Disk drive employing comb filter for repeatable fly height compensation

1. A disk drive comprising:

a disk comprising a plurality of tracks;

a head actuated over the disk;

a fly height actuator operable to adjust a fly height of the head; and

control circuitry operable to:

generate a fly height measurement (FHM) for the head;

generate a fly height error (FHE) as a difference between the FHM and a target fly height;

using a comb filter to generate fly height compensation values in response to the FHE;

generate a fly height control signal in response to the FHE and the fly height compensation values; and

apply the fly height control signal to the fly height actuator.

2. The disk drive as recited in claim 1 , wherein the control circuitry is operable to generate the fly height control signal from a feed-forward compensation value generated in response to the fly height compensation values.

3. The disk drive as recited in claim 1 , wherein the control circuitry is operable to adjust the FHE in response to the fly height compensation values.

4. The disk drive as recited in claim 1 , wherein:

each track comprises N servo sectors;

the comb filter comprises N registers each corresponding to a servo sector; and

one of the fly height compensation values is generated for each servo sector in response to a value stored in the corresponding register of the comb filter.

5. The disk drive as recited in claim 4 , wherein the comb filter comprises a frequency response comprising:

H

⁡

(

ⅇ

j

⁢

⁢

ω

)

⁢

β

·

1

1

-

α

⁢

⁢

ⅇ

j

⁢

⁢

ω

⁢

⁢

N

wherein 1/N represents a rotation frequency of the disk.

6. The disk drive as recited in claim 5 , wherein a magnitude of the comb filter frequency response extracts the rotation frequency from the FHE and harmonics of the rotation frequency from the FHE.

7. The disk drive as recited in claim 1 , wherein the control circuitry is operable to:

generate the fly height compensation values over multiple disk revolutions; and

after generating the fly height compensation values, post process the fly height compensation values to attenuate a frequency.

8. The disk drive as recited in claim 7 , wherein the attenuated frequency comprises a harmonic of a rotation frequency of the disk.

9. A method of operating a disk drive comprising a head actuated over a disk comprising a plurality of tracks, and a fly height actuator operable to adjust a fly height of the head, the method comprising:

generating a fly height measurement (FHM) for the head;

generating a fly height error (FHE) as a difference between the FHM and a target fly height;

using a comb filter to generate fly height compensation values in response to the FHE;

generating a fly height control signal in response to the FHE and the fly height compensation values; and

applying the fly height control signal to the fly height actuator.

10. The method as recited in claim 9 , further comprising generating the fly height control signal from a feed-forward compensation value generated in response to the fly height compensation values.

11. The method as recited in claim 9 , further comprising adjusting the FHE in response to the fly height compensation values.

12. The method as recited in claim 9 , wherein:

each track comprises N servo sectors;

the comb filter comprises N registers each corresponding to a servo sector; and

one of the fly height compensation values is generated for each servo sector in response to a value stored in the corresponding register of the comb filter.

13. The method as recited in claim 12 , wherein the comb filter comprises a frequency response comprising:

H

⁡

(

ⅇ

j

⁢

⁢

ω

)

⁢

β

·

1

1

-

α

⁢

⁢

ⅇ

j

⁢

⁢

ω

⁢

⁢

N

wherein 1/N represents a rotation frequency of the disk.

14. The method as recited in claim 13 , wherein a magnitude of the comb filter frequency response comprises a plurality of peaks for extracting the rotation frequency from the FHE and harmonics of the rotation frequency from the FHE.

15. The method as recited in claim 14 , further comprising adjusting at least one of the β and α coefficients to adjust at least one of a width and magnitude of the peaks.

16. The method as recited in claim 15 , further comprising adjusting at least one of the β and α coefficients within predetermined limits to achieve stability.

17. The method as recited in claim 9 , further comprising:

generating the fly height compensation values over multiple disk revolutions; and

after generating the fly height compensation values, post processing the fly height compensation values to attenuate a frequency.

18. The method as recited in claim 17 , wherein the attenuated frequency comprises a harmonic of a rotation frequency of the disk.