Patent Publication Number: US-8542458-B2

Title: Magnetic disk drive and method for controlling microactuator in magnetic disk drive

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2010-288831, filed Dec. 24, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a magnetic disk drive comprising a microactuator and a method for controlling the microactuator in the magnetic disk drive. 
     BACKGROUND 
     As is well known, a magnetic disk drive comprises a magnetic disk as a recording medium. The magnetic disk is hereinafter sometimes simply referred to as a disk. Servo data (servo patterns) is recorded, for example, on both surfaces of the disk. The servo data is used to position a head at a target position on the disk. 
     Recent magnetic disk drives have a function (what is called a self-servo writing function) in which the magnetic disk drive itself writes servo data to both surfaces of the disk. The self-servo writing is generally carried out as follows. First, it is assumed that the disk comprises a first surface and a second surface and that original servo data is already recorded on the first surface. It is also assumed that a first head and a second head are arranged in association with the first surface and the second surface, respectively. In this case, the first head is positioned at a target position on the first surface (that is, tracking is performed) based on the original servo data recorded on the first surface. That is, the first surface is used as a tracking surface. In this state, the first head and the second head simultaneously write the servo data to the first surface and the second surface, respectively. The second surface, which is not used for tracking, is hereinafter referred to as the non-tracking surface or the servo writing surface. Additionally, the self-servo writing, in which the servo data is written simultaneously to each of the opposite surfaces of the disk, is hereinafter referred to as servo writing using a bank write method. 
     Furthermore, in recent years, magnetic disk drives have emerged which comprise microactuators (micromotion actuators) associated with the respective surfaces of the disk. Compared to a primary actuator (coarse motion actuator, VCM actuator), each of the microactuators enables the corresponding head to make micromotion independently of the other head. Thus, in the magnetic disk drive comprising microactuators, a first operation amount is provided to a microactuator (first microactuator) associated with the first surface (tracking surface) of the disk. On the other hand, a second operation amount that is different from the first operation amount is provided to a microactuator (second microactuator) associated with the second surface (non-tracking surface) of the disk. 
     As described above, in the magnetic disk drive comprising the first and second microactuators, the first and second microactuators are driven independently. Thus, the first and second microactuators are provided independently with the first and second operation amounts, respectively. 
     Here, it is assumed that data is written simultaneously to both surfaces of the disk as is the case with the servo writing using the bank write method. In such a case, if an operation amount obtained by inverting the polarity of the first operation amount is used as the second operation amount, positioning errors reverse in phase between the first surface and the second surface of the disk can be inhibited. However, positioning errors the same in phase on between the first surface and the second surface of the disk increase on the second surface. Thus, there has been a demand to also reduce positioning errors on the second surface of the disk if the first surface of the disk is used as a tracking surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A general architecture that implements the various features of the embodiments will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate the embodiments and not to limit the scope of the invention. 
         FIG. 1  is a block diagram showing an exemplary configuration of a magnetic disk drive according to an embodiment; 
         FIG. 2  is a block diagram showing an exemplary configuration of a servo controller applied in the embodiment; and 
         FIG. 3  is a block diagram showing an exemplary configuration of a servo controller applied in a modification of the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments will be described hereinafter with reference to the accompanying drawings. 
     In general, according to one embodiment, a magnetic disk drive comprises a disk, a first microactuator, a second microactuator and a servo controller. The disk comprises a first surface and a second surface. The first microactuator is configured to allow a first head associated with the first surface to make micromotion based on a first operation amount. The second microactuator is configured to allow a second head associated with the second surface to make micromotion based on a second operation amount. The servo controller is configured to provide a third operation amount to the first microactuator as the first operation amount and to provide an operation amount obtained by inverting a polarity of a frequency component with a particular frequency contained in the third operation amount, to the second microactuator as the second operation amount, if the third operation amount is generated as an operation amount to be provided to the first microactuator in a particular mode in which the first surface is used as a tracking surface. 
       FIG. 1  is a block diagram showing an exemplary configuration of a magnetic disk drive according to an embodiment. A magnetic disk drive (hereinafter referred to as an HDD) shown in  FIG. 1  comprises a disk (magnetic disk)  11 , heads (magnetic head)  12 - 1  and  12 - 2 , a spindle motor (SPM)  13 , a main actuator  14 , microactuators  15 - 1  and  15 - 2 , a voice coil motor (VCM)  16 , a driver IC  17 , a head IC  18 , and a main controller  19 . 
     The disk  11  is a magnetic recording medium comprising an upper disk surface (first surface)  11 - 1  and a lower disk surface (second surface)  11 - 2 . Disk surfaces  11 - 1  and  11 - 2  form recording surfaces on which data is magnetically recorded. The disk  11  is rotated at high speed by the SPM  13 . The SPM  13  is driven by a drive current (or a drive voltage) supplied by the driver IC  17 . 
     Head (first head)  12 - 1  is arranged in association with disk surface  11 - 1  of the disk  11 . Head (second head)  12 - 2  is arranged in association with disk surface  11 - 2  of the disk  11 . That is, heads  12 - 1  and  12 - 2  are associated with disk surfaces  11 - 1  and  11 - 2 , respectively. Each of heads  12 - 1  and  12 - 2  comprises a read element and a write element (not shown in the drawings). Head  12 - 1  is used to write data to disk surface  11 - 1  of the disk  11  and to read data from disk surface  11 - 1 . Head  12 - 2  is used to write data to disk surface  11 - 2  of the disk  11  and to read data from disk surface  11 - 2 . In the configuration in  FIG. 2 , the HDD is assumed to comprise the single disk  11 . However, a plurality of the disks  11  may be arranged in the HDD in a stacked manner. 
     The main actuator  14  comprises arms  140 - 1  and  140 - 2  associated with disk surfaces  11 - 1  and  11 - 2  of the disk  11 . Heads  12 - 1  and  12 - 2  are attached to the tips of suspensions  141 - 1  and  141 - 2  (more specifically, head sliders provided at the tips of suspensions  141 - 1  and  141 - 2 ) extending from arms  140 - 1  and  140 - 2 , respectively, of the main actuator  14 . 
     Microactuator (first microactuator)  15 - 1  is attached to suspension  141 - 1  (more specifically, to between suspension  141 - 1  and a head slider) in proximity to head  12 - 1 . Similarly, microactuator (second microactuator)  15 - 2  is attached to suspension  141 - 2  in proximity to head  12 - 2 . Microactuators  15 - 1  and  15 - 2  are independently driven based on operation amounts u MA1  and u MA2  (more specifically, for example, drive voltages designated by operation amounts u MA1  and u MA2 , respectively) provided by the main controller  19  via the driver IC  17 . Thus, microactuators  15 - 1  and  15 - 2  allow the corresponding heads  12 - 1  and  12 - 2  to make micromotion. 
     The main actuator  14  is supported so as to move pivotally around a pivotal axis  142 . The main actuator  14  comprises the VCM  16 . The VCM  16  is a drive source for the main actuator  14 . The VCM  16  is driven based on an operation amount u VCM  (more specifically, for example, a drive current designated by operation amount u VCM ) provided by the main controller  19  via the driver IC  17  to allow the main actuator  14  to move pivotally around the pivotal axis  142 . That is, the VCM  16  simultaneously moves arms  140 - 1  and  140 - 2  of the main actuator  14  in the radial direction of disk surfaces  11 - 1  and  11 - 2  of the disk  11 . Thus, heads  12 - 1  and  12 - 2  are also moved in the radial direction of disk surfaces  11 - 1  and  11 - 2  of the disk  11 . 
     The driver IC  17  drives the SPM  13 , VCM  16  (that is, the main actuator  14 ), and microactuators  15 - 1  and  15 - 2  under the control of the main controller  19 . The head IC  18  amplifies a signal (read signal) read by head  12 - 1  or  12 - 2 . The head IC  18  also converts write data transferred by the main controller  19  into a write current and outputs the write current to head  12 - 1  or  12 - 2 . 
     The main controller  19  is implemented by, for example, a system LSI where a plurality of elements including a microprocessor unit (MPU) and memories are integrated into a single chip. The main controller  19  controls the SPM  13  via the driver IC  17  in order to rotate the disk  11  at high speed. 
     The main controller  19  also functions a disk controller. The main controller  19  transmits and receives signals to and from a host. Specifically, the main controller  19  receives commands (write commands, read commands, and the like) transferred by the host via a host interface  200 . The main controller  19  also controls the data transfer between the host and the main controller  19 . The main controller  19  further controls the data transfer between the disk  11  and the main controller  19 . 
     The main controller  19  also functions as a read/write channel. The main controller  19  converts a read signal output by the head IC  18 , into digital data. The main controller  19  then decodes read data from the digital data. The main controller  19  extracts servo data required to position head  12 - 1  or  12 - 2 , from the digital data. The main controller  19  also encodes write data. 
     The main controller  19  comprises a servo controller  20 . The servo controller  20  controls the VCM  16  via the driver IC  17  in order to position heads  12 - 1  and  12 - 2  at target positions on disk surfaces  11 - 1  and  11 - 2 , respectively, of the disk  11 . Here, controlling the VCM  16  is equivalent to controlling the main actuator  14  comprising the VCM  16 . Thus, the main actuator  14  is hereinafter referred to as the VCM actuator  14 . It is assumed that the servo controller  20  controls the VCM actuator  14 . The main controller  19  further controls microactuators  15 - 1  and  15 - 2  independently via the diver IC  17  in order to fine-tune heads  12 - 1  and  12 - 2 , respectively. 
       FIG. 2  is a block diagram showing an exemplary configuration of the servo controller  20  applied in the embodiment. The servo controller  20  shown in  FIG. 2  has a configuration compatible with a particular mode in which disk surface (first surface)  11 - 1  of the disk  11  is used as a tracking surface. More specifically, the servo controller  20  has a configuration compatible with the servo writing using the bank write method, in which servo data is written simultaneously to disk surface (first surface)  11 - 1  and disk surface (second surface)  11 - 2  of the disk  11 . Here, it is assumed that such original servo data as described in, for example, Japanese Patent No. 4227111 is already recorded on disk surface  11 - 1 . 
     Based on the original servo data recorded on disk surface  11 - 1 , the servo controller  20  controls the VCM actuator  14  and microactuators  15 - 1  and  15 - 2  in order to position heads  12 - 1  and  12 - 2  at the target positions. For this control, the servo controller  20  forms what is called a dual stage actuator following control system. That is, the servo controller  20  uses disk surface  11 - 1  as a tracking surface to control the VCM actuator  14 , thus coarsely adjusting the positions of heads  12 - 1  and  12 - 2 . The servo controller  20  also controls microactuators  15 - 1  and  15 - 2  individually to finely adjust the positions of heads  12 - 1  and  12 - 2 . 
     Thus, in the dual stage actuator following control system, the VCM actuator  14  and microactuators  15 - 1  and  15 - 2  are to be controlled. Thus, in  FIG. 2 , the VCM actuator  14  is denoted as P VCM , and microactuators  15 - 1  and  15 - 2  are denoted as P MA1  and P MA2 . The servo controller  20  comprises a subtractor  22 , a microactuator controller (C MA )  23 , a filter unit  24 , a microactuator model (M MA )  25 , an adder  26 , and a VCM actuator controller (C VCM )  27 . 
     In  FIG. 2 , a symbol y at an addition point  21  indicates the position of head  12 - 1  (head position) corresponding to the tracking surface. Here, displacement of of microactuator (P MA1 )  15 - 1  corresponding to the tracking surface is denoted by y MA . In this case, the sum (y VCM +y MA ) of displacement y VCM  and displacement y MA  is observed as the head position y. The subtractor  22  calculates the difference of the head position y from the target position r to be a deviation e (=r−y). Based on the deviation e, the microactuator controller  23  generates an operation amount (third operation amount) u MA  to be provided to microactuator  15 - 1 . 
     The filter unit  24  outputs operation amount (third operation amount) u MA  without any change as an operation amount (first operation amount) u MA1 . The filter unit  24  also outputs, as an operation amount (second operation amount) u MA2 , an operation amount obtained by inverting the polarity of a frequency component with a particular frequency contained in operation amount (third operation amount) u MA . Operation amounts u MA1  and u MA2  are used to drive microactuators  15 - 1  and  15 - 2 , respectively. The filter unit  24  comprises a band elimination filter (F)  241 , a bandpass filter (1−F)  242 , an adder  243 , and an adder  244 . 
     The band elimination filter (F)  241  eliminates a frequency component with a particular frequency from operation amount u MA . An operation amount (fourth operation amount) obtained by eliminating the frequency component with the particular frequency from operation amount u MA  is hereinafter referred to as operation amount F·u MA . The bandpass filter (1−F)  242  allows passage of a frequency component with a particular frequency contained in operation amount u MA . An operation amount (fifth operation amount) obtained by allowing the passage of the frequency component with the particular frequency contained in operation amount u MA  is hereinafter referred to as operation amount (1−F)·u MA . 
     The adder (first adder)  243  adds operation amount F·u MA  output by the band elimination filter  241  to operation amount (1−F)·u MA  output by the bandpass filter  242 . The addition result from the adder  243  is used as u MA1 . The adder (second adder)  244  adds operation amount F·u MA  to an operation amount −(1−F)·u MA  obtained by inverting the polarity of operation amount (1−F)·u MA . The addition result from the adder  244  is used as u MA2 . Instead of the adder  244 , a subtractor may be used which subtracts operation amount (1−F)·u MA  from operation amount F·u MA . 
     Based on operation amount u MA1  provided to microactuator  15 - 1  by the filter unit  24 , the microactuator model  25  obtains the displacement of microactuator  15 - 1 . The adder  26  adds the displacement obtained by the microactuator model  25  to the deviation e calculated by the subtractor  22 . The VCM actuator controller  27  generates an operation amount u VCM  to be provided to the VCM actuator  14  based on an output from the adder  26 . Thus, in the dual stage actuator following control system, the displacement obtained by the microactuator model (M MA )  25  is added to the deviation e. The addition result is input to the VCM actuator controller (C VCM )  27 . That is, the dual stage actuator following control system forms a non-interference control system. 
     Now, the operation of the embodiment will be described taking servo writing using the bank write method, as an example. In the embodiment, it is assumed that the servo writing using the bank write method is carried out in the particular mode in which disk surface  11 - 1  of the disk  11  is used as a tracking surface. Here, in particular, generation of operation amounts u MA1  and u MA2  provided to microactuators  15 - 1  and  15 - 2 , respectively, will be described. In the servo writing using the bank write method, operation amounts u MA1  and u MA2  are used to accurately position heads  12 - 1  and  12 - 2  at the target positions on disk surfaces  11 - 1  and  11 - 2 , respectively, of the disk  11 . 
     First, the microactuator controller  23  generates an operation amount u MA  to be provided to microactuator  15 - 1 , based on the deviation e. Microactuator  15 - 1  corresponds to disk surface  11 - 1  of the disk  11 , which is used as the tracking surface. Operation amount u MA  contains a frequency component with a particular frequency which inhibits positioning errors caused by disk flutter (disk flutter disturbance) when head  12 - 1  corresponding to disk surface  11 - 1  (tracking surface) is positioned at the target position. Thus, the particular frequency is almost equal to the frequency of the positioning errors caused by the disk flutter (this frequency is hereinafter referred to as the flutter component of the positioning errors). 
     The flutter component of the positioning errors depends on the resonance characteristics of the disk, the rotation speed of the disk  11 , and the like. The resonance characteristics of the disk  11  depend on the material, size, and holding condition (the manner of holding the disk) of the disk  11 . In common HDDs such as 2.5-inch HDDs and 3.5-inch HDDs, the flutter component of the positioning errors is distributed, for example, between one and several kilohertz. Furthermore, within the frequency range between one and several kilohertz, a low-frequency component with a frequency of, for example, between 1 and 2 kHz is larger than the other frequency components. 
     Thus, in the embodiment, operation amount u MA  generated by the microactuator controller  23  contains, for example, a frequency component with a frequency of between 1 and 2 kHz which significantly impacts disk positioning and which corresponds to the frequency component with the particular frequency inhibiting positioning errors on the tracking surface caused by disk flutter. Here, the positioning errors caused by the disk flutter are reverse in phase between disk surface  11 - 1  (tracking surface) and disk surface  11 - 2  (non-tracking surface) of the disk  11  as also described in Japanese Patent No. 4227111. 
     Furthermore, operation amount u MA  also contains a frequency component with a frequency which is different from the above-described particular frequency and which inhibits positioning errors caused by disturbance other than the disk flutter. The runout of the disk  11  (disk runout) and a fluid originated force are known as the disturbance other than the disk flutter. The fluid originated force is a force exerted, on a control target, by a flow of air generated in conjunction with rotation of the disk  11 . For the positioning errors caused by the disturbance other than the disk flutter, the frequency component of positioning errors caused by disturbance significantly impacting head positioning is different from that with the above-described particular frequency. For example, the frequency component of positioning errors caused by the disk runout or the fluid originated force is lower than 1 kHz. The positioning errors caused by the disturbance such as the disk runout or the fluid originated force, which is different from the disk flutter and which significantly impacts the head positioning, are the same in phase between disk surface  11 - 1  and disk surface  11 - 2  of the disk  11 . 
     Operation amount u MA  generated by the microactuator controller  23  is input to each of the band elimination filter (F)  241  and bandpass filter (1−F)  242  of the filter unit  24 . 
     The band elimination filter  241  eliminates the frequency component with the above-described particular frequency (in the embodiment, between 1 and 2 kHz) from operation amount u MA . That is, the band elimination filter  241  allows the frequency components of operation amount u MA  other than that with the particular frequency to pass through. Thus, the band elimination filter  241  outputs operation amount F·u MA . 
     On the other hand, the bandpass filter  242  allows a frequency component with a particular frequency contained in operation amount u MA  to pass through. That is, the bandpass filter  242  eliminates the frequency components other than that with the particular frequency from operation amount u MA . Thus, the bandpass filter  242  outputs operation amount (1−F) u MA . 
     The adder  243  adds operation amount F·u MA  and operation amount (1−F)·u MA  together. The adder  243  then outputs the addition result as operation amount u MA1 . Operation amount u MA1  is expressed by: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     Operation amount u MA1  output by the adder  243  (=operation amount u MA ) is provided to microactuator (P MA1 )  15 - 1 . On the other hand, the adder  244  adds operation amount F·u MA  to operation amount −(1−F)·u MA , obtained by inverting the polarity of operation amount (1−F)·u MA . The adder  244  then outputs the addition result as operation amount u MA2 . Operation amount u MA2  is expressed by:
 
 u   MA2   =F·u   MA −(1− F )· u   MA   (2)
 
     Operation amount u MA2  output by the adder  244  is provided to microactuator (P MA2 )  15 - 2 . As is apparent from the above description, operation amount u MA1  output to microactuator (P MA1 )  15 - 1  by the adder  243  is equal to operation amount u MA  output by the microactuator controller (C MA )  23 . On the other hand, operation amount u MA2  output to microactuator (P MA2 )  15 - 2  by the adder  244  corresponds to a component obtained by adding (superimposing) component F·u MA  passing through the band elimination filter (F)  241  to component −(1−F)·u MA , obtained by inverting the polarity of component (1−F)·u MA  passing through the bandpass filter (1−F)  242 . 
     As is apparent from the above description, the band elimination filter (F)  241  and bandpass filter (1−F)  242  assumed in the embodiments are characterized as follows. 
     (1) The frequency component allowed to pass through the band elimination filter (F)  241  inhibits the positioning errors which are the same in phase between disk surface  11 - 1  (tracking surface) and disk surface  11 - 2  (non-tracking surface) of the disk  11 . Thus, condition 1 needs to be met in order to inhibit the positioning errors having the same phase on disk surface  11 - 2 . Condition 1 is such that operation amount u MA2  contains the frequency component allowed to pass through the band elimination filter (F)  241  with the polarity of the component not inverted. 
     (2) The frequency component (with the particular frequency) allowed to pass through the bandpass filter (F)  242  inhibits the positioning errors reverse in phase between disk surface  11 - 1  and disk surface  11 - 2  of the disk  11  (more specifically, the positioning errors on disk surface  11 - 1 ). Thus, condition 2 needs to be met in order to inhibit the positioning errors having the reverse phase on disk surface  11 - 2 . Condition 2 is such that operation amount u MA2  contains the frequency component allowed to pass through the bandpass filter (1−F)  242  with the polarity of the component inverted. 
     In the embodiment, the frequency of the frequency component eliminated by the band elimination filter (F)  241  and the frequency of the frequency component allowed to pass through the bandpass filter (1−F)  242  are both set to the value of the frequency of the positioning errors caused by the disk flutter. Thus, operation amount u MA2 , that is, operation amount u MA2  provided to microactuator  15 - 1  by the adder  244 , meets conditions 1 and 2, as is also apparent from Equation (2) described above. That is, in the embodiment, the servo controller  20  can generate operation amount u MA2  characterized as follows. First, operation amount u MA2  contains the frequency component inhibiting the positioning errors which are caused by the disturbance other than the disk flutter and which are the same in phase between disk surface  11 - 1  and disk surface  11 - 2  of the disk  11  (more specifically, the positioning errors on disk surface  11 - 2 ). Second, operation amount u MA2  contains the frequency component inhibiting the positioning errors which are caused by the disk flutter and which are reverse in phase between disk surface  11 - 1  and disk surface  11 - 2  of the disk  11  (more specifically, the positioning errors on disk surface  11 - 2 ). 
     As described above, operation amount u MA2  is provided to microactuator (P MA2 )  15 - 2 . On the other hand, operation amount u MA1  is provided to microactuator (P MA1 )  15 - 1 . That is, the servo controller  20  controls microactuator (P MA1 )  15 - 1  based on operation amount u MA1 , and simultaneously controls microactuator (P MA2 )  15 - 2  based on operation amount u MA2 . Thus, over disk surfaces  11 - 1  and  11 - 2  of the disk  11 , heads  12 - 1  and  12 - 2 , respectively, can be accurately positioned at the target positions with the same relative position. 
     In the embodiment, generation of operation amounts u MA1  and u MA2  is applied to the servo writing using the bank write method described in Japanese Patent No. 4227111. In the servo writing using the bank write method, disk surface  11 - 1  of the disk  11  is assumed to be used as a tracking surface (more specifically, both a tracking surface and a servo writing surface). On the other hand, disk surface  11 - 2  of the disk  11  is assumed to be used as a servo writing surface (more specifically, both a non-tracking surface and a servo writing surface). This enables an effective reduction not only in displacement of head  12 - 1  over disk surface  11 - 1  (tracking surface) but also in displacement of head  12 - 2  over disk surface  11 - 2  (servo writing surface). Therefore, the embodiment can contribute to improving servo writing quality. 
     If unlike in the case of the embodiment, the operation amount obtained simply by inverting the polarity of operation amount u MA  is used as operation amount u MA2 , condition 2 described above is met, but condition 1 described above is not met. In this case, the positioning errors reverse in phase between disk surface  11 - 1  and disk surface  11 - 2  of the disk  11  are inhibited on both disk surfaces  11 - 1  and  11 - 2 . However, the positioning errors the same in phase between disk surface  11 - 1  and disk surface  11 - 2  of the disk  11  increase on disk surface  11 - 2 . That is, the displacement of head  12 - 2  increases on disk surface  11 - 2  (servo writing surface). 
     [Modification] 
     Now, a modification of the embodiment will be described with reference to the drawings. The modification is characterized in that the configuration of the servo controller  20  is simplified. First, Equation (2) described above can be expressed by:
 
 u   MA2 =(2 F− 1)· u   MA   (3)
 
     Here, if G=2F−1, Equation (3) can be expressed by:
 
 u   MA2   =G·u   MA   (4)
 
     Equation (4) indicates that generation of operation amount u MA2  based on operation amount u MA  can be achieved by a single filter. On the other hand, Equation (1) indicates that operation amount u MA  can be used as operation amount u MA1  without any change. In the modification of the embodiment, a filter unit for the servo controller  20  is configured in view of the above-described points. 
       FIG. 3  is a block diagram showing an exemplary configuration of the servo controller applied in the modification of the embodiment. Elements in  FIG. 3  which are equivalent to corresponding ones in  FIG. 2  are denoted by the same reference numerals. The servo controller  20  shown in  FIG. 3  is different from that shown in  FIG. 2  in that a filter unit  28  is used instead of the filter unit  24 . The filter unit  28  comprises filters  281  and  282 . 
     The filter  281  allows operation amount u MA  generated by the microactuator controller  23  to pass through without any change as operation amount u MA1 . As is apparent from Equation (1) described above, the relationship between operation amounts u MA  and u MA1  is such that u MA =1·u MA1 =u MA1 . The filter  281  is a virtual filter introduced in order to make the relationship between operation amounts u MA  and u MA1  easily understood. The filter  281  is thus physically unnecessary. On the other hand, the filter  282  outputs the operation amount obtained by inverting the polarity of the frequency component with the particular frequency contained in operation amount u MA , as operation amount u MA2 . That is, the filter  282  allows the frequency components of operation amount u MA  other than the one with the particular frequency to pass through. As is the case with the embodiment, the servo controller  20  configured as shown in  FIG. 3  can generate operation amount u MA2  that meets conditions 1 and 2 described above. Furthermore, the configuration of the filter unit can be simplified. 
     At least one of the embodiments can provide a magnetic disk drive with a microactuator in which if a first surface of the disk is used as a tracking surface, positioning errors on the second surface of the disk can also be reduced, as well as a method for controlling the microactuator in the magnetic disk drive. 
     The various modules of the systems described herein can be implemented as software applications, hardware and/or software modules, or components on one or more computers, such as servers. While the various modules are illustrated separately, they may share some or all of the same underlying logic or code. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.