Abstract:
A disk drive has multiple feedforward controllers for handling external disturbances, such as rotational vibration (RV), that have different external disturbance frequency spectra. Each feedforward controller is designed to be optimal for a canceling a specific associated RV spectrum. The actual RV spectrum acting on the disk drive is determined and the proper feedforward controller is then selected and used to generate a compensation signal for canceling the RV. Each feedforward controller may be tested when the disk drive is experiencing the RV, and the resulting compensation signal and PES measured. The feedforward controller that produces the best external disturbance cancellation is then selected as the feedforward controller. A signal from a RV sensor may be used to detect the peak frequency of the actual RV spectrum. This detected peak frequency is then matched to a peak frequency in a plurality of peak frequencies in a lookup table, and the feedforward controller associated with the matching peak frequency is selected as the feedforward controller.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to disk drives, and more particularly to a magnetic recording disk drive that includes a system for canceling the effects of rotational vibration. 
     2. Description of the Related Art 
     Magnetic recording hard disk drives (HDDs) use an actuator, typically a rotary voice-coil-motor (VCM) type of 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. 
     Disk drives experience rotational vibration and disturbance 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. 
     Rotational vibration (RV) cancellation is a method that uses sensors (typically accelerometers) to detect rotational vibration and improve the PES by canceling the off-track motion induced by the rotational vibration. The RV sensor signal is input to a feedforward controller that creates a feedforward compensation signal that is summed with the control signal to the VCM actuator. The use of a RV sensor and feedforward compensation in this manner is well-known, as 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. 
     The spectral content of external disturbances from customer environments vary as a function of the disk drive mounting geometry, as well as the location of the disk drive and the number of adjacent disk drives if the disk drive is in a disk array system. Consequently, a single feedforward controller design for RV cancellation can not optimally reject such a wide variety of external disturbances. Thus what is typically used is a feedforward controller that provides the best disturbance rejection over a broad range of frequencies. 
     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. 6,580,579 B1 describes a disk drive with a RV sensor and an adaptive filter that adjusts its parameters in real-time from the PES using an estimate of the VCM plant transfer function. 
     What is needed is a disk drive with an adaptive method for RV cancellation in which the best available feedforward controller design is selected based on the RV sensor signal or the PES. 
     SUMMARY OF THE INVENTION 
     The invention is a disk drive with multiple feedforward controllers for handling rotational vibration (RV) having different RV spectra. Each feedforward controller is designed to be optimal for a canceling a specific associated RV spectrum. The actual RV spectrum acting on the disk drive is determined and from this determination the proper feedforward controller is selected and used to generate a compensation signal for canceling the RV. In one approach for selecting the appropriate feedforward controller, each feedforward controller is tested when the disk drive is experiencing the RV, and the resulting compensation signal and PES are measured and stored in memory. The feedforward controller that produces the best external disturbance cancellation, e.g., the largest compensation signal or smallest PES, is then selected as the feedforward controller. In another approach for selecting the appropriate feedforward controller, the RV sensor signal is used to detect the peak frequency of the actual RV spectrum. This detected peak frequency is then matched to a peak frequency in a plurality of peak frequencies in a lookup table, and the feedforward controller associated with the matching peak frequency is selected as the feedforward controller. 
     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 feedforward compensation according to the prior art. 
         FIG. 2  is the control system loop for the prior art system of  FIG. 1 . 
         FIG. 3  is the RV frequency spectra for two different disk drive systems. 
         FIG. 4  is the RV rejection frequency response for two different feedforward controllers compared to the case without feedforward compensation. 
         FIG. 5  is the PES power spectra for two different feedforward controllers compared to the case without feedforward compensation for the disk drive in a first system. 
         FIG. 6  is the PES power spectra for the same two feedforward controllers as in  FIG. 5  compared to the case without feedforward compensation for a disk drive in a second system different from the system of  FIG. 5 . 
         FIG. 7  is the PES standard deviation for the same two feedforward controllers for each a disk drive in each of the two systems compared to the case without feedforward compensation for each system. 
         FIG. 8  is a control system loop for illustrating one method of selecting which of two feedforward controllers should be selected, when the invention has two feedforward controllers. 
         FIG. 9  is a flow chart showing the method for selecting the optimal feedforward controller from a plurality of N feedforward controllers. 
         FIG. 10  is a control system loop showing an embodiment wherein the peak frequency of the external disturbance is detected and used to determine which feedforward controller is to be selected. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a block diagram of a magnetic recording HDD according to the prior art. 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 pattern in the recording layer that defines 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) 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 an actuator  14 , such as a rotary voice coil motor (VCM) type of actuator, 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 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  provides an output  191  to VCM driver  192  that 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 read/write 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 control signal  191  to VCM driver  192 . The control algorithm is a feedback “controller”  116 , which includes program instructions and 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 feedback controller parameters are coefficients used in the multiplication. 
     The disk drive is subject to rotational disturbances, as shown by arrows  70 , that arise both internally, such as from motion of the VCM  14 , and 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 disturbances cause track misregistration (TMR) of the read/write heads. Thus it is known to incorporate into the disk drive a rotational vibration (RV) sensor  200  that detects rotational disturbances. RV sensor  200  may be 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 two-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 . 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 a compensation signal that is used to provide the input  191  to the VCM driver  192 . The 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 feedforward controller may be implemented with analog circuitry that converts the signal from the RV sensor to the compensation signal, with the compensation signal then being summed with the control signal to the VCM driver  192  after the servo control processor has calculated the control signal. 
     The control system loop is shown in  FIG. 2 . 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 D(s) and thus creates a disturbance added to the control signal to the VCM. However, the RV sensor detects the rotational vibration and the feedforward controller compensates for the disturbance by generating a compensation signal W(s) that is summed with the VCM control signal from the VCM controller. The feedforward controller may be implemented in analog circuitry, but is more commonly calculated by the servo control processor. F(s) represents the feedforward controller transfer function. Thus W(s)=R(s)F(s), and with the RV feedforward compensation the actual PES is given by: 
     
       
         
           
             
               
                 
                   
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     As shown in  FIG. 2 , the RV sensor is also subject to noise input. When the RV sensor output includes a large amount of noise not related to the rotational vibration, the RV compensation may degrade the PES. Source of noise in the RV sensor output may be electrical or sensor-related. Examples of sensor-related noise include 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. In addition, when the RV sensor uses a pair of linear accelerometers, like  201 ,  202  in  FIG. 1 , the gain mismatch of the two accelerometers also creates an undesirable compensation signal and degrades the PES. 
     The optimal feedforward controller design is generally unrealizable, due to delays in the system and non-minimum phase zeros in the actuator dynamics. Therefore a compromise must be made in the controller design such that the realizable feedforward controller fits well at frequencies with large disturbances and has more error at frequencies where there are fewer disturbances. However, a problem arises when disk drives with the same single feedforward controller design are placed in different operating environments. For example, the disturbances experienced by a server-class drive in one storage subsystem may be quite different from disturbances in another model of storage subsystem. Even different locations in the same subsystem or removal and re-installation of the drive may change the disturbance characteristics. 
       FIG. 3  shows examples of the measured RV spectra for two different subsystems in which the disk drive is mounted. System # 1  has relatively large amplitude vibrations that are largest around 500 Hz, as shown by RV spectrum  100 . System # 2  has lower amplitude vibrations that are largest around 800 Hz, as shown by RV spectrum  200 .  FIG. 3  illustrates that if the disk drive has a single feedforward controller, it will have to be a compromised design that results in less than optimal rejection for both RV spectra  100  and  200 . 
     The disk drive according to this invention has multiple feedforward controllers, each associated with a RV spectrum. Each feedforward controller is a set of parameters stored in memory and accessible by the processor for calculating a compensation signal. The feedforward controller associated with the RV spectrum that most closely matches the actual RV disturbance is then selected and used to generate the compensation signal to be summed with the VCM control signal.  FIG. 4  shows the disturbance rejection for two different feedforward controller designs. Design # 1 , shown by line  110 , is optimized for System # 1  having the RV spectrum  100  ( FIG. 3 ) and Design # 2 , shown by line  210 , is optimized for System # 2  having the RV spectrum  200  ( FIG. 3 ). Both are compared to the disturbance rejection without feedforward compensation (line  310 ). Designs # 1  and # 2  in  FIG. 4  each show maximum disturbance attenuation at a single frequency, at approximately 500 Hz for Design # 1  and at approximately 800 Hz for Design # 2 . 
       FIG. 5  shows a simulation result based on measured data when both feedforward controller Designs # 1  and # 2  are applied to System # 1 . Although both feedforward controller designs are better than the case without the feedforward control (line  320 ), Design # 1  (line  120 ), which was optimized for System # 1 , clearly works better on System # 1  than Design # 2  (line  220 ).  FIG. 6  shows the same type of simulation for System # 2 . Again, both feedforward controller designs are better than the case without feedforward control (line  340 ), but Design # 2  (line  240 ), which was optimized for System # 2 , performs better than Design # 1  (line  140 ). This is further emphasized in  FIG. 7 , which shows the PES standard deviation for the six cases in  FIGS. 5 and 6 . As expected from the higher magnitude of the disturbance levels for System # 1 , as shown in  FIG. 3 , the PES is higher for the three cases in System # 1  than for the corresponding three cases in System # 2 . However, for each of System # 1  and System # 2 , the feedforward controller design that was optimized for that system shows the lowest levels of PES, and thus the best performance. 
       FIG. 8  is a control system loop for illustrating one method of selecting which of the two feedforward controllers (Design # 1  or Design # 2 ) should be selected, when the invention has two feedforward controllers. The RV sensor output is directed to two different bandpass filters (BPF 1  and BPF 2 ). BPF 1  has a center frequency near the center of the RV spectrum for System # 1  and BPF 2  has a center frequency near the center of the RV spectrum for System # 2 . The bandpass filter outputs are sent to a comparator. As shown in  FIG. 8  the absolute values (ABS) of the RV sensor signals are used in the comparison. Also, the RV sensor signals may optionally be filtered, as shown by low-pass filters (LPFs), to remove resonances known to be unrelated to rotational disturbances. The comparator output controls the selection of which feedforward controller (Design # 1  or Design # 2 ) is used. Thus, for Example, if the actual RV acting on the disk drive most closely matches the RV spectrum of System # 1 , with a center frequency around 500 Hz, then the output from BPF 1  would be greater than the output from BPF 2  and feedforward controller Design # 1  would be selected. The parameter set associated with that feedforward controller would be recalled from memory and used by the processor to calculate the compensation signal to be summed with the VCM control signal. 
     In one embodiment of the invention there are N feedforward controllers, each having a design optimized to attenuate an associated RV spectrum. The feedforward controller that is selected is the one that has the design to attenuate the RV spectrum that most closely matches the actual RV disturbance acting on the disk drive. This selection occurs in real time and is based on the RV sensor output or the PES.  FIG. 9  is a flow chart illustrating a trial and error approach for selecting a feedforward controller from among N feedforward controllers. In block  500 , the method is initiated when n is set to 1, and in block  510  the nth of the N feedforward controllers is implemented when the processor recalls the nth feedforward controller parameter set from memory and runs the feedforward controller algorithm to calculate the nth compensation signal. In block  520 , the PES and the nth feedforward controller output (the compensation signal) are stored in memory. The stored values are the PES and compensation signal that result when the disk drive is experiencing an actual external disturbance and the disturbance is being compensated by the nth feedforward controller. The stored values may be the average, root-mean-square (RMS) or some other statistical measurement taken over multiple samples of the PES and compensation signal. In block  530 , the counter is incremented and the next feedforward controller (feedforward controller n+1) is implemented and the corresponding PES and feedforward controller output are stored in memory. This continues until all N feedforward controllers have been tested (block  540 ) in the presence of the actual external disturbance. In block  550  the processor then searches the stored values to find the feedforward controller associated with the RV spectrum that most closely matches the actual RV disturbance acting on the disk drive. This may be the feedforward controller that produced the smallest PES value or the largest compensation signal, or some combination of PES value and compensation signal. This feedforward controller is then selected and used as the feedforward controller (block  560 ). 
       FIG. 10  is a control system loop showing an embodiment wherein the peak frequency of the external disturbance is detected and used to determine which feedforward controller is to be selected. The RV sensor signal is sampled, a discrete Fourier transform (DFT) is performed by the processor and the peak frequency is detected from the DFT. The disk drive has a plurality of feedforward controllers, each associated with an RV spectrum having a peak frequency. The peak frequency values are stored in a lookup table in memory. After the peak frequency of the external disturbance is detected, the processor selects the appropriate feedforward controller by finding the closest match to the detected peak frequency in the peak frequency lookup table. The feedforward controller associated with this peak frequency is the selected feedforward controller. The processor then recalls the stored parameter set associated with the selected feedforward controller and performs the calculations to generate the compensation signal. 
     Because most disk drives use rotary actuators that are typically sensitive to RV, the invention has been described as a disk drive with multiple selectable feedforward controllers for compensating primarily RV external disturbances. However, the invention is also applicable when the disk drive is exposed to non-RV external disturbances, such as linear vibrations and various types of shocks. 
     The invention has been described as implemented in a magnetic recording HDD. However, the invention is applicable to other types of disk drives, such as optical disk drives, for example, CD and digital versatile disk (DVD) types of read-only and writable disk drives, that use optical disks and optical read or read/write heads. 
     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.