Patent Publication Number: US-7595953-B1

Title: Magnetic recording disk drive with switchable compensation for mechanical and electrical disturbances

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to magnetic recording disk drives, and more particularly to a disk drive that includes a head-positioning servo control system that has compensation for generally high-frequency mechanical and electrical disturbances. 
     2. Description of the Related Art 
     Magnetic recording hard disk drives use an actuator, typically a rotary voice-coil-motor (VCM) actuator, for positioning the read/write heads on the data tracks of the recording disks. The disk drive has a servo control system that receives a position error signal (PES) from servo positioning information read by the heads from the data tracks and generates a VCM control signal to maintain the heads on track and move them to the desired track for reading and writing of data. 
     The servo control system is designed for optimal response to generally low-frequency mechanical disturbances. However, disk drives may experience high-frequency mechanical and electrical disturbances to which the servo control system cannot adequately respond. Also, the servo control system may detect such a high-frequency mechanical disturbance (MD) or electrical disturbance (ED) and respond as if it were a low-frequency disturbance, a problem referred to as “aliasing”. 
     A typical MD that occurs at a frequency outside the design range of the servo control system is rotational vibration (RV). RV may arise internally, such as from motion of the VCM actuator, or externally, such as from shocks to the frame supporting the disk drive or from the movement of other disk drives when the drives are mounted together in a disk array system. RV compensation is a method that uses sensors (typically accelerometers) to detect RV and improve the PES by canceling the off-track motion induced by the RV. The RV sensor signal is input to a RV feedforward controller that creates a RV feedforward compensation signal that is summed with the control signal to the VCM actuator. The use of a RV sensor and feedforward compensation is described by Jinzenji et al., “Acceleration Feedforward Control Against Rotational Disturbance in Hard Disk Drives,”  IEEE Transactions on Magnetics , Vol. 37, No. 2, March 2001, pp. 888-893; and M. T. White et al., “Increased Disturbance Rejection in Magnetic Disk Drives by Acceleration Feedforward Control,”  Proceedings of the  13 th Triennial IFAC World Congress, Jun.  30-Jul. 5, 1996, San Francisco, Calif., pp. 489-494. 
     However, the RV sensor output may include noise not related to RV. The source of noise may be electrical or sensor-related, such as non-rotational vibrations detected by the sensor&#39;s cross-axis sensitivity, and/or spurious signals generated as a result of physical distortion of the sensor itself. Thus if the RV compensation is enabled when the disk drive is not being subjected to RV disturbances, the servo control system performance may be degraded. 
     A typical ED that occurs at a frequency outside the design range of the servo control system is noise in the power supply voltage applied to the VCM driver that sends control current to the VCM actuator. This noise, typically a ripple voltage of the nominal power supply voltage, will cause the VCM driver to generate a control current with noise, resulting in undesirable mechanical movement of the VCM actuator. Pending application Ser. No. 12/036,478, filed Feb. 25, 2008 and assigned to the same assignee of this application, describes a disk drive with ED feedforward compensation for an ED to the VCM driver from power supply voltage noise. However, if this ED compensation is enabled when the disk drive is not subjected to noise from the power supply voltage, or the signal is an aliased version of an existing high-frequency noise due to the lack of a pre-designed or correct anti-aliasing filter, the servo control system performance may be degraded. 
     An additional problem has been discovered if both an MD in the form of RV, and an ED in the form of power supply voltage noise, are present simultaneously. Specifically, the movement of the VCM actuator and/or direct coupling into the RV sensor system induced by power supply voltage noise can cause the RV sensor to erroneously sense RV. This causes the RV feedforward compensation to inject noise into the servo control system. 
     What is needed is a disk drive that has compensation for both mechanical and electrical high-frequency disturbances outside the design range of the servo control system, but wherein either or both of the MD and ED compensations can be disabled when not needed. 
     SUMMARY OF THE INVENTION 
     The invention relates to a magnetic recording disk drive that has compensation for both mechanical disturbances and electrical disturbances. The disk drive&#39;s servo control system includes mechanical disturbance (MD) feedforward compensation and electrical disturbance (ED) feedforward compensation, of which either or both can be selectively enabled and disabled. The MD may be a rotational vibration (RV) disturbance, in which case the disk drive will include an RV sensor and RV feedforward compensation. 
     In the operation of the disk drive, it is first determined that there is some disturbance that is causing track misregistration (TMR). The TMR can be tested by calculating the frequency of recent write-inhibits, as compared against a write-inhibit frequency (WIF) threshold. The WIF threshold may be a predetermined number of write-inhibit flags per write commands. The system is then tested to determine if the ED is greater than an ED threshold by measuring an averaged ED sensor signal. If ED is greater than the threshold, the system is then tested for a reduction in TMR by selectively turning RV compensation and ED compensation on and off, and determining changes in the position error signal (PES). Depending on the test results from PES measurements one or both of RV compensation and ED compensation is left on, and maintained on until the TMR falls below the predetermined threshold. In the preferred embodiment, only if there is no ED greater than the ED threshold is the RV compensation turned on. This avoids the problem of coupling of the ED disturbance into the RV sensor. Since there is no ED disturbance, the RV compensation responds to actual RV sensed by the RV sensor. 
     In the preferred method of operation, the method is initiated by periodically testing the WIF. However, the method may also be initiated on a regularly scheduled basis instead of or in addition to testing the WIF. For example, the method may be initiated as a scheduled task each time the disk drive is powered on and/or each time the disk drive enters an idle state, i.e., a period during which the hard disk controller (HDC) is not processing read or write commands from the host. 
     For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic top view of a magnetic recording hard disk drive with a rotational vibration (RV) sensor and RV feedforward compensation and with an electrical disturbance (ED) sensor and ED feedforward compensation according to the invention. 
         FIG. 2  is generalized version of the control system loop for the system of  FIG. 1 . 
         FIG. 3  is a schematic of the VCM driver, which can be considered a transconductance (G m ) amplifier. 
         FIG. 4  is a flow chart of the logic representing the method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a block diagram of a magnetic recording hard disk drive according to the invention. The disk drive includes a magnetic recording disk  12  that is rotated about an axis of rotation  13  in direction  15  by a spindle motor (not shown) mounted to the disk drive housing or base  16 . The disk  12  has a magnetic recording layer patterned into magnetizable blocks that define concentric data tracks, such as typical tracks  50 ,  51 , and servo sectors, such as typical servo sectors  60 ,  61 ,  62 . The servo sectors extend generally radially across the concentric data tracks so that each data track has a plurality of equally-angularly spaced servo sectors that extend around the track. Each of the servo sectors in a data track contains a servo timing mark (STM), also called a servo address mark (SAM), that indicates the start of the servo sector, a track identification (TID) code, and a portion of a pattern of magnetized blocks or high-frequency bursts that are decoded to provide a head position error signal (PES). 
     The disk drive also includes a rotary voice coil motor (VCM) actuator  14  supported on the base  16 . The actuator  14  pivots about axis  17  and includes a rigid actuator arm  18 . A generally flexible suspension  20  includes a flexure element  23  and is attached to the end of arm  18 . A head carrier or air-bearing slider  22  is attached to the flexure  23 . A magnetic recording read/write (R/W) head  24  is formed on the trailing surface  25  of slider  22 . The flexure  23  and suspension  20  enable the slider to “pitch” and “roll” on an air-bearing generated by the rotating disk  12 . 
     As the disk  12  rotates in the direction  15 , the positioning information in the servo sectors is read by the read head, amplified at pre-amplifier  102  and sent to R/W electronics  113 . The servo electronics  112  receives input from R/W electronics  113  and provides digital signals to servo control processor  115 . The servo control processor  115  includes a microprocessor  117 , associated memory  118  and a control algorithm or controller  116 , and provides a digital VCM control signal  191  to a digital-to-analog converter (DAC)  190 . The output of DAC  190  is input to VCM driver  192 . VCM driver  192  operates as a transconductance amplifier that is controlled in part by a reference voltage related to the voltage V from the disk drive&#39;s power supply voltage (not shown). VCM driver  192  controls current to the VCM  14  to move the read/write head  24  to the desired data track and maintain it on track for reading and writing of data. 
     Within the servo electronics  112 , the STM decoder  160  receives a clocked data stream from the read/write electronics  113 . Once an STM has been detected, an STM found signal is generated. The STM found signal is used to adjust timing circuit  170 , which controls the operating sequence for the remainder of the servo sector. After detection of an STM, the track identification (TID) decoder  180  receives timing information from timing circuit  170 , reads the clocked data stream, which is typically Gray-code encoded, and then passes the decoded TID information to servo control processor  115 . Subsequently, the PES decoder  190  (also called the servo demodulator) captures the position information from R/W electronics  113  and passes a position error signal (PES) to servo control processor  115 . 
     The servo control processor  115  includes a microprocessor  117  that uses the PES as input to a control algorithm to generate the digital control signal  191  to VCM driver  192 . The control algorithm recalls from memory  118  a “controller”  116 , which is a set of parameters based on the static and dynamic characteristics of the “plant” being controlled, i.e., the VCM  14 . The control algorithm is essentially a matrix multiplication algorithm, and the controller parameters are coefficients used in the multiplication and stored in memory  118  accessible by the microprocessor  117 , as is well known in the art. 
     The servo control processor  115  is depicted as part of the hard disk controller (HDC)  110 . The HDC  110  performs many of the disk drive&#39;s functions, including passing of read/write commands from the host computer to the R/W electronics  113  and passing of data back to the host. Disk drives typically monitor commands to write data and inhibit writing if there is track misregistration (TMR), i.e., if the PES is too large (greater than some predetermined threshold). When this occurs the servo control processor  115  triggers a status bit or flag that represents a write fault or write inhibit and records this event in memory  118 . 
     Disk drives experience mechanical disturbance (MD) forces during normal operation. These disturbances arise internally, such as from motion of the VCM actuator, as well as externally, such as from shocks to the frame supporting the disk drive or from the movement of other disk drives when the drives are mounted together in a disk array system. These MDs are typically linear or rotational vibration disturbances that cause track misregistration (TMR) of the read/write heads. In the present invention the disk drive detects MDs with an MD sensor, determines if there is potential TMR, and then enables or disables MD compensation of the VCM control signal  191  as necessary. In  FIG. 1 , the disk drive is illustrated as being subjected to rotation vibration (RV), but the invention is applicable to other types of MDs. In  FIG. 1 , RV sensor  200  detects rotational disturbances. RV sensor  200  is preferably a rotational accelerometer, shown as two linear accelerometers  201 ,  202 , each attached to a respective side of base  16 . The linear accelerometers  201 ,  202  are commercially available single-axis piezoelectric accelerometers, such as Murata Model PKGS-00LD accelerometers. The accelerometer is shown schematically as being attached directly to the base  16 , but it may also be attached to a printed circuit board or card (not shown) that contains the disk drive electronics, which is secured to the base  16 . The rotational accelerometer may also be a single-piece angular accelerometer. Other types of rotational accelerometers are commercially available from STMicroelectronics and Delphi. 
     The difference in output of the two linear accelerometers  201 ,  202  is summed at differential amplifier  203 , so that together the linear accelerometers and the differential amplifier  203  function as a rotational accelerometer. The output of differential amplifier  203  is passed through a conditioning filter  204  and then to microprocessor  117  in the servo control processor  115 . The servo control processor  115  uses the signal from RV sensor  200  to cancel the off-track motion induced by rotational vibration through a feedforward control method that creates an MD compensation signal that is used to provide the digital control signal input  191  to the VCM driver  192 . The RV feedforward controller is calculated by the microprocessor  117  using parameters and program instructions stored in memory  118 . It is also well known in the art that the RV feedforward controller may be implemented with analog circuitry that converts the signal from the RV sensor  200  to the MD compensation signal, with the MD compensation signal then being summed with the control signal to the VCM driver  192  after the servo control processor has calculated the control signal. U.S. Pat. No. 5,663,847 describes a disk drive with a RV sensor and a feedforward controller whose gain is adapted to accommodate changes in RV sensor sensitivity, and a threshold detector for turning off the adaptive gain feature. U.S. Pat. No. 6,414,813 B2 describes a disk drive with a RV sensor and a feedforward controller with multiple sets of adjustable gains, where a gain set is selected if the RV sensor output and the PES exceed certain thresholds. U.S. Pat. No. 7,177,113 B1 describes a disk drive with switchable RV cancellation. 
     Disk drives also experience electrical disturbance (ED) forces during normal operation. A common source of ED is caused by noise, typically represented as a ripple voltage, from the power supply. This ED is reflected as noise in the reference voltage V R  used by VCM driver  192 , which is then reflected as noise on the current applied to VCM  14  by VCM driver  192 . In the present invention the disk drive detects EDs with an ED sensor, determines if there is potential TMR, and then enables or disables ED compensation of the VCM control signal  191  as necessary. In  FIG. 1 , the disk drive is illustrated as being subjected to an ED on the voltage reference applied to VCM driver  192 , but the invention is applicable to other types of EDs. The reference voltage V R  applied to the VCM driver  192  is directly related to the power supply voltage V and either it or the power supply voltage is monitored and output in digital form by analog-to-digital converter (ADC)  104  to microprocessor  117  of the servo control processor  115 . The servo control processor  115  uses the signal from ADC  104  to cancel the off-track motion induced by the ED through a feedforward control method that creates an ED compensation signal that is used to provide the digital control signal input  191  to the VCM driver  192 . The ED feedforward controller is calculated by the microprocessor  117  using parameters and program instructions stored in memory  118 . It is also well known in the art that the ED feedforward controller may be implemented with analog circuitry that converts the signal from ADC  104  to the ED compensation signal, with the ED compensation signal then being summed with the control signal to the VCM driver  192  after the servo control processor has calculated the control signal. 
       FIG. 2  is a generalized depiction of the control system loop of the present invention. The reference position of the head during track following is represented as input “r”. P(s) is the VCM actuator or “plant” transfer function, where s is the Laplace transform variable. This transfer function is known from modeling (e.g., finite element modeling) during the disk drive design process, verified through standard frequency response measurement techniques during the disk drive testing process, and can be tested on each individual disk drive during manufacturing or recalibration operations. C(s) represents the VCM feedback controller transfer function, which is determined during the disk drive design process. It can also be verified through standard frequency response measurement techniques during testing process, and can also be tested on each individual disk drive during manufacturing or recalibration operations. 
     The rotational vibration R(s) affects the disk drive dynamics and thus creates a MD to the VCM  14 . However, the RV sensor  200  detects the rotational vibration and the RV feedforward controller compensates for the MD by generating a compensation signal y(s) that is summed with the VCM control signal u(s) from the VCM controller. The RV feedforward controller may be implemented in analog circuitry or calculated by the servo control processor. 
     The power supply voltage, represented as V(s), creates an ED to the VCM driver  192  that affects the VCM driver&#39;s reference voltage V R , which it turn affects the VCM actuator  14 . However, the ADC  104  detects the ED of the power supply voltage and the ED feedforward controller compensates for the ED by generating a compensation signal w(s) that is summed with the VCM control signal u(s) from the VCM controller. The ED feedforward controller may be implemented in analog circuitry or calculated by the servo control processor. The ED feedforward controller includes a conventional lead-lag filter that filters the frequency content of the disturbance and provides an output to gain compensation. The parameters of the ED feedforward controller are calculated in the conventional manner of control system design by measuring the response of the transconductance amplifier in VCM driver  192  to the power supply voltage. 
       FIG. 3  is a schematic of VCM driver  192 , which can be considered a transconductance (G m ) amplifier  192 . G m  amplifier  192  provides a current I that is proportional to a voltage that it receives. G m  amplifier  192  is commonly used in control systems for controlling an apparatus such as VCM  14 . DAC  190  converts a digital voltage of a control command  191  to an analog voltage of a control command, and presents it to G m  amplifier  192  for processing. Inherent in G m  amplifier  192  is a voltage reference V R    215  which provides at least one reference voltage to error amplifier  220 , power amplifier ( 230   a ,  230   b ), and/or current sensing amplifier  240 . V R    215  is typically half of the voltage V delivered by the power supply. Since V R    215  is electrically coupled with the power supply voltage, the performance of error amplifier  220 , power amplifier ( 230   a ,  230   b ), and current sensing amplifier  240  is affected by a disturbance in the power supply voltage. Associated with each amplifier is an array of electrical components. For example, associated with sensing amplifier  240  are sense feedback resistors Rsf, and sense input resistors Rsi. There are similarly associated components for error amplifier  220  and power amplifier ( 230   a ,  230   b ). Each of these components has its own inherent performance or tolerance for its expected electrical parameter. In turn, the tolerance of each electrical component affects the performance of the associated amplifiers, and the performance of G m  amplifier  192 . The performance of G m  amplifier  192  has specified tolerances. 
     With reference to  FIG. 2 , the relational equations which link elements of the servo control loop are expressed include elements defined as follows: PES is position error signal; r is the reference position; G m  is the transconductance gain which translates from voltage input to current output from transconductance amplifier  192 ; I is VCM current; B is a mechanical bias; U is the control command; ED is the electrical disturbance; and V is the input voltage from the power supply. 
     Under closed servo loop control, when PES=0 during tracking, the VCM is compensating for mechanical bias B. There is no motion of VCM and therefore
 
 I+B= 0  (1)
 
The relationship of VCM current to transconductance amplifier output, disturbance and control command is
 
 I=Gm× ( ED+U ).  (2)
 
Substituting equation (2) into equation (1) presents
 
( Gm ×( ED+U ))+ B= 0  (3)
 
during tracking on reference position r.
 
     ED is a function of the input voltage V from the power supply. ED is also a function of the common mode rejection ratio (CMRR) of the sense amplifier ( FIG. 4 ). CMRR is proportional to the ratio of a change in disturbance ED to a change in input voltage V. CMRR is a metric by which current sense amplifier  240  is characterized for rejecting a common mode voltage. Common mode gain of an amplifier enables an amplifier such as current sense amplifier  240  to detect differences between signals with small amplitudes. The common mode gain is specified for any given amplifier, but is only known once it is built and operational. The common mode gain is dependent upon the consistency of matching its components, such as the associated resistors of the sense amplifier  240  presented in the schematic of  FIG. 3 . 
     With continued reference to  FIG. 3 , the CMRR of sense amplifier  240  is affected by the consistency of matching the components associated with sense amplifier  240 . The CMRR affects how disturbances in the voltage from the power supply and provided to transconductance (G m ) amplifier  192 , become manifested in VCM current I. Voltage reference (V R )  215  provides a reference voltage to current sensing amplifier  240  as well as error amplifier  220 , power amplifier ( 230   a ,  230   b ). V R    215  is typically half of the voltage delivered by the power supply. Since V R    215  is electrically coupled with power supply voltage V, the performance of current sensing amplifier  240 , as well as error amplifier  220 , and power amplifier ( 230   a ,  230   b ), is affected by the disturbance ED in power supply voltage V. 
     The operation of ED feedforward compensation will now be explained. A first power setting V 1  for power supply is presented to the servo control system. A disturbance ED 1  is caused by V 1  interacting with current sensing amplifier  240  coupled with G m  amplifier  192 . Disturbance ED 1  is associated with the effect of V 1  on CMRR associated with current sensing amplifier  240 . This is associated with CMRR because the mechanics of the VCM are constant, therefore mechanical bias B has not changed, and the servo control loop is track following a constant reference position r. The control command, U 1  for the V 1  power setting is then measured. 
     A second power setting V 2  for the power supply is presented to the servo control system. A disturbance ED 2  is caused by V 2  interacting with current sensing amplifier  240  coupled with G m  amplifier  192 . Disturbance ED 2  is associated with the effect of V 2  on CMRR associated with current sensing amplifier  240  for the same above-described reasons, i.e., constant mechanical bias B and constant reference position r. The control command, U 2  for the V 2  power setting is measured. Recalling equation (3):
 
( Gm ×( ED+U ))+ B= 0,  (4)
 
ED can be expressed as:
 
 ED =(− B/Gm )− U.   (5)
 
     The disturbance ED is whatever bias B that is needed mechanically to stay at reference position r transformed back through the transconductance amplifier to match the control command current, but in the opposite sign. The CMRR is proportional to the differences between the two disturbances (ED 1 −ED 2 ) and the differences in voltage (V 1 −V 2 ) in the power supply. It is appreciated that from a control system standpoint that for the above set of conditions a signal is provided that matches the disturbance ED. Mathematically it is possible to state that 
     
       
         
           
             
               
                 
                   
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     The proportionality constant for CMRR enables optimal gain compensation to be derived for the ED feedforward controller. The amplitude for the disturbance ED in the power the power supply voltage V is derived and hence is used to compute gain compensation. 
     In the present invention either or both of MD compensation and ED compensation can be selectively enabled and disabled. This is depicted in  FIG. 2  as logic block  300  which represents computer program instructions that implement an algorithm run in microprocessor  117  of the servo control processor  115 . The logic block  300  receives as input the PES and the ADC  104  output of digital voltage and provides output commands to enable/disable MD and ED compensation, as depicted in  FIG. 2  by “switches”  302 ,  304  to the RV feedforward controller and ED feedforward controller, respectively. 
       FIG. 4  is a flow chart of the logic  300  representing the method of the present invention. The method may be implemented in software or firmware, and may be stored as computer program instructions in a computer-readable storage medium, such as memory  118 , for execution by a computer processor, such as microprocessor  117 . At block  305 , RV compensation and ED compensation are both disabled (turned off). At block  310 , the frequency of recent write-inhibits (WI) is measured against a write-inhibit frequency (WIF) threshold. The WIF threshold may be a predetermined number of write-inhibit flags per write commands, for example 10 percent of write commands. A WIF greater than the threshold would indicate some likely disturbance to the disk drive is causing TMR. If WIF is less than the WIF threshold the method returns to block  305 . If WIF is greater than the WIF threshold the method proceeds to block  315 . At block  315  an averaged ED sensor signal (averaged voltage from ADC  104 ) is calculated and compared to an ED threshold. The ED threshold may be determined by design criteria for the noise tolerance level of the servo system, e.g. 250 mV AC. The averaged voltage may be an actual average, a weighted power average, or any statistical type of average, such as a root-mean-square, taken over a number of digital samples from ADC  104  over the AC components of ED. If the ED is less than the ED threshold, then at block  320  RV compensation is enabled (turned on) and the method returns to block  310 . If the ED is greater than the ED threshold at block  315 , then at block  325 , if both ED compensation and RV compensation are not already off, they are turned off. Then, with both ED compensation and RV compensation off, a first measurement of PES is made (PES 1 ), preferably as an averaged PES over a number of samples. Then at block  330 , ED compensation is turned on and with ED compensation on, a second measurement of averaged PES is made (PES 2 ). At block  335 , PES 2  is compared with PES 1  to determine if there has been a significant improvement in track following. As with the averaged ED, the averaged PES may be an actual average, a weighted power average, or any statistical type of average, such as a root-mean-square, taken over a number of digital samples from PES. If the averaged PES with ED compensation on (PES 2 ) is less than the averaged PES with ED compensation off (PES 1 ) by an amount greater than a predetermined first improvement threshold (IMPT 1 ), this indicates that the disturbance was an ED and has been corrected by switching in the ED feedforward compensation. IMPT 1  may be selected by experiment. For example, by characterizing the transfer functions of  FIG. 2  and typical measurement repeatability tolerances, margin can be applied to establish specific improvement thresholds based on the drive sensitivity to electronic disturbances. This is represented in block  335  as (PES 1 -PES 2 ) being greater than the first improvement threshold IMPT 1 . If there is no improvement, then at block  340  ED compensation is turned off and the method proceeds to block  350 . If there is an improvement then at block  345  ED compensation is kept on and the method proceeds to block  350 . At block  350 , RV compensation is turned on and a third measurement of averaged PES is made (PES 3 ). PES 1 , PES 2  and PES 3  may each be an actual average, a weighted power average, or any statistical type of average, such as a root-mean-square, taken over a number of digital samples. At block  355 , PES 3  is compared with PES 2  to determine if there has been a significant improvement in track following with RV compensation turned on. If the averaged PES with RV compensation on (PES 3 ) is less than the averaged PES with RV compensation off (PES 2 ) by an amount greater than a predetermined second improvement threshold (IMPT 2 ), this indicates that the disturbance was a RV disturbance and has been corrected by switching in the RV feedforward compensation. IMPT 2  may be selected by experiment. For example, by characterizing the transfer functions of  FIG. 2  and typical measurement repeatability tolerances, margin can be applied to establish specific improvement thresholds based on the drive sensitivity to RV disturbances. If there is no improvement at block  355 , then at block  360  RV compensation is turned off and the method proceeds to block  310  where the method continues with the comparison of WIF to a WIF threshold. If there is an improvement at block  355 , then at block  365  RV compensation is kept on and the method proceeds back to block  310 . 
     In the preferred embodiment of the method of the present invention, after it has been determined that there is some disturbance that is causing TMR (WIF greater that the WIF threshold at block  310 ), then the system is tested at block  315  to determine if there is an ED greater than the ED threshold. Only if there is no ED greater than the ED threshold is RV compensation turned on at block  320  and the method returns to block  310 . This sequence avoids the problem of coupling of the ED disturbance into the RV sensor. Since there is no ED disturbance, the RV compensation responds to actual RV sensed by the RV sensor. 
     In the preferred embodiment of the method as described above, if the test of block  315  is YES then the system is tested for improvement first by ED compensation by measuring averaged PES values before and after turning ED compensation on (blocks  325 ,  330 ,  335 ). However, as an alternative sequence, if the test of block  315  is YES then the system can be tested for improvement first by RV compensation by measuring averaged PES values before and after turning RV compensation on. In this alternative approach, after block  315 , the sequence would be blocks  325 ,  350 ,  355 . 
     In the preferred embodiment of the method as described above, the method is initiated by periodically testing the WIF at block  310 . However, the method may also be initiated on a regularly scheduled basis instead of or in addition to testing the WIF. For example, the method may be initiated as a scheduled task each time the disk drive is powered on and/or each time the disk drive enters an idle state, i.e., a period during which the HDC  110  is not processing read or write commands from the host. In such an embodiment the method would proceed from block  305  directly to block  315 . 
     While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.