Active damping of actuator bearing translational mode

Apparatus and method for improving operational performance of a disc drive by detecting and compensating for translational vibration modes in a disc drive actuator. The actuator is configured to support a head adjacent a recording disc and pivots about a stationary shaft by way of a cartridge bearing assembly having a pair of ball bearing assemblies. A proximity probe detects translational movement of the actuator assembly relative to the shaft due to, for example, abrupt acceleration and deceleration of the head during a seek operation, or externally applied mechanical shocks. The output of the proximity probe is used to generate a bearing translation signal which is fed into an actuator control servo circuit to modify the amount of current applied to an actuator motor to compensate for the translational vibrations during track following and seek operations.

FIELD OF THE INVENTION
 This invention relates generally to the field of magnetic data storage
 devices, and more particularly, but not by way of limitation, to improving
 disc drive operational performance by detecting and canceling
 translational motion of the actuator due to actuator bearing deformation.
 BACKGROUND
 Disc drives are used as primary data storage devices in modem computer
 systems and networks. A typical disc drive comprises one or more rigid
 magnetic storage discs which are journaled about a rotary hub of a spindle
 motor to form a disc stack. An array of read/write transducing heads are
 supported adjacent the disc stack by an actuator to transfer data between
 tracks of the discs and a host computer in which the disc drive is
 mounted.
 Conventional actuators employ a voice coil motor to position the heads with
 respect to the disc surfaces. The heads are mounted via flexures at the
 ends of a plurality of arms which project radially outward from an
 actuator body. The actuator body pivots about a shaft mounted to the disc
 drive housing at a position closely adjacent the outer extreme of the
 discs. The pivot shaft is parallel with the axis of rotation of the
 spindle motor and the discs, so that the heads move in a plane parallel
 with the surfaces of the discs.
 The actuator voice coil motor includes a coil mounted on the side of the
 actuator body opposite the head arms so as to be immersed in the magnetic
 field of a magnetic circuit comprising one or more permanent magnets and
 magnetically permeable pole pieces. When current is passed through the
 coil, an electromagnetic field is set up which interacts with the magnetic
 field of the magnetic circuit to cause the coil to move in accordance with
 the well-known Lorentz relationship. As the coil moves, the actuator body
 pivots about the pivot shaft and the heads move across the disc surfaces.
 The control of the position of the heads is typically achieved with a
 closed loop servo system such as disclosed in U.S. Pat. No. 5,262,907
 issued to Duffy et al. and assigned to the assignee of the present
 invention. A typical servo system utilizes servo information (written to
 the discs during the disc drive manufacturing process) to detect and
 control the position of the heads through the generation of a position
 error signal (PES) which is indicative of the position of the head with
 respect to a selected track. The PES is generated by the servo system by
 comparing the relative signal strengths of burst signals generated from
 precisely located magnetized servo fields in the servo information on the
 disc surface.
 The servo system primarily operates in one of two selectable modes: seeking
 and track following. A seek operation entails moving a selected head from
 an initial track to a destination track on the associated disc surface
 through the initial acceleration and subsequent deceleration of the head
 away from the initial track and toward the destination track. A velocity
 control approach is used whereby the velocity of the head is repeatedly
 estimated (based on measured position) and compared to a velocity profile
 defining a desired velocity trajectory for the seek. Corrections to the
 amount of current applied to the coil during the seek are made in relation
 to the difference between the estimated velocity and the desired velocity.
 At such time that the head reaches a predetermined distance away from the
 destination track (such as one track away), the servo system transitions
 to a settling mode wherein the head is settled onto the destination track.
 Thereafter, the servo system enters a track following mode of operation
 wherein the head is caused to follow the destination track until the next
 seek operation is performed.
 Disc drive designs thus typically use proximate time optimal control with a
 velocity profile to control a selected head during a seek, a state
 estimator based controller with relatively slow integration to settle the
 head onto the destination track, and the same state estimator based
 controller with relatively fast integration for track following.
 Typically, disc drive designers have employed ball bearing cartridges for
 journaling the actuator assembly about the pivot point. These bearing
 assemblies are subject to very rapid, repetitive movements of the actuator
 arm about the pivot point as the heads are radially moved from track to
 track. The precision of seeking and track following operations is
 dependent upon the performance of the actuator bearing assembly. As the
 storage capacity of modern disc drives continues to increase, the
 precision required by the rotation of the actuator arm about the bearing
 assembly also increases.
 Despite the requirements for precise movement, ball bearing assemblies are
 subject to mechanical limitations that can adversely affect their use in
 today's high-performance disc drives. More specifically, conventional ball
 bearing assemblies are subject to metal wear, increased vibrational
 resonance and friction, and lubricant outgassing. Each of these
 limitations increases the presence of extraneous motion exhibited by the
 ball bearing assembly during rotation.
 In concert with these mechanical limitations, ball bearing assemblies also
 provide an undesirable translational degree of freedom in the X-Y plane
 (i.e. a plane intersecting the assemblies and normal to the axes about
 which the assemblies rotate). This translation is caused primarily by the
 deflection of the ball bearings within the inner and outer races of the
 bearing assembly. The deflection of the ball bearings results from a
 lateral force applied to the actuator during a seek or track following
 operation. During deflection, the ball bearings exhibit a "spring-like"
 response to the laterally applied force. The natural frequency of the
 resulting bearing translation is dependent on the mass of the actuator arm
 and the spring stiffness of the bearing assembly. This vibration mode is
 often referred to as the bearing translation mode.
 A variety of solutions have been proposed to limit the presence of
 translational modes of vibration in disc drive actuator bearings. For
 instance, adding mass to the actuator arm tends to reduce the frequency of
 the bearing translational mode. U.S. Pat. No. 4,812,935 issued to Sleger
 teaches the limitation of bearing translational modes through use of mass
 dampers. However, adding mass to the actuator arm has the unwanted side
 effect of slowing seek operations and limiting servo bandwidth. Other
 proposed solutions include absorbing the vibratory energy through use of
 elastomeric components within the bearing assembly, as taught by U.S. Pat.
 No. 5,983,485 issued to Misso and assigned to the assignee of the present
 invention. Although absorption components may reduce translational
 vibration, the additional space required for installing these components
 is prohibitive in modern, compact disc drives.
 In light of the deficiencies presented by the prior art solutions, there
 continues to be a pressing need to develop a compact means for limiting
 the presence of bearing translation while improving the overall
 performance of actuator movement.
 SUMMARY OF THE INVENTION
 The present invention provides an apparatus and method for improving the
 operational performance of a disc drive through the detection of a
 translational mode of vibration within a bearing assembly of the disc
 drive.
 As exemplified by presently preferred embodiments, the disc drive includes
 a base deck supporting a rotatable disc and a rotary actuator. The
 actuator supports a head adjacent a recording surface of the disc and is
 controllably rotated about a bearing assembly by an actuator motor. A
 servo control circuit applies current to the actuator motor to position
 the head in relation to servo signals obtained from the disc recording
 surface.
 A proximity probe, such as a capacitance probe, is provisioned adjacent the
 actuator, or inside the cartridge bearing assembly, and is used to detect
 extraneous movement of the actuator caused by a translation within the
 bearing assembly. The bearing translation signal can be representative of
 a deflection within the bearing assembly caused by the application of
 current to the actuator motor. Additionally, the bearing translation
 signal can be representative of an externally applied shock to the disc
 drive.
 The proximity probe measures the bearing translation by monitoring the
 position of the actuator relative to a stationary component in the disc
 drive. The proximity probe outputs a bearing translation signal indicative
 of the direction and magnitude of the bearing translation. The bearing
 translation signal is used to compensate the servo control of the actuator
 motor for the disturbance caused by bearing translation.
 These and other features and advantages which characterize the present
 invention will be apparent from a reading of the following detailed
 description and a review of the associated drawings.

DETAILED DESCRIPTION
 In order to set forth a detailed description of various presently preferred
 embodiments of the present invention, reference is first made to FIG. 1
 which shows a top plan view of a disc drive 100 used to store computer
 data. The disc drive 100 includes a head-disc assembly (HDA) 101 and a
 printed wiring assembly (PWA) supporting control electronics used by the
 disc drive 100. The PWA is mounted to the underside of the HDA 101 and
 hence, is not visible in FIG. 1.
 The HDA 101 includes a base deck 102 which supports a spindle motor 104
 used to rotate a plurality of discs 106 at a constant high speed. Although
 not shown, it will be understood that tracks are defined on each of the
 disc surfaces using servo data written to the disc drive 100 during
 manufacturing in a conventional manner. A disc clamp 108 secures the discs
 106 and a series of disc spacers disposed between adjacent discs (not
 visible in FIG. 1) to the spindle motor 104. A top cover (not shown) mates
 with the base deck 102 to provide a contained environment for the HDA 101.
 A rotary actuator 110 is configured for pivotal rotation about a cartridge
 bearing assembly 112 (hereinafter referred to as "bearing assembly 112")
 supported by the base deck 102. An E-block 115 comprises the central
 portion of the actuator 110 and serves as the mount for a plurality of
 actuator arms 116. The plurality of actuator arms 116 project from the
 E-block 115 and support flexure assemblies 118 which, in turn, support a
 plurality of corresponding heads 120 over the surfaces of the discs 106.
 The actuator 110 is rotated through controlled application of current to
 an actuator coil 113 of a voice coil motor (VCM) 114.
 A latch/stop assembly 122 secures the heads over texturized landing zones
 (not designated) at the inner diameters of the discs 106 when the disc
 drive is not in use and includes limit stops (not separately designated)
 to limit the radial extent (stroke) of the actuator 110. A flex circuit
 assembly 124 and a preamplifier/driver (preamp) 126 facilitate electrical
 communication between the actuator 110 and the disc drive PWA.
 Now referring to FIG. 2, shown therein is a side cross-sectional view of
 the bearing assembly 112 and a preferred embodiment of a proximity probe
 130. As discussed below, the proximity probe 130 is used to detect bearing
 translations. The bearing assembly 112 includes a stationary shaft 132
 which is rigidly supported by a top cover 134 and the base deck 102. The
 stationary shaft 132 is affixed to the base deck 102 by a retaining nut
 136. At the close of the fabrication process, a top screw 138 secures the
 stationary shaft 132 to the top cover 134. The stationary shaft 132 is
 annular having an outer diameter, an inner diameter and a height (not
 separately designated).
 The bearing assembly 112 further comprises a plurality of ball
 bearings/race assemblies (hereafter referred to as "bearings 140") which
 are affixed to the outer circumference of the stationary shaft 132. FIG. 2
 shows a preferred configuration of the bearings 140 about the stationary
 shaft 132 and includes two sets of bearings 140 disposed at substantially
 the top and bottom of the stationary shaft 132. Other configurations
 include, but are not limited to, placing additional sets of bearings 140
 about the outer circumference of the stationary shaft 132. It will be
 understood that the bearings 140 comprise various known components
 including an inner race, an outer race, ball bearings and lubricants (not
 separately shown).
 Continuing with FIG. 2 and the bearing assembly 112, an outer sleeve 142 is
 connected to the bearings 140 such that the outer sleeve 142 freely
 rotates about the stationary shaft 132 with limited vertical movement. The
 outer sleeve 142 is generally cylindrical and has inner and outer
 circumferences (not separately shown). There are numerous and well known
 means for connecting the moveable and fixed components of a bearing
 assembly. Often, the outer sleeve is held in place with a sufficiently
 strong compressive force that is generated by preloaded the outer sleeve.
 Other well known means include using retaining rings or chamfered inner
 and outer bearing races.
 The outer circumference of the outer sleeve 142 is rigidly affixed to the
 E-block 115 in a conventional manner. An ordinarily skilled artisan will
 recognize that there are numerous and nonexclusive means for attaching the
 E-block 115 to the outer sleeve 142. Many designs employ laterally
 engaging retaining screws for securing the E-block to the bearing
 assembly. Alternative designs make use of strong adhesives or pressure
 fittings.
 With continued reference to FIG. 2, shown therein is a preferred provision
 of the proximity probe 130 within the bearing assembly 112. In the
 embodiment depicted in FIG. 2, the proximity probe 130 is affixed to the
 outer circumference of the stationary shaft 132. As shown, the proximity
 probe 130 is disposed in the space provided by the bearings 140 between
 the outer sleeve 142 and the stationary shaft 132. A set of signal wires
 144 create the electrical connection between the proximity probe 130 and
 the PWA (not numerically designated). The signal wires 144 are directed
 through an aperture in the stationary shaft 132 and run through the
 annular center of the stationary shaft 132.
 In a preferred embodiment, the proximity probe 130 is a capacitance probe
 which outputs an analog voltage signal representative of a corresponding
 change in capacitance. The change in capacitance is induced by relative
 displacement of the proximity probe 130 relative to the outer sleeve 142.
 A suitable proximity probe 130 is model HPB-40 commercially available from
 Capacitec Inc., Ayer, Mass., USA.
 The principle underlying proximity measurement through capacitance is based
 on detecting a change in the capacitance exhibited between a pair of
 ferromagnetic plates. The capacitance of a parallel plate capacitor can be
 determined using the following equation:
 ##EQU1##
 where .epsilon..sub.0 represents the constant of permittivity of free
 space, A represents the area of the two parallel plates, d represents the
 distance of separation between the parallel plates and where C represents
 the capacitance exhibited by the parallel plate capacitor.
 Equation (1) demonstrates that capacitance is inversely proportional to the
 distance between the parallel plates. The proximity probe 130 detects a
 change in the proximity of the outer sleeve 142 relative the stationary
 shaft 132 by measuring the capacitance held between the proximity probe
 130 and the outer sleeve 142. The proximity probe 130 outputs a bearing
 translation signal (B.sub.T signal) having a magnitude and polarity
 indicative of a change in measured capacitance.
 Turning now to FIG. 3, shown therein are graphical depictions of bearing
 deflection (curve 146) and B.sub.T signal output (curve 148) plotted
 against an elapsed time horizontal axis 147 and common vertical axis 149.
 In a normal state, that is a bearing assembly 112 exhibiting no deflection,
 the magnitude of the B.sub.T signal output by the proximity probe 130 will
 be a nominal baseline value, such as zero (note the respective values of
 curves 146 and 148 at the beginning of the time period). If bearing
 deflection increases the distance between the outer sleeve 142 and the
 proximity probe 130, the proximity probe 130 outputs a B.sub.T signal
 having a negative polarity and a magnitude that is directly proportional
 to the increasing distance (see the first half of curves 146 and 148
 respectively). When, on the other hand, the deflection causes the outer
 sleeve 142 to approach the proximity probe 130, the capacitance increases
 and the proximity probe 130 outputs a B.sub.T signal having a positive
 polarity and a magnitude that is directly proportional to the decreasing
 distance (see the second half of curves 146 and 148 respectively).
 Turning now to FIG. 4, shown therein is a top cross-sectional view of the
 bearing assembly 112 and the proximity probe 130. The pictured
 cross-section is taken at a level flush with the placement of the
 proximity probe 130. As depicted in FIG. 4, the bearings 140 include a
 plurality of individual ball bearings (one denoted at 151) spaced about
 the outer circumference of the stationary shaft 132.
 Because the proximity probe 130 measures a change in proximity in a
 direction normal to its face (illustrated by an x-axis 150), the amount of
 deflection registered by the proximity probe 130 varies with the angular
 position of the rotary actuator 110. In other words, during rotation of
 the rotary actuator 110, the proximity probe 130 registers only a single
 component of the two-dimensional vector representing total bearing
 deflection.
 The application of current to the voice coil motor 113 creates a resultant
 force that is realized at the bearing assembly 112 and which acts
 substantially perpendicular to centerline of the actuator 110. Centerline
 axis 152 represents the rotary actuator 110 in a position in which the
 centerline axis 152 overlaps a y-axis 154 defined to be perpendicular to
 the x-axis 150. In the position represented by centerline 152, a
 deflection represented by a vector V.sub.1 is comprised completely of
 x-axis coordinates. As such, the proximity probe 130 registers the entire
 deflection vector V.sub.1.
 In contrast, centerline 156 illustrates the position of the actuator at the
 limit of its angular rotation (stroke). The angle .alpha. represents
 one-half of the total stroke of the rotary actuator 110. In most
 applications, one-half of the total actuator stroke is approximately
 15.degree.. When the rotary actuator 110 is in a position represented by
 centerline 156, a deflection represented by a vector V.sub.2 acts in a
 direction perpendicular to the centerline 156 and has both x axis and y
 axis components. Because the proximity probe 130 only registers the x-axis
 component of the deflection vector V.sub.2, that portion of the deflection
 vector V.sub.2 attributable to y-axis component is undetected.
 Because the proximity probe only measures unidirectional bearing
 deflection, there is some error associated with the output of the
 proximity probe 130 as the rotary actuator 110 rotates away from the
 centerline 152. The maximum expected error can be approximated by
 determining the amount of bearing deflection not detected by the proximity
 probe 130. This error can be expressed mathematically through the
 following series of equations:
 ##EQU2##
 where F is the vector representing the total deflection, F.sub.x is the
 vector representing the x-axis component of the vector F and where E
 represents the percent error associated with detecting only the x-axis
 components of the total deflection. Using elementary trigonometric
 properties, it can be shown that:
EQU Fx=F(cos.alpha.) (3)
 where .alpha. represents one-half of the total stroke of the rotary
 actuator 110. Substituting equation (3) into equation (2) provides the
 following equation:
 ##EQU3##
 Reducing equation (4) yields the following expression:
EQU 1-cos.alpha.=E (5)
 Substituting .alpha. with one-half of the total angular stroke of the
 rotary actuator 110 and solving for E gives the maximum expected error in
 the measurement of the proximity probe 130. In a typical disc drives,
 substituting .alpha. with 15.degree. gives a percent error of 3.4%. In
 other words, there is a maximum difference between the actual bearing
 deflection and the bearing deflection registered by the proximity probe
 130 of 3.4%. This limited error will generally be acceptable in most
 applications.
 However, in some embodiments of the present invention, it may be desirable
 to take into consideration the angular position of the rotary actuator
 when measuring bearing deflection. A correction factor can be applied to
 the B.sub.T signal which takes into consideration the angular position of
 the rotary actuator 110 with respect to the proximity probe 130. The
 angular position of the rotary actuator 110 can be easily determined from
 the radial position of the head 120 using servo data located on the discs
 106.
 Now turning to FIG. 5, shown therein is a generalized functional block
 diagram of relevant portions of the disc drive 100 of FIG. 1, including
 circuitry disposed on the aforementioned disc drive PWA. The disc drive
 100 is shown to be operably coupled to a host device 160 with which the
 disc drive 100 is associated. For example, the host device 160 can
 comprise a personal computer (PC).
 A control processor 162 provides top level control of the operation of the
 disc drive 100 in accordance with programming and parameter values stored
 in dynamic random access memory (DRAM) 164 and flash memory 166. An
 interface circuit 168 includes a data buffer (not separately shown) for
 the temporary buffering of transferred data, and a sequence controller
 ("sequencer," also not separately shown) which directs the operation of a
 read/write channel 170 and the preamp 126 during data transfer operations.
 The preamp 126 is preferably mounted to the actuator 110, as shown in FIG.
 1.
 A spindle circuit 172 is provided to control the rotation of the discs 106
 through back electromotive force (bemf) commutation of the spindle motor
 104. A servo circuit 176 controls the position of the selected head 120
 relative to the disc 106.
 FIG. 6 provides a block diagram of the servo circuit 176 of FIG. 5, in
 conjunction with proximity probe circuitry to be described as follows.
 During disc drive operation, servo information stored to the discs 106 is
 supplied to an automatic gain control (AGC) block 178 which adjusts the
 input signal amplitude to a range suitable for remaining portions of the
 circuit. A demodulator (demod) 180 conditions the servo information,
 including analog-to-digital (A/D) conversion, and provides the same to a
 digital signal processor (DSP) 182.
 In response to the servo information, commands provided by the control
 processor 162 (FIG. 5) and programming stored in DSP memory (MEM) 184, the
 DSP 182 outputs a current command signal to a coil driver circuit 186
 which in turn applies a current I.sub.c to the voice coil 113 in order to
 position the selected head 120 relative to the tracks on the corresponding
 disc 106. With reference to both FIGS. 5 and 6, a primary servo path (or
 loop) is thus established by the head 120, preamp 126, AGC 178, demod 180,
 DSP 182, coil driver 186 and voice coil 113.
 Additionally, FIG. 6 shows the proximity probe 130 to be operably connected
 to an amplifier (amp) 188, which outputs the B.sub.T (bearing translation)
 signal in relation to a measured change in proximity between the proximity
 probe 130 and outer sleeve 142. The B.sub.T signal is converted to digital
 form by way of an analog-to-digital (A/D) converter 190. The digital
 signal, representative of a translated bearing position (and accordingly
 designated as X.sub.B), is provided to the DSP 182, as well as to a
 differentiator 192.
 The X.sub.B signal is differentiated by a differentiator 192 and filtered
 by a lead-lag filter 194 to provide a bearing velocity signal V.sub.B to
 the DSP 182. A secondary, bearing velocity path is thus established by the
 proximity probe 130, amp 188, A/D 190, differentiator 192 and filter 194.
 For reference, at least the amp 188 will sometimes also be referred to as
 "proximity probe circuitry," as it is used to detect bearing translation
 in relation to the bearing translation voltage output by the proximity
 probe 130. However, it will be appreciated that other configurations of
 circuitry can readily be used to detect a bearing translation through use
 of a proximity probe 130, so that the circuitry of FIG. 6 is merely
 illustrative and is not limiting to the scope of the claims provided
 below.
 Turning now to FIG. 7, shown therein is a functional block diagram
 representing the programming of the DSP 182 for carrying out velocity
 controlled seeks. Initially, FIG. 7 shows a plant block 196,
 representative of electrical and mechanical portions of the disc drive 100
 including the VCM 114, the head 120 and the preamp 126. An observer 198,
 configured to provide a mathematical model of the operation of the plant
 196, outputs estimates of head position, velocity and bias (X.sub.E,
 V.sub.E and W.sub.E) on respective paths 200, 202 and 204. Bias is
 representative of forces that tend to move the head 120 off-track, such as
 windage effects from the air currents established by the rotation of the
 discs 106 and spring forces from the flex circuit 124. Bias will often be
 position dependent.
 During a seek, the number of tracks to go is input on path 206 to a
 profiler 208. As discussed above, the tracks to go is the physical
 distance remaining in the seek and is determined in relation to the
 difference between the position of the head 120 and the location of the
 destination track. In response, the profiler outputs the appropriate
 demand velocity on path 210 through, for example, interpolation techniques
 or from values stored in a look-up table. The bearing velocity signal
 V.sub.B from is output along signal path 212 to a summing junction 214.
 The difference between the demand velocity, the estimated velocity V.sub.E
 and the bearing velocity signal V.sub.B is determined using summing
 junction 214. It will be understood that the polarity of the bearing
 translation velocity V.sub.B signal depends on whether the translation
 increases or decreases the distance between the outer sleeve 142 and the
 proximity probe 130. This difference from summing block 214 is referred to
 as velocity error and is provided to gain block 216 having gain K.sub.AL
 to carry out an acceleration limiting function and then through a notch
 filter 218. At the same time, the destination track location is provided
 on input path 220 to a bias prediction block 222, which predicts an amount
 of bias which is summed with the estimated bias at summing junction 224.
 The output on path 226 is summed at the summing junction 228 with the
 output from the notch filter 218, as well as a second summing junction
 230, to be discussed shortly.
 The output of the summing junction 228 is provided to a gain block 232
 having gain K.sub.T, used to compensate for nonlinear torque
 characteristics of the VCM 114. The output is summed at summing junction
 234 with a current null signal on path 236, used to null out current. The
 resulting signal on path 238 comprises a current demand signal which is
 provided to the plant 196 to adjust the position of the selected head 120.
 In response, the plant provides a sense output on path 240; servo data are
 provided to a demodulation (demod) block 242 and current level is provided
 to summing junction 244. After demodulation, the servo data are linearized
 using linearization block 246 to give a position sample X.sub.SAMP on path
 248, which is differenced at summing junction 250 with the position
 estimate X.sub.E to provide an observer error O.sub.ERR on path 252. Also
 summed at summing block 250 is the bearing position signal X.sub.B output
 by the A/D 190 along signal path 254. In this manner, the operation of the
 observer 198 is maintained nominally that of the plant 300 while taking
 into consideration the X.sub.B signal. It will be understood to one of
 ordinary skill in the art that, although "summed" at summing block 250,
 the polarity of the bearing position signal X.sub.B signal varies with
 specific bearing deflection.
 The current input to the summing junction 244 is used for saturation
 compensation and is accordingly summed with a saturation null input from
 path 256. Gain block 258 applies a saturation gain K.sub.SAT and the
 output is differenced with the bias sum from path 230. Finite response
 filter (FIR) block 260 provides the desired time delay to the output of
 the notch filter 218, so that the observer 198 receives a switched input
 from either the FIR 260 or the saturation loop, depending upon whether the
 coil is in saturation.
 Accordingly, when large changes in current are applied to the VCM coil 113
 during a seek to quickly accelerate and decelerate the head 120, provision
 of the X.sub.B and V.sub.B signals to the DSP 182 enables the servo
 circuit 176 to compensate for the resulting bearing translation.
 Now turning to FIG. 8, shown therein is a functional block diagram of the
 programming of the DSP 182 during a position controlled, or track
 following operation. A plant block 262 is presented representative of
 selected electrical and mechanical aspects of the disc drive 100. For
 reference, the plant 262 generally includes portions of the primary loop
 established by the servo circuit 176 (see FIG. 6). The plant block 262
 receives as an input a current command (I.sub.CMD) signal on path 264 and,
 in response, outputs a position error signal (PES) on path 266 indicative
 of positional error in the selected head 120.
 FIG. 8 further shows an observer (OBS) block 268, which generally provides
 a mathematical model of the plant 262 and periodically outputs estimates
 of head position (X.sub.E), velocity (V.sub.E) and bias (W.sub.E) on paths
 270, 272 and 274, respectively (similar to the observer 198 in FIG. 8). As
 before, bias will be understood as indicative of forces that tend to move
 the heads away from a selected position, such as spring forces applied by
 the flex circuit 124 (FIG. 1) and windage effects caused by air currents
 set up by the rotation of the discs 106.
 The X.sub.E on path 270 is summed at a summing junction 276 with a
 reference position (indicative of desired head position) and with the
 bearing position signal X.sub.B output by the A/D 190 along signal path
 254. The output from summing junction 276 on path 278 is applied to a
 position gain block 280 having gain K.sub.x. The Ve signal is summed with
 the bearing velocity signal V.sub.B, output by the lead-lag filter 194
 along signal path 212, at summing junction 282. The output of summing
 junction 282 applied to a velocity gain block 284 having gain K.sub.v. The
 outputs of the position and velocity gain blocks 280, 284 are brought to a
 summing junction 286 by way of paths 288, 290, respectively. The output
 (on path 292) is summed at a summing junction 294 with the W.sub.E from
 path 274 to generate the I.sub.CMD signal on path 402.
 The output on path 292 is further applied to gain block 296 and fed to the
 observer 268. It will be noted that the sign designation for the various
 inputs to the summing junctions 276, 282, 286 and 294 have been generally
 arbitrarily assigned and could be modified with corresponding changes in
 polarity of the respective signals. Moreover, it will be understood that
 the polarity of the bearing position X.sub.B and bearing velocity V.sub.B
 signals vary with specific translational modes.
 Accordingly, during disc drive operation the bearing position X.sub.B and
 bearing velocity V.sub.B signals are generated on a steady-state basis and
 provided to the servo circuit 176 to minimize the effects of bearing
 translation upon the disc drive 100 during track following.
 To further explain the interrelated operation of proximity probe 130 and
 the servo circuit 176, FIGS. 9 and 10 respectively show seek and track
 following optimization routines.
 Turning now to FIG. 9, shown therein is a seek optimization routine 300.
 The seek optimization routine 300 begins at step 302 with the initiation
 of a seek routine. At step 304, the profiler 208 calculates a reference
 velocity from the tracks to go signal 206. Once the seek is in motion,
 step 306 shows that the proximity probe 130 registers translation present
 within the bearing assembly 112 and outputs the bearing translation
 B.sub.T signal indicative of the direction and magnitude of the bearing
 translation. At step 308, the B.sub.T signal is sent through the A/D 190
 to produce the bearing position X.sub.B signal which is then sent to the
 DSP 182 along signal path 254 and also to the differentiator 192.
 Next, at step 310, the X.sub.B signal is differentiated with respect to
 time and filtered by the lead-lag filter 194 to produce the bearing
 velocity V.sub.B signal output to the DSP 182 along signal path 212. The
 bearing velocity V.sub.B and bearing position X.sub.B signals are used to
 compensate the reference velocity for the presence of bearing translation
 at step 312. More specifically, the bearing velocity V.sub.B signal is
 summed with the Ve output from the observer 198 and with the updated
 reference velocity from the profiler 208. The bearing position X.sub.B is
 summed with the position output from the plant 196 and sent to the
 observer 198. In this way, the bearing position signal X.sub.B affects
 reference velocity by altering the Ve output from the observer 198.
 Once the reference velocity has been compensated for bearing translation, a
 corrected command current is calculated at step 314. At step 316 the
 corrected command current is applied to the voice coil 113 of the voice
 coil motor 114. It will be understood that the seek optimization routine
 300 operates on a steady state basis and serves to decrease seek time by
 reducing the amount of time required to settle the head 120 on a specific
 track in the presence of bearing translation.
 Turning now to FIG. 10, shown therein is a track following optimization
 routine 320. The track following optimization routine 320 begins at step
 322 with the initiation of a track following operation. As mentioned
 above, the track following operation is used after settling the head 120
 onto a selected track of the disc 106. The track following operation
 begins at step 324 by estimating the position (Xe), velocity (Ve) and bias
 (We) through use of the observer 268. At step 326, the proximity probe 130
 measures bearing translation and outputs the bearing translation B.sub.T
 signal. While the proximity probe 130 is used to measure bearing
 translation in the presently preferred embodiment, it will be understood
 that alternative methods and devices for measuring bearing translation
 exist and are considered applicable to the present optimization routine.
 Next, at step 328, the proximity probe circuitry (amp 188, A/D 190)
 calculates the bearing position signal X.sub.B. The bearing position
 signal X.sub.B is sent to the DSP 182 along signal path 254 and also to
 the differentiator 192. At step 330, the X.sub.B signal is differentiated
 with respect to time to produce the bearing velocity signal V.sub.B. The
 bearing velocity signal V.sub.B is output to the DSP 182 along signal path
 212.
 The track following optimization routine 320 continues at step 332 where
 the position estimate signal Xe output from the observer 268 is summed
 with the bearing position signal X.sub.B to produce a compensated position
 signal. Likewise, at step 334, the velocity estimate signal Ve is summed
 with the bearing velocity signal V.sub.B to produce a compensated velocity
 signal. The compensated position and velocity signals are used to
 calculate a corrected command current at step 336. Finally, at step 338,
 the corrected command current is applied to the voice coil 113 of the
 voice coil motor 114.
 Typically, the head velocity is kept relatively low during a track
 following operation. As such, the amount of command current being applied
 to the voice coil motor 114 will also be relatively low. Therefore, the
 presence of bearing deflection during a track following operation is more
 likely the result of an externally applied mechanical shock to the disc
 drive 100 than from the application of command current to the voice coil
 motor 114. The track following optimization routine 320 is performed on a
 steady-state operation to enable a track following operation which is more
 resistant to externally applied mechanical shocks.
 In a preferred embodiment of the present invention, the proximity probe 130
 is provisioned adjacent the stationary shaft 132 of the bearing assembly
 112 (see FIGS. 2 and 4). However, alternate configurations of the
 proximity probe 130 relative the bearing assembly 112 exist and are
 contemplated as within the scope of the present invention. For example,
 FIG. 11 provides a top plan view of a disc drive 100 showing an alternate
 embodiment of the present invention in which the proximity probe 130 is
 located external to the bearing assembly 112. The proximity probe 130 is
 mounted on a mounting bracket 340 which is rigidly affixed to the base
 deck 102.
 To ensure proper measurement of bearing translation as the rotary actuator
 110 sweeps through a plurality of angular positions, a rounded face 342 is
 included on the side of the E-block 115 closest to the proximity probe
 130. As the rotary actuator 110 pivots about the stationary shaft 132, the
 rounded face 342 remains in constant proximity with the proximity probe
 130. As such, any change in the proximity between the rounded face 342 and
 the proximity probe 130 is attributable to bearing translation. All other
 aspects of the alternate embodiment shown in FIG. 11, including the
 proximity probe 130 operation and interrelation to the servo circuit 176,
 are identical to those disclosed in reference to the first described
 embodiment of the present invention.
 From the foregoing discussion, it will be clearly understood that the
 present invention is directed to a proximity probe 130 and associated
 method for improving servo control in a disc drive. As exemplified by
 presently preferred embodiments, a disc drive 100 includes a rotary
 actuator 110 supporting a head 120 adjacent a rotatable disc 106 and an
 actuator coil 113 immersed in a magnetic field of a voice coil motor 114.
 A bearing assembly 112 is used to pivot the actuator 110 and a proximity
 probe 130 is provisioned adjacent the bearing assembly 112 and generally
 comprises a stationary shaft 132, a plurality of bearings 140 and an outer
 sleeve 142. A proximity probe 130 is used to register a change in the
 position of the outer sleeve 142 relative the stationary components (i.e.
 the stationary shaft 132 or base deck 102) of the disc drive 100. In
 response to a change in the relative position of the outer sleeve 142, the
 proximity probe 130 outputs an analog bearing translation signal B.sub.T
 representative of the direction and magnitude of the translation.
 The proximity probe 130 includes proximity probe circuitry which generates
 a bearing position signal X.sub.B and a bearing velocity signal V.sub.B
 used to enhance the performance of the servo circuit 176. The servo
 circuit 176 applies current to the actuator coil to position the head
 relative to the disc recording surface in relation to servo information
 transduced by the head, as well as in relation to the X.sub.B and V.sub.B
 signals.
 For purposes of the appended claims, the terms "circuit" and "block" will
 be understood to be realize in either hardware or software, in accordance
 with the foregoing discussion. The phrase "host device" will be understood
 to describe any device which communicates with the claimed disc drive,
 such as, but not limited to, the personal computer discussed above.
 Although method steps have been set forth in a particular order, such
 ordering is not necessarily limiting to the scope of the claims.
 It will be clear that the present invention is well adapted to carry out
 the objects and attain the ends and advantages mentioned as well as those
 inherent therein. While presently preferred embodiments have been
 described for purposes of this disclosure, numerous changes may be made
 which will readily suggest themselves to those skilled in the art and
 which are encompassed in the spirit of the invention disclosed and as
 defined in the appended claims.