Disk drive selecting disturbance signal for feed-forward compensation

A disk drive is disclosed comprising a disk, a head, and control circuitry comprising a servo control system operable to actuate the head over the disk. A plurality of disturbance signals is generated in response to a vibration. A plurality of correlations is generated in response to each disturbance signal and an error signal of the servo control system. At least one of the disturbance signals is selected in response to the correlations. A feed-forward compensation value is generated in response to the selected disturbance signal, and the feed-forward compensation value is applied to the servo control system to compensate for the vibration.

BACKGROUND

FIG. 1shows a prior art disk format2comprising a number of servo tracks4defined by concentric servo sectors60-6Nrecorded around the circumference of each servo track, wherein data tracks are defined relative to the servo tracks4. Each servo sector6icomprises a preamble8for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark10for storing a special pattern used to synchronize to a servo data field12. The servo data field12stores 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 sector6ifurther comprises groups of servo bursts14(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 data track centerline. The groups of servo bursts14provide 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.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 2Ashows a disk drive according to an embodiment of the present invention comprising a disk16, a head18, and control circuitry20comprising a servo control system operable to actuate the head18over the disk16. The control circuitry20executes the flow diagram ofFIG. 2B, wherein a plurality of disturbance signals is generated in response to a vibration (step22). A plurality of correlations is generated in response to each disturbance signal and an error signal of the servo control system (step24). At least one of the disturbance signals is selected in response to the correlations (step26). A feed-forward compensation value is generated in response to the selected disturbance signal (step28), and the feed-forward compensation value is applied to the servo control system to compensate for the vibration (step30).

In the embodiment ofFIG. 2A, the disk16comprises embedded servo sectors320-32Nthat define a plurality of servo tracks34. The control circuitry20processes a read signal36emanating from the head18to demodulate the servo sectors320-32Nand 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 control circuitry20filters the PES using a suitable compensation filter to generate a control signal38applied to a voice coil motor (VCM)40which rotates an actuator arm42about a pivot in order to actuate the head18radially over the disk in a direction that reduces the PES. The servo sectors320-32Nmay 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 inFIG. 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 head18in fine movements radially over the disk16. The microactuator may be implemented in any suitable manner, such as a microactuator that actuates a suspension relative to the actuator arm42, or a microactuator that actuates a head gimbal relative to the suspension. In one embodiment, feed-forward compensation values may be generated in response to a selected disturbance signal for use in the microactuator servo control system in addition to, or instead of, generating feed-forward compensation values for the VCM servo control system.

In one embodiment, the head18may 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 circuitry20may 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 signal44(FIG. 2A) similar to the servo control system that controls the radial position of the head. In one embodiment, feed-forward compensation values are generated in response to a selected disturbance signal 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). Using a suitable sensor a disturbance signal can be generated that represents the disturbance; however, since the disturbance may be caused by a number of different sources, in embodiments of the present invention a number of sensors are employed each corresponding to a possible source of vibration. The disturbance signals generated by the sensors are evaluated in order to select the optimal disturbance signal(s), that is, the disturbance signal(s) that best represent(s) the actual vibration. In the embodiment ofFIG. 2B, the disturbance signal having the best correlation with an error signal in the servo control system is selected to generate the feed-forward compensation values. In other embodiments described below with reference toFIGS. 7Aand7B, the disturbance signal that generates a smallest residual error (after feed-forward compensation) is selected to generate the feed-forward compensation values applied to the actual servo control system.

FIG. 3Ashows a servo control system according to an embodiment of the present invention, including to select a disturbance signal based on a correlation with an error signal in the servo control system. A suitable actuator46(radial or vertical) actuates the head18over the disk16in response to an actuator control signal48. An estimated position50of the head18is subtracted from a reference position52to generate an error signal54. A suitable compensator56processes the error signal54to generate a control signal58that is combined with a feed-forward compensation value60to generate the actuator control signal48. A plurality of correlators621-62Ncorrelate the error signal54with respective disturbance signals641-64N. A selector66applies the disturbance signal64ithat best correlates with the error signal54to a feed-forward algorithm68that generates the feed-forward compensation values60that compensate for the vibration disturbing the servo control system.

Any suitable sensor may be used to generate the disturbance signals641-64Nin the embodiments of the present invention, including an electronic sensor and/or a sensor implemented in firmware.FIG. 3Bshows an embodiment wherein the sensors include a first electrical sensor701(e.g., an accelerometer) for generating a first disturbance signal641representing a linear vibration, and a second electrical sensor702for generating a second disturbance signal642representing a rotational vibration. Also in this embodiment a firmware sensor70Ngenerates a disturbance signal64N, for example, in response to the read signal36emanating from the head, or in response to a back electromotive force (BEMF) signal generated by the VCM40or a spindle motor that rotates the disk16, or in response to any other suitable signal that may affect the servo control system(s).

FIG. 3Balso illustrates an embodiment wherein the selector66may apply a plurality of the disturbance signals (M disturbance signals) to the feed-forward algorithm for generating the feed-forward compensation values60. For example, the selector66may select the best M out of the N disturbance signals, or the M disturbance signals that satisfy a selection criteria (e.g., exceed a threshold).

Any suitable algorithm may be employed by the correlators621-62Nto correlate the disturbance signals641-64Nwith the error signal54.FIG. 4shows an example correlation algorithm where e represents the error signal and x represents the disturbance signal. Other embodiments may employ a different algorithm to compute the correlation, such as computing a Euclidean Distance between the error signal and the disturbance signals.

In an embodiment illustrated in the flow diagram ofFIG. 5, the gain of at least one of each disturbance signal and the error signal may be adjusted (step72) prior to performing the correlation (step24). In one embodiment, the gain may be adjusted to a number of different values for each disturbance signal and the corresponding correlation computed. Also in the embodiment ofFIG. 5, the phase of at least one of each disturbance signal and the error signal may be adjusted (step72) prior to performing the correlation (step24). For example, the phase may be adjusted to a number of different values for each disturbance signal and the corresponding correlation computed. The disturbance signal(s) that best correlates with the error signal at each of the amplitude and phase values is selected to generate the feed-forward compensation values applied to the servo control system. In an alternative embodiment, the correlation is computed as a normalized correlation (e.g., using the equation shown inFIG. 4) to account for a difference in gain between each disturbance signal and the error signal.

Any suitable algorithm may be employed to generate the feed-forward compensation values in response to the selected disturbance signal.FIG. 6Ashows an embodiment of the present invention for adaptively generating feed-forward compensation values. The selected disturbance signal S164icomprises a sequence of digital values x(n) that is filtered by a finite impulse response (FIR) filter76to generate feed-forward compensation values y(n)60applied to the plant C80representing the actuator46. The output yc(n)50of the plant C80is subtracted from a reference to generate an error signal e(n)54(e.g., the PES of the VCM servo control system). The digital values x(n) of the selected disturbance signal S164iare applied to a model C*84of the plant C80to generate a sequence of digital values XC*(n)86representing the estimated effect the digital values x(n)74have on the plant C80. An adaptive algorithm88processes the digital values XC*(n)86and the error signal e(n)54in order to adapt the FIR filter76toward a state that minimizes the error signal e(n)54. In one embodiment, the goal is to minimize a cost function J(n)=E[e(n)2], where:

y⁡(n)=wT⁡(n)⁢x⁡(n)e⁡(n)=d⁡(n)-yc⁡(n)xc*⁡(n)=[∑i=0I-1⁢ci*⁢x⁡(n-i)∑i=0I-1⁢ci*⁢x⁡(n-i-1)⋮∑i=0I-1⁢ci*⁢x⁡(n-i-M-1)]
In the above equations, d(n) represents the reference signal an w represents the vector of coefficients in the FIR filter76. To find the optimal coefficients of the FIR filter the gradient method is used as described by:
∇w(n)J(n)=2E[e(n)∇w(n)e(n)]
which results in
w(n+1)=γw(n)+μxx*(n)e(n)
where γ represents the leakage factor and μ represents the step size. The above described adaption algorithm is based on a known filtered-X LMS algorithm. However, the feed-forward compensation values may be generated using any suitable algorithm.

FIG. 6Bshows an embodiment of the present invention wherein the feed-forward compensation values are generated in response to two selected disturbance signals S1and S2. The embodiment ofFIG. 6Buses the same adaptive feed-forward algorithm as inFIG. 6Afor each of the disturbance signals S1and S2, and the outputs of the respective FIR filters761and762are combined78to generate the feed-forward compensation values60applied to the actuator46(the plant80).

In another embodiment of the present invention, the optimal disturbance signal(s) that will optimize the feed-forward compensation are selected by evaluating a residual error of the servo control system. First feed-forward compensation values are generated in response to a first disturbance signal and an error signal of the servo control system. A first residual error is generated in response to the first feed-forward compensation values and an output of the servo control system. Second feed-forward compensation values are generated in response to a second disturbance signal and the error signal of the servo control system. A second residual error is generated in response to the second feed-forward compensation values and the output of the servo control system. At least one of the disturbance signals is selected in response to the first and second residual errors. Third feed-forward compensation values are generated in response to the selected disturbance signal, and the third feed-forward compensation values are applied to the servo control system to compensate for a vibration.

FIG. 7Ashows an embodiment of the present invention for selecting the disturbance signal(s) that will optimize the feed-forward compensation based on a residual error of the servo control system. In this embodiment, each of the disturbance signals641-64Nis selected82one at a time (by configuring multiplexer83) and used to generate68the feed-forward compensation values60applied to the servo control system. A residual error is generated based on the error signal54after performing feed-forward compensation for a period of time. After selecting each disturbance signal to generate the feed-forward compensation values60and generating a corresponding residual error, the disturbance signal(s) that generates the smallest residual error is selected82to generate68the feed-forward compensation values60during normal operation.

FIG. 7Bshows an alternative embodiment of the present invention for selecting the disturbance signal(s) that will optimize the feed-forward compensation based on a residual error of the servo control system. In this embodiment, first feed-forward compensation values901are generated in response to a first disturbance signal641and an error signal54of the servo control system. The first feed-forward compensation values901are applied to a model92of at least part of the servo control system to generate a first model output941. A first residual error961is generated in response to an output50of the servo control system and the first model output941. Second feed-forward compensation values902are generated in response to a second disturbance signal642and the error signal54of the servo control system. The second feed-forward compensation values902are applied to the model92of at least part of the servo control system to generate a second model output942. A second residual error962is generated in response to the output50of the servo control system and the second model output942. At least one of the disturbance signals641-64Nis selected98in response to the residual errors941-94N. A feed-forward compensation value60is generated in response to the selected disturbance signal64i, and the feed-forward compensation value60is applied to the servo control system to compensate for a vibration.

In the embodiment ofFIG. 7B, the feed-forward compensation values901-90Ngenerated by the feed-forward algorithms1001-100Ndrive the model outputs941-94Ntoward the output of the servo system50. The disturbance signal(s) that correlate well with the error signal54will generate the model output94closest to the output50of the servo control system, thereby minimizing the residual error96. Accordingly, in one embodiment the disturbance signal that generates the smallest residual error96, or the M disturbance signals that generate a residual error96smaller than a threshold, are selected to generate the feed-forward compensation values60applied to the servo control system.

In one embodiment, when executing the algorithm for selecting the disturbance signals641-64N, the feed-forward compensation of the servo control system is disabled. With the feed-forward compensation disabled, the effect of a vibration on the servo control system will manifest directly in the error signal54so that, for example, each of the disturbance signals641-64Nmay be correlated directly with the error signal54as illustrated in the embodiment ofFIG. 3A. After selecting the optimal disturbance signal(s) in response to the correlations, the feed-forward compensation is enabled using the selected disturbance signal(s).

In another embodiment, the feed-forward compensation is enabled while evaluating the disturbance signals641-64N. For example, a first disturbance signal may be selected for feed-forward compensation while the disk drive is subjected to a first type of vibration. Over time the type of vibration may change (due a change in operating conditions) so that the first disturbance signal may no longer correlate well with the error signal54. Accordingly, in one embodiment the control circuitry may execute the algorithm for selecting the disturbance signals641-64Nwhile the disk drive is operating normally, and change the selected disturbance signal(s) over time to adapt to changes in operating conditions. However, when the feed-forward compensation is enabled there will be at least some compensation of a different vibration using the currently selected disturbance signal. Therefore, in an embodiment shown inFIG. 8a disturbance observer102may be employed to estimate the effect of a vibration on the servo control system. The disturbance observer102evaluates the control signal48applied to the actuator46in order to estimate the degree to which the feed-forward compensation is compensating for the current vibration. The degree to which the feed-forward compensation is not compensating for the vibration will be reflected as a residual error in the error signal54. The disturbance observer102combines these signals to generate an error signal104that is used by the disturbance signal selection algorithm106to change the selected disturbance signal(s) over time to account for changes in the vibrations.