Disk drive employing offset compensation for velocity control of a voice coil motor

1. A disk drive comprising:

a disk;

a head;

a voice coil motor (VCM) comprising a voice coil operable to actuate the head radially over the disk, the voice coil comprising a first end and a second end;

a VCM control loop comprising a voltage detector operable to detect a back EMF voltage across the voice coil, a current detector operable to detect a current flowing through the voice coil, and a control voltage generator, responsive to a command input and the detected back EMF voltage, operable to generate a control voltage applied to the voice coil; and

control circuitry operable to:

(a) initialize the control voltage;

(b) inject a transient into the control loop and detect a corresponding current flowing through the voice coil;

(c) adjust the control voltage in response to a polarity of the detected current;

(d) repeat elements (b) and (c) at least once; and

(e) store an offset value in response to the adjusted control voltage.

2. The disk drive as recited in claim 1 , wherein the control voltage generator is responsive to the stored offset value to generate the control voltage.

3. The disk drive as recited in claim 1 , wherein the control circuitry adjusts the control voltage relative to a delta value.

4. The disk drive as recited in claim 3 , wherein the control circuitry adjusts the delta value when the polarity of the detected current changes sign.

5. The disk drive as recited in claim 4 , wherein the control circuitry decreases the delta value when the polarity of the detected current changes sign.

6. The disk drive as recited in claim 1 , wherein the control circuitry injects the transient into the VCM control loop by injecting a transient into the detected back EMF voltage.

7. The disk drive as recited in claim 6 , wherein the voltage detector comprises:

an IR voltage detector, responsive to the detected current, operable to detect a resistive voltage due to a resistance of the voice coil; and

a voltage compensator for adjusting the detected back EMF voltage in response to the detected resistive voltage;

wherein the control circuitry injects the transient into the detected back EMF voltage by adjusting the IR voltage detector.

8. The disk drive as recited in claim 7 , wherein the IR voltage detector comprises a variable resistor representing a resistance estimate of the voice coil, and the control circuitry injects the transient into the detected back EMF voltage by adjusting the variable resistor.

9. The disk drive as recited in claim 8 , wherein the control circuitry injects the transient into the detected back EMF voltage by increasing a resistance of the variable resistor.

10. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to tristate the voice coil for a tristate interval after detecting the current flowing through the voice coil.

11. The disk drive as recited in claim 1 , wherein the current detector comprises a variable resistor, and the control circuitry injects the transient into the VCM control loop by adjusting the variable resistor.

12. The disk drive as recited in claim 1 , wherein the control circuitry injects the transient into the VCM control loop by adjusting a gain of an amplifier.

13. A method of operating a voice coil motor (VCM) control loop in a disk drive, the disk drive comprising a disk, a head, a VCM comprising a voice coil operable to actuate the head radially over the disk, the voice coil comprising a first end and a second end, the method comprising:

(a) initializing a control voltage applied to the voice coil;

(b) detecting a back EMF voltage across the voice coil;

(c) injecting a transient into the VCM control loop and detecting a corresponding current flowing through the voice coil;

(d) adjusting the control voltage in response to a polarity of the detected current;

(e) repeating elements (b) through (d) at least once; and

(f) storing an offset value in response to the adjusted control voltage.

14. The method as recited in claim 13 , further comprising generating the control voltage in response to the stored offset value.

15. The method as recited in claim 13 , wherein adjusting the control voltage comprises adjusting the control voltage relative to a delta value.

16. The method as recited in claim 15 , further comprising adjusting the delta value when the polarity of the detected current changes sign.

17. The method as recited in claim 16 , further comprising decreasing the delta value when the polarity of the detected current changes sign.

18. The method as recited in claim 13 , wherein injecting the transient into the VCM control loop comprises injecting a transient into the detected back EMF voltage.

19. The method as recited in claim 18 , wherein detecting the back EMF voltage comprises:

detecting a resistive voltage due to a resistance of the voice coil; and

adjusting the detected back EMF voltage in response to the detected resistive voltage;

wherein injecting the transient into the detected back EMF voltage comprises adjusting the resistive voltage.

20. The method as recited in claim 19 , wherein adjusting the resistive voltage comprises adjusting a variable resistor.

21. The method as recited in claim 20 , wherein adjusting the resistive voltage comprises increasing a resistance of the variable resistor.

22. The method as recited in claim 13 , further comprising tristating the voice coil for a tristate interval after detecting the current flowing through the voice coil.

23. The method as recited in claim 13 , wherein injecting the transient into the VCM control loop comprises adjusting a variable resistor used to detect the current flowing through the voice coil.

24. The method as recited in claim 13 , wherein injecting the transient into the VCM control voltage comprises adjusting a gain of an amplifier.

25. A disk drive comprising:

(a) a disk;

(b) a head;

(c) a voice coil motor (VCM) comprising a voice coil operable to actuate the head radially over the disk, the voice coil comprising a first end and a second end;

(d) a VCM control loop comprising

a means for detecting a back EMF voltage across the voice coil;

a means for detecting a current flowing through the voice coil;

a means for generating a control voltage applied to the voice coil in response to a command input and the detected back EMF voltage;

(e) a means for initializing the control voltage;

(f) a means for injecting a transient into the VCM control loop and adjusting the control voltage;

(g) a means for tristating the voice coil for a tristate interval;

(h) a means for repeating elements (f) and (g) at least once; and

(i) a means for storing an offset value in response to the adjusted control voltage.