Compensating for thermal expansion when writing spiral tracks to a disk of a disk drive

1. A method of writing M spiral tracks (i=1 to M) to a disk of a disk drive, the method comprising:

positioning a head over a first radial location;

writing a first concentric reference track at the first radial location, the first concentric reference track comprising N concentric servo sectors;

prior to writing one of the spiral tracks:

positioning the head over the first concentric reference track, reading the concentric servo sectors in the first concentric reference track, and generating a position error signal first_PES 0 (n) for each servo sector (n=1 to N) representing an offset from the first radial location;

positioning the head over the first concentric reference track and reading the first concentric reference track;

generating a position error signal first_PES i (j) for at least one of the servo sectors j in the first concentric reference track, wherein the first_PES i (j) represents an offset of the head from the first radial location, and the servo sector j corresponds to a circumferential location of the spiral track;

generating a distance delta D_Δ relative to a difference between the first_PES 0 (j) and the first_PES i (j), wherein at least one of a starting radial location and a velocity profile is adjusted in response to the distance delta D_Δ; and

writing the spiral track to the disk using the starting radial location and the velocity profile.

2. The method as recited in claim 1 , wherein adjusting at least one of the starting radial location and the velocity profile in response to the first_PES i (j) compensates for uniform and non-uniform thermal expansion.

3. The method as recited in claim 1 , further comprising:

positioning the head over a second radial location;

writing a second concentric reference track at the second radial location, the second concentric reference track comprising N concentric servo sectors;

prior to writing one of the spiral tracks:

positioning the head over the second concentric reference track and generating a position error signal second_PES i (j) for at least one of the servo sectors j in the second concentric reference track representing an offset from the second radial location, the servo sector j corresponding to a circumferential location of the spiral track; and

wherein at least one of the starting radial location and the velocity profile is adjusted in response to the first_PES i (j) and the second_PES i (j).

4. The method as recited in claim 3 , wherein:

the first radial location is near an outer diameter of the disk; and

the second radial location is near an inner diameter of the disk.

5. The method as recited in claim 1 , further comprising:

comparing the first_PES i (j) to a threshold; and

if the first_PES i (j) exceeds the threshold, regenerating the first_PES i (j) for at least one of the servo sectors corresponding to a circumferential location of the spiral track.

6. The method as recited in claim 5 , wherein the first_PES i (j) is regenerated during a subsequent revolution of the disk using the same servo sector.

7. The method as recited in claim 5 , wherein the first_PES i (j) is regenerated using a different servo sector.

8. The method as recited in claim 7 , wherein the first_PES i (j) is regenerated during the same revolution of the disk.

9. The method as recited in claim 1 , further comprising:

saving the distance delta D_Δ generated for a first spiral track;

generating a difference between the distance delta D_Δ generated for a second spiral track and the distance delta D_Δ generated for the first spiral track; and

if the difference exceeds a threshold:

adjusting the at least one of the starting radial location and the velocity profile in response to the distance delta D_Δ generated for the first spiral track; and

writing the second spiral track to the disk using the starting radial location and the velocity profile.

10. A disk drive comprising:

a disk;

a head actuated over the disk; and

control circuitry operable to write M spiral tracks (i=1 to M) to the disk by:

positioning a head over a first radial location;

writing a first concentric reference track at the first radial location, the first concentric reference track comprising N concentric servo sectors;

prior to writing one of the spiral tracks:

positioning the head over the first concentric reference track, reading the concentric servo sectors in the first concentric reference track, and generating a position error signal first_PES 0 (n) for each servo sector (n=1 to N) representing an offset from the first radial location;

positioning the head over the first concentric reference track and reading the first concentric reference track;

generating a position error signal first_PES i (j) for at least one of the servo sectors j in the first concentric reference track, wherein the first_PES i (j) represents an offset of the head from the first radial location, and the servo sector j corresponds to a circumferential location of the spiral track;

generating a distance delta D_Δ relative to a difference between the first_PES 0 (j) and the first_PES i (j), wherein at least one of a starting radial location and a velocity profile is adjusted in response to the distance delta D_Δ; and

writing the spiral track to the disk using the starting radial location and the velocity profile.

11. The disk drive as recited in claim 10 , wherein adjusting at least one of the starting radial location and the velocity profile in response to the first_PES i (j) compensates for uniform and non-uniform thermal expansion.

12. The disk drive as recited in claim 10 , wherein the control circuitry is further operable to write the plurality of spiral tracks (i=1 to M) to the disk by:

positioning the head over a second radial location;

writing a second concentric reference track at the second radial location, the second concentric reference track comprising N concentric servo sectors;

prior to writing one of the spiral tracks:

positioning the head over the second concentric reference track and generating a position error signal second_PES i (j) for at least one of the servo sectors j in the second concentric reference track representing an offset from the second radial location, the servo sector j corresponding to a circumferential location of the spiral track; and

wherein at least one of the starting radial location and the velocity profile is adjusted in response to the first_PES i (j) and the second_PES i (j).

13. The disk drive as recited in claim 12 , wherein:

the first radial location is near an outer diameter of the disk; and

the second radial location is near an inner diameter of the disk.

14. The disk drive as recited in claim 10 , wherein the control circuitry is further operable to write the plurality of spiral tracks (i=1 to M) to the disk by:

comparing the first_PES i (j) to a threshold; and

if the first_PES i (j) exceeds the threshold, regenerating the first_PES i (j) for at least one of the servo sectors corresponding to a circumferential location of the spiral track.

15. The disk drive as recited in claim 14 , wherein the first_PES i (j) is regenerated during a subsequent revolution of the disk using the same servo sector.

16. The disk drive as recited in claim 14 , wherein the first_PES i (j) is regenerated using a different servo sector.

17. The disk drive as recited in claim 16 , wherein the first_PES i (j) is regenerated during the same revolution of the disk.

18. The disk drive as recited in claim 10 , further comprising:

saving the distance delta D_Δ generated for a first spiral track;

generating a difference between the distance delta D_Δ generated for a second spiral track and the distance delta D_Δ generated for the first spiral track; and

if the difference exceeds a threshold:

adjusting the at least one of the starting radial location and the velocity profile in response to the distance delta D_Δ generated for the first spiral track; and

writing the second spiral track to the disk using the starting radial location and the velocity profile.