Disk drive computing data center off-track from repeatable runout of write servo track and read servo track

1. A disk drive comprising:

a disk comprising a plurality of servo tracks that define a plurality of data tracks;

a head actuated over the disk by a voice coil motor (VCM), wherein the head comprises a read element offset radially from a write element; and

control circuitry operable to:

position the read element over a first servo track and the write element over a second servo track;

measure a first repeatable runout (RRO) of the first servo track;

position the read element over the second servo track;

measure a second RRO of the second servo track; and

compute a data center off-track in response to the first RRO and the second RRO.

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

servo the head in response to feedforward compensation values;

adapt the feedforward compensation values; and

disable the feedforward compensation values when measuring the first RRO and the second RRO.

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

servo the head in response to feedforward compensation values;

adapt the feedforward compensation values; and

freeze the adapting of the feedforward compensation values when measuring the first RRO and the second RRO.

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

the VCM is controlled according to an approximate open loop impulse response h; and

the data center off-track is computed in response to the first RRO, the second RRO, and the open loop impulse response h.

5. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to:

compute a first position Y W associated with the movement of the read element while following the first servo track by convolving the open loop impulse response h with the first RRO;

compute a second position Y R associated with the movement of the read element while following the second servo track by convolving the open loop impulse response h with the first RRO; and

compute the data center off-track as a difference between Y W and Y R .

6. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to compute the data center off-track by convolving the open loop impulse response h with a difference between the first RRO and the second RRO.

7. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to relocate at least one of the data tracks to a spare data track when the data center off-track exceeds a threshold.

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

generate wedge RRO values used to compensate for eccentricity when the read element is positioned over at least one of the first and second servo tracks; and

adjust at least one of the wedge RRO values in response to the data center off-track.

9. A method of operating a disk drive, the disk drive comprising a disk comprising a plurality of servo tracks that define a plurality of data tracks, and a head actuated over the disk by a voice coil motor (VCM), wherein the head comprises a read element offset radially from a write element, the method comprising:

positioning the read element over a first servo track and the write element over a second servo track;

measuring a first repeatable runout (RRO) of the first servo track;

positioning the read element over the second servo track;

measuring a second RRO of the second servo track; and

computing a data center off-track in response to the first RRO and the second RRO.

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

servoing the head in response to feedforward compensation values;

adapting the feedforward compensation values; and

disabling the feedforward compensation values when measuring the first RRO and the second RRO.

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

servoing the head in response to feedforward compensation values;

adapting the feedforward compensation values; and

freezing the adapting of the feedforward compensation values when measuring the first RRO and the second RRO.

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

the VCM is controlled according to an approximate open loop impulse response h; and

the data center off-track is computed in response to the first RRO, the second RRO, and the open loop impulse response h.

13. The method as recited in claim 12 , further comprising:

computing a first position Y W associated with the movement of the read element while following the first servo track by convolving the open loop impulse response h with the first RRO;

computing a second position Y R associated with the movement of the read element while following the second servo track by convolving the open loop impulse response h with the first RRO; and

computing the data center off-track as a difference between Y W and Y R .

14. The method as recited in claim 12 , wherein the data center off-track is computed by convolving the open loop impulse response h with a difference between the first RRO and the second RRO.

15. The method as recited in claim 12 , further comprising relocating at least one of the data tracks to a spare data track when the data center off-track exceeds a threshold.

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

generating wedge RRO values used to compensate for eccentricity when the read element is positioned over at least one of the first and second servo tracks; and

adjusting at least one of the wedge RRO values in response to the data center off-track.