Disk drive filtering disturbance signal and error signal for adaptive feed-forward compensation

Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a VCM servo controller to control the actuator arm as it seeks from track to track.

FIG. 1 shows a prior art disk format 2 comprising a number of servo tracks 4 defined by concentric servo sectors 6 0 - 6 N recorded around the circumference of each servo track, wherein data tracks are defined relative to the servo tracks 4 . Each servo sector 6 i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to synchronize to a servo data field 12 . The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6 , further comprises groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a servo track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.

An air bearing forms between the head and the disk due to the disk rotating at high speeds. Since the quality of the write/read signal depends on the fly height of the head, conventional heads (e.g., a magnetoresistive heads) may comprise an actuator for controlling the fly height. Any suitable fly height actuator may be employed, such as a heater which controls fly height through thermal expansion, or a piezoelectric (PZT) actuator. A dynamic fly height (DFH) servo controller may measure the fly height of the head and adjust the fly height actuator to maintain a target fly height during write/read operations.

Certain conditions may affect the ability of the VCM servo controller to maintain the head along the centerline of a target data track and/or the ability of the DFH servo controller to maintain the target fly height. For example, an external vibration applied to the disk drive or degradation and/or malfunction of the spindle motor that rotates the disks may induce a disturbance in the servo control systems. The degradation caused by such a disturbance may be ameliorated using a feed-forward compensation algorithm.

FIG. 1 shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors.

FIG. 2A shows a disk drive according to an embodiment of the present invention comprising a head actuated over a disk by control circuitry comprising a servo control system.

FIG. 2B is a flow diagram according to an embodiment of the present invention wherein feed-forward compensation values are generated for the servo control system in response to a filtered disturbance signal and a filtered error signal.

FIG. 3 shows a servo control system according to an embodiment of the present invention, including feed-forward compensation values that compensate for a disturbance.

FIG. 4A shows an embodiment of the present invention wherein filtering the disturbance signal and the error signal boosts a high frequency component of the disturbance signal.

FIG. 4B shows an embodiment of the present invention wherein filtering the disturbance signal and the error signal extends a bandwidth of an electronic sensor for sensing the disturbance signal.

FIG. 2A shows a disk drive according to an embodiment of the present invention comprising a disk 16 , a head 18 , and control circuitry 20 comprising a servo control system ( FIG. 3 ) operable to actuate the head 18 over the disk 16 . The control circuitry 20 executes the flow diagram of FIG. 2B , wherein a disturbance signal 21 is generated in response to a vibration (block 22 ), and an error signal 24 of the servo control system is generated (block 26 ). The disturbance signal 21 is filtered with a first filter 28 comprising a frequency response to generate a filtered disturbance signal 30 (block 32 ). The filtered disturbance signal 30 is filtered with a model of the servo control system 34 to generate a compensated disturbance signal 36 (block 38 ). The error signal 24 is filtered with a second filter 40 comprising the frequency response to generate a filtered error signal 42 (block 44 ). An adaptation control signal 46 is generated 48 in response to the compensated disturbance signal 36 and the filtered error signal 42 (block 50 ). An adaptive filter 52 is adapted in response to the adaptation control signal 46 (block 54 ), wherein the adaptive filter 52 filters the disturbance signal 21 to generate feed-forward compensation values 56 (block 58 ). The feed-forward compensation values 56 are applied to the servo control system to compensate for the vibration (block 60 ).

In the embodiment of FIG. 2A , the disk 16 comprises embedded servo sectors 62 0 - 62 N that define a plurality of servo tracks 64 . The control circuitry 20 processes a read signal 66 emanating from the head 18 to demodulate the servo sectors 62 0 - 62 N and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. The servo control system 20 filters the PES using a suitable compensation filter to generate a control signal 68 applied to a voice coil motor (VCM) 70 which rotates an actuator arm 72 about a pivot in order to actuate the head 18 radially over the disk 16 in a direction that reduces the PES. The servo sectors 62 0 - 62 N may comprise any suitable position information, such as a track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as the amplitude-based servo pattern shown in FIG. 1 , or a suitable phase-based servo pattern.

In one embodiment, the disk drive comprises a suitable microactuator, such as a suitable piezoelectric actuator, for actuating the head 18 in fine movements radially over the disk 16 . The microactuator may be implemented in any suitable manner, such as a microactuator that actuates a suspension relative to the actuator arm 72 , or a microactuator that actuates a head gimbal relative to the suspension. In one embodiment, feed-forward compensation values 56 may be generated in response to the disturbance signal 21 for use in the microactuator servo control system in addition to, or instead of, generating feed-forward compensation values 56 for the VCM servo control system.

In one embodiment, the head 18 may comprise a suitable fly height actuator, such as a heater or a piezoelectric actuator, operable to actuate the head vertically over the disk in order to maintain a target fly height. The control circuitry 20 may comprise a servo control system operable to compare a measured fly height to a target fly height to generate a fly height error used to generate a dynamic fly height (DFH) control signal 73 ( FIG. 2A ) similar to the servo control system that controls the radial position of the head. In one embodiment, feed-forward compensation values 56 are generated in response to the disturbance signal 21 for use in the fly height servo control system.

An external vibration applied to the disk drive or degradation and/or malfunction of the spindle motor that rotates the disks may induce a disturbance in one or more of the servo control systems that actuate the head over the disk (radially or vertically). The vibration may be detected in any suitable manner, such as by evaluating the read signal 66 emanating from the head 18 , or by using a suitable electronic sensor 74 as shown in the embodiment of FIG. 3 . Any suitable electronic sensor 74 may be employed in the embodiments of the present invention, such as a suitable accelerometer which detects a force (e.g., linear and/or rotational) applied to the disk drive that causes a corresponding vibration.

In one embodiment, it may be desirable to boost a high frequency component of the disturbance signal. Accordingly in this embodiment, the frequency response of the first filter 28 and the second filter 40 in FIG. 3 comprises a unity gain up to a target frequency and a boosting gain beyond the target frequency as illustrated in FIG. 4A . In one embodiment, the phase response of the first filter 28 and the second filter 40 is substantially unity for all frequencies to avoid inducing a delay into the filtered disturbance signal 30 and filtered error signal 42 .

In one embodiment it may be desirable to boost the high frequency component of the disturbance signal 21 generated by an electronic sensor 74 in order to increase the effective bandwidth of the electronic sensor 74 . This embodiment is illustrated in FIG. 4B wherein filtering the disturbance signal 21 and the error signal 24 may increase the bandwidth of the electronic sensor 74 from frequency response 76 to frequency response 78 . Increasing the effective bandwidth of the electronic sensor 74 may improve the performance of the servo control system 84 by improving the disturbance compensation provided by the feed-forward compensation values 56 .

In the embodiment of FIG. 3 , a measured position 80 of the head 18 (radial or vertical) is compared to a reference position 82 to generate the error signal 24 . The error signal 24 may be input into the servo control system 84 as a feedback control signal to a suitable compensator. The feed-forward compensation values 56 may be used to adjust any suitable control signal within the servo control system 84 , such as by modifying the error signal 24 input into the compensator, or by modifying the output of the compensator (the control signal applied to an actuator).

Any suitable adaptive algorithm 48 may be employed to adapt the adaptive filter 52 in response to the filtered disturbance signal 30 and the filtered error signal 42 . In one embodiment, the adaptive algorithm 48 implements a Filtered-X Least Mean Square (LMS) algorithm with attempts to minimize the following equation:

E ( e f ( k ) 2 )= E [( F ( z −1 ) e ( k )) 2 ].

Any suitable adaptive filter 52 may also be employed, wherein in one embodiment the adaptive filter 52 comprises a finite impulse response (FIR) filter having coefficients θ(k) wherein the control circuitry 20 is operable to adapt the adaptive filter 52 according to:

θ( k+ 1)=θ( k )+μ( k )φ f ( k ) e f ( k )

where μ represents a learning coefficient, φ f (k) represents the compensated disturbance signal 36 , and e f (k) represents the filtered error signal 42 . In one embodiment, the learning coefficient μ is updated according to:

μ( k )=β/(ε+φ f ( k ) T φ f ( k ))

if μ( k )>μ max ,μ( k )=μ max ; else if μ( k )<μ min ,μ( k )=μ min

where β is a suitable scalar, and ε is a small positive number to avoid division by zero. With the filtered (weighted) error signal 42 and the adaptation regressor φ f (k) 36 , the modified Filtered-X LMS algorithm in the above-described embodiment may be considered a Filtered-X weighted-error normalized LMS (WE-NLMS) algorithm.

Any suitable control circuitry may be employed to implement the flow diagrams in the embodiments of the present invention, such as any suitable integrated circuit or circuits. For example, the control circuitry may be implemented within a read channel integrated circuit, or in a component separate from the read channel, such as a disk controller, or certain operations described above may be performed by a read channel and others by a disk controller. In one embodiment, the read channel and disk controller are implemented as separate integrated circuits, and in an alternative embodiment they are fabricated into a single integrated circuit or system on a chip (SOC). In addition, the control circuitry may include a suitable preamp circuit implemented as a separate integrated circuit, integrated into the read channel or disk controller circuit, or integrated into a SOC.

In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one embodiment, they may be stored on a non-volatile semiconductor memory external to the microprocessor, or integrated with the microprocessor in a SOC. In another embodiment, the instructions are stored on the disk and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry.