Abstract:
A method and apparatus to safely repress disturbances from being applied to a system from the surroundings in which the system is used. The method of adaptively repressing disturbances includes estimating frequency components of a disturbance applied to a system in a user environment; and optimizing parameter values determining characteristics of a disturbance compensating servo control loop including a disturbance observer, based on the estimated frequency components of the disturbance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. §119(a) from Korean Patent Application No. 10-2006-0076725, filed on Aug. 14, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present general inventive concept relates to a disturbance compensation method and apparatus, and more particularly, to a method and apparatus to safely repress disturbances from being applied to a system from the surroundings in which the system is used. 
   2. Description of the Related Art 
   The present general inventive concept is related to inventions disclosed in Korean Patent Publication No. 2001-017826 and Japanese Patent Publication No. 1993-242509, which are hereby incorporated by reference. 
   Korean Patent Publication No. 2001-017826 relates to technology that measures the amount of disk unbalance by analyzing a vibration frequency of a hard disk drive, and Japanese Patent Publication No. 1993-242509 relates to technology that improves anti-disturbance characteristics of an optical disk drive. 
   A conventional hard disk drive is a data storage system that contributes to the operation of a computer system by reading data written on a disk through a magnetic head or writing data on the disk through the magnetic head. As hard disk drives increase in terms of capacity, density, and compactness, the density in a rotating direction of the disk of the hard disk drive (bits per inch (BPI)) and the density in a radial direction of the disk of the hard disk drive (tracks per inch (TPI)) have increased, and thus hard disk drives require finer operating controlling mechanisms. 
   The purpose of the controlling mechanisms is to follow the track of a hard disk drive by keeping the magnetic head in the center of a target track of the hard disk drive. However, many disturbances to the hard disk drive can cause a tracking error. Particularly, a microdrive of a portable device is especially susceptible to such disturbances such as a vibration. 
   Thus, when a hard disk drive experiences disturbances, the effects of the disturbances cause a position error signal to immediately appear, and reduce the read/write performance of the hard disk drive. Accordingly, the conventional hard disk drive is designed with a built-in controller to detect a characteristic of an applied disturbance and to compensate for the applied disturbance in order to reliably maintain the read/write performance when the hard disk drive is subjected to the disturbances. 
   However, external vibrations being applied to a hard disk drive often have different frequency ranges and phases depending on the conditions and circumstances. Even when a controller with a high gain is used to remove the effects of these external vibrations, the use of the controller compromises the stability of the entire system. 
   A conventional “Q filter” disturbance observer, which is used to remove the effects of the disturbances, has fixed frequency characteristics for stable system operation, but the “Q filter” disturbance observer is unable to effectively remove the effects of the disturbances applied to the system from the surroundings in which the system is used. 
   SUMMARY OF THE INVENTION 
   The present general inventive concept provides an adaptive disturbance repressing method and apparatus to minimize a decrease in system capacity caused by disturbances while providing system stability in various operating environments, and a disk drive apparatus using the method and apparatus. The present general inventive concept also provides a computer readable medium to store a program to execute the above method. 
   Additional aspects and utilities of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept. 
   The foregoing and/or other aspects and utilities of the present general inventive concept may be achieved by providing a method of adaptively repressing disturbances, including estimating one or more frequency components of a disturbance applied to a system in a user environment and optimizing one or more parameter values corresponding to one or more characteristics of a disturbance compensating servo control loop including a disturbance observer, based on the estimated one or more frequency components of the disturbance. 
   The estimating operation may also include detecting a servo signal that responds to a servo control of the system and is outputted, with the disturbance observer in a disabled state, Fast Fourier Transforming the detected servo signal and estimating a largest frequency component of the Fast Fourier Transformed detected servo signal to be a disturbance frequency component. 
   The optimizing operation may also include reading the one or more parameter values required to optimize a gain margin of a servo control system corresponding to the one or more frequency components estimated in the estimating operation and one or more disturbance repressing characteristics, from a look-up table and varying the one or more parameter values required to determine a transfer function of a disturbance compensating control loop from the one or more parameter values read in the reading operation. 
   The one or more parameter values may correspond to one or more characteristics of a disturbance sensitivity transfer function. 
   The parameter values may correspond to one or more characteristics of a variable filter included in the disturbance observer. 
   The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieve by providing an adaptive disturbance repressing apparatus including a disturbance observer to receive a control input signal to control a plant and a servo output signal of the plant corresponding to the control input signal, and to estimate a disturbance value applied to the plant as a variable disturbance sensitivity transfer function according to one or more parameter values of a variable filter disposed at a feedback of a control loop, a subtracter to subtract the disturbance value estimated by the disturbance observer from a control signal generated in the control loop, and to generate the control input signal to control the plant and a controller to estimate a frequency component of a disturbance applied to the plant in a user environment, and to vary the one or more parameter values of the variable filter based on the frequency component of the disturbance. 
   The disturbance observer may also include a nominal plant inverse modeling tool having response characteristics, to receive a servo output signal of the plant and output a sum of the control input signal of the plant and the disturbance value applied to the plant, a subtracter to subtract the control input signal of the plant from the output signal of the nominal plant inverse modeling tool and a variable filter to receive and to filter the subtracted output signal of the subtracter with one or more frequency characteristics corresponding to the varied one or more parameter values of the variable filter. 
   The one or more parameter values may correspond to a pole, zero, and a gain of the variable filter. 
   The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a disk drive apparatus including a disk to store data, a head to write data on the disk or to read the data from the disk, a servo controller to estimate head movement data values including a position value, a speed value, and a bias value of the head from a servo signal read by the head, and to generate a control signal based on the estimated head movement data values, a disturbance observer using a tool designed as an inverse of a function modeling a head driving system, to estimate a disturbance value applied to the head driving system from the servo signal through one or more disturbance sensitivity transfer function characteristics that vary according to one or more parameter values of a variable filter disposed at a feedback of a control loop, a subtracter to subtract the disturbance value estimated by the disturbance observer from the control servo signal, and to generate a disturbance compensating control signal, a VCM (voice coil motor) driver to generate a current to move the head according to the disturbance compensating control signal and a system controller to estimate a disturbance frequency component applied to the head driving system in a user environment, and to optimize the one or more parameter values of the variable filter based on the frequency component of the estimated disturbance applied to the head driving system in the user environment. 
   The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a computer readable medium containing computer readable codes to perform an adaptive disturbance repressing method, the method including estimating frequency components of a disturbance applied to a system in a user environment and optimizing parameter values corresponding to characteristics of a disturbance compensating servo control loop including a disturbance observer, based on the estimated frequency components of the disturbance. 
   The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a disturbance repressing apparatus, including a variable filter having a plurality of parameters and a controller coupled to the variable filter to analyze one or more disturbance frequency components of a disturbance and to vary at least one of the plurality of parameters of the variable filter based on the analyzed one or more disturbance frequency components to repress the disturbance. 
   The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a disk drive apparatus, including a disk to store data, a head to read the data from and to write the data to the disk, and a disturbance repressing apparatus, including a variable filter having a plurality of parameters and a controller coupled to the variable filter to analyze one or more disturbance frequency components of a disturbance and to vary at least one of the plurality of parameters of the variable filter based on the analyzed one or more disturbance frequency components in order to repress the disturbance and to maintain reliability of the read and write performance of the head. 
   The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a method of operating a disk drive apparatus, the method including writing data to a disk, reading data from the disk, analyzing one or more disturbance frequency components of a disturbance capable of reducing reliability of the writing operation and the reading operation and varying at least one of a plurality of parameters of a variable filter based on the analyzed disturbance frequency components in order to repress the disturbance and to maintain the reliability of the writing operation and the reading operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and/or other aspects and utilities of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
       FIG. 1  illustrates a plan view of a head disk assembly of a hard disk drive according to an embodiment of the present general inventive concept; 
       FIG. 2  is a block diagram illustrating an electronic circuit of a hard disk drive according to an embodiment of the present general inventive concept; 
       FIG. 3  is a block diagram illustrating an electronic circuit of a conventional disturbance repressing device used in a servo control system; 
       FIG. 4  is an equivalent circuit diagram of the electronic circuit of the conventional disturbance repressing device as illustrated in  FIG. 3 ; 
       FIG. 5  is a block diagram illustrating an electronic circuit of a disturbance repressing device, according to an embodiment of the present general inventive concept; 
       FIG. 6  is a flowchart illustrating a disturbance repressing method according to an embodiment of the present general inventive concept; 
       FIG. 7  is a graph illustrating characteristics of a sensitivity transfer function of a control loop when a conventional disturbance observer with a fixed low pass filter is used; 
       FIG. 8  is a graph illustrating characteristics of a disturbance sensitivity transfer function of a control loop when a conventional disturbance observer with a fixed low pass filter is used; 
       FIG. 9  is a graph illustrating exemplary transfer function characteristics of a variable filter used in a disturbance observer according to an embodiment of the present general inventive concept; 
       FIG. 10  is a graph illustrating characteristics of a sensitivity transfer function of a control loop when a disturbance observer with a variable filter according to an embodiment of the present general inventive concept is used; 
       FIG. 11  is a graph illustrating characteristics of a disturbance sensitivity transfer function of a control loop when a disturbance observer with a variable filter according to an embodiment of the present general inventive concept is used; and 
       FIG. 12  is a graph illustrating respective characteristics of disturbance sensitivity transfer function when parameters of a variable filter of a disturbance observer corresponding to different disturbance frequencies (200 Hz and 700 Hz) are optimized according to an embodiment of the present general inventive concept. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures. 
   A hard disk drive according to an embodiment of the present general inventive concept, such as a mechanical device, is composed of a head disk assembly (HDA)  10  coupled to an electronic circuit. 
     FIG. 1  is a plan view of the HDA  10  of a hard disk drive, according to an embodiment of the present general inventive concept. The HDA  10  includes at least one magnetic disk  12  that is rotated by a spindle motor  14 . The HDA  10  also includes a transducer  16  disposed proximate to a surface of the magnetic disk  12 . 
   The transducer  16  detects magnetic fields of each magnetic disk  12  and magnetizes each magnetic disk  12  to read or write data on the magnetic disk  12 . A conventional transducer is coupled to the surface of each magnetic disk. Although a single transducer is being described as the transducer  16 , the transducer  16  can also be a combination of a separate writing transducer to magnetize the magnetic disk  12  and a reading transducer that detects the magnetic field of the magnetic disk  12 . The reading transducer is a magneto-resistive (MR) device. Also, the transducer  16  can be called a head. 
   The transducer  16  may be combined with a slider  20 . The slider  20  is formed in a structure that acts as an air bearing between the transducer  16  and the surface of the magnetic disk  12 . The slider  20  is coupled to a head gimbal assembly  22 . The head gimbal assembly  22  is attached to an actuator arm  24  having a voice coil  26 . The voice coil  26  of a voice coil motor (VCM)  30  is proximally disposed to a magnetic assembly  28 . A current supplied to the voice coil  26  of the VCM generates a torque to turn the actuator arm  24  relative to the bearing assembly  32 . The rotation of the actuator arm  24  moves the transducer  16  across the surface of the magnetic disk  12 . 
   Data is stored in annular tracks  34  of the magnetic disk  12 . Each of the tracks  34  usually includes a plurality of sectors. Each of the sectors includes a data field and an identification field. The identification field in each of the sectors includes a gray code to identify each of the sectors and each of the tracks  34  (cylinder). The transducer  16  moves across the surface of the magnetic disk  12  to read or write data on other tracks  34  on the surface of the magnetic disk  12 . 
     FIG. 2  is a block diagram illustrating an electronic circuit  40  to control a hard disk drive, according to an embodiment of the present general inventive concept. The electronic circuit  40  includes a magnetic disk  12 , a head  16 , a controller  42 , a read/write (R/W) channel circuit  44 , a pre-amplifier  45 , a VCM drive unit  48 , a read only memory (ROM) device  50 , a random access memory (RAM) device  52 , and a host interface circuit  54 . 
   The ROM device  50  stores various commands and data used by the controller  42  to execute a software routine. The ROM device  50  also stores the programs to perform the adaptive disturbance repressing method according to an embodiment of the present general inventive concept as illustrated in the flowchart of  FIG. 6 . 
   The ROM device  50  also stores lookup table data that divides frequency bands of the disturbances into a plurality of sections, and sets parameter values to have optimum disturbance inhibiting characteristics for each of the frequency bands. In an embodiment of the present general inventive concept, the lookup table data may be stored on a system cylinder region of the magnetic disk  12 . The system cylinder region is a region on which data related to the hard disk drive is stored, that is, a region that cannot be accessed by a user, and can be called a maintenance cylinder region. A detailed description of the above lookup table will be given below. 
   The RAM device  52  stores data read from the ROM device  50  or magnetic disk  12  required to operate the hard disk drive. The RAM device  52  also stores data that is generated during the operation of the hard disk drive according to an embodiment of the present general inventive concept. 
   The controller  42  analyzes a command received from a host device (not Illustrated) through the host interface circuit  54 , and performs an operation according to the analysis results of the controller  42 . Then, the controller  42  supplies a control signal to the VCM drive unit  48  to control the VCM and the movement of the head  16 . 
   A general description of the operation of a disk drive will be provided below. 
   In data read mode, the disk drive amplifies an electrical signal from the magnetic disk  12  detected through the head  16  by the pre-amplifier  45 . Then, the R/W channel circuit  44  encodes the signal read from the magnetic disk  12  into a digital signal according to the timing of a read sector pulse generated by the controller  42 , and converts the digital signal to stream data and sends the stream data through the host interface circuit  54  to a host device (not illustrated). 
   In data write mode, the disk drive receives data from the host device through the host interface circuit  54 , the controller  42  adds error correcting parity symbols to the data, the data stored in a buffer is sequentially outputted and converted through the R/W channel circuit  44  to a binary data stream compatible with the write channels of the magnetic disk  12 . The head  16  then writes the binary data stream on the magnetic disk  12  in relation to a sector pulse using a write current amplified by the pre-amplifier  45 . 
   A description of a conventional disturbance repressing device will now be provided below. 
   Referring to  FIG. 3 , the disturbance repressing device includes subtracters  310 A,  310 B, and  310 C, a servo controller  320 , adders  330 A and  330 B, a plant  340 , a nominal plant inverse modeling tool  350 , and a Q filter  360 . The plant  340 , for example, may be a head driving system. 
   The conventional disturbance observer  1000  includes the nominal plant inverse modeling tool  350 , the Q filter  360 , the subtracter  310 C, and the adder  330 B. 
   Also, the adders  330 A and  330 B equivalently represent a disturbance (d) applied to the plant  340  and noise (n) existing in the plant  340 . 
   In a dynamic system such as a disk drive that includes a mechanically rotating part, disturbances are potentially overriding elements that usually reside in low frequency ranges. Accordingly, when designing a servo control system of a disk drive, a high gain of an open loop transfer function in a low frequency is commonly designed. The increase of the gain of the open loop transfer function in a low frequency is simply accomplished by increasing the gain of the entire servo control system. In such an increase in the gain of the entire servo control system, however, the margin of system stability is reduced, and errors occur. A controller, in addition to the main controller servo controller  320 , may be incorporated to compensate for the disturbances. The conventional disturbance observer  1000  is widely known to employ a method of increasing the gain of the open loop transfer function in a low frequency. 
   The important parameters to consider when designing the conventional disturbance observer  1000  are the model type of the servo control system and the Q filter  360  used to output signals. The Q filter  360  used to output signals is designed with noise reduction features and a non-casual characteristic according to a relative degree of the servo control system. 
   The basic principle of a disturbance observer is based on the theory that if the input applied to the servo control system, the actual output, and the plant are known, then a disturbance applied to the servo control system can be calculated. However, in actuality, the plant cannot be completely modeled, and noise cannot completely be removed, so that an error-free calculation of the disturbance on the system is unlikely. Moreover, an inverse of a transfer function in a nominal model of a conventional dynamic system becomes a non-causal system, so that in order to calculate the inverse transfer function in the nominal model, a so-called Q filter having a degree higher than a relative degree of a plant nominal model (a degree derived by subtracting a numerator degree from a denominator degree of the nominal model transfer function) must be used. 
   A conventional disturbance observer uses a low pass filter with a degree higher than a relative degree of a plant nominal model in order to avoid the above restrictions and more accurately observe the disturbances. 
   The disturbance d^ observed by the conventional disturbance observer  1000  in  FIG. 3  can be expressed by Equation 1 below. 
   
     
       
         
           
             
               
                 
                   
                     
                       
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   where, 
   G ud^  is a transfer function from a control input u to observed disturbance d^, 
   G dd^  is a transfer function from an actual inputted disturbance d to observed disturbance d^, and 
   G nd^  is a transfer function from measured noise n to observed disturbance d^. 
   Here, it is assumed that the plant nominal model is the same as the plant  340  of the disturbance observer  1000 . Also, in Equation 1, when Q=1, the result is the relation expressed in Equation 2 below:
 
G u{circumflex over (d)} =0, G d{circumflex over (d)} =1, G n{circumflex over (d)} =P n   −1   Equation 2
 
   Here, in Equation 1, when Q=0, the result is the relationship in Equation 3 below:
 
G u{circumflex over (d)} =0, G d{circumflex over (d)} =0, G n{circumflex over (d)} =0  Equation 3
 
   In a conventional dynamic system, measured noise n resides in a high frequency range, and mechanical disturbances reside in a low frequency range. Thus, when Q in Equations 2 and 3 is designed as a low pass filter, the measured noise n can be reduced while the mechanical disturbances are observed. 
   Moreover, in a conventional low frequency range in plant modeling and that accurate modeling is possible, the actual mechanical disturbances of the low frequency range can easily be observed through Equation 2. Accordingly, the low pass filter is used with a filter gain of 1 for the low frequency range, so that disturbances or mechanical disturbances in the low frequency range can be accurately observed and the measured noise n in the high frequency range can be reduced at the same time. 
   Likewise, a conventional disturbance observer is designed as an add-on type of system that is used to improve the performance of an existing main control loop. 
   If an existing main control loop has been designed to have a sufficiently high control gain in a low frequency range, a disturbance that is to be compensated for by the disturbance observer resides in a frequency range between low and high frequency ranges, and a fixed low pass filter is added to a frequency range appropriate to the disturbance observer, and thus an unnecessary increase arises in the control gain of the low frequency range. This substantially reduces the stability margin of the system, so that there is a limit imposed on the bandwidth range of a disturbance that can be compensated for by the disturbance observer. 
     FIG. 4  is an equivalent circuit diagram of the electronic circuit of the conventional disturbance repressing device as illustrated in  FIG. 3 . For example, the electronic circuit of  FIG. 4  includes subtracters  410 A and  410 B, adders  430 A and  410 C, a plant  440 , a Q filter  460 , and controllers  420  and  470 . The calculation of the inverse of the actual nominal model and the Q filter  360  in  FIG. 3  is performed with an additional controller, so that if the calculation of the inverse of the actual nominal model and the Q filter  360  are similarly reconfigured in  FIG. 4 , the calculation of the inverse of the actual nominal model and the Q filter  360  can be expressed by Equation 4 below. Here, the equivalent transfer function of the additional controller is expressed in Equation 4. 
   
     
       
         
           
             
               
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   where, 
   Pn is transfer function of a nominal plant model, 
   Q is transfer function of Q filter, and 
   C is transfer function of a controller. 
   When the Q filter  360  is designed as a conventional low pass filter and the frequency thereof is 0 Hz, the filter value is (1,0). In this case, the DC gain is infinity, so that the reduction characteristics of low frequency disturbances improve. However, a conventional feedback controller is designed from the perspective of a sensitivity transfer function to improve the reduction characteristics of disturbances in a low frequency range at the cost of not improving disturbances in a high frequency range and compromising the stability margin of the system. 
     FIG. 7  is a graph illustrating characteristics of a sensitivity transfer function of the conventional disturbance observer  1000  in an ON/OFF state with the Q filter  360  installed as a low frequency-pass filter, and  FIG. 8  is a graph illustrating the disturbance sensitivity transfer function of the conventional disturbance observer  1000  in an ON/OFF state with the Q filter  360  installed as a low pass filter. 
   Referring to  FIG. 7 , when a nominal controller sufficiently reduces low frequency disturbances and is coupled to the conventional disturbance observer  1000  with an additional low pass filter, the low frequency characteristic of the system is unnecessarily worsened by over inhibiting the low frequency disturbance characteristics and the stability margin is also highly decreased, so that the sensitivity transfer function of the conventional disturbance observer  1000  is substantially amplified to a frequency range of several KHz. 
   Referring to  FIG. 8 , when the disturbance sensitivity transfer function of the conventional disturbance observer  1000  unnecessarily limits the low frequency range, there is no significant improvement in a frequency range greater than 500 Hz. 
   To offset this unfavorable condition, the present general inventive concept uses a variable filter capable of varying the filtering characteristics of the variable filter in terms of a center frequency, bandwidth, gain, etc., according to user conditions of a disk drive, instead of a low pass filter with a fixed bandwidth using the Q filter  360  in the disturbance observer  1000 . 
     FIG. 5  is a block diagram illustrating an electronic circuit of an adaptive disturbance repressing apparatus, according to an embodiment of the present general inventive concept. 
   Referring to  FIG. 5 , the adaptive disturbance repressing apparatus according to an embodiment of the present general inventive concept includes subtracters  510 A,  510 B, and  510 C, a servo controller  520 , adders  530 A and  530 B, a plant  540 , a nominal plant inverse modeling tool  550 , a variable filter  560 , a system controller  570 , a ROM device  50 , and a RAM device  52 . The plant  540 , for example, may be a head driving system. 
   A disturbance observer  2000  includes the nominal plant inverse modeling tool  550 , the variable filter  560 , the adder  530 B, and the subtracter  510 C. Also, the servo controller  520  and the system controller  570  are controllers included in the controller  42  of  FIG. 2 . 
   Also, the adders  530 A and  530 B equivalently represent disturbance (d) applied to the plant  540  and noise (n) existing in the plant  540 . 
   The adaptive disturbance repressing apparatus according to an embodiment of the present general inventive concept, which is applied to a disk drive servo controlling system as an embodiment herein, is not limited thereto and may be applied to a variety of servo controlling systems. 
   When the present general inventive concept is applied to a servo controlling system of a disk drive, the plant  340  may be a head driving system. 
   The servo controller  520  receives as an input an error signal (e) according to a difference between a reference signal (r) of the system controller  570  and a servo output signal (y) of the plant  540 , calculates position, velocity, and bias values, and uses the calculated position, velocity, and bias values to calculate a control input signal (u). The error signal (e) in the disk drive may be a position error signal in an on-track mode. 
   The actual plant (P) characteristics of a conventional system cannot accurately be determined, so that a modeled plant (Pn) is modeled after a lower degree model for designing convenience. The difference between the modeled plant (Pn) and an actual plant (P) is usually largely concentrated in the higher frequency ranges, and the difference between the modeled plant (Pn) and the actual plant (P) in the lower frequency ranges is insignificant. Therefore, when it is assumed that the modeled plant (Pn) is the same as the actual plant (P) and measured noise (n) does not exist, a disturbance (d) may be obtained by calculating the difference between the product of the control input signal (u) and the product of the control output signal (y) and the inverse of the modeled plant (Pn −1 ). However, in a conventional physical system, because the degree of a denominator of a transfer function is greater than the degree of a numerator of the transfer function of the modeled plant (Pn), and in order to calculate the inverse of the modeled plant (Pn −1 ), the measured output from the plant must be differentiated by an amount equal to the degree of the denominator of the transfer function minus the degree of the numerator of the transfer function of the modeled plant (Pn). However, in an actual system, high frequency noise (n) exists, so that it is very unlikely to derive an inverse of the modeled plant (Pn −1 ) through the differential. To solve this problem, in an embodiment of the present general inventive concept, a variable filter  560  is added. The degree of the denominator minus the degree of the numerator of the transfer function of the variable filter  560  is designed to be greater than the degree of the denominator minus the degree of the numerator of the transfer function of the modeled plant (Pn), so that a differential can be avoided. When the variable filter  560  is designed as a band pass filter having a high degree, the system controller  570  is designed to variably adapt according to the user environment. 
   Using this method, the nominal plant inverse modeling tool  550  and the variable filter  560  are designed accordingly. 
     FIG. 9  illustrates frequency response characteristics of a band pass filter used as the variable filter  560  of  FIG. 5  according to an embodiment of the present general inventive concept. The disturbance observer  2000  is designed to not have an actual disturbance reflected as is by a calculated disturbance in a pass bandwidth through the variable filter  560 . Instead, the parameters of the variable filter  560  are varied so that all the servo characteristics including those of the disturbance observer  2000  can optimally reduce disturbances and retain a high degree of reliability. 
   The transfer function Q of the variable filter  560  can be expressed in Equation 5 below. 
   
     
       
         
           
             
               
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                         ⁢ 
                         
                             
                         
                         ⁢ 
                         ⋯ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           ( 
                           
                             z 
                             - 
                             
                               p 
                               n 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
             
             
               
                 Equation 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 5 
               
             
           
         
       
     
       
       
         
           where, g is gain of a Q filter, p 1 , . . . , p n  are values of pole points, and z 1 , . . . , z m  are values of zero points. 
         
       
     
  
   Here, a pole, zero, and gain of the variable filter  560  have bandwidth pass characteristics in a region largely effected by external disturbances, and are determined in order to increase stability. 
   The system controller  570  varies the parameters of the pole, zero, and gain of the variable filter  560  according to disturbance frequency components. 
   The parameter values of the pole, zero, and gain of the variable filter  560  are determined to have a sensitivity transfer function of a servo control loop and a disturbance sensitivity transfer function to effectively inhibit disturbances without reducing system stability according to each disturbance bandwidth and the determined parameter values store in the ROM device  50  or a system cylinder of the magnetic disk  12  as a lookup table type. The optimum parameter values of the pole, zero, and gain of the variable filter  560  to determine the characteristics of a variable filter  560  corresponding to disturbance frequencies frequency components may be derived during the system design process through testing. 
   The system controller  570  uses a lookup table with set parameter values to optimize the variable filter  560  according to frequency bandwidth, and varies the characteristics of the variable filter  560  as follows. 
   In an embodiment of the present general inventive concept, the system controller  570  analyzes the frequency components of a disturbance applied to the system. Provided are two methods of detecting the frequency components of the disturbance by the system controller  570 . 
   The first method is to disable the disturbance observer  2000  and transform servo signals outputted from the plant  540  using the Fast Fourier Transformation (FFT), and designate the largest frequency components from the FFT transformed servo signals as disturbance frequency components. Accordingly, when the plant  540  is the head driving system of a disk drive, the servo signals outputted from the plant  540  may be a position error signal, for example. 
   The second method is by setting the frequency bandwidth of the disturbance observer  2000  as a wide bandwidth, transforming disturbance signals detected by the disturbance observer  2000  using the FFT, and designating the largest frequency components from the transformed detected disturbance signals as disturbance frequency signals. 
   After the disturbance frequency components are analyzed by the system controller  570 , the parameter values to optimize the variable filter  560  corresponding to the analyzed disturbance frequency components are searched for in the lookup table. 
   The system controller  570  updates the parameter values to optimize the variable filter  560  set in the variable filter  560  of the disturbance observer  2000  with those found in the lookup table, so that the disturbance applied to the system can effectively be inhibited while not reducing the system&#39;s stability according to the environment in which the disk drive is used. 
     FIG. 6  is a flowchart illustrating an adaptive disturbance repressing method according to an embodiment of the present general inventive concept. 
   In operation  610 , a system with a disturbance observer determines whether the system has been changed into a condition to optimize the disturbance observer. A condition to optimize the disturbance observer may be set as the initial condition of the system. Also, the condition to optimize the disturbance observer may also be set during an idle state of the system. 
   When it is determined in operation S 610  that the system has been changed into the condition to optimize the disturbance observer, frequency components of the disturbances applied to the system are analyzed in operation S 620 . 
   In an embodiment of the present general inventive concept, the analysis of the disturbance frequency components involves a Fast Fourier Transforming of a servo signal that respond to a control signal and are outputted, with the disturbance observer of the system turned OFF, and designating the largest frequency components of the Fast Fourier transformed servo signal as disturbance frequency components. When the controlled system is a head driving system of a disk drive, the servo signal may be a position error signal in one instance. In another embodiment of the present general inventive concept, after setting the frequency bandwidth of the disturbance observer as a wide bandwidth, the disturbance signals detected by the disturbance observer are transformed using a FFT, and the largest frequency components from the transformed detected disturbance signals are designated as disturbance frequency signals. 
   In operation S 630 , parameter values to optimize the characteristics of the disturbance observer according to the disturbance frequency components detected in operation S 620  are set. Accordingly, the parameters to optimize the characteristics of the disturbance observer may specifically be parameters corresponding to the pole, zero, and gain of the transfer function of the variable filter included in the disturbance observer. These parameter values may be taken from a lookup table with parameter value settings to optimize the variable filter included in the disturbance observer according to the disturbance frequency bandwidths. 
   The parameter values to optimize the characteristics of the disturbance observer that were obtained in operation S 630  are applied to the disturbance observer in order to optimize the disturbance observer in operation S 640 . Accordingly, the obtained parameter values corresponding to the detected disturbance frequency components are used to update the parameters to determine the pole, zero, and gain of the variable filter included in the disturbance observer. 
   The characteristics of a variable filter included in a disturbance observer may thus be set to optimize the controlling of the system in accordance with the user environment. 
   For example, assuming that a large disturbance of 200-300 Hz caused by an external vibration is detected by the system controller  570 , the lookup table is used to determine varying parameter values of the pole, zero, and gain of the variable filter to obtain the transfer function characteristics of the variable filter as illustrated in  FIG. 9 , so that the characteristics of the sensitivity transfer function illustrated in  FIG. 10  and the characteristics of the disturbance sensitivity transfer function illustrated in  FIG. 11  can be derived. 
   Comparing  FIGS. 7 and 10 , the controller used in the present general inventive concept sufficiently maintains a reliability margin while not generating unnecessary amplification for the sensitivity transfer function. Also, when comparing  FIGS. 8 and 11 , the controller used in the present general inventive concept has a superior disturbance repressing performance in a several hundred-Hertz bandwidth. 
   Table 1 below compares the disturbance repressing performance and the reliability margin of a conventional fixed low pass filter of a disturbance observer and a variable filter of a disturbance observer of the present general inventive concept that varies in terms of pole, zero, and gain according to the user environment 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               DSF 
               GM 
               PM 
               SPL 
               SPH 
             
             
                 
               (dB) 
               (dB) 
               (deg) 
               (dB) 
               (dB) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               CONVENTIONAL 
               11.6 
               3.24 
               36.18 
               10.79 
               1.84 
             
             
               LOW PASS 
             
             
               VARIABLE FILTER 
               7.1 
               3.98 
               37.69 
               8.72 
               1.60 
             
             
                 
             
           
        
       
     
   
   In Table 1, DSF is the value of the disturbance sensitivity function at 300 Hz, GM is the stability margin, PM is the phase margin value, SPL is the maximum value of the sensitivity transfer function, and SPH is the maximum value of the sensitivity transfer function in a high frequency range higher than a first resonating frequency. 
   In a conventionally configured controller using a compensating device such as a deflector, mutual interference between two controllers can make the system unstable. However, in the present general inventive concept, only the range needing disturbance prevention is sufficiently compensated for, so that the problems arising from interference between the two controllers does not occur. 
   Modern disk drive systems are used not only in their traditional roles as auxiliary memory devices in home and office desktop computers, but as stacked multi-disk disk drives used in servo format memory devices such as network attached storage (NAS), digital video recorders (DVR), set top boxes, high definition televisions (HDTVs), and other auxiliary memory devices for AV machines. Such diverse user environments of disk drives necessitates that the specifications of the disk drives be suitable for the various requirements of consumers. Accordingly, the disturbance repressing characteristics capable of repressing strong vibration-inflicted disturbances having a wide bandwidth between several hundred hertz to 1 KHz may be required, for example, with NAS. In contrast, in a disk drive for a desktop computer or DVR or when a hard disk drive vibrates due to a power supplying device, the disturbance repressing characteristics capable of repressing such strong vibration-inflicted disturbances having a bandwidth below 300 Hz may be required. 
   However, repressing disturbances having bandwidths of different frequencies can be fulfilled because the present general inventive concept incorporates the Q filter as a variable bandwidth pass filter in the disturbance observer. 
     FIG. 12  is a graph illustrating respective characteristics of disturbance sensitivity transfer functions when parameters of variable filters of disturbance observers corresponding to two different disturbance frequencies are used, according to an embodiment of the present general inventive concept. The disturbances in the frequency ranges in  FIG. 12  can effectively be prevented. 
   The present general inventive concept can also be embodied as computer-readable codes on a computer-readable medium. The computer-readable medium may include a computer-readable recording medium and a computer-readable transmission medium. The computer-readable recording medium is any data storage device that can store data that can be thereafter read by a computer system. Examples of the computer-readable recording media include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet). The computer-readable recording medium can also be distributed over network-coupled computer systems so that the computer-readable code is stored and executed in a distributed fashion. Also, functional programs, codes, and code segments to accomplish the present general inventive concept can be easily construed by programmers skilled in the art to which the present general inventive concept pertains. 
   As described above, a filter in the disturbance observer according to several embodiments of the present general inventive concept is designed to adaptively vary according to the user environment, so that even when the user environment changes, the stability of the system is not compromised and the performance of the system can effectively be prevented from decreasing due to the disturbances. 
   Although a few embodiments of the present general inventive concept have been illustrated and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.