Disk drive tuning dual stage actuator servo loop gains from open loop response at target frequency

1. A disk drive comprising:

a disk surface;

a head coupled to a distal end of an actuator arm;

a dual stage actuator (DSA) servo loop comprising a voice coil motor (VCM) servo loop and a microactuator servo loop operable to actuate the head over the disk surface; and

control circuitry operable to:

disable the microactuator servo loop;

after disabling the microactuator servo loop:

inject a sinusoid into the VCM servo loop, wherein the sinusoid comprises a target frequency; and

compute a first open loop response of the VCM servo loop;

enable the microactuator servo loop;

after enabling the microactuator servo loop:

inject the sinusoid into the DSA servo loop; and

compute a second open loop response of the DSA servo loop; and

tune a microactuator servo loop gain and a VCM servo loop gain in response to the first and second open loop responses.

2. The disk drive as recited in claim 1 , wherein the control circuitry is operable to tune the microactuator servo loop gain and the VCM servo loop gain simultaneously to satisfy a target gain and phase at the frequency of the injected sinusoid.

3. The disk drive as recited in claim 1 , wherein the target frequency substantially matches a nominal bandwidth frequency of the DSA servo loop.

4. The disk drive as recited in claim 1 , wherein the DSA servo loop comprises an open loop response of the form:

k1·k2·Gc1·Gvcm·Gc2·G′uact+k1·Gc1·Gvcm+k2·Gc2·Guact

where:

k 1 and k 2 are scalars;

an absolute value |k 1 ·Gc 1 | represents the VCM servo loop gain;

an absolute value |k 2 ·Gc 2 | represents the microactuator servo loop gain;

G′uact represents a microactuator estimator;

Gvcm represents the VCM; and

Guact represents the microactuator.

5. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to determine Gc 1 ·Gvcm at the target frequency in response to the first open loop response.

6. The disk drive as recited in claim 5 , wherein the control circuitry is further operable to determine a nominal Gc 2 ·G′uact at the target frequency.

7. The disk drive as recited in claim 6 , wherein the control circuitry is further operable to determine Gc 2 ·Guact at the target frequency in response to the second open loop response, the determined Gc 1 ·Gvcm, and the determined Gc 2 ·G′uact.

8. The disk drive as recited in claim 7 , wherein the control circuitry is further operable to compute k 1 and k 2 in response to the determined Gc 2 ·Guact, the determined Gc 1 ·Gvcm, the determined Gc 2 ·G′uact, and a target open loop response.

9. The disk drive as recited in claim 8 , wherein the target open loop response comprises a target real part and a target imaginary part corresponding to a target magnitude and a target phase at the target frequency.

10. A method of operating a disk drive, the disk drive comprising a disk surface, a head coupled to a distal end of an actuator arm, and a dual stage actuator (DSA) servo loop comprising a voice coil motor (VCM) servo loop and a microactuator servo loop operable to actuate the head over the disk surface, the method comprising:

disabling the microactuator servo loop;

after disabling the microactuator servo loop:

injecting a sinusoid into the VCM servo loop, wherein the sinusoid comprises a target frequency; and

computing a first open loop response of the VCM servo loop;

enabling the microactuator servo loop;

after enabling the microactuator servo loop:

injecting the sinusoid into the DSA servo loop; and

computing a second open loop response of the DSA servo loop; and

tuning a microactuator servo loop gain and a VCM servo loop gain in response to the first and second open loop responses.

11. The method as recited in claim 10 , further comprising tuning the microactuator servo loop gain and the VCM servo loop gain simultaneously to satisfy a target gain and phase at the frequency of the injected sinusoid.

12. The method as recited in claim 10 , wherein the target frequency substantially matches a nominal bandwidth frequency of the DSA servo loop.

13. The method as recited in claim 10 , wherein the DSA servo loop comprises an open loop response of the form:

k1·k2·Gc1·Gvcm·Gc2·G′uact+k1·Gc1·Gvcm+k2·Gc2·Guact

where:

k 1 and k 2 are scalars;

an absolute value |k 1 ·Gc 1 | represents the VCM servo loop gain;

an absolute value |k 2 ·Gc 2 | represents the microactuator servo loop gain;

G′uact represents a microactuator estimator;

Gvcm represents the VCM; and

Guact represents the microactuator.

14. The method as recited in claim 13 , further comprising determining Gc 1 ·Gvcm at the target frequency in response to the first open loop response.

15. The method as recited in claim 14 , further comprising determining a nominal Gc 2 ·G′uact at the target frequency.

16. The method as recited in claim 15 , further comprising determining Gc 2 ·Guact at the target frequency in response to the second open loop response, the determined Gc 1 ·Gvcm, and the determined Gc 2 ·G′uact.

17. The method as recited in claim 16 , further comprising computing k 1 and k 2 in response to the determined Gc 2 ·Guact, the determined Gc 1 ·Gvcm, the determined Gc 2 ·G′uact, and a target open loop response.

18. The method as recited in claim 17 , wherein the target open loop response comprises a target real part and a target imaginary part corresponding to a target magnitude and a target phase at the target frequency.