Patent Publication Number: US-6222336-B1

Title: Rotational vibration detection using spindle motor velocity sense coils

Description:
RELATED APPLICATIONS 
     This application claims the benefit of United States Provisional Application No. 60/088,077, filed Jun. 5, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to improving data transfer performance of a disc drive by using velocity sense coils in a disc drive spindle motor to detect and compensate for the application of rotational vibration to the disc drive. 
     BACKGROUND OF THE INVENTION 
     Disc drives are digital data storage devices which enable users of computer systems to store and retrieve large amounts of data in a fast and efficient manner. Disc drives of the present generation have data storage capacities in excess of several gigabytes (GB) and can transfer data at sustained rates of several megabytes (MB) per second. 
     A typical disc drive is provided with a plurality of magnetic recording discs which are mounted to a rotatable hub of a spindle motor for rotation at a constant, high speed. An array of read/write heads are disposed adjacent surfaces of the discs to transfer data between the discs and a host computer. The heads are radially positioned over the discs by a closed loop, digital servo system, and are caused to fly proximate the surfaces of the discs upon air bearings established by air currents set up by the high speed rotation of the discs. A plurality of nominally concentric tracks are defined on each disc surface to accommodate the storage of user data. 
     A preamplifier/driver circuit (preamp) generates write currents that are used by the head to selectively magnetize the tracks during a data write operation. The preamp further amplifies read signals detected by the head during a data read operation. A read/write channel and interface circuit are operably connected to the preamp to transfer the data between the discs and the host computer. 
     A rigid housing is provided to support the spindle motor and the actuator and to form an internal controlled environment to minimize particulate contamination of the discs and heads. A printed circuit board is mounted to the exterior of the housing to accommodate the disc drive control electronics (including the aforementioned servo circuit, read/write channel and interface circuit). 
     Disc drives are often used in a stand-alone fashion, such as in a typical personal computer (PC) or portable data processing/communication device where a single disc drive is utilized as the primary data storage peripheral. However, in applications requiring vast amounts of data storage capacity or high input/output (I/O) bandwidth, a plurality of drives can be arranged into a multi-drive array, sometimes referred to as a RAID (“Redundant Array of Inexpensive Discs”; also “Redundant Array of Independent Discs”). A seminal article proposing various RAID architectures was published in 1987 by Patterson et al., entitled “A Case for Redundant Arrays of Inexpensive Discs (RAID)”, Report No. UCB/CSD 87/391, December 1987, Computer Science Division (EECS), University of California, Berkeley, Calif. 
     Since their introduction, RAIDs have found widespread use in a variety of applications requiring significant data transfer and storage capacities. It is presently common to incorporate several tens, if not hundreds, of drives into a single RAID. While advantageously facilitating generation of large scale data storage systems, however, the coupling of multiple drives within the same enclosure can also set up undesirable vibrations from excitation sources within the drives, such as spindle motors used to rotate the discs and actuators used to move the heads to various tracks on the discs. Such vibrations can be transmitted from drive to drive through chassis mounts used to secure the drives within the enclosure. 
     Vibrational components can be characterized as translational, or rotational in nature. Translational vibrations tend to move a disc drive housing back and forth along a plane of the drive, whereas rotational vibrations tend to rotate a disc drive housing about an axis normal to a plane of the drive. Because attempts are made to provide nominally balanced actuators, translational vibrations will generally have little effect upon the ability of the actuator to maintain the heads at a selected position with respect to the discs, as the discs and the actuator will both respond to the movement induced by such translational vibrations. 
     However, such is not usually true with rotational vibrations. Even with a nominally balanced actuator, rotational vibrations will tend to move the discs relative to the actuator because the actuator, acting as a free body, remains essentially undisturbed due to inertial effects while the discs, mounted to the housing, are displaced by imparted rotational vibration. When sufficiently severe, such movement will cause an “off-track” condition whereby a head is moved away from a selected track being followed. Such off-track conditions can adversely affect the ability of the drive to transfer data between the discs and host device. 
     The problems associated with rotational vibrations are well known in the disc drive art. Compensation attempts have included use of sensors that can detect the presence of rotational vibration in a disc drive, such as discussed in U.S. Pat. No. 5,235,472 issued Aug. 10, 1993 to Smith, assigned to the assignee of the present invention. Efforts to both detect and compensate rotational vibration using feedforward control include discussions by White and Tomizuka, “Increased Disturbance Rejection in Magnetic Disk Drives by Acceleration Feedforward Control,” and Abramovitch, “Rejecting Rotational Disturbances on Small Disk Drives Using Rotational Accelerometers.” Both of these papers were presented at the 13 th  Triennial World Congress, San Francisco, U.S.A., 1996. 
     While operative, there are limitations with these and other prior art approaches to minimizing the effects of rotational vibration in a disc drive. Sensors that specifically detect rotational vibration are commercially available, but are often prohibitively expensive for use in low cost disc drive designs and are also often difficult to properly calibrate for a particular drive application. Such sensors may include a piezoelectric polymer film disposed between metallic layers that detects rotational vibration in response to torsion induced on the film, as disclosed by the aforementioned Smith U.S. Pat. No. 5,235,472 patent; another construction uses multiple piezoelectric transducers within a single component enclosure to detect rotation in relation to differences in detected motion among the transducers. 
     Alternatively, rotational sensors can be formed from two or more discrete linear accelerometers which detect rotational vibration in response to differences in the detected motion between the devices. While potentially less expensive to implement than an integrated rotational sensor, commercially available discrete linear accelerometers (piezo or similar construction) can have significant part-topart output gain variation characteristics, making such unsuitable for use in a drive to detect rotational vibration without special screening and trimming operations to obtain matched sets of accelerometers. 
     By way of example, the aforementioned White et al. and Abramovitch references are illustrative of conventional approaches requiring use of relatively precise (and therefore expensive) accelerometers, as well as a calibration routine requiring use of a shaker table to impart vibrations of known characteristics. Such considerations make these approaches undesirable for high volume disc drive manufacturing, and prevent future adaptation of the response characteristics of a given drive to its subsequent field environment. 
     These references are also limited to compensating for rotational effects and do not address translational effects. However, translational effects have also been found to contribute to off-track errors due to actuator imbalance. In practice, induced vibration is seldom purely rotational or translational, but rather usually includes a combination of both. 
     Accordingly, as disc drive track densities and performance requirements continue to increase, there remains a continual need for improved approaches in the art to compensating for the effects of vibration in a disc drive using inexpensive and easily configured vibration sensor circuitry. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and method for detecting application of rotational vibration to a disc drive. 
     As exemplified by presently preferred embodiments, a disc drive spindle motor controllably rotates a data storage disc  106  using a set of driver coils which magnetically interact with a plurality of circumferentially extending driver magnets when electrical currents are applied to the driver coils. A separate set of sense coils is provided to output voltages indicative of rotational velocity of the disc to detect the application of rotational vibration to the disc drive. The sense coils preferably magnetically interact with the driver magnets, although separate sense magnets can be alternatively provided. 
     A spindle motor driver circuit applies the electrical currents to the set of driver coils and outputs a velocity error signal V ERR  indicative of error in rotational velocity of the disc, with the spindle motor driver circuit adjusting the electrical currents in relation to the velocity error signal. A spindle velocity sense circuit generates a motor velocity signal V M  indicative of the velocity of the spindle motor in response to the set of voltages output by the set of sense coils. A processor having associated programming detects the application of rotational vibration to the disc drive in relation to the velocity error signal V ERR  and the motor velocity signal V M . 
     The disc drive further preferably comprises an actuator supporting a head adjacent the disc, the actuator having a coil of an actuator motor. A servo circuit applies current to the coil to position the head relative to the disc, wherein the servo circuit compensates for the application of rotational vibration to the disc drive by adjusting the current applied to the coil in relation to the motor velocity signal. 
    
    
     These and various other features as well as advantages which characterize the present invention as claimed below will be apparent from a reading of the following detailed description and a review of the associated drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 provides a top plan view of a disc drive constructed in accordance with a preferred embodiment of the present invention. 
     FIG. 2 provides an elevational, cross-sectional representation of the disc drive of FIG. 1, illustrating a preferred internal construction of the spindle motor used to rotate a plurality of discs and having a set of velocity sense coils. 
     FIG. 3 provides a generalized functional block diagram of the disc drive of FIG.  1 . 
     FIG. 4 provides a functional block diagram of the spindle velocity sense circuit of FIG.  3 . 
     FIG. 5 provides a functional block diagram of the spindle motor driver circuit of FIG.  3 . 
     FIG. 6 provides a generalized flow chart for a DATA TRANSFER INTERRUPTION routine, representative of programming utilized by the DSP to temporarily interrupt a data transfer operation between the disc drive and a host device when a magnitude of rotational vibration exceeds a predetermined threshold. 
     FIG. 7 is a control diagram representative of programming utilized by a digital signal processor (DSP) of the servo circuit of FIG.  3 . 
     FIG. 8 provides a generalized flow chart for a GAIN CALIBRATION routine, representative of steps carried out to set a gain of a gain block of the control diagram of FIG.  7 . 
     FIG. 9 provides a representation of an operational environment rich in rotational and translational vibration, useful for the flow of FIG.  8 . 
     FIG. 10 provides a generalized flow chart for a ROTATIONAL VIBRATION COMPENSATION routine, representative of steps carried out by the disc drive in compensating for the effects of applied rotational vibration. 
    
    
     DETAILED DESCRIPTION 
     The scope of the invention disclosed herein will be defined by the appended claims; however, in order to provide sufficient information to enable those skilled in the art to practice the claimed invention, various preferred embodiments thereof will now be discussed. It will be understood that many of the following features and aspects are provided merely for purposes of illustration and to describe environments in which the claimed invention can be advantageously practiced. 
     Referring first to FIG. 1, shown therein is 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) which is mounted to the underside of the HDA and is thus not visible in FIG.  1 . The PWA includes electronics used to control the operation of the HDA  101 , as discussed below. 
     A top cover, omitted from FIG. 1 to reveal interior portions of the HDA  101 , mates with a base deck  102  of the HDA  101  to provide an environmentally controlled environment for the HDA  101 . A spindle motor (generally designated at  104 ) is supported by the base deck  102  and rotates a plurality of discs  106  at a constant high speed. A disc clamp  108  secures the discs  106  to the spindle motor  104  using a number of fasteners  109 . 
     The discs  106  include recording surfaces (not separately designated) to which user data are written by way of a rotary actuator  110 , which rotates about a cartridge bearing assembly  112  in response to the application of current to an actuator coil  113  of a voice coil motor (VCM)  114 . As will be recognized, the VCM includes a magnetic circuit (not separately designated) which establishes a magnetic field in which the actuator coil  113  is immersed. The passage of current through the actuator coil  113  sets up a magnetic field which interacts with the magnetic field of the magnetic circuit to rotate the actuator  110  about the cartridge bearing assembly  112 . 
     A plurality of rigid arms  116  extend from the actuator  110 , each of which supports a corresponding flexible suspension  118 . A plurality of heads  120  are supported by the suspensions  118  over the recording surfaces of the discs  106  by air bearings established by air currents set up by the high speed rotation of the discs  106 . The heads  120  are preferably characterized as magneto-resistive (MR) heads, each having a thin film inductive write element and an MR read element. 
     A latch  122  secures the actuator  110  when the disc drive  100  is deactivated, so that the heads  120  come to rest upon texturized landing zones  123  at a common inner radius of the discs  106 . A flex circuit assembly  124  facilitates electrical interconnection between the actuator  110  and the disc drive PWA, and comprises a flexible, laminated plastic ribbon with embedded conductive paths to pass write and read signals to and from the head, as well as VCM drive currents to the coil  113 . 
     To understand the operation of the disc drive  100  in accordance with preferred embodiments of the present invention, it will first be helpful to review the manner in which rotational vibration can adversely affect the operation of the disc drive  100 . As mentioned above, vibrational effects can be characterized as translational, or rotational in nature. By way of example, translational vibration, illustrated by acceleration vectors  126  and  128  in FIG. 1, tends to move the base deck  102  laterally along a selected plane of the disc drive  100  (in this case, along a plane generally parallel to a plane of the top disc  106 ). Because the actuator  110  is nominally balanced about the cartridge bearing assembly  112 , both the discs  106  and the heads  120  will tend to move together, resulting in minimal head/disc displacement. 
     On the other hand, rotational vibration, illustrated by acceleration vectors  126  and  130 , causes movement of the base deck  102  about an axis normal to a plane along which the top disc  106  extends. The discs  106  accordingly move along with the base deck  102 , but the actuator  110 , as a free body, remains essentially stationary in space. The resulting displacement can adversely affect a data transfer operation between the selected head  120  and the corresponding disc  106 ; for example, should the displacement occur during a write operation, data intended to be written to a particular track on the disc  106  might be overwritten onto an adjacent track, irretrievably corrupting the user data stored on the disc  106 . Such rotational vibration will also tend to change the rotational velocity of the spindle motor  104 , which can be detected as discussed below. 
     FIG. 2 provides an elevational, cross-sectional view of the disc drive of FIG. 1, showing a preferred internal construction of the spindle motor  104 . FIG. 2 includes the aforementioned top cover (which is designated at  132 ) and disc drive PWA (which is designated at  134 ). 
     The spindle motor  104  comprises a rotatable hub portion  136  which rotates about a stationary stator portion  138  by way of a pair of bearing assemblies  140 . The hub portion  136  includes a plurality of permanent magnets  142  and a circumferentially extending flange  144 . The discs  106  and a corresponding array of disc spacers  146  are clamped between the flange  144  and the disc clamp  108 , as shown. 
     The permanent magnets  142  (also referred to as driver magnets”) magnetically interact with a set of spindle motor driver coils  148  which, as discussed below, are electrically commutated to cause the hub portion  136 , and hence the discs  106 , to rotate at a nominally constant speed. The permanent magnets  142  further preferably magnetically interact with a set of spindle motor velocity sense coils  150  which provide a measure of rotational velocity of the spindle motor  104  in order to facilitate detection and compensation of rotational vibration applied to the disc drive  100 . In a preferred embodiment, the driver coils  148  are wound about the velocity sense coils  150 , as shown in FIG. 2, although other configurations are readily contemplated, such as placement of the velocity sense coils  150  about the driver coils  148 , as well as placement of the velocity sense coils  150  apart from the driver coils  148  with a second set of permanent magnets (not shown, also referred to as “sense magnets”) to interact with the velocity sense coils  150 . The spindle motor  104  is preferably a three phase motor, so that there are a total of three driver coils  148  and three sense coils  150  wound about a total of 12 poles (two of which are represented at  151 ) arranged circumferentially about the stator portion  138 . 
     The poles  151  are supported by a spindle motor shaft  152 , the shaft defining a central axis about which the discs  106  rotate. The shaft  152  is affixed to the top cover  132  and the base deck  102  using fasteners  154  and  156 , respectively. 
     The general manner in which the velocity sense coils  150  are used to detect and compensate rotational vibration can be understood beginning with a review of FIG. 3, which provides a generalized functional block diagram of the control electronics disposed on the disc drive PWA  134  and used to control the operation of the disc drive  100 . A control processor  158  directs overall operation of the disc drive  100  in accordance with programming stored in memory (MEM)  160 . An interface circuit  162  controls the transfer of data between the discs  106  and a host computer (not shown) in which the disc drive  100  is mounted in a user environment. 
     During a data write operation, a read/write channel  164  encodes and serializes data to be written to disc and passes the same to a preamp/driver circuit  166  which applies a series of write currents to a write element of a selected head  120  to selectively magnetize the corresponding disc  106 . During a data read operation, a read element of the head  120  transduces the selective magnetization of the disc  106  and outputs a read signal which is preamplified by the preamp/driver  166  and passed to the read/write channel  164  for reconstruction of the data. The preamp/driver  166  is preferably affixed to the actuator assembly  110  within the confines of the HDA  101 , as shown in FIG.  1 . 
     A servo circuit  168  uses servo information stored on the discs  106  to provide closed loop head positional control. The servo circuit  168  includes a digital signal processor (DSP)  170  having associated programming to carry out both seeks (movement of the selected head from an initial track to a destination track) and track following operations (controlled positioning of the selected head over a particular track). The disc drive  100  is contemplated as utilizing an embedded servo scheme, so that both user data and servo information are written to each of the disc surfaces, although such a configuration is not limiting to the scope of the claimed invention. 
     Continuing with FIG. 3, a spindle motor driver circuit  172  operates to apply drive currents to the driver coils  148  to rotate the discs  106 . For reference, it will be noted that the spindle motor  104  is characterized as a three-phase inductive motor with the phases connected in a delta-configuration, although the claimed invention is not so limited. During operation, the spindle motor driver circuit  172  generates a velocity error signal V ERR  which is supplied to the control processor  158  on path  174 . 
     A spindle velocity sense circuit  176  likewise detects voltages induced in the velocity sense coils  150  and preferably outputs a pair of signals to the control processor  158 : an actual velocity signal V M  on path  178  and an actual rotational acceleration signal A M  on path  180 , which provide measures of the rotational velocity and acceleration, respectively, of the spindle motor  104 . 
     FIG. 4 provides a functional block diagram of the spindle velocity sense circuit  176  of FIG. 3, in conjunction with the velocity sense coils  150 . The circuit of FIG. 3 includes a full wave rectifier  182  and filter  184  which operate to provide an analog voltage generally indicative of the rotational velocity of the spindle motor  104 . In this regard, the sense coils  150  and the sense circuit  176  operate as a voltage generator. For a related discussion of the use of spindle driver coils to generate a voltage when power is removed from a disc drive, see U.S. Pat. No. 4,679,102 issued Jul. 7, 1987 to Wevers et al., assigned to the assignee of the present invention. 
     The output of the filter  184  is digitized by an analog to digital (A/D) converter  186  to provide the V M  signal on path  178 . The V M  signal is further differentiated by a differentiator (DIFF)  188  and filtered by a lead/lag filter  190  to produce the A M  signal on path  180 . Use of these two signals in accordance with preferred embodiments will be discussed after a brief review of the spindle motor driver circuit  172  of FIG.  3 . 
     Referring to FIG. 5, the spindle motor driver circuit  172  operates to provide commutation timing and rotational speed control for the driver coils  148  of the spindle motor  104 . As discussed more fully in U.S. Pat. No. 5,631,999 issued May 20, 1997 to Dinsmore, assigned to the assignee of the present invention, commutation involves sequentially energizing the driver coils  148  to induce magnetic fields which interact with the permanent magnets  142  to cause the hub portion  136  to rotate in the desired direction at the desired speed. The circuit of FIG. 5 rotates through a series of commutation steps over each revolution of the spindle motor  104 , with the time between successive commutation steps defining a commutation period. 
     A transconductance amplifier and driver circuit  202  sequentially supplies current to, sinks current from, and holds at a high impedance each of three connection nodes A, B and C of the driver coils  148  at each of the commutation steps in response to commutation timing signals from commutation logic  204 . Back electromotive force (emf) sensing is accomplished using a back emf sense circuit  206 , which outputs zero crossing (ZX) pulses when the voltage on the node held to high impedance crosses over the voltage at the node sinking the current. The ZX pulse will nominally occur at the mid-point of each commutation period. 
     Continuing with FIG. 5, the ZX pulses are fed to a commutation pulse generator  208  which generates the necessary timing for the commutation logic circuit  204  to apply the appropriate sequence of inputs to the driver portion of the transconductance amplifier and driver circuit  202 . This is preferably implemented in hardware with top level processor control. 
     The ZX pulses are further provided to a commutation frequency (F COM ) generator  210 , which outputs F COM  pulses in response thereto (the F COM  pulses can occur at the same time, or can be delayed with respect to, the ZX pulses). The F COM  pulses, as well as a reference frequency F REF  from a reference clock  212  are provided to a phase error detector  214 , which determines phase error between these two signals. The F REF  frequency nominally corresponds to the operational speed of the spindle motor  104 ; for example, for a three-phase, twelve-pole spindle motor rotating at 7,200 revolutions/minute (120 revs/second), F REF  would be: 
     
       
         F REF =(no. of phases)(no. of poles)(revs/second) 
       
     
     
       
          =(3)(12)(120)=4.32 Hz.  (1) 
       
     
     The phase detector generates pump up and pump down pulses P U  and P D , respectively, based on the relative timing of the F REF  and F COM  pulses. A P U  pulse is generated when an F COM  pulse occurs after the corresponding F REF  pulse and has a duration equal to the phase difference between the F COM  and F REF  pulses. A P U  pulse indicates that the motor is running too slowly. Similarly, a P D  pulse indicates that the motor is running too fast and is generated when an F COM  pulse occurs before the corresponding F REF  pulse and has a duration equal to the phase difference therebetween. 
     The P U  and P D  pulses are provided to a charge pump circuit  216  which outputs a P OUT  signal having a voltage magnitude determined by the application of the P U  and P D  pulses. In the absence of a P U  and a P D  pulse, P OUT  is a reference voltage; P OUT  is increased to a first value above the reference voltage for the duration of a P U  pulse and is decreased to a second value below the reference voltage for the duration of a P D  pulse. 
     The P OUT  signal is provided to a compensation filter  218 , which includes a low pass filter stage (not separately shown) used to filter out sample frequencies related to the P U  and P D  pulses. A lead-lag filter stage (also not separately shown) receives the output of the low pass filter state and provides desired phase margin to achieve stability of the control loop. 
     The output of the compensation filter  218  is an analog velocity error signal indicative of the velocity error of the spindle motor  104 . The signal is provided to the transconductance amplifier and driver circuit  202  to adjust the amount of current applied to the driver coils  148 , as well as to an analog to digital (A/D) converter  220  to generate the digital V ERR  signal on path  174 . 
     Referring now to FIG. 6, shown therein is a DATA TRANSFER INTERRUPTION routine  230 , which sets forth steps carried out by the disc drive  100  to interrupt the transfer of data when the magnitude of rotational vibration applied to the disc drive is sufficiently severe. The routine of FIG. 6 is representative of programming stored in the control processor memory (MEM)  160  and utilized by the control processor  158 . It will be appreciated that the routine of FIG. 6 is a top level routine performed as part of other continuously executing programming steps of the control processor  158  during disc drive operation. 
     As shown at step  232 , the routine first determines the magnitude of the motor velocity signal V M  obtained from the sense coils  150 . As discussed above, this value is provided to the control processor  158  by way of the path  178 . It will be noted that the V M  signal is indicative of actual relative movement between the sense coils  150  and the permanent magnets  142  (and hence, the relative movement between the discs  106  and the base deck  102 ); that is, the V M  signal is a composite of both the rotation of the discs  106  caused by the commutation of the driver coils  148 , as well as any rotational component induced by the application of rotational vibration to the disc drive. 
     The velocity error V ERR  signal is next determined at step  234  (as provided on path  174  in FIGS. 3 and 5) and the control processor  158  determines the velocity of the spindle motor  104  as a result of the application of rotational vibration, referred to as V RV , through an algebraic combination of V ERR  and V M  as follows: 
     
       
         V RV .=V M −V ERR .  (2) 
       
     
     The magnitude (absolute value) of the rotational vibration velocity V RV  is compared to a predetermined threshold T at decision step  238 , with the threshold T preferably being selected to correspond to a level above which the servo circuit  168  cannot adequately reject the applied rotational vibration; that is, the threshold T preferably identifies when a magnitude of the rotational vibration applied to the disc drive  100  exceeds a specified magnitude, such as, for example, 21 radians per second 2  (rads/sec 2 ), over a frequency range of interest, such as, for example, from 20 hertz (Hz) to 800 Hz. 
     At such time that the magnitude exceeds the threshold T, a data transfer operation (such as a read or a write operation) is temporarily interrupted, as indicated by step  240 ; likewise, step  212  resumes data transfer once the rotational vibration returns to an acceptable level (and represents normal operation of the drive in the absence of applied rotational vibration). 
     In a further preferred embodiment, the disc drive  100  operates to compensate for the effects of rotational vibration. Referring now to FIG. 7, shown therein is a block diagram representation of such operation of the servo circuit  168 . More particularly, the block diagram representation includes modules existing in programming utilized by the DSP in providing positional control while minimizing the effects of rotational vibration upon the drive. 
     As shown in FIG. 7, a plant block  300  is presented representative of selected electrical and mechanical aspects of the disc drive  100 . For reference, the plant  300  generally corresponds to the electromechanical servo loop established by the head  120 , the preamp/driver  166 , the read/write channel  164 , the servo circuit  168 , and the actuator coil  113 . The plant  300  receives as an input a current command (I CMD ) signal on path  302  and, in response, outputs a position error signal (PES) on path  304  indicative of positional error in the selected head  120 . 
     FIG. 7 further shows an observer block  306 , which generally provides a mathematical model of the plant  300  and periodically outputs estimates of head position (X EST ), velocity (V EST ) and bias (W EST ) on paths  308 ,  310  and  312 , respectively. 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 . Bias will usually be dependent upon radial position of the head with respect to the disc. The estimates output by the observer  306  are formed in relation to an observer error (O ERR ) signal on path  314  generated by a summing junction  316  as the difference between the PES and the X EST . 
     The X EST  on path  308  is further summed at a summing junction  318  with a reference position (indicative of desired head position) and the output on path  320  is applied to a position gain block  322  having gain K X . The V EST  on path  310  is similarly applied to a velocity gain block  324  having gain K V . The outputs of the position and velocity gain blocks  322 ,  324  are brought to a summing junction  326  by way of paths  328 ,  330 , respectively. 
     The manner in which the A M  signal is preferably used to control head position (i.e., control the amount of current applied to the coil  113 ), thereby compensating for the effects of rotational vibration applied to the disc drive  100 , will now be discussed. Particularly, FIG. 7 shows the A M  signal being applied to a gain block  330  having a gain K A . It is contemplated that the A M  signal is passed from the control processor  158  to the servo DSP  170  (in accordance with the functional configuration of FIG.  3 ), although in alternative configurations the A M  signal can be provided directly to the DSP  170 . It will be noted that the use of two processors, the control processor  158  and the DSP  170 , is merely for purposes of illustrating a preferred embodiment; the claimed invention can be readily practiced in drives incorporating the use of a single processor that carries out both overall drive management and head positional control. 
     The gain K A  is preferably selected in a manner discussed below, but generally is set to a value that maximizes the ability of the servo circuit  168  to compensate for the effects of rotational vibration on the disc drive  100 . The output of the K A  gain block  330  is provided on path  338  and is likewise summed by the summing junction  326 . 
     The output of the summing junction  326  is provided along path  340  to a summing junction  342 , which further receives the W EST  from the observer  306  on the path  312 . The resulting signal, impressed on path  344 , is generally proportional to the current to be applied to the coil  113  and is provided to the observer  306  as shown. To maintain the operation of the observer  306  nominally that of the plant  300 , however, the signal of path  344  is also passed through a gain block  346  with gain K P  to generate the aforementioned current command signal I CMD . 
     To present a preferred method for setting the gain K A  of the gain block  330  to an optimum value, FIG. 8 has been provided which shows a general flow chart for a GAIN CALIBRATION routine  350 , carried out in accordance with a preferred embodiment of the present invention. The routine of FIG. 8 generally corresponds to programming utilized by the DSP  170 . It is contemplated that the routine will be carried out during disc drive manufacturing, but can also be subsequently carried out during data processing use of the disc drive  100  (i.e., by an end-user of the drive). 
     As shown at step  352 , the disc drive  100  is first placed and operated within a rich vibrational environment, wherein a broad spectrum of translational and rotational vibrational components is applied to the disc drive  100 . Such an environment is shown schematically in FIG.  9 . 
     More particularly, FIG. 9 illustrates an enclosure  354  housing a plurality of hard disc drives ( 12  in this example, identified as HDD 1 -HDD 12 ) nominally identical to the disc drive  100  of FIG.  1 . The disc drives are mechanically coupled together in such a manner so as to maximize transfer of vibrational components from drive to drive during operation. The enclosure  354  can correspond to an environmental chamber used to perform manufacturing dynamic burn-in (DBI) testing wherein the drives are operated over an extended period of time (such as 48 hours) while being subjected to a variety of environmental conditions, such as different temperatures, ambient pressures, etc. Alternatively, the enclosure  354  can correspond to a RAID housing so that the disc drives form a multi-drive array and are operated as a single data storage system. 
     A plurality of mechanical vibration sources  356  such as motors (“MOT”) can also be coupled to the disc drives, as shown in FIG.  9 . These vibration sources  356  inject additional amounts of vibration into the disc drives, and can represent cooling fans used to enhance convective cooling of the drives (such as commonly used in RAID housings); alternatively, when the routine of FIG. 8 is carried out during DBI, the vibration sources  356  can be motors that are specifically placed within the environmental chamber. 
     Returning to FIG. 8, the routine next proceeds to identify an optimum value for the gain K A  which results in a minimum average PES magnitude; in other words, the optimum gain setting for K A  minimizes the effects of vibration on head position, as reflected by PES magnitude. This is preferably accomplished by first setting the gain K A  to an initial value, as indicated by step  358 , measuring the average PES, step  360 , and repeating for each new increment of K A , as indicated by decision step  362  and step  364 . Steps  358 ,  360 ,  362  and  364  accordingly comprise a sweep of the gain K A  from its minimum to maximum value while measuring the average PES for each increment, using an accumulation function or other suitable methodology to capture the average PES in each case. The optimum value of K A  is next selected at step  366  in relation to the value that provided the minimum average PES, and this optimum value is thereafter used by the gain block  330  (FIG. 7) until the next execution of the routine of FIG.  8 . Once the optimum value is selected, the routine ends at step  368 . 
     The routine of FIG. 8 thus presents an efficient methodology for determining the optimum value for the gain K A  and possesses several additional advantages over the prior art. For example, unlike some conventional prior art approaches, it is unnecessary to place the disc drive  100  onto a shaker table in order to apply carefully controlled amounts of vibration to the drive; instead, the routine can be carried out during existing conventionally applied manufacturing steps (such as DBI) with little or no modification to the test routine. The rich vibrational environment used to calibrate the gains during manufacturing (i.e., multi-drive DBI chamber) will often be representative of the actual operational environment in which the drive will ultimately be operated, ensuring better correlation with actual field use. 
     Moreover, the adaptive capabilities of the routine of FIG. 8 allow the disc drive  100  to further optimize the gain K A  in relation to the vibrations experienced in each particular operational environment. For example, it is contemplated that the routine of FIG. 8 can be performed by the disc drive during field use on a periodic basis to maintain optimal performance of the drive. The routine can also be specifically performed by the drive after the occurrence of a sufficient number of off-track faults, in an effort to better optimize present settings of the drive. 
     Having completed the foregoing discussion of the manner in which the gain K A  is preferably selected to minimize the effects of rotational vibration, reference is now made to FIG. 10 which sets forth a generalized flow chart for a ROTATIONAL VIBRATION COMPENSATION routine  400 , carried out in accordance with a preferred embodiment during operation of the disc drive  100 . The routine of FIG. 10 is preferably performed in conjunction with other top level disc drive operational routines. 
     As the driver coils  148  magnetically interact with the permanent magnets  142  to rotate the spindle motor  104 , the voltages induced by the passage of the permanent magnets  142  proximate the spindle velocity coils  150  undergo full-wave rectification and filtering by the respective circuits  182  and  184  of FIG. 4, as indicated by step  402 . A/D conversion is next applied by the A/D converter  186 , step  404 , thus generating the measured spindle velocity signal V M  discussed above. The V M  signal is differentiated and filtered by the circuits  188 ,  190  of FIG. 4, as shown by steps  406  and  408  in FIG. 10, to generate the aforementioned measured acceleration signal A M . It will be noted that during normal operation of the disc drive  100 , in the general absence of rotational vibration (and negligible amounts of spindle motor speed variation), the A M  signal will be nominally zero, and accordingly have a nominal effect upon the operation of the servo circuit  168 . 
     The gain K A  is next applied to the A M  signal, step  410  of FIG. 10, as carried out by the gain block  330  of FIG.  7 . The resulting product (A M )(K A ) is then fedforward into the servo loop (step  412  of FIG. 10, at the summing junction  326  of FIG.  7 ), to minimize the effects of rotational vibration upon the disc drive  100 . The routine of FIG. 10 loops back as shown, thereby continuing in like manner during continued operation of the drive. 
     In view of the foregoing discussion, it will now be clear that the present invention is directed to a method and apparatus for detecting and compensating for the effects of rotational vibration on a disc drive. As exemplified by preferred embodiments, a disc drive spindle motor  104  controllably rotates a data storage disc  106  using a set of driver coils  148  which magnetically interact with a plurality of circumferentially extending driver magnets  142  when electrical currents are applied to the driver coils. A separate set of sense coils  150  output a set of voltages indicative of rotational velocity of the disc to detect the application of rotational vibration to the disc drive. The sense coils preferably magnetically interact with the driver magnets, although separate sense magnets can be alternatively provided. 
     A spindle motor driver circuit  172  applies the electrical currents to the set of driver coils and outputs a velocity error signal V ERR  indicative of error in rotational velocity of the disc, with the spindle motor driver circuit adjusting the electrical currents in relation to the velocity error signal. A spindle velocity sense circuit  176  generates a motor velocity signal V M  indicative of the velocity of the spindle motor in response to the set of voltages output by the set of sense coils. A processor  158  having associated programming detects the application of rotational vibration to the disc drive in relation to the velocity error signal V ERR  and the motor velocity signal V M . 
     The disc drive further preferably comprises an actuator  110  supporting a head  120  adjacent the disc, the actuator having a coil  113  of an actuator motor  114 . A servo circuit  168  applies current to the coil to position the head relative to the disc, wherein the servo circuit compensates for the application of rotational vibration to the disc drive by adjusting the current applied to the coil in relation to the motor velocity signal. 
     For purposes of the appended claims, the terms “circuit” will be understood to be realizable in either hardware or software, in accordance with the foregoing discussion. 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 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.