal to the disk drive 10' has a shaft 130 that extends through a sealed bearing 131 to an end outside the housing 12', so that its mechanical angular position is available outside the housing 12'. The angular position of the shaft 130 is directly indicative of the arm and head positions. Consequently, a mirror 28' may be temporarily attached by a fixture 132, such as a magnetic or threaded element, to the shaft 130 end. The mirror 28' can bisect the central axis of the actuator shaft, or it can be at a radius, but in each instance the plane of the mirror should be parallel to the axis. While the mirror 28' can obviously be adjusted by trial and error, an alignment unit (not shown) may be used to place the mirror 28' at a precise angle when the actuator is in a predetermined position, such as at the "home" stop. Preferably, however, an index slot (not shown) will be incorporated in the shaft 130 transverse to the actuator axis, so as to enable the fixture 132 to seat itself with the mirror 28' at a precise angular orientation. It should be noted that the shaft 130 need not extend flush with the housing wall but can be below it, as long as the mirror 28' can receive the beam. The bearing seal 131 can be assured by a replaceable cover (not shown) which further protects against leakage into the housing interior.

With this arrangement, the disk drive 10' with attached external fixture 132 and mirror 28' on the shaft 130 is seated on a holder so as to be in the path of the focused laser beam from the translator 42' and positioner 46' system Initialization and incrementing, with writing of position reference information at each position, thus proceeds as previously described. When writing is complete the fixture 132 and mirror 28' are removed from the shaft 130 and the disk drive 10' is ready for data transfer operations.

A feature of the invention is that the capability for externally sensing an internal position can be used in other modes as well. As seen in FIGS. 7(a) and (b), a disk 14' in a sealed disk drive 10' may be manufactured with precisely scribed index marks 140 on its periphery. A window 32' in the housing wall 12' is in line with the periphery, so that a beam from a fixed laser 142 passes through the window 32' to be reflected back out to a detector cell 144. With a number of approximately equally spaced index marks 140, the cell 144 generates a train of pulses at a given periodicity. These are applied to processor circuits 146 which then generate timing, clocking or velocity control signals as desired. In accordance with known techniques, for example, a high frequency train of clock signals is synchronized to the index pulses. The system therefore provides an accurate means of identifying the instantaneous angular position of the disk.

While there have been described above and illustrated in the drawings various forms and alternatives in accordance with the invention, it will be appreciated that the invention is not limited thereto but encompasses all modifications in accordance with the scope of the appended claims.