Abstract:
A filter for attenuating a resonance mode without substantial phase margin loss. The filter comprises a transfer function having an additional pole, such that the phase of the input signal is advanced. The phase-advanced filter substantially attenuates all frequencies above a selected center frequency while avoiding substantial phase margin loss.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority of U.S. provisional application Serial No. 60/326,769, filed Oct. 2, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This application relates generally to disc drives and more particularly to phase-advanced filtering of mechanical resonance.  
         BACKGROUND OF THE INVENTION  
         [0003]    Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium on a disc. Modern disc drives comprise one or more discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers (“heads”) mounted to an actuator for radial movement of the heads relative to the discs. Each of the concentric tracks is generally divided into a plurality of separately addressable data sectors. The read/write transducer, e.g. a magnetoresistive read/inductive write head, is used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment. Critical to both of these operations is the accurate locating of the head over the center of the desired track.  
           [0004]    A problem in disc drives that limits drive performance in general and head position accuracy specifically is component vibration or resonance. Components in the disc drive exhibit resonance modes that adversely affect the performance of disc drive components. For example, because of resonance in the actuator arm, the transducer heads may not be directly over the desired tracks indicated by the servo control of the disc drive. This problem is exacerbated by the recent push to increase the tracks-per-inch (TPI) on the disc surfaces. When TPI is increased, the room for margin in head placement becomes disproportionately smaller, and servo positioning errors more frequent.  
           [0005]    Unfortunately, component resonance cannot be completely eliminated without extreme cost. Traditional approaches to the problem of component resonance have involved utilizing a filter to compensate for the component resonances. Typically, a filter is implemented in the signal path having filter parameters such as pass band and attenuation parameters associated with a known resonance mode of a component receiving the signal. The typical filter that is used is a notch filter exhibiting an attenuation band in a generally high frequency associated with the resonance mode of the component. However, every component in a disc drive exhibits resonance modes at different frequencies. Furthermore, the resonant frequency of a given component will vary with temperature. Thus, fine-tuning a notch filter to accommodate vibrations at these many frequencies is difficult, if not impossible. Traditional approaches have included widening the attenuation band in the notch filter to attenuate a wider range of frequencies. Unfortunately, as is well known, this approach leads to significant phase margin loss as the attenuation band is widened. Phase margin loss generally means phase loss occurring at the open loop gain crossover frequency (i.e., the frequency where the open loop gain is 0 decibels (dB)).  
           [0006]    Further aggravating the problem, is the use of Mask Read Only Memory (ROM) for disc drive boot software. Mask ROM is a one-time burn-in ROM that is typically a lower cost than other types of ROM such as flash EPROM and is frequently used to store disc drive boot code. Disc drive manufacturers typically provide an integrated circuit (IC) vendor with boot code and the IC vendor burns the boot code into a Mask ROM. The boot code in Mask ROM is run on power up to perform basic initial power up operations of the disc drive servo controller, including seeks and track following to load code from the disc. During power up, reserve tracks are typically accessed to gather or load data and/or executable code stored on the reserve tracks. To precisely position the head over the reserve tracks, a filter is employed to filter out the resonance modes previously discussed. Filter parameters are stored in Mask ROM along with the boot code.  
           [0007]    Due to time-to-market demands, sufficient time must be given to the IC vendor to burn the boot code into the Mask ROM. Typically, disc drive manufacturers provide the IC vendor with the boot code three to four months prior to final testing and manufacture of the disc drive. Between the time that the boot code is given to the IC vendor and the time for manufacture, changes are frequently made to components in the disc drive. Changes to components result in changes in resonance modes associated with those components. Thus, the filter parameters included in the boot code given to the IC vendor may not, and frequently do not, exactly correspond to the resonance modes of components that are ultimately used in the disc drive. Consequently, notch filters in boot code often do not adequately attenuate resonant frequencies. One suggested solution is to choose filter parameters in the boot code such that the notch filter or notch filters exhibit a wide attenuation band. However, as discussed above, a wider attenuation band leads to significant phase margin loss during disc drive operation. It is well known in the art that phase margin loss leads to deterioration in disc drive performance, including, but not limited to, increased run-out and reduced servo controller bandwidth.  
           [0008]    Accordingly, there is a need for a method and system for effectively attenuating unwanted component resonance in a disc drive, while limiting or avoiding phase margin loss.  
         SUMMARY OF THE INVENTION  
         [0009]    Against this backdrop the present invention has been developed. An embodiment of the present invention is a unique system for attenuating selected resonance modes due to component vibration in a disc drive. More specifically, an embodiment is a phase-advanced filter filtering a selected frequency corresponding to a resonance mode. Still further, an embodiment is a unique low-pass filter providing significant gain attenuation at selected resonance modes, while preventing significant phase margin loss.  
           [0010]    Embodiments of the invention may be implemented as a computer process, a computing system or as an article of manufacture such as a computer program product or computer readable media. The computer program product may be a computer storage media readable by a computer system and encoding a computer program of instructions for executing a computer process. The computer program product may also be a propagated signal on a carrier readable by a computing system and encoding a computer program of instructions for executing a computer process.  
           [0011]    These and various other features as well as advantages which characterize embodiments of the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a plan view of a disc drive incorporating a preferred embodiment of the present invention showing the primary internal components.  
         [0013]    [0013]FIG. 2 is a simplified functional block diagram of a servo controller and a plant of the disc drive shown in FIG. 1.  
         [0014]    [0014]FIG. 3 is a bode plot showing mechanical resonance in a disc drive.  
         [0015]    [0015]FIG. 4 illustrates a portion of the servo controller and the plant shown in FIG. 2.  
         [0016]    [0016]FIG. 5 is a bode plot showing the effects of a phase-advanced filter on the mechanical resonance shown in FIG. 3.  
         [0017]    [0017]FIG. 6 is a bode plot showing the response of the phase-advanced filter to the control signal transmitted from the servo control module of FIG. 4.  
         [0018]    [0018]FIG. 7 is a process flow diagram illustrating exemplary operations employed by the portion of the servo loop illustrated in FIG. 4. 
     
    
     DETAILED DESCRIPTION  
       [0019]    Embodiments of the invention are described in detail below with reference to the drawing figures. When referring to the figures, like structures and elements shown throughout are indicated with like reference numerals.  
         [0020]    A disc drive  100  constructed in accordance with a preferred embodiment of the present invention is shown in FIG. 1. The disc drive  100  includes a base  102  to which various components of the disc drive  100  are mounted. A top cover  104 , shown partially cut away, cooperates with the base  102  to form an internal, sealed environment for the disc drive in a conventional manner. The components include a spindle motor  106 , which rotates one or more discs  108  at a constant high speed. Information is written to and read from tracks on the discs  108  through the use of an actuator assembly  110 , which rotates during a seek operation about a bearing shaft assembly  112  positioned adjacent the discs  108 . The actuator assembly  110  includes a plurality of actuator arms  114  which extend towards the discs  108 , with one or more flexures  116  extending from each of the actuator arms  114 . Mounted at the distal end of each of the flexures  116  is a head  118 , which includes an air bearing slider, enabling the head  118  to fly in close proximity above the corresponding surface of the associated disc  108 .  
         [0021]    During a seek operation, the track position of the heads  118  is controlled through the use of a voice coil motor (VCM)  124 , which typically includes a coil  126  attached to the actuator assembly  110 , as well as one or more permanent magnets  128  which establish a magnetic field in which the coil  126  is immersed. The controlled application of current to the coil  126  causes magnetic interaction between the permanent magnets  128  and the coil  126  so that the coil  126  moves in accordance with the well-known Lorentz relationship. As the coil  126  moves, the actuator assembly  110  pivots about the bearing shaft assembly  112 , and the heads  118  are caused to move across the surfaces of the discs  108 .  
         [0022]    The spindle motor  106  is typically de-energized when the disc drive  100  is not in use for extended periods of time. The heads  118  are typically moved over park zones  120  near the inner diameter of the discs  108  when the drive motor is de-energized. The heads  118  are secured over the park zones  120  through the use of an actuator latch arrangement, which prevents inadvertent rotation of the actuator assembly  110  when the heads are parked.  
         [0023]    A flex assembly  130  provides the requisite electrical connection paths for the actuator assembly  110  while allowing pivotal movement of the actuator assembly  110  during operation. The flex assembly includes a printed circuit board  132  to which head wires (not shown) are connected; the head wires being routed along the actuator arms  114  and the flexures  116  to the heads  118 . The printed circuit board  132  typically includes circuitry for controlling the write currents applied to the heads  118  during a write operation and a preamplifier for amplifying read signals generated by the heads  118  during a read operation. The flex assembly terminates at a flex bracket  134  for communication through the base deck  102  to a disc drive printed circuit board (not shown) mounted to the bottom side of the disc drive  100 . The disc drive  100  further includes a drive controller  210  (FIG. 2), which is operable to be coupled to a host system or another controller that controls a plurality of drives. In an illustrative embodiment, the drive controller  210  is a microprocessor, or a digital signal processor. The drive controller  210  is either mountable within the disc drive  100 , or is located outside of the disc drive  100  with suitable connection to the actuator assembly  110 .  
         [0024]    Shown in FIG. 2 is a functional block diagram of what is commonly referred to as the servo loop  200  of the disc drive  100 . In general, the servo loop  200  includes a servo controller  202  (small dashed lines) and a plant  204  (large dashed lines). The servo controller  202  includes a disc drive microprocessor  210  having an associated memory  212 , a transconductance amplifier  216 , a read/write channel  218 , and a Mask Read Only Memory (ROM)  230 . The plant generally includes the actuator assembly  110 , the discs  108 , the VCM  124 , the heads  118 , and the spindle motor  106 . In operation, the microprocessor  210  typically receives a seek command from a host computer (not shown) or Mask ROM  230  that indicates that a particular track on the discs  108  is to be accessed. In response to the seek command, the microprocessor  210  determines an appropriate velocity or seek profile to move the head from its current position to the track that is to be accessed. The seek profile is then sent to the transconductance amplifier  216  for amplification. The transconductance amplifier  216  then provides a driving current corresponding to seek profile to the coil  126 . In response to the driving current, the actuator assembly  110  accelerates toward the target track and then decelerates and stops the actuator assembly  110  when the head  118  is over the target track and the seek operation is completed.  
         [0025]    While reading or writing data from or to the target track, the servo loop  200  performs a track follow operation to keep the head  118  as close to the center of the target track as possible. During a track follow operation, the microprocessor  210  receives a position error signal (PES) from servo control wedges on the disc  108 . The PES indicates how far the head  118  is from the center of the target track. In response to receipt of the PES, the microprocessor  210  generates an adjustment signal to adjust the position of the head  118  to be closer to the center of the target track.  
         [0026]    One or more reserve tracks  228  on the disc  108  contain data and software employed during runtime operation. The runtime software is copied from the reserve tracks  228  into a high-speed memory, such as the memory  212 , during power up, or boot up. Computer-executable instructions residing in a Mask ROM  230  are executed at power up and referred to as boot code. The boot code in Mask ROM  230  contains instructions for seeking to and track following the reserve tracks  228  during power up. Seek commands are embedded in the boot code to cause the transducer head  118  to be positioned over a reserve track  228  and follow the reserve track  228 . During seek and track follow operations, a digital phase-advanced filter in the boot code filters out mechanical resonance. Thus, the phase-advanced filter is particularly suitable for improving performance during track following over the reserve tracks  228  during power up. After the head  118  settles on a reserve track, the phase-advanced filter filters out unwanted resonance to keep the head  118  as close to the center of the track as possible, without substantial loss of phase-margin.  
         [0027]    For a given seek command, the microprocessor  210  computes a seek profile comprising various steps, referred to herein as servo states. In this embodiment, the seek profile includes five servo states. A first “start move” state provides the appropriate signal to start the actuator moving towards the desired track. A second “constant acceleration” state provides the appropriate signal to drive the actuator through the constant acceleration portion of the seek. A third “velocity profile/square root” state provides the appropriate signal for controlling the actuator velocity to a predefined profile. A fourth “linear” state provides the appropriate signal to drive the actuator through the linear portion of the seek. Finally, a fifth “settle” state provides the appropriate signal for controlling the final “settling” of the actuator, and thus the head, in position over the desired track. That is, the settle state provides the appropriate signal to transition the actuator from track seeking to track following. It will be understood to one skilled in the art that the precise number, nomenclature, and/or function of each of the states in a seek profile may vary from one disc drive to another and/or from one manufacturer to another. As such, it will be understood that the five-state seek profile described herein is but one example of a possible seek profile that may be used in conjunction with the present invention.  
         [0028]    Most relevant to the present invention is filtering unwanted resonance after the seek operation and during track following. The head  118  settles on the target track at the end of the seek operation. Then, the microprocessor  210  executes a track follow operation. During the track follow operation, the microprocessor  210  executes servo control code (e.g., servo control module  400  in FIG. 4) to hold the read or write element of the head  118  as close to the center of the target track as possible while data is read from and/or written to the target track. The microprocessor  210  senses servo control data from the target track. Servo control data on the track includes a position error signal (PES) that the microprocessor  210  uses to monitor how far the head  118  is from the center of the track. In response to a deviation from the center of the track, the microprocessor  210  sends a control signal to the VCM  124  (FIG. 1) to correct for the deviation. The control signal is filtered in accordance with an embodiment of the present invention before being sent to the VCM  124 . More particularly, before the VCM  124  receives the control signal, the microprocessor  210  executes phase-advanced filter code (e.g., phase-advance filter module  412  in FIG. 4) filtering frequencies out of the control signal that would otherwise excite resonance in the VCM  124 .  
         [0029]    Each of the mechanical components of the disc drive  100  may have various resonant modes that, if excited by an external energy source, will cause the mechanical components to oscillate at the natural resonance frequencies of the component. FIG. 3 illustrates an open loop Bode plot  300  showing a mechanical resonance in a disc drive. For clarity, the phase information has been removed from the Bode plot  300  and the plot is not shown to scale. The x-axis  310  represents the frequency of the excitation energy, while the y-axis  312  represents the open loop system velocity gain in decibels (dB). The open loop system velocity gain  314  generally drops at the rate of 20 dBs per decade. However, as shown in FIG. 3, a mechanical resonance causes a sharp increase  316  in the system gain. The increase  316  in the system gain caused by the mechanical resonance depicted is centered at a center frequency  318  and has a peak amplitude  320 .  
         [0030]    The memory  212  and the Mask ROM  230  of FIG. 2 include computer executable instructions for implementing a servo control module (e.g., servo control module  400  of FIG. 4) and a phase-advanced filter (e.g., phase advanced filter module  412  of FIG. 4). The computer executable instructions are executed by the microprocessor  210  to perform the seeking and track following operations necessary to accurately position the read or write element on the head  118  over a target track.  
         [0031]    [0031]FIG. 4 illustrates a simplified block diagram of the operational environment of a phase-advanced filter module  412  according to an illustrative embodiment. In this embodiment, and other embodiments described herein, the logical operations of the phase-advanced filter module  412  and the servo control module  400  may be implemented as a sequence of computer implemented steps or program modules running on a microprocessor, such as, microprocessor  210 . It will be understood to those skilled in the art that the phase-advanced filter module  412  and the servo control module  400  may also be implemented as interconnected machine logic circuits or circuit modules within a computing system.  
         [0032]    Additionally, the servo control module  400  and the phase-advanced filter  412  may be implemented in separate components of the disc drive  100 , such as a dedicated servo controller and a dedicated phase-advanced filter  412 , respectively. The implementation is a matter of choice dependent on the performance and design requirements of the disc drive  100 . As such, it will be understood that the operations, structural devices, acts, and/or modules described herein may be implemented in software, in firmware, in special purpose digital logic, and/or any combination thereof without deviating from the spirit and scope of the present invention as recited within the claims attached hereto. Furthermore, the various software routines or software modules described herein may be implemented by any means as is known in the art. For example, any number of computer programming languages, such as “C”, “C++”, Pascal, FORTRAN, assembly language, Java, etc., may be used. Furthermore, various programming approaches such as procedural, object oriented or artificial intelligence techniques may be employed.  
         [0033]    In this embodiment, the computer implemented steps and corresponding digital data that comprise the operations of the phase-advanced filter  412  and the servo control module  400  are preferably stored on computer readable media. As used herein, the term computer-readable media may be any available media that can be accessed by a processor or component that is executing the functions, steps and/or data of the phase-advanced filter  412 . By way of example, and not limitation, computer-readable media might include computer storage media that includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, RAM, ROM, EPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store the desired information and that can be accessed by the computer or processor which is executing the operating code. An embodiment of the phase-advanced filter  412  is particularly beneficial for, but not limited to, use in Mask ROM boot code. Boot code is employed to perform basic seek and track following operations to load code and data from the disc  108  during power up of a disc drive  100 .  
         [0034]    Computer-readable media might include communication media that embodies computer-readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared, and other wireless media. Combinations of any of the above should also be included within the scope of computer-readable media. Computer-readable media may also be referred to as computer program product.  
         [0035]    Processing of a typical seek or track follow operation is shown in FIG. 4. The servo control module  400  receives a command  408  and generates, in response, a control signal  410  (during either a seek or track follow operation) composed of frequency components that range from direct current (DC) to multiple kilohertz or higher components. During a track follow operation, the control signal  410  is generally related to a position error signal (PES) detected on the disc. The PES indicates to the servo control module  400  how far the head  118  is from center of the track being followed. The servo control module  400  generates the control signal  410  designed to adjust the position of the head  118  closer to the track center. The control signal  410  is provided to a phase-advanced filter  412 , which reduces the frequency components in the signal that are at or near the resonance frequencies of the disc drive mechanical structure. The phase-advanced filter  412  then provides a filtered control signal  414  to the transconductance amplifier  216  which amplifies the filtered control signal  414 . The amplifier  216  then provides a driving current  416  to the coil  126  of the actuator VCM  124  to position the head  118  at the desired location over the disc  108 . The filtered control signal  414  reduces risk of exciting the disc drive mechanical structure into oscillation.  
         [0036]    Various methods of implementing a phase-advanced filter  412  may be used with respect to this embodiment. One illustrative method of implementing a phase-advanced filter  412  is to employ the discrete-time domain transfer function shown in equation (1):  
                 H        (   z   )       =         B   0     +       B   1          z     -   1         +       B   2          z     -   2         +       B   3          z     -   3             1   +       A   1          z     -   1         +       A   2          z     -   2         +       A   3          z     -   3               ,           (   1   )                               
 
         [0037]    where A 1 , A 2 , A 3 , B 0 , B 1 , B 2 , and B 3  are low-pass filter constants describing the frequency, depth, and width of the band pass. These constants may be predetermined before the manufacture of the disc drive. Alternatively, these constants may be determined experimentally during the manufacture of the disc drive. The equation may be given by the more general form:  
                 H        (   z   )       =           b   0     +       b   1        z     +   ⋯   +       b   m          z   m             a   0     +       a   1        z     +   ⋯   +       a   n          z   n                  (     z   +   1     )       n   -   m           ,           (   2   )                               
 
         [0038]    where n−m&gt;=1. The exemplary transfer function H(z) is generally referred to as a low-pass filter. More specifically, it is a digital filter having a factor of (z+1) in the numerator. In an alternative embodiment, the factor (z+1) may be replaced with a factor 2z. Using the factor 2z advances the phase by ωT/2, where T is the sampling period, while the gain is almost unaffected. Thus, the degree of phase advancement is proportional to the frequency. For example, when ω=2π*f N , where f N  is the Nyquist frequency, the phase of the signal output from the phase-advance filter is advanced by 90° compared to the phase of the input signal.  
         [0039]    In the analog, or s-domain, the transfer function may be characterized generally as having more poles than zeros. An analog filter in an embodiment of the present invention would employ a transfer function of the form illustrated as follows in Equation 3:  
               H        (   s   )       =         b   0     +       b   1        s     +   ⋯   +       b   m          s   m             a   0     +       a   1        s     +   ⋯   +       a   n          s   n                   (   3   )                               
 
         [0040]    The phase-advanced filter  412  may be implemented in software. An exemplary portion of assembly code that implements a software phase-advance filter  412  in one embodiment is shown in Table 1.  
                                                         TABLE 1                           @*define PhaseAdvanceLowPassFilter(VarPointer,CoeffPointer,Order)    @local(loop05)                movw   word ptr (@VarPointer+@Order*2),R3           movw   IDX0,#@VarPointer           movw   R2,#@CoeffPointer           comul   [IDX0+],[R2+]           movw   MRW,#(@Order*2-1)            @loop05:     -usr1   comacm [IDX0+],[R2+]                jmpa     cc_nusr1,@loop05           coshl     #(16-LP_QSCALE)           movw     R3,MAH           movw     word ptr (@VarPointer+@Order*4),R3           @endd                      
 
         [0041]    The software algorithm of Table 1 is a routine implementing a digital low-pass phase-advanced filter  412 . Generally, the routine accepts a list of input parameters, including a pointer to one or more variables (VarPointer), a pointer to one or more coefficients (CoeffPointer), and an order (Order), representing the order of the filter  412 . The routine processes the input data by iteratively multiplying each coefficient with its associated variable. The output of each iteration is saved for input to the next iteration. The values output by each iteration are used to develop a state matrix. The number of states in the state matrix depends on the order of the phase-advanced filter  412 . The code shown in Table 1 is executed by a fixed-point microprocessor, so the output is scaled in the “coshl” step. The exemplary software in Table 1 is assembly code but any software language known in the art may be used for implementing the phase-advanced filter  412 . The algorithm illustrated in Table 1 is one exemplary method of implementing the phase-advanced filter  412 , and it is envisioned that other algorithms may be utilized that fall within the scope of the present invention.  
         [0042]    The effect of the phase-advanced filter  412  can be seen in the summation Bode plot of FIG. 5, which for clarity&#39;s sake does not show the phase response and is not shown to scale. The x-axis  510  represents the frequency of the driving energy, while the y-axis  512  represents the system gain in decibels (dB). FIG. 5 shows a mechanical resonance  502  (larger, dashed lines) centered at center frequency  504 . Also shown is the frequency response  506  (short, dashed lines) of the phase-advanced filter  412 . It can be seen that phase-advanced filter  412  appreciably attenuates the driving energy about the mechanical resonance  502 , with the maximum attenuation occurring at the center frequency  504  of the mechanical resonance. FIG. 5 further shows the open loop frequency response  514  with the phase-advanced filter  412  active. When the phase-advanced filter  412  is activated, the open loop response  514  is a summation of the original response and the phase-advanced filter  412  response. That is, the phase-advanced filter  412  works on the principle of superposition. It can be seen that the peak amplitude  512  at center frequency  504  is now well below 0 dB.  
         [0043]    When the phase-advanced filter response  506  is properly located, as in FIG. 5, the driving force energy at the center frequency  504  of the mechanical resonance  502  can be reduced so that there will be little or no energy made available to excite the mechanical structure. It will be understood that one phase-advanced filter  412  may be used to eliminate a number of resonance modes in the disc drive  100 . Advantageously, and unlike when utilizing traditional notch filters, it is not necessary to center multiple phase-advanced filters  412  at multiple center frequencies corresponding to mechanical resonance modes. One phase-advanced filter  412  can guarantee attenuation substantially equal to −15 decibels (dB) above a selected frequency (e.g., center frequency  504 ).  
         [0044]    [0044]FIG. 6 is a Bode plot  600  showing a closed-loop response of the phase-advanced filter  412  as compared to a closed-loop response of a typical notch filter. The Bode plot  600  includes a phase-advanced filter magnitude response  601  (upper solid line) and a phase-advanced filter phase response  602  (lower solid line). The phase-advanced responses  601  and  602  illustrated in the Bode plot  600  are responses exhibited by an exemplary embodiment of a phase-advanced filter  412 . For clarity, the Bode plot  600  is not shown to scale. As illustrated by the phase-advanced filter magnitude response  601 , the phase-advanced filter  412  applies substantial attenuation to the control signal  410  around the center frequency  504  to advantageously filter out the mechanical resonance  502 . Furthermore, the magnitude response  601  provides −20 dB of attenuation at frequencies substantially greater than the center frequency  504 . Experimentation has shown that the phase-advanced filter  412  can guarantee at least −15 dB gain attenuation above 10 KHz when the center frequency is located at 11 KHz. Thus, the phase-advanced filter  412  is robust above 10 KHz, in that only one phase-advanced filter  412  is generally required to attenuate multiple resonance modes above 10 KHz.  
         [0045]    A traditional notch filter magnitude response  604  is shown in FIG. 6 for illustrative purposes. In contrast to the phase-advance filter magnitude response  601 , a traditional notch filter magnitude response  604  (dashed line) includes an attenuation notch around the center frequency  504 . Unlike a phase-advanced filter  412 , a traditional notch filter generally attenuates only around the center frequency  504 , but not at frequencies substantially above the center frequency  504 . Thus, utilizing notch filters, in order to attenuate resonant modes above the center frequency  504 , more notch filters are required; i.e., a notch filter for each unwanted resonant mode. As is known, the addition of notch filters undesirably reduces the phase margin. However, embodiments of the phase-advanced filter  412  substantially reduce or eliminate phase margin loss, as is shown by the phase-advanced filter phase response  602 .  
         [0046]    The phase-advanced filter phase response  602  illustrates the difference in phase between the filtered control signal  414  and the control signal  410  with respect to frequency. It can be seen that the phase does not decrease substantially between 0 Hz and the open loop gain cross over frequency  630 . For comparison, a traditional notch filter phase response  606  (lower dashed line) is shown. The notch filter phase response  606  drops substantially at the open loop gain crossover frequency  630 . The difference in phase loss  610  is directly related to the difference in phase margin loss created by the phase-advanced filter  412  and the notch filter. The phase-advanced filter  412  substantially reduces or eliminates phase margin loss. The difference  610  (and thus the difference in phase margin loss) has been experimentally shown to be around 10°. Additionally, to achieve the same attenuation at frequencies above the center frequency  504  as the attenuation of the phase-advanced filter  412 , more than one notch filter would be required. Adding more notch filters will further increase the difference in phase loss  610 . Thus, one phase-advance filter  412  substantially attenuates resonant frequencies above the center frequency  504 , while simultaneously substantially reducing or eliminating any phase margin loss. Reduction of phase margin loss reduces the risk of run-out and reduces the loss in bandwidth of the servo loop  200 .  
         [0047]    [0047]FIG. 7 is a process flow diagram  800  illustrating exemplary operations employed by the portion of the servo loop  200  illustrated in FIG. 4. In a receiving operation  802 , the microprocessor  210  receives a seek command to position the head  118  at a target track. In response to receipt of the seek command, the microprocessor  210  performs a seeking operation  803 , wherein a seek profile signal is generated as discussed above, to accelerate the head  118  toward the target track. Control transfers to a generating operation  804  wherein a control signal  410  is generated to adhere to the seek profile signal of the seeking operation  803 . The generating operation  804  may be implemented by the microprocessor  210  executing a servo control module  400  shown in FIG. 4.  
         [0048]    Control then transfers to a filtering operation  806  wherein the control signal  410  is filtered by a phase-advanced filter  412  to generate a filtered control signal  414 . The filtering operation  806  substantially attenuates frequencies above a cut-off frequency and advances the phase of the control signal  410  so that phase margin loss is substantially reduced or avoided as discussed above. The transfer function associated with the filtering operation  806  is given in equations 1, 2 and/or 3. Control then transfers to an amplifying operation  808  wherein the filtered control signal  414  is amplified to create a driving signal  416  for driving the voice coil motor  124 . Control then transfers to a driving operation  810  wherein the driving signal  416  is transmitted to the voice coil motor  124  whereby the position of the actuator arm is adjusted in response to the voice coil motor being energized by the drive current. Control then transfers to a determining operation  812  wherein it is determined whether the head  118  is positioned over the target track. Typically the determination is made by detecting position signals read from the disc during the seek operation and feeding those position signals back to the microprocessor  210 . If the head  118  is not positioned over the target track, control transfers to the seeking operation  803 , wherein the seeking operation continues until the head  118  is positioned over the target track.  
         [0049]    If, on the other hand, it is determined in the determining operation  812  that the head is positioned over the target track, control transfers to a track following operation  814 , wherein a track following mode is entered. Entering the track following mode, involves settling the head  118  over the target track. After settling over the target track, PES is read from the disc  108  and the head position is substantially continuously adjusted so that the head is as close to the center of the track as possible. Control then transfers to the generating operation  804  wherein a control signal  410  related to the PES is generated. The control signal  410  that is generated includes frequency components as described above. Control then transfers to a filtering operation  806  wherein the control signal  410  is filtered by a phase-advanced filter  412 . The filtering operation  806  substantially attenuates frequencies above a selected center frequency and advances the phase of the control signal so that phase margin loss is substantially reduced or avoided as described in embodiments above. The transfer function associated with the filtering operation  806  is given in equations 1, 2 and/or 3. Control then transfers to an amplifying operation  808  wherein the filtered signal is amplified to create a drive current for driving the voice coil motor. Control then transfers to a driving operation  810  wherein the drive current is transmitted to the voice coil motor to adjust the position of the actuator arm more closely to the center of the target track. Thus, the phase-advanced filter  412  may be used to perform filtering during either a seek operation or a track following operation.  
         [0050]    In summary, an embodiment may be viewed as a disc drive (such as  100 ) that has a data disc (such as  108 ) with concentrically arranged data tracks (such as  228 ) and a voice coil motor (such as  124 ) for positioning a transducer head (such as  118 ) over a selected track (such as  228 ). The disc drive (such as  100 ) includes a servo control module (such as  400 ) that receives a seek command (such as  408 ) and transmits a control signal (such as  410 ) to the voice coil motor (such as  124 ). The embodiment further includes phase-advanced filter (such as  412 ) that filters the control signal (such as  410 ) such that a range of frequencies above a predetermined center frequency (such as  504 ) are substantially attenuated and phase margin loss is substantially avoided.  
         [0051]    An embodiment may also be viewed as a method of reducing component resonance (such as  320 ) in a disc drive (such as  100 ) by receiving a seek command (such as  408 ) to seek to a target track (such as  228 ), generating (such as  320 ) a seek control signal (such as  410 ) to move the a head (such as  118 ) to the target track (such as  228 ), and filtering (such as  806 ) the seek control signal (such as  410 ) with a phase-advanced filter (such as  412 ) to generate a filtered seek control signal (such as  414 ). The method additionally includes driving a voice coil motor (such as  124 ) with a driving current (such as  416 ) based on the filtered seek control signal (such as  414 ) to position the head (such as  118 ) over the target track (such as  228 ).  
         [0052]    The method may further include entering a track follow mode (such as  814 ). The track follow mode typically involves settling the head over the target track (such as  228 ) and detecting a position error signal (PES) from the target track (such as  228 ). During track follow mode, the method involves generating (such as  804 ) a track follow control signal (such as  410 ) based on the PES. Following the track further involves filtering (such as  806 ) the track follow control signal (such as  410 ) with a phase-advanced filter (such as  412 ) to generate a filtered track follow control signal (such as  414 ) before driving (such as  810 ) a voice coil motor (such as  124 ) with a driving current (such as  416 ) based on the filtered track follow control signal (such as  414 ).  
         [0053]    The logical operations of the various embodiments of the present invention are implemented (1) as a sequence of computer implemented acts or program modules running on a computing system and/or (2) as interconnected machine logic circuits or circuit modules within the computing system. The implementation is a matter of choice dependent on the performance requirements of the computing system implementing the invention. Accordingly, the logical operations making up the embodiments of the present invention described herein are referred to variously as operations, structural devices, acts or modules. It will be recognized by one skilled in the art that these operations, structural devices, acts and modules may be implemented in software, in firmware, in special purpose digital logic, and any combination thereof without deviating from the spirit and scope of the present invention as recited within the claims attached hereto.  
         [0054]    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 a presently preferred embodiment has been described for purposes of this disclosure, various changes and modifications may be made which are well within the scope of the present invention. For example, the phase-advanced filter could be employed in other (non-disc drive) environments where mechanical resonance modes arise and reduce performance of servo control. Additionally, analog versions of the phase-advanced filter may be suitable for analog environments and may be readily apparent to those skilled in the art. Numerous other 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.