Regulating tuning rate of adaptive filter coefficients for feed-forward disturbance rejection in a servo control loop

• US 7,633,704 B2
• Filed: 04/28/2008
• Issued: 12/15/2009
• Est. Priority Date: 04/28/2008
• Status: Expired due to Fees

• Current Assignee: Seagate Technology LLC (Seagate Technology Holdings Plc)
• Original Assignee: Seagate Technology LLC (Seagate Technology Holdings Plc)
• Inventors: Supino, Louis, Bernett, Frank W.
• Primary Examiner(s): Olson; Jason C
• Application Number:
  US12/110,957
• Publication Number:
  US 20090268340A1
• Time in Patent Office: 596 Days
• Field of Search: None
• US Class Current: 360/75
• CPC Class Codes: G11B 19/042 due to external shock or vibration
• CPC Class Codes: G11B 5/5582 system adaptation for working during or after external perturbation, e.g. in the presence of a mechanical oscillation caused by a shock
• CPC Class Codes: G11B 5/596 for track following on disks G11B5/5526, G11B5/5552, G11B5/5565, G11B5/5582 take precedence

Abstract

In a servo control loop, the rate of change of filter coefficients used by an adaptive filter to counteract external disturbances to head position is regulated in response to a characteristic of an acceleration signal. Regulating the rate of change of the filter coefficients may improve the stability of the servo control loop.

Claims

1. A circuit comprising:

a filter coefficient adaptation circuit that responds to an acceleration signal and a position error signal, which is indicative of head position error, by tuning filter coefficients of an adaptive filter to reduce the position error signal, the filter coefficient adaptation circuit responding to a characteristic of the acceleration signal to constrain a rate at which the filter coefficients are changed to track changes of the acceleration signal.

2. The circuit of claim 1, further comprising:

an adaptive filter that responds to the filter coefficients to filter the acceleration signal, which is indicative of mechanical vibration, to generate a feed-forward signal that controls a head actuator to counteract disturbances to head position caused by the vibration.

3. The circuit of claim 2, wherein:

the adaptive filter comprises a finite impulse response (FIR) filter that filters the acceleration signal to generate the feed-forward signal; and

the filter coefficient adaptation circuit tunes filter coefficients applied to each of a plurality of time-delay taps of the FIR filter in response to the acceleration signal and a position error signal, and constrains rate of change of the filter coefficients in response to a characteristic of the acceleration signal.

4. The circuit of claim 3, wherein the filter coefficient adaptation circuit constrains rate of change of the filter coefficients in response to power level of the acceleration signal.

5. The circuit of claim 4, wherein the filter coefficient adaptation circuit increases the rate at which the filter coefficients are changed tracking changes of the acceleration signal and the position error signal in response to decreased power level of the acceleration signal, and decreases the rate of change of the filter coefficients tracking changes of the acceleration signal and the position error signal in response to increased power level of the acceleration signal.

6. The circuit of claim 3, wherein the filter coefficient adaptation circuit uses a least means squares (LMS) algorithm to repetitively tune the FIR filter coefficients in response to a result of multiplication of the acceleration signal and the position error signal.

7. The circuit of claim 6, wherein the filter coefficient adaptation circuit scales the product of the acceleration signal and the position error signal by a learning rate value that is varied inversely proportional to power level of the acceleration signal to generate the FIR filter coefficients.

8. The circuit of claim 7, wherein the filter coefficient adaptation circuit repetitively updates each of the FIR filter coefficients by scaling a previous value of a selected one of the FIR filter coefficients by a leaky term value that provides a defined decay rate over time on effect of past values of the selected FIR filter coefficient on the previous value of the selected FIR filter coefficient, and sums the scaled previous value of the selected FIR filter coefficient to the scaled product of the acceleration signal and the position error signal to generate a present updated value of the selected FIR filter coefficient.

9. The circuit of claim 8, wherein size of the leaky term value is varied over time proportional to the power level of the acceleration signal.

10. The circuit of claim 1, wherein the filter coefficient adaptation circuit constrains rate at which the filter coefficients are changed to track changes of the acceleration signal and the position error signal inversely proportional to a square of the acceleration signal.

11. The circuit of claim 10, wherein the filter coefficient adaptation circuit constrains rate of change of the filter coefficients in response to a ratio of a defined normalized step size value divided to the square of the acceleration signal.

12. The circuit of claim 1, wherein the filter coefficient adaptation circuit constrains the rate at which the filter coefficients are changed to track changes of the acceleration signal and the position error signal to within a defined range of values.

13. The circuit of claim 1, further comprising a phase normalization filter that conditions the vibration signal to counteract gain and phase lag introduced by at least some circuitry between a vibration sensor, which responds to the vibration by generating the vibration signal, and an input of the phase normalization filter, the phase normalization filter outputting a conditioned vibration signal to the filter coefficient adaptation circuit.

14. A method comprising:

tuning filter coefficients of an adaptive filter circuit in response to an acceleration signal, which is indicative of mechanical vibration, and a position error signal, which is indicative of head position error, to reduce the position error signal;

operating the adaptive filter circuit responsive to the filter coefficients to filter the acceleration signal to generate a feed-forward signal that controls a head actuator to counteract disturbances to head position caused by the vibration; and

regulating, responsive to a characteristic of the acceleration signal, a rate of change at which the filter coefficients are changed to track changes of the acceleration signal.

15. The method of claim 14, wherein:

operating the adaptive filter circuit comprises operating a finite impulse response (FIR) filter to filter the acceicration signal to generate the feed-forward signal; and

tuning the filter coefficients comprises tuning filter coefficients applied to each of a plurality of time-delay taps of the FIR filter in response to the acceleration signal and a position error signal.

16. The method of claim 15, wherein:

tuning the FIR filter coefficients comprises increasing the rate at which the FIR filter coefficients are changed tracking changes of the acceleration signal and the position error signal in response to decreased power level of the acceleration signal, and decreasing the rate of change of the FIR filter coefficients tracking changes of the acceleration signal and the position error signal in response to increased power level of the acceleration signal.

17. The method of claim 15, wherein tuning of the FIR filter coefficients comprises:

using a least means squares (LMS) algorithm to repetitively tune the FIR filter coefficients in response to a result of multiplication of the acceleration signal and the position error signal; and

scaling the product of the acceleration signal and the position error signal by a learning rate value that is varied inversely proportional to power level of the acceleration signal to generate the filter coefficients.

18. The method of claim 17, further comprising:

repetitively updating each of the FIR filter coefficients by scaling a previous value of a selected one of the FIR filter coefficients by a leaky term value that provides a defined decay rate over time on effect of past values of the selected FIR filter coefficient on the previous value of the selected FIR filter coefficient, and summing the scaled previous value of the selected FIR filter coefficient to the scaled product of the acceleration signal and the position error signal to generate a present updated value of the selected FIR filter coefficient.

19. A servo control circuit comprising:

a finite impulse response (FIR) filter that filters an acceleration signal, which is indicative of mechanical vibration, using a plurality of coefficients weights that are applied to a plurality of time-delay filter taps of the acceleration signal to generate a feed-forward signal;

a filter coefficient adaptation circuit that responds to the acceleration signal and a position error signal, which is indicative of head position error, by tuning the coefficients weights of the FIR filter to reduce the position error signal, the filter coefficient adaptation circuit responding to a characteristic of the acceleration signal to regulate a rate of change at which the coefficients weights are changed to track changes of the acceleration signal; and

a head actuator that responds to the feed-forward signal to control head position to counteract disturbances to head position caused by the vibration.