Dual-stage actuator disk drive with method for secondary-actuator failure detection and recovery using a relative-position sensor while track following

1. A method for operating a dual-stage actuator disk drive, the disk drive including (a) a rotatable magnetic recording disk having a plurality of concentric data tracks containing servo positioning information; (b) a recording head movable across the disk, the head being capable of reading data and servo positioning information in the data tracks; (c) a voice-coil-motor (VCM) for moving the head; (d) a secondary actuator connected to the VCM, the head being connected to the secondary actuator; (e) a relative-position sensor for detecting the position of the secondary actuator relative to its neutral position; and (f) a servo control processor responsive to servo information read by the head and comprising (i) a dual-stage controller having a degraded-stability VCM controller with relatively high low-frequency gain and a secondary actuator controller for simultaneously generating a VCM control signal and a secondary actuator control signal, and (ii) a selectable single-stage controller for generating only a VCM control signal, the processor-implemented method comprising:

selecting the single-stage controller if a potential failure of the secondary actuator is detected;

generating from the single-stage controller a VCM control signal to cause the head to follow a data track;

generating a first test signal to the secondary actuator;

receiving a relative-position signal (RPS) from the relative-position sensor in response to the first test signal;

calculating the stroke of the secondary actuator from the first test signal and the RPS; and

posting a secondary actuator failure signal if the calculated stroke is outside a predetermined stroke range.

2. The method of claim 1 further comprising, if the calculated stroke is within the predetermined range, generating a series of second test signals to the secondary actuator, each of the second test signals in the series being at a frequency different from the other second test signals in the series; receiving a series of second relative-position signals in response to the series of second test signals; calculating the frequency response of the secondary actuator from the second relative-position signals; recording the gains and corresponding frequencies from each of the second relative-position signals; and posting a secondary actuator failure signal if the maximum recorded gain is outside a predetermined range.

3. The method of claim 2 further comprising calculating the coherence of the calculated frequency response of the secondary actuator, and wherein a secondary-actuator failure signal is posted if the coherence is less than a predetermined value.

4. The method of claim 2 further comprising, if the maximum recorded gain is within a predetermined range, posting a secondary actuator failure signal if the maximum recorded gain is at a recorded frequency outside a predetermined frequency range.

5. The method of claim 2 further comprising, if the maximum recorded gain is at a recorded frequency within said predetermined frequency range, modifying the parameters of the controller from the recorded gains and corresponding frequencies.

6. The method of claim 5 further comprising terminating subsequent modification of the controller parameters if the controller parameters have been modified more than a predetermined number of times within a predetermined time period.

7. The method of claim 1 further comprising, if the calculated stroke is within the predetermined range, generating a series of second test signals to the secondary actuator, each of the second test signals in the series being at a frequency different from the other second test signals in the series; receiving a series of second relative-position signals in response to the series of second test signals; and recording the phases and corresponding frequencies from each of the second relative-position signals.

8. The method of claim 1 wherein the servo control processor also includes a model of the dynamic response of the VCM and a model of the dynamic response of the secondary actuator, and wherein the processor-implemented method further comprises:

providing a modeled head-position output when the control signals from the dual-stage controller are input to the models; and wherein detecting a potential failure of the secondary actuator comprises determining if the difference between the modeled head-position output and the measured head position from the servo information read by the head is greater than a predetermined value.

9. The method of claim 1 wherein the servo control processor also includes a model of the dynamic response of the secondary actuator, and wherein the processor-implemented method further comprises:

providing a modeled output of the position of the secondary actuator relative to the VCM when the secondary actuator control signal from the dual-stage controller is input to the secondary actuator model; and wherein detecting a potential failure of the secondary actuator comprises determining if the difference between the modeled secondary actuator relative position and the measured relative secondary actuator position from the relative-position sensor is greater than a predetermined value.

10. The method of claim 1 wherein the secondary actuator is an electrostatic microactuator having a first set of movable and fixed electrodes and a second set of movable and fixed electrodes, and wherein receiving a RPS comprises measuring the change in capacitance across each set of electrodes in response to the test signal.

11. The method of claim 1 wherein the secondary actuator is a piezoelectric actuator.

12. A method for operating a dual-stage actuator disk drive, the disk drive including (a) a rotatable magnetic recording disk having a plurality of concentric data tracks containing servo positioning information; (b) a recording head movable across the disk, the head being capable of reading data and servo positioning information in the data tracks; (c) a voice-coil-motor (VCM) for moving the head; (d) a secondary actuator connected to the VCM, the head being connected to the secondary actuator; (e) a relative-position sensor for detecting the position of the secondary actuator relative to its neutral position; and (f) a servo control processor responsive to servo information read by the head and comprising (i) a dual-stage controller for simultaneously generating a VCM control signal and a secondary-actuator control signal, and (ii) a selectable single-stage controller for generating only a VCM control signal, the processor-implemented method comprising:

selecting the single-stage controller if a potential failure of the secondary actuator is detected;

generating from the single-stage controller a VCM control signal to cause the head to follow a data track;

generating a series of dynamic test signals, each of the dynamic test signals in the series having a fixed frequency different from the other dynamic test signals in the series;

receiving, in response to each dynamic test signal in the series, a relative-position signal (RPS) from the relative-position sensor;

calculating the frequency response of the secondary actuator from the RPS received for each dynamic test signal in the series;

recording the gains and corresponding frequencies from each of the calculated secondary-actuator frequency responses; and

posting a secondary-actuator failure signal if the maximum recorded gain is outside a predetermined range.

13. The method of claim 12 further comprising, if the maximum recorded gain is within a predetermined range, posting a secondary-actuator failure signal if the maximum recorded gain is at a recorded frequency outside a predetermined frequency range.

14. The method of claim 12 further comprising:

calculating the coherence of the calculated frequency response of the secondary actuator for each dynamic test signal in the series;

calculating an average of the calculated coherences; and

posting a secondary-actuator failure signal if the average is less than a predetermined value.

15. A magnetic recording disk drive comprising:

a rotatable magnetic recording disk having a plurality of concentric data tracks containing servo positioning information;

a recording head movable across the disk, the head being capable of reading data and servo positioning information in the data tracks;

a voice-coil-motor (VCM) for moving the head;

a secondary actuator connected to the VCM, the head being connected to the secondary actuator;

a relative-position sensor for generating a relative-position signal (RPS) representing the position of the secondary actuator relative to its neutral position; and

a servo control processor responsive to servo information read by the head and to the RPS and comprising (i) a dual-stage controller for simultaneously generating a VCM control signal and a secondary-actuator control signal, (ii) a single-stage controller for generating only a VCM control signal, and (iii) a program of instructions readable by the processor for undertaking method acts comprising:

detecting a potential failure of the secondary actuator;

generating from the single-stage controller a VCM control signal to cause the head to follow a data track;

generating a series of dynamic test signals to the secondary actuator, each of the dynamic test signals in the series having a fixed frequency different from the other dynamic test signals in the series;

receiving the RPS in response to each dynamic test signal in the series;

calculating the frequency response of the secondary actuator from the RPS for each dynamic test signal in the series;

recording the gains and corresponding frequencies from each of the calculated secondary-actuator frequency responses; and

posting a secondary-actuator failure signal if the maximum recorded gain is outside a predetermined range.

16. The disk drive of claim 15 wherein the program includes instructions for undertaking method acts further comprising:

calculating the coherence of the calculated frequency response of the secondary actuator for each dynamic test signal in the series;

calculating an average of the calculated coherences; and

posting a secondary-actuator failure signal if the average is less than a predetermined value.