Abstract:
A control system for an actuator arm assembly of a hard disk drive reduces the Non-Repeatable Run-out (NRRO) caused from external sources exciting higher-frequency actuator arm assembly modes. The actuator arm assembly includes a primary actuator and a secondary actuator. The control system includes a primary control loop controlling the primary actuator and a secondary control loop controlling the secondary actuator. The secondary control loop includes at least one peak filter at a frequency corresponding to at least one frequency that is greater in frequency than the primary mode of the actuator arm assembly. The primary actuator can be any type of primary actuator. Similarly, the secondary actuator can be any type of actuator that is located between the primary actuator and a read/write head.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to hard disk drives (HDDs). More particularly, the present invention relates to a technique for reducing Non-Repeatable Run-Out (NRRO) of an actuator arm assembly of an HDD caused by high-frequency actuator and arm (actuator/arm) modes. 
     2. Description of the Related Art 
       FIG. 1  shows an exemplary hard disk drive (HDD)  100  having a dual-stage servo system for positioning a slider assembly  101  over a selected track on a magnetic disk  102  for writing data to and/or reading data from the surface of disk  102 . The dual-stage servo system of HDD  100  includes a primary actuator  104 , such as a rotary voice-coil motor (VCM), for coarse positioning an actuator arm  105  and a read/write head suspension  106 , and a secondary actuator (not shown in  FIG. 1 ), such as a microactuator or micropositioner, for fine positioning slider assembly  101  over a selected track. A microactuator (or a micropositioner), as used herein, is a small actuator that is typically placed between a suspension and a slider and moves the slider relative to the suspension, but can be placed on the suspension or other locations within a dual-stage servo system. Slider assembly  101  includes a read/write head (not shown) having a read element and a write element that respectively read data from and write data to disk  102 . While HDD  100  is shown as having only a single magnetic disk  102 , HDDs typically have a plurality of stacked, commonly rotated rigid magnetic disks and a corresponding number of actuator arms, read/write head suspensions, secondary actuators and slider assemblies. 
       FIG. 2  depicts a cross-section of an exemplary suspension and rotary microactuator arrangement  200  that can be part of a dual-stage servo system. Suspension and microactuator arrangement  200  includes a suspension  201 , a microactuator  205  and a slider  209 . Suspension  201  includes a load beam  202 , a dimple  203  and a flexure  204 . Microactuator  205  includes a substrate  206 , a microactuator structure  207  and at least one flexure element  208 . Substrate  206  is the stationary structure of microactuator  205 . Microactuator structure  207  is the movable structure of microactuator  205 . Slider  209  includes a read element  210  and a write element  211  that is offset from read element  210 . 
       FIG. 3  is a schematic block diagram showing an exemplary actuator arm assembly  301  that can be used for the actuator arm assembly shown in  FIG. 1 . Actuator arm assembly  301  includes a primary actuator  302  (corresponding to VCM  104 ), an actuator arm portion  303  (corresponding to actuator arm  105 ), a read/write head suspension portion  304 , (corresponding to suspension  106 ) and a slider assembly  305  (corresponding to slider assembly  101 ). 
     Actuator arm assembly  301  is controlled by an exemplary conventional control system  306  that includes a control circuit  307  that generates a signal  308  that is output to a primary amplifier  309  that, in turn, drives primary actuator  302 . When primary actuator  302  is a rotary-type VCM, actuator arm assembly  301  rotates (as indicated by arrows  310 ), about a pivot  311  under the force generated by primary actuator  302 . Control circuit  307  also generates a signal  312  that is output to a secondary amplifier  313  that, in turn, drives a secondary actuator (not shown in  FIG. 3 ). A position signal  314  representing the position of slider assembly with respect to a disk is input to control circuit  307 . 
     The position of a read/write head in relation to data on a disk is affected by the effects of VCM  302 , external disturbances  315 , resonant modes of actuator arm assembly  301 , and motion of the disk.  FIG. 4A  is a graph of Non-Repeatable Run-Out (NRRO) as a function of frequency based on representative data that is typical for the currently available generation of actuator arm assemblies.  FIG. 4B  is a graph of cumulative NRRO as a function of frequency corresponding to the graph of  FIG. 4A . The abscissa for both  FIGS. 4A and 4B  is the frequency, and the ordinate of both  FIGS. 4A and 4B  is NRRO. It should be understood that all of the graphs depicted herein are simulations that are based on data that is representative for the currently available generation of actuator arm assemblies for the enterprise class of hard disk drives. It should also be understood that graphs depicted herein could be based on data that is representative for other currently available classes of hard disk drives, such as desktop hard disk drives, mobile hard disk drives and consumer electronics hard disk drives. In  FIG. 4A , NRRO caused by operational vibration (an external disturbance  315 ) is indicated at  401 . NRRO caused by disk flutter (another external disturbance  315 ) is indicated at  402 . NRRO caused by high-frequency actuator and arm (actuator/arm) mode effects of actuator arm assembly  301  is indicated at  403 . The large amplitude motion at the high-frequency resonances of actuator arm assembly  301  may result in the inability of the read/write head to read or write data at the appropriate location on a disk. 
     Many of the resonant modes of actuator arm assembly  301  are greater than the bandwidth of the control loop for VCM  302  because the control bandwidth of VCM  302  is generally limited to be below the first main resonance of VCM  302 . For example,  FIGS. 5A and 5B  respectively show a magnitude and phase response as a function of frequency for primary amplifier  309  and VCM  302 . The first main resonance, or primary mode, of VCM  302 , commonly referred to as the butterfly mode, is indicated at  501  in  FIG. 5A . Higher-frequency actuator and arm (actuator/arm) modes corresponding to high-frequency actuator/arm mode effects  403  in  FIG. 4A  are indicated at  502  in  FIG. 5A . 
       FIGS. 6A and 6B  respectively show a magnitude and phase response as a function of frequency for the primary control portion of conventional control circuit  307 , that is, the portion of control circuit  307  that controls VCM  302 .  FIGS. 7A and 7B  respectively show an open-loop magnitude and phase response as a function of frequency for the primary control portion of control circuit  307 , primary amplifier  309  and VCM  302 . The butterfly mode can be observed at  701  and the higher-frequency actuator/arm modes can be observed at  702 .  FIG. 8  shows a closed-loop magnitude response of the VCM error rejection as a function of frequency for the primary control portion of control circuit  307 , primary amplifier  309  and VCM  302 . At higher frequencies corresponding to the frequencies of the actuator/arm modes, portions of the VCM open-loop frequency response corresponding to the higher-frequency actuator/arm modes are greater than 0 dB, as indicated by  702  in  FIG. 7A . This generally results in portions of the VCM error rejection corresponding to the higher-frequency actuator/arm modes that are less than 0 dB, as indicated by  801  in  FIG. 8 . Magnitudes of the error rejection frequency response that are less than 0 dB indicate desirable disturbance rejection. Higher-frequency actuator/arm modes that have VCM open-loop frequency response magnitudes that are greater than 0 dB, however, are difficult or impossible to stabilize and often lack robustness to manufacturing tolerances, parameter variations, and other factors. Thus, a conventional primary control loop for a VCM does not adequately compensate for higher-frequency actuator/arm modes. 
     One conventional approach to reduce the NRRO that occurs at the higher-frequency actuator/arm modes has been to use notch filters in the primary control loop to prevent primary actuator  302  from exciting the resonant modes of actuator arm assembly  301 .  FIGS. 9A and 9B  respectively show a magnitude and phase response as a function of frequency for the primary control portion of control circuit  307  when control circuit  307  includes notch filters. The effect on the frequency response by the notch filters are shown at  901 .  FIGS. 10A and 10B  respectively show an open-loop magnitude and phase response as a function of frequency for the primary control portion of control circuit  307 , primary amplifier  309  and VCM  302  when control circuit  307  includes notch filters. The attenuating effect of the notch filters on the higher-frequency actuator/arm modes is shown at  1001 .  FIG. 11  shows a closed-loop magnitude and phase of the VCM error rejection as a function of frequency for the primary control loop portion of control circuit  307 , primary amplifier  309  and VCM  302  when control circuit  307  includes notch filters. The magnitudes of the VCM open-loop frequency response at the higher-frequency actuator/arm modes are well below 0 dB, as indicated by  1001  in  FIG. 10 , so there are no stability issues associated with the higher-frequency actuator/arm modes and the higher-frequency actuator/arm modes should be excited only weakly by the motion of the actuator arm assembly  1201 . The magnitudes of the VCM error rejection frequency response at the higher-frequency actuator/arm modes, however, is nearly flat at 0 dB, as indicated by  1101  in  FIG. 11 . This means that the higher-frequency actuator/arm modes are very susceptible to excitation by other types of disturbances, such as airflow. 
     Thus, the decreased open-loop gain caused by the notch filters decreases the disturbance rejection of control loop for primary actuator  302  at the frequencies of the higher-frequency actuator/arm modes, thereby making the resonant modes more susceptible to excitation caused by external disturbances  315 . One technique to increase the disturbance rejection at a particular frequency is to introduce a peak filter. See, for example, U.S. Pat. Nos. 6,339,512 and 6,487,028, both to Sri-Jayantha et al. Introducing peak filters in to the primary control loop at the higher-frequency actuator/arm modes, however, would cancel the stabilizing effects of the notch filters and suffer the same instability and robustness issues as the case without the notch filters shown in  FIGS. 7A and 7B . 
     Another technique to improve disturbance rejection is to increase the open loop bandwidth. Increasing the open loop bandwidth of the primary control loop also has limited effectiveness in reducing the adverse effects of the higher-frequency actuator/arm modes. The Bode Integral Theorem mathematically proves that all feedback loops have a region of disturbance attenuation and a region of disturbance amplification. See, for example, H. W. Bode, Network Analysis and Feedback Amplifier Design, Princeton, N.J.: Van Nostrand, 1945. Moreover, the ratio of the attenuation to the amplification regions is fixed. Consequently, regardless of how a higher bandwidth is achieved for the primary control loop, the disturbance amplification region will still exist and will generally be pushed to a higher frequency. Because the bandwidth of the primary control loop is generally limited to be below the butterfly mode, the negative affects of the higher frequency actuator/arm modes will potentially be exacerbated. 
     Yet another approach for reducing NRRO is based on design of shrouding and mechanics for reducing actuator/arm mode excitation caused by airflow, but often provides limited effectiveness and may have negative implications on other aspects of the HDD. 
     Consequently, what is needed is a technique for reducing the adverse effects of high-frequency actuator/arm modes, while maintaining stability and robustness, thereby reducing total off-track motion of a read/write head and enabling a higher track density for an HDD. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a technique for significantly reducing the adverse effects of high-frequency actuator/arm modes, while maintaining stability and robustness, thereby reducing total off-track motion of a read/write head and enabling a higher track density for an HDD. 
     The advantages of the present invention are provided by a control system for an actuator arm assembly for a hard disk drive. The actuator arm assembly includes a primary actuator. The primary actuator can be any type of primary actuator, such as a rotary-type primary actuator or a linear-type primary actuator. The actuator arm assembly also includes a secondary actuator. The secondary actuator can be any type of secondary actuator, such as an undamped rotary-type micro-electro-mechanical-system (MEMS) microactuator, a damped rotary-type MEMS microactuator, an undamped linear-type MEMS microactuator, a damped linear-type MEMS microactuator, or a PZT-based secondary actuator. A primary control loop controls the primary actuator, and a secondary control loop controls the secondary actuator. According to the present invention, the secondary control loop includes at least one peak filter at a frequency that is greater in frequency than a primary mode of the actuator arm assembly. For example, at least one peak filter can have a frequency corresponding to a mode of the actuator arm assembly. Alternatively, at least one peak filter can have a frequency corresponding to a frequency of a disturbance that is external to the actuator arm assembly and that affects a position of the actuator arm assembly, such as an airflow disturbance. Further, at least one peak filter can have a frequency corresponding to off-track motion induced by the primary actuator, a frequency corresponding to a frequency of a disturbance that causes non-repeatable runout, and/or a frequency corresponding to a frequency of a disturbance that causes repeatable runout. The secondary control loop includes a controller portion and a feedback portion in which at least one peak filter is included in the controller portion of the secondary control loop. Alternatively, at least one peak filter is included in the feedback portion of the secondary control loop. 
     The present invention also provides a hard disk drive having an actuator arm that includes a primary actuator and a secondary actuator, and a control system having a primary control loop controlling the primary actuator and a secondary control loop controlling the secondary actuator. The actuator arm assembly includes a primary actuator. The primary actuator can be any type of primary actuator, such as a rotary-type primary actuator or a linear-type primary actuator. The actuator arm assembly also includes a secondary actuator. The secondary actuator can be any type of secondary actuator, such as an undamped rotary-type micro-electro-mechanical-system (MEMS) microactuator, a damped rotary-type MEMS microactuator, an undamped linear-type MEMS microactuator, a damped linear-type MEMS microactuator, or a PZT-based secondary actuator. A primary control loop controls the primary actuator, and a secondary control loop controls the secondary actuator. According to the present invention, the secondary control loop includes at least one peak filter at a frequency that is greater in frequency than a primary mode of the actuator arm assembly. For example, at least one peak filter can have a frequency corresponding to a mode of the actuator arm assembly. Alternatively, at least one peak filter can have a frequency corresponding to a frequency of a disturbance that is external to the actuator arm assembly and that affects a position of the actuator arm assembly, such as an airflow disturbance. Further, at least one peak filter can have a frequency corresponding to off-track motion induced by the primary actuator, a frequency corresponding to a frequency of a disturbance that causes non-repeatable runout, and/or a frequency corresponding to a frequency of a disturbance that causes repeatable runout. The secondary control loop includes a controller portion and a feedback portion in which at least one peak filter is included in the controller portion of the secondary control loop. Alternatively, at least one peak filter is included in the feedback portion of the secondary control loop. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example and not by limitation in the accompanying figures in which like reference numerals indicate similar elements and in which: 
         FIG. 1  shows an exemplary hard disk drive (HDD) having a dual-stage servo system; 
         FIG. 2  depicts a cross-sectional view of an exemplary suspension and rotary microactuator arrangement for a dual-stage servo system; 
         FIG. 3  is a schematic block diagram showing an exemplary actuator arm assembly and an exemplary primary control system that can be used for the actuator arm assembly shown in  FIG. 1 ; 
         FIG. 4A  is a graph of Non-Repeatable Run-Out (NRRO) as a function of frequency for an exemplary actuator arm assembly, such as the actuator arm assembly shown in  FIG. 3 ; 
         FIG. 4B  is a graph of cumulative NRRO as a function of frequency corresponding to the graph of  FIG. 4A . 
         FIGS. 5A and 5B  respectively show a magnitude and phase response as a function of frequency for the primary amplifier and the VCM shown in  FIG. 3 ; 
         FIGS. 6A and 6B  respectively show a magnitude and phase response as a function of frequency for the primary control loop portion of the control circuit shown in  FIG. 3 ; 
         FIGS. 7A and 7B  respectively show an open-loop magnitude and phase response as a function of frequency for the primary control loop portion of the control circuit, the primary amplifier and the VCM shown in  FIG. 3 ; 
         FIG. 8  shows a closed-loop magnitude response of the VCM error rejection as a function of frequency for the primary control loop portion of the control circuit, the primary amplifier and the VCM shown in  FIG. 3 ; 
         FIGS. 9A and 9B  respectively show a magnitude and phase response as a function of frequency for the primary control portion of the control circuit shown in  FIG. 3  when the control circuit includes notch filters; 
         FIGS. 10A and 10B  respectively show an open-loop magnitude and phase response as a function of frequency for the primary control loop portion of the control circuit, the primary amplifier and the VCM shown in  FIG. 3  when the control circuit includes notch filters; 
         FIG. 11  shows a closed-loop magnitude of the VCM error rejection as a function of frequency for the primary control loop portion of the control circuit, the primary amplifier and the VCM shown in  FIG. 3  when the control circuit includes notch filters; 
         FIG. 12  is a schematic block diagram showing an exemplary actuator arm assembly and a simplified version of an exemplary control scheme according to the present invention; 
         FIG. 13  shows a functional block diagram of the exemplary actuator arm assembly and the exemplary control scheme shown in  FIG. 12  according to the present invention; 
         FIGS. 14A and 14B  respectively show a magnitude and phase response as a function of frequency of a secondary amplifier and an undamped rotary MEMS microactuator; 
         FIGS. 15A and 15B  respectively show a magnitude and phase response as a function of frequency for a secondary amplifier and a damped rotary MEMS microactuator; 
         FIGS. 16A and 16B  respectively show a magnitude and phase response as a function of frequency of a secondary microactuator controller; 
         FIGS. 17A and 17B  respectively show an open-loop magnitude and phase response as a function of frequency of a secondary microactuator controller having no peak filters, a secondary amplifier and an undamped rotary MEMS microactuator; 
         FIGS. 18A and 18B  respectively show a magnitude and phase response as a function of frequency of a secondary controller having peak filters according to the present invention; 
         FIGS. 19A and 19B  respectively show an open-loop magnitude and phase response as a function of frequency of a secondary microactuator controller having peak filters according to the present invention, a secondary amplifier and an undamped rotary MEMS microactuator; 
         FIGS. 20A and 20B  respectively show an open-loop magnitude and phase response as a function of frequency of a secondary controller having peak filters according to the present invention, a secondary amplifier and a damped rotary MEMS microactuator; 
         FIGS. 21A and 21B  respectively show open-loop magnitude and phase responses as functions of frequency for a VCM and for a secondary controller having no peak filters, a secondary amplifier and an undamped rotary MEMS microactuator; 
         FIG. 21C  shows magnitudes of the error rejection frequency responses for a primary control loop that includes a VCM and for a secondary controller having no peak filters, a secondary amplifier and an undamped rotary MEMS microactuator; 
         FIGS. 22A and 22B  respectively show open-loop magnitude and phase responses as functions of frequency for a VCM and for a secondary controller having peak filters according to the present invention, a secondary amplifier and an undamped rotary MEMS microactuator; 
         FIG. 22C  shows magnitudes of the error rejection frequency responses for a primary control loop that includes a VCM and for a secondary controller having peak filters according to the present invention, a secondary amplifier and a damped rotary MEMS microactuator; 
         FIGS. 23A and 23B  respectively show open-loop magnitude and phase responses as a function of frequency for a VCM and for a secondary controller having peak filters according to the present invention, a secondary amplifier and a damped rotary MEMS microactuator; 
         FIG. 23C  shows a magnitude of the error rejection frequency response for a primary control loop that includes a VCM and for a secondary controller having peak filters according to the present invention, a secondary amplifier and a damped rotary MEMS microactuator; 
         FIG. 24A  is a graph of NRRO as a function of frequency for an exemplary actuator arm assembly in which a secondary controller includes peak filters according to the present invention and an undamped rotary MEMS microactuator; 
         FIG. 24B  is a graph of cumulative NRRO as a function of frequency corresponding to the graph of  FIG. 24A . 
         FIG. 25A  is a graph of NRRO as a function of frequency for an exemplary actuator arm assembly in which a secondary controller includes peak filters according to the present invention and a damped rotary MEMS microactuator; and 
         FIG. 25B  is a graph of cumulative NRRO as a function of frequency corresponding to the graph of  FIG. 25A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a technique for significantly reducing the adverse effects of high-frequency actuator/arm modes by adding localized regions of high open-loop gain, or peak filters, to the control loop for a secondary actuator that are at frequencies that are near the frequencies of the high-frequency actuator/arm modes. Consequently, total off-track motion of a read/write head is reduced and higher track density for an HDD is enabled. 
       FIG. 12  is a schematic block diagram showing an exemplary actuator arm assembly  1201  and a simplified version of an exemplary control scheme  1206  according to the present invention. Actuator arm assembly  1201  includes a primary actuator  1202 , an actuator arm portion  1203 , a read/write head suspension portion  1204 , and a slider assembly  1205 . Similar to the actuator arm assembly shown in  FIG. 3 , actuator arm assembly  1201  is controlled by an exemplary control scheme  1206  that includes a control circuit  1207  that generates a signal  1208  that is output to a primary amplifier  1209  that, in turn, drives primary actuator  1202  to affect the position of slider assembly  1205  in relation to data on a disk (not shown). External disturbances  1215 , such as operational vibration and disk flutter, applied to actuator arm assembly  1201  also affect the position of slider assembly  1205  in relation to data on the disk. Primary actuator  1202  can be a rotary-type actuator or a linear-type actuator. Because rotary-type primary actuators are commonly used in HDDs, the following description of the present invention is directed toward rotary-type primary actuators. It should be understood, though, that the present invention is applicable to both rotary-type and linear-type primary actuators. When primary actuator  1202  is a rotary VCM, actuator arm assembly  1201  rotates (as indicated by arrows  1210 ) about a pivot  1211  under the force generated by primary actuator  1202 . Control circuit  1207  also generates a signal  1212  that is output to a secondary amplifier  1213  that, in turn, drives a secondary actuator (not shown in  FIG. 12 ). 
       FIG. 13  shows a functional block diagram of exemplary actuator arm assembly  1201  and exemplary control system  1206  according to the present invention. As shown in  FIG. 13 , control system  1206  includes at least two closed loops. A primary control loop includes a primary controller  1216 , primary amplifier  1209  and a primary actuator  1202 , such as a VCM. A secondary control loop includes a secondary controller  1217 , secondary amplifier  1213 , and a secondary actuator  1218 , such as a microactuator. Exemplary control system  1206  can potentially include two more closed control loops. For example, a secondary actuator motion signal  1219  that represents the motion of secondary actuator  1218  that is output from a motion sensor  1220  that may be included in slider assembly  1205  can be input to primary controller  1216  and/or secondary controller  1217 . Alternatively, when a suspension-mounted PZT milliactuator is used, motion sensor  1220  would be mounted on read/write head suspension  1204 . As yet another alternative, an estimated value of secondary actuator motion signal  1219  can be used. Such an estimated secondary actuator motion signal may be based on signals  1212 ,  1208  and  1222  and the models for primary actuator  1202 , primary amplifier  1209 , secondary actuator  1218 , and secondary amplifier  1213 . 
     A desired position signal  1221  of read/write heads (not shown) is compared to a signal  1222  representing the position of the read/write head to generate an error signal  1223 . Error signal  1223  is input into primary controller  1216  and/or secondary controller  1217 . Considering the primary control loop first, primary controller  1216  outputs signal  1208  in response to error signal  1223  (and in response to secondary actuator motion signal  1219  when motion sensor  1220  is used) that is input to primary amplifier  1209 . Primary amplifier  1209  outputs a drive signal  1224  that drives VCM  1202 . External disturbances  1215  are also applied to actuator arm assembly  1201  and VCM  1202 . An actuator arm assembly position signal  1225  representing the position of actuator arm assembly  1201  is generated as part of the primary control loop. Secondary controller  1217  outputs signal  1212  in response to error signal  1223  (and in response to secondary actuator motion signal  1219  when motion sensor  1220  is used) that is input to secondary amplifier  1213 . Secondary amplifier  1213  outputs a drive signal  1226  that drives secondary actuator  1218 . Secondary actuator position signal  1227  represents the relative position of secondary actuator  1218  and may be measured, estimated, or unknown. Secondary actuator position signal  1227  combines with signal  1225  representing the position of actuator arm assembly  1201  to form measured position signal  1222 . 
     The control bandwidth of secondary actuator  1218  is significantly higher than the control bandwidth of primary actuator  1202 , in addition to being higher than many of the resonant frequencies of actuator arm assembly  1201 . Peak filters at the resonant frequencies of the actuator arm assembly  1201  modes are generally stable when added to secondary actuator controller  1217  and do not suffer from the robustness issues that may occur if the peak filters were added to the primary actuator controller  1216 . Adding peak filters to secondary actuator controller  1217  enables secondary actuator  1218  to compensate for motion caused by the resonant modes of actuator arm assembly  1201 , regardless whether the resonant modes of actuator arm assembly are excited by the primary control loop or by external sources, such as air flow caused by disk motion and/or by spindle motion. The result is improved disturbance rejection and the ability to maintain the desired position of the read/write heads over the disk surface. According to the invention, the peak filters that included in the control loop for the secondary actuator can be embodied as active and/or passive components in the secondary actuator feedback loop and/or by digital signal processing (DSP) techniques. 
     The techniques of the present invention are applicable to rotary micro-electro-mechanical-system—(MEMS) type microactuators and other types of microactuators. Because rotary MEMS-type microactuators seem to have many advantages, the description of the present invention is directed toward rotary MEMS-type microactuators. It should be understood, though, that the techniques of the present invention are applicable to linear-type microactuators and to any type of secondary actuator that is located between the VCM and the read/write head, such as PZT-based secondary actuators and linear MEMS-type secondary actuators. There are at least two types of rotary MEMS microactuators for which the present invention is applicable. The first type is an undamped rotary MEMS microactuator. The second type is a damped (either actively and/or passively) MEMS rotary microactuator in which the first resonance has been damped (either actively and/or passively). An undamped rotary MEMS microactuator has a first mode at a relatively low frequency, typically between 2 kHz to 5 kHz. Above the first mode, there is a relatively wide frequency range, typically up to 80 kHz, in which there are no resonances. Both types of rotary MEMS microactuators will be described to illustrate the present invention. 
       FIGS. 14A and 14B  respectively show a magnitude and phase response as a function of frequency of secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . It should be understood that all of the graphs depicted herein are simulations that are based on data that is representative for the currently available generation of actuator arm assemblies for the enterprise class of hard disk drives. It should also be understood that graphs depicted herein could be based on data that is representative for other currently available classes of hard disk drives, such as desktop hard disk drives, mobile hard disk drives and consumer electronics hard disk drives. Moreover, it should also be understood that graphs depicted herein could be based on data that is representative for future classes of hard disk drives, such as desktop hard disk drives, mobile hard disk drives and consumer electronics hard disk drives. In  FIG. 14A , the first resonance mode of undamped microactuator  1218  is indicated at  1401 . There are no additional resonance modes until very high frequencies that are beyond the range of the graphs shown in  FIGS. 14A and 14B .  FIGS. 15A and 15B  respectively show a magnitude and phase response as a function of frequency for secondary amplifier  1213  and a damped rotary MEMS microactuator  1218 . The first resonance mode of the microactuator has been damped, as indicated at  1501 . 
       FIGS. 16A and 16B  respectively show a magnitude and phase response as a function of frequency of secondary microactuator controller  1217  having no peak filters. 
     As a comparative baseline,  FIGS. 17A and 17B  respectively show an open-loop magnitude and phase response as a function of frequency of secondary microactuator controller  1217  having no peak filters, secondary amplifier  1213  and undamped rotary MEMS microactuator  1218 . The first resonance peak of undamped MEMS microactuator  1218  is indicated at  1701 . 
       FIGS. 18A and 18B  respectively show a magnitude and phase response as a function of frequency of secondary controller  1217  having peak filters according to the present invention. The peak filters have frequencies corresponding to the frequencies of the higher-frequency primary actuator/arm modes, indicated as  502  in  FIG. 5A . The effect on the magnitude response of secondary controller  1217  caused by the peak filters can be observed at  1801 . 
     In contrast to  FIGS. 17A and 17B ,  FIGS. 19A and 19B  respectively show an open-loop magnitude and phase response as a function of frequency of secondary microactuator controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . The undamped resonance is indicated at  1901 . The increased open-loop gain at the higher frequency actuator/arm modes caused by the peak filters is indicated at  1902 . 
       FIGS. 20A and 20B  respectively show an open-loop magnitude and phase response as a function of frequency of secondary controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and a damped rotary MEMS microactuator  1218 . No resonance is visible in the response because the microactuator has been damped. The increased open-loop gain at the higher frequency actuator/arm modes caused by the peak filters is indicated at  2001 . 
     As another comparative base line,  FIGS. 21A and 21B  respectively show open-loop magnitude and phase responses as functions of frequency for primary controller  1216 , primary amplifier  1209 , and VCM  1202  and for secondary controller  1217  having no peak filters, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . Curve  2101  in  FIG. 21A  represents the open-loop magnitude response of primary controller  1216 , primary amplifier  1209 , and VCM  1202 . Curve  2102  in  FIG. 21B  represents the open-loop phase response of primary controller  1216 , primary amplifier  1209 , and VCM  1202 . Curve  2103  in  FIG. 21A  represents the open-loop magnitude response of secondary controller  1217  having no peak filters, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . Curve  2104  in  FIG. 21B  represents the open-loop phase response of secondary controller  1217  having no peak filters, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . Curve  2105  in  FIG. 21A  represents the combination of the open-loop magnitude response for primary controller  1216 , primary amplifier  1209 , and VCM  1202  and secondary controller  1217  having no peak filters, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . Curve  2106  in  FIG. 21B  represents the combination of the open-loop phase response for primary controller  1216 , primary amplifier  1209 , and VCM  1202  and secondary controller  1217  having no peak filters, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . 
       FIG. 21C  shows a magnitude of the error rejection frequency response. Curve  2107  is the closed-loop error rejection response for the primary control loop that includes primary controller  1216 , primary amplifier  1209 , and VCM  1202 . Curve  2108  is the closed-loop error rejection response for secondary controller  1217  having no peak filters, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . 
       FIGS. 22A and 22B  respectively show open-loop magnitude and phase responses as functions of frequency for primary controller  1216 , primary amplifier  1209 , and VCM  1202  and for secondary controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . Curve  2201  in  FIG. 22A  represents the open-loop magnitude response of primary controller  1216 , primary amplifier  1209 , and VCM  1202 . Curve  2202  in  FIG. 22B  represents the open-loop phase response of primary controller  1216 , primary amplifier  1209 , and VCM  1202 . Curve  2203  in  FIG. 22A  represents the open-loop magnitude response for secondary controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . Curve  2204  in  FIG. 22B  represents the open-loop phase response for secondary controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . The effect of the peak filters on the magnitude response can be seen at  2207 . Curve  2205  in  FIG. 22A  represents the combination of the open-loop magnitude response for primary controller  1216 , primary amplifier  1209 , and VCM  1202  and secondary controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . Curve  2206  in  FIG. 22B  represents the combination of the open-loop phase response for primary controller  1216 , primary amplifier  1209 , and VCM  1202  and secondary controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . 
       FIG. 22C  shows the magnitude of the error rejection frequency response. Curve  2208  is the closed-loop error rejection response for the primary control loop that includes primary controller  1216 , primary amplifier  1209 , and VCM  1202 . Curve  2209  is the closed-loop error rejection response for secondary controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and an undamped rotary MEMS microactuator  1218 . 
       FIGS. 23A and 23B  respectively show open-loop magnitude and phase responses as functions of frequency for primary controller  1216 , primary amplifier  1209 , and VCM  1202  and for secondary controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and a damped rotary MEMS microactuator  1218 . Curve  2301  in  FIG. 23A  represents the open-loop magnitude response of primary controller  1216 , primary amplifier  1209 , and VCM  1202 . Curve  2302  in  FIG. 23B  represents the phase response of primary controller  1216 , primary amplifier  1209 , and VCM  1202 . Curve  2303  in  FIG. 23A  represents the magnitude response of secondary controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and a damped rotary MEMS microactuator  1218 . Curve  2304  in  FIG. 23B  represents the phase response of secondary controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and a damped rotary MEMS microactuator  1218 . 
       FIG. 23C  shows a magnitude of the error rejection frequency response. Curve  2307  is the closed-loop error rejection response for the primary control loop that includes primary controller  1216 , primary amplifier  1209 , and VCM  1202 . Curve  2308  is the closed-loop error rejection response for secondary controller  1217  having peak filters according to the present invention, secondary amplifier  1213  and a damped rotary MEMS microactuator  1218 . 
       FIG. 24A  is a graph of NRRO as a function of frequency for exemplary actuator arm assembly  1201  in which secondary controller  1217  includes peak filters according to the present invention and secondary actuator  1218  is an undamped rotary MEMS microactuator.  FIG. 24B  is a graph of cumulative NRRO as a function of frequency corresponding to the graph of  FIG. 24A . The abscissa for both  FIGS. 24A and 24B  is the frequency, and the ordinate of both  FIGS. 24A and 24B  is NRRO. The effect of the present invention is evident in the reduced amplitude of the NRRO caused by high-frequency actuator and arm (actuator/arm) mode effects of actuator arm assembly  1201  indicated at  2403  compared to the amplitudes indicated at  403  in  FIG. 4A  for actuator arm assembly  301 . 
       FIG. 25A  is a graph of NRRO as a function of frequency for exemplary actuator arm assembly  1201  in which secondary controller  1217  includes peak filters according to the present invention and secondary actuator  1218  is a damped rotary MEMS microactuator.  FIG. 25B  is a graph of cumulative NRRO as a function of frequency corresponding to the graph of  FIG. 25A . The abscissa for both  FIGS. 25A and 24B  is the frequency, and the ordinate of both  FIGS. 24A and 24B  is NRRO. The effect of the present invention is evident in the reduced amplitude of the NRRO caused by high-frequency actuator and arm (actuator/arm) mode effects of actuator arm assembly  1201  indicated at  2503  compared to the amplitudes indicated at  403  in  FIG. 4A  for actuator arm assembly  301 . 
     A variety of control loop configurations have been suggested for dual-stage servo systems. In particular, active-damping techniques for the main microactuator resonance, often using the relative position error signal (RPES) of a read/write head, have been suggested. The techniques of the present invention of adding peak filters to the control loop for the secondary actuator may be applied to any of the suggested control loop configurations regardless of whether the relative position error signal is used. In that regard, the present invention can utilize feedback control signals that are based on velocity and/or acceleration. 
     While the present invention has been described as providing peak filters as part of the secondary controller, such as secondary controller  1217  in  FIG. 13 , it should be understood that the peak filters can be located other than within the secondary controller. For example, when the secondary controller receives a secondary actuator motion signal, such as signal  1219 , the secondary actuator motion signal can have been conditioned using peak filters before being input to the secondary controller. That is, the peak filters can be part of the transfer function for the feedback. 
     Further, while the present invention has been described in connection with NRRO, the present invention is applicable to repeatable runout (RRO). 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced that are within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.