Apparatus and Method for Damping Head Positioners for High Performance Disc Drivers

This invention relates generally to the control of magnetic head movement in high performance magnetic disk drives. More particularly, the invention is directed to the stabilization and control of mechanical resonances which occur in head positioners for such drives.

A disk drive is a rotating magnetic storage device capable of storing data which may by accessed at high speeds. This device includes a rotating, flat magnetic surface onto which data can be recorded and later retrieved, usually in milliseconds because of the high rotational rate of the disk and the fast radial accessing capability of the head positioner. The head positioner of the disk drive assembly functions to locate and maintain the magnetic read write heads at a commanded track position above the disk, and this head positioner also provides precision support and stability which is necessary for the heads to function properly. Most disk drive designs are based upon the moving head principle as opposed to the faster, but more costly, "head per track" concept. Usually more than one disk or disk surface is used per drive, and in this case one or two separate heads are used for accessing each disc surface. The heads for each surface are mounted on aligned arms of the positioner such that they all move in unison. The resulting head positioner structure looks like a comb and is often referred to the "head comb". The head comb is typically aligned and moved by a support and motor structure which is operative with either linear or rotary motion.

When a head positioner in a disk drive performs a very fast see many resonant or ringing codes of vibration can be excited and unless properly damped, will tend to ring for long periods of time, typically), on the order of 100 milliseconds or more. Often the amplitude of these modes of vibration at the magnetic head position is a large percentage of the width of a data track on the magnetic disk. If this is the case, data receding or writing will be disabled until the ringing amplitude drops to a value sufficiently low to permit accurate reading or writing.

Disk drives having a high track density typically employ positioning servo schemes in order to ensure accurate and fast track locating by the magnetic heads. However, the accuracy and speed of such servo systems are highly dependant upon the mechanical system resonances. A servo system is a negative feedback control system in which part of the system is mechanical. An ideal control loop for such a servo system feeds the output signal of the system back to the input of the system and out of phase with the input thereto to force the system to exhibit a unity gain transfer function.

When the servo system mechanics are introduced, mechanical resonances produce output signals greater than the input signals as well as cause shifted output signal phase relationships relative to the input signal. Some compensation is customarily introduced into the servo control loop to provide improved stability with the mechanical system. But in spite of this compensation, mechanical resonances of the above type produced by head positioner vibrational resonances often are large enough and provide a phase shift of sufficient magnitude to create a positive feedback for the system at greater than unity gain. When this happens, the servo loop becomes unstable at that resonance frequency.

In the low performance disc drive industry, one practice has been to employ rotational dampers on stepper motors which are used to drive head positioners for the disc drives. These dampers are used to increase the stability of the stepper motor per se. However, as presently known, these damping techniques of the prior art are not directed to the damping of vibrational resonances of an entire head positioner used in high performance track following servo disc drive actuators, as in the case with the present invention.

The general purpose of this invention is to provide a new and improved solution to the above problems resulting from vibrational resonances in head positioners for disk drives. This solution involves reducing the amplitude of resonance modes of vibration in the head positioner by providing a chosen movable floating mass at a selected location on the head positioner, and this location exhibits large motions of the vibrating resonant modes in the head positioner. This mass is positioned in a liquid film of known viscosity, and the mass and film are located within a housing securely fixed to the head positioner at the selected location thereon. As the head positioner traverses its seeking path and abruptly comes toward a rest position, vibrational mode energy is coupled from the head positioner to the mass to thus cause the mass to move across the liquid film, thereby creating shear forces at the mass-film interface. These shear forces and the distance over which they act represent damper of energy loss that is removed from the vibration mode energy of the head positioner.

The specific sequence of steps that occur in our novel damping method include viscously coupling vibrational mode energy from a head positioning member to a movable mass, (e.g. steel balls), and then moving the mass relative to the head positioning member to produce a energy dissipation in the viscous coupling. The above action causes a concurrent reduction in the vibrational mode energy in the head positioning member, and such concurrent reduction in vibrational mode energy in turn produces a desirable reduction in magnitude in the vibrational modes of the head positioning member.

In a first embodiment of invention, the mass is a metal cylindrical slug which is centered via a soft-spring between opposite end walls of the housing, and vibrational energy received by the housing is viscously coupled to the mass at frequencies close or equal to the vibrational resonant frequencies of the head positioner. The spring only serves to keep the mass away from the end walls of the mass-housing, so that the damper mass remains functional regardless of its orientation relative to gravity.

In another preferred second embodiment of the invention, the damper member includes a plurality of spherical steel balls which are located on a viscous film within a damper housing and are adapted for movement back and forth on the viscous film during operation of the damper. As in the first embodiment above, the damper in this second embodiment is mounted on the track accessing arms of the disc drive at a predetermined distance from the rotational axis thereof. This second embodiment of the invention lends itself to rapid, high volume and low cost production and provides relaxed tolerance requirements relative to the high dimensional tolerance requirements for constructing the single slug-type damper.

Accordingly, an object of the present invention is to provide a novel damper apparatus which is easy to construct and which is particularly and uniquely adapted and well-suited for mounting on a head positioner for a disc drive.

Another object is to provide a new and improved damper apparatus of the type described which may be constructed at high yields, with relatively low manufacturing costs and requiring relatively low manufacturing tolerances in one embodiment thereof.

Another object is to provide a new and improved damping method and apparatus which allows faster disk drive track locating times and wider bandwidth and higher performance of servo positioning systems associated therewith.

Another object is to provide a damping method and apparatus of the type described which dissipates energy from modes of head positioner vibration to thereby reduce settling time after a seek movement. Thus subsequent earlier reading and writing operations are achieved.

A further object is to provide a new and improved method and apparatus of the type described which improves associated servo control loop stability.

Another object is to provide a new and improved method and apparatus of the type described operative to improve servo and seek performance without the need to increase resonant frequencies which, in some cases, may not be possible.

A novel feature of this invention is the provision of either an elongated cylindrical metal slug or a plurality of steel balls which are movably mounted on a viscous oil within an elongated damper housing. Both of these masses are caused to oscillate back and forth in the housing during a damping operation to in turn produce a significant reduction in magnitude of the vibrational modes of a head positioning member.

These and other objects and features of this invention will become more readily apparent in the following description of the accompanying drawing.

FIGS. 1a and 1b are isometric views (FIG. 1b is partially cut away). of a head positioner and damping apparatus according to the invention.

FIG. 2 is a cross-section view of the damping apparatus of FIG. 1.

FIG. 3 is a graph plotting resonant mode amplitude versus resonant frequency for the damped (D) and undamped (U) head positioner to illustrate the reduction in resonant mode amplitude produced by the present invention.

FIG. 4A is a partially cut-away isometric view illustrating the spherical ball damper arrangement according to an alternative embodiment of the invention.

FIG. 4B is a cross-sectional view taken along lines B--B of FIG. 4A.

Referring now to FIG. 1, there is shown a rotary head positioner including a motor and arm support structure 12 with front and back openings 14 and 16 respectively. The support structure 12 serves as a housing for a magnetic head accessing arm structure 13 which includes a plurality of accessing head arms 22, 23, 24, 25, 26 and 27 extending as shown out of the front opening 14 and having a plurality of magnetic heads 28, 29, 30, 31, 32 and 33 located on the ends of the accessing arms. The accessing head arm structure 18 is pivotally mounted within the support structure 12 on a vertical axis 34 about which it is driven with rotary motion at high speeds during a fast seek operation. Magnetic discs (not shown) are positioned in the openings between the magnetic heads 28, 29, 30, 31, 32 and 33, and these heads are driven to preselected positions on certain tracks of these discs during a fast seek operation.

The accessing head arm structure 18 is driven by a motor coil 36 which is mounted inside the housing 12 and on the head arm structure 18. This motor coil 36 is positioned as shown at the rear center of the accessing head arm structure 18. The motor coil 36 is also centered between upper and lower motor magnets 38 and 40, and includes a pair of end flanges 42 and 44 between which the motor coil is wound. A floating mass damper 46 is mounted as shown between these flanges 42 and 44.

Referring now to FIG. 2, the floating mass damper 46 is shown in more detail in a cross section view thereof and includes a cylindrical housing 48, typically of magnesium, having an end wall 50 and another removable wall or cap 52 which permits the loading therein of a floating damper mass 54. The floating damper mass 54 is typically non-magnetic stainless steel and has a central passage 56 along the longitudinal center line thereof and is centered in the housing 48 with a pair of soft centering springs 58 and 60. These springs extend between the opposite end walls 50 and 52 of the housing 48 and recessed or cavity regions 62 and 64 in the respective ends of the floating damper mass 54.

A viscous damping film 66 is provided as shown between the outer surface of the floating damper mass 54 and the interior surface of the housing 48, and this viscous damping film 66 will typically consist of silicone oil.

During a fast seek operation, the ends of the accessing head arms 22, 24 and 26 will typically undergo a resonant mode vibrational amplitude (greatly exaggerated) as indicated by the distance A in FIG. 1. When undergoing such a resonant mode vibration, the rotary head positioner 10 cannot become electrically useable until the value of A is reduced to near zero as the head arm vibration ceases. In accordance with the present invention, the floating mass damper 46 provides an elegantly simple solution to reducing the time required for the value of A to be reduced. In accordance with this invention, the floating mass damper 46 is mounted at the rear of the accessing head arm structure 18 and in a vertical plane with the dimension B. This dimension B corresponds to the maximum resonant mode vibration which the rear of the accessing head arm structure 18 will undergo during a fast seek operation and corresponding to the displacement A of the accessing head arms 22, 24 and 26. During the resonant mode vibration which produces the displacement B as indicated, the housing 48 is rapidly driven laterally back and forth across the viscous damping film 66. This moment of the housing 48 in cooperation with less movement of the floating damper mass 54 produces lateral shear forces which serve to substantially dissipate vibrational mode energy in the rotating accessing head arm structure 18. This energy dissipation thus quickly reduces the value of dimension B and maintains the value of B at a level much lower than it would otherwise have been in the absence of the floating mass damper 46.

Referring now to FIG. 3, there is shown a graph of the vibrational magnitude of the accessing lead arm structure 18 as a function of vibration frequency Line U of the curve in FIG. 3 represents the vibrational magnitude M without damping, whereas line D indicates the variation of M with frequency after damping. As shown in FIG. 3, the maximum point on line D curve will be slightly shifted (lowered) in frequency by a certain amount as a result of the damping action. Thus, this reduction in the magnification factor M as a result of employing the floating mass damper 46 substantially reduces the magnitude of the resonance values at A and B in FIG. 1 to more stably locate the magnetic heads 28, 30 and 32 on preselected tracks of the magnetic discs previously described.

Thus, there has been described a new and improved method and apparatus which is easy to construct and which is reliable and durable in operation and adds a relatively small increase in cost to the rotary head positioner 10. In contrast to the vastly more complex viscous damping systems of the prior art, the floating mass damper 46 of the present invention employs only a single floating damper mass 54 which is spring centered using only a pair of springs 58 and 60 and generates substantial shear forces as the housing 48 moves over the viscous damping film 66. It has been determined that the reduction in settling time of the damped head positioner has been 5 to 10 times that of the undamped head positioner.

Referring now to FIGS. 4A and 4B, in this embodiment of the invention the movable mass slug shown in FIG. 2 has been replaced with a plurality of spherical steel balls 70 which are movably mounted inside a housing shell 72. As in the case of the movable slug-type damper of FIG. 2, this embodiment in FIGS. 4A and 4B employs a viscous fluid 74 such as a silicone oil with a viscosity of about 30,000.00 centistokes and previously filtered to remove therefrom particles of 25 microns or greater in diameter. The steel balls are non-magnetic series 300 stainless steel balls which are typically about 1/4 inch in diameter and weigh about one (1) gram each. In present disk drives, five (5) of these steel balls are employed inside the damper housing 72. On the interior housing walls at each end of the housing 72 is a soft ethylene-propylene rubber disk which is used to prevent direct ball contact to the ends of the shell housing.

The advantage that this embodiment of the invention has over the embodiment of FIG. 2 resides in lessening the dimensional critically of the straightness of the outer surface of the slug 54 in FIG. 2 and the corresponding straightness of the interior walls of the housing 48 which face the slug in the closely spaced arrangement shown and separated by the thin viscous film 66. Thus, if there is a slight warping in either or both of the slug surface or the housing walls, this variation in design tolerance will cause a corresponding undesirable variation of the damped curve D in FIG. 3 and thus in turn produce an undesirable performance variation from disk drive to disk drive. This requirement for high tolerance manufacturing and mating part surface straightness for the embodiment of FIG. 2 ultimately increases the manufacturing cost of this damper and lowers the yields of the manufacturing processes used to make the damper.

On the other hand, however, when using the plurality of steel balls as indicated in FIG. 4, these balls will move back and forth approximately as wall in a non-linear or slightly warped path as they would in a straight or linear path and thus produce much less damping variability of the damped curved D in FIG. 3. Thus, the design of the damper in FIG. 4 is much more forging of tolerance variations in surface straightness of the interior housing walls adjacent to which the spherical balls 70 move back and forth.

Another advantage that the embodiment of FIG. 4 has over the embodiment of FIG. 2 is that it takes only about one minute for the balls 70 in FIG. 4 to come to a rest position when the damper is turned upside down. With the embodiment of FIG. 2, it may take up to ten hours or more for the silicone oil to move around the outer surface of the single cylindrical slug member 54 and resettle to a position of gravity when the damper unit is turned upside down. Thus, not only does the damper design of FIG. 4 exhibit an overall improvement in operational performance with respect to the performance of the damper in FIG. 2, but it is also easier and less expensive to manufacture than the damper in FIG. 2.

Various modifications may be made in the above described embodiments of the invention without departing from the scope of the appended claims. For example, for reasons of ease of assembly, it is often preferable to use a 3-piece construction for the accessing head arm structure 18. Thus, one piece may consist of the arms 22, 23, 24, 25, 26 and 27 integrally joined at one end; a second or center interlocking piece may be rotatably mounted at the axis 34 and mechanically interlocked with the accessing arms 22, 23, 24, 25, 26 and 27; and an end or motor coil support piece 41 may be mounted at the rear of center interlocking piece to facilitate bringing the motor coil 36 within the housing 12 and between the upper and lower motor magnets 38 and 40, respectively.