Disk drive employing pivot friction compensation

1. A disk drive comprising:

(a) a disk comprising a plurality of servo tracks, wherein:

each servo track comprises a plurality of servo sectors forming N servo wedges; and

a wedge time period (WTP) occurs between each servo wedge;

(b) a head coupled to an actuator arm;

(c) a voice coil motor (VCM) for rotating the actuator arm about a pivot to actuate the head over the disk; and

(d) control circuitry for generating a VCM control signal applied to the VCM by:

generating an estimated WTP by detecting the servo sectors;

estimating a pivot velocity in response to the estimated WTP, the pivot velocity representing a velocity of the actuator arm about the pivot;

generating a pivot friction compensation in response to the estimated pivot velocity; and

generating the VCM control signal in response to the pivot friction compensation.

2. The disk drive as recited in claim 1 , wherein the estimated WTP is generated relative to:

RTP+â*cos(2πk/N)+{circumflex over (b)}*sin(2πk/N)

where RTP is a reference time period corresponding to a nominal WTP, k is an index representing one of the servo wedges, and {â,{circumflex over (b)}} are adjustable coefficients.

3. The disk drive as recited in claim 2 , wherein the coefficients {â,{circumflex over (b)}} for generating the estimated WTP are adjusted according to:

â ( k+ 1)={circumflex over ( a )}( k )− G*e ( k )*cos(2 πk/N )

{circumflex over (b)} ( k+ 1)={circumflex over ( b )}( k )− G*e ( k )*sin(2 πk/N )

wherein:

G is a gain;

e(k) is a wedge time error generated in response to the difference between the estimated WTP and an actual WTP; and

the actual WTP is generated by detecting the servo sectors.

4. The disk drive as recited in claim 1 , wherein the pivot velocity is estimated relative to a derivative of the estimated WPT.

5. The disk drive as recited in claim 1 , wherein the pivot friction compensation is generated relative to:

g c (t)*sign(v)

where g c (t) is a gain and sign(v) represents a sign of the estimated pivot velocity.

6. The disk drive as recited in claim 5 , wherein the gain g c (t) is adaptable.

7. The disk drive as recited in claim 6 , wherein the gain g c (t) is adaptable relative to:

g c ( k+ 1)= g c ( k )−μ*PES( k )* P ( k )

wherein:

k is an index representing one of the servo sectors;

μ is a gain;

PES(k) is a position error signal representing a radial location of the head and generated in response to the servo sector at index k; and

P(k) is a train of pulses having an alternating sign and representing the effect of the pivot friction on the PES(k).

8. The disk drive as recited in claim 7 , wherein P(k) is generated according to:

sign(v(k))−sign(v(k−M))

wherein v(k) is the estimated pivot velocity and M is a width of the pulses.

9. A method of compensating for pivot friction in a disk drive, the disk drive comprising a disk having a plurality of servo tracks, a head coupled to an actuator arm, and a voice coil motor (VCM) for rotating the actuator arm about a pivot to actuate the head over the disk, wherein each servo track comprises a plurality of servo sectors forming N servo wedges, and a wedge time period (WTP) occurs between each servo wedge, the method comprising:

generating an estimated WTP by detecting the servo sectors;

estimating a pivot velocity in response to the estimated WTP, the pivot velocity representing a velocity of the actuator arm about the pivot;

generating a pivot friction compensation in response to the estimated pivot velocity;

generating a VCM control signal in response to the pivot friction compensation; and

applying the VCM control signal to the VCM.

10. The method as recited in claim 9 , wherein the estimated WTP is generated relative to:

RTP+â*cos(2πk/N)+{circumflex over (b)}*sin(2πk/N)

where RTP is a reference time period corresponding to a nominal WTP, k is an index representing one of the servo wedges, and {â,{circumflex over (b)}} are adjustable coefficients.

11. The method as recited in claim 10 , wherein the coefficients {â,{circumflex over (b)}} for generating the estimated WTP are adjusted according to:

â ( k+ 1)={circumflex over ( a )}( k )− G*e ( k )*cos(2 πk/N )

{circumflex over (b)} ( k+ 1)={circumflex over ( b )}( k )− G*e ( k )*sin(2 πk/N )

wherein:

G is a gain;

e(k) is a wedge time error generated in response to the difference between the estimated WTP and an actual WTP; and

the actual WTP is generated by detecting the servo sectors.

12. The method as recited in claim 9 , wherein the pivot velocity is estimated relative to a derivative of the estimated WPT.

13. The method as recited in claim 9 , wherein the pivot friction compensation is generated relative to:

g c (t)*sign(v)

where g c (t) is a gain and sign(v) represents a sign of the estimated pivot velocity.

14. The method as recited in claim 13 , wherein the gain g c (t) is adaptable.

15. The method as recited in claim 14 , wherein the gain g c (t) is adaptable relative to:

g c ( k+ 1)= g c ( k )−μ*PES( k )* P ( k )

wherein:

k is an index representing one of the servo sectors;

μ is a gain;

PES(k) is a position error signal representing a radial location of the head and generated in response to the servo sector at index k; and

P(k) is a train of pulses having an alternating sign and representing the effect of the pivot friction on the PES(k).

16. The method as recited in claim 15 , wherein P(k) is generated according to:

sign(v(k))−sign(v(k−M))

wherein v(k) is the estimated pivot velocity and M is a width of the pulses.

17. A disk drive comprising:

(a) a disk comprising a plurality of servo tracks;

(b) a head coupled to an actuator arm;

(c) a voice coil motor (VCM) for rotating the actuator arm about a pivot to actuate the head over the disk; and

(d) control circuitry for generating a VCM control signal by:

estimating a pivot velocity representing a velocity of the actuator arm about the pivot; and

generating a pivot friction compensation relative to:

g c (t)*sign(v)

wherein:

sign(v) represents a sign of the estimated pivot velocity; and

g c (t) is a gain adaptable relative to:

g c ( k+ 1)= g c ( k )−μ*PES( k )* P ( k )

where:

k is an index representing one of the servo sectors;

μ is a gain;

PES(k) is a position error signal representing a radial location of the head and generated in response to the servo sector at index k; and

P(k) is a train of pulses having an alternating sign and representing the effect of the pivot friction on the PES(k).

18. The disk drive as recited in claim 17 , wherein P(k) is generated according to:

sign(v(k))−sign(v(k−M))

wherein v(k) is the estimated pivot velocity and M is a width of the pulses.

19. A method of compensating for pivot friction in a disk drive, the disk drive comprising a disk having a plurality of servo tracks, a head coupled to an actuator arm, and a voice coil motor (VCM) for rotating the actuator arm about a pivot to actuate the head over the disk, the method comprising:

estimating a pivot velocity representing a velocity of the actuator arm about the pivot; and

generating a pivot friction compensation relative to:

g c (t)*sign(v)

wherein:

sign(v) represents a sign of the estimated pivot velocity; and

g c (t) is a gain adaptable relative to:

g c ( k+ 1)= g c ( k )−μ*PES( k )* P ( k )

where:

k is an index representing one of the servo sectors;

μ is a gain;

PES(k) is a position error signal representing a radial location of the head and generated in response to the servo sector at index k; and

P(k) is a train of pulses having an alternating sign and representing the effect of the pivot friction on the PES(k).

20. The method as recited in claim 19 , wherein P(k) is generated according to:

sign(v(k))−sign(v(k−M))

wherein v(k) is the estimated pivot velocity and M is a width of the pulses.