Disk drive determining head touchdown threshold based on curve fitting prediction error

1. A disk drive comprising:

a disk;

a head actuated over the disk;

a dynamic fly height (DFH) actuator operable to control a fly height of the head over the disk in response to a DFH setting;

a touchdown (TD) sensor operable to generate an analog TD signal; and

control circuitry operable to:

compare the analog TD signal to a TD threshold in order to generate a calibrated TD threshold for a plurality of different DFH settings;

curve fit the calibrated TD thresholds and corresponding DFH settings to an estimated curve;

adjust at least one of the TD threshold and the DFH setting and predict a corresponding next point in the estimated curve;

determine an actual next point corresponding to the adjustment;

generate a prediction error based on a difference between the predicted next point and the actual next point; and

detect a touchdown of the head on the disk when the prediction error exceeds a prediction threshold.

2. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to generate the calibrated TD threshold for a corresponding DFH setting by:

counting a number of times the analog TD signal exceeds the TD threshold over a predetermined window; and

determining the calibrated TD threshold for the corresponding DFH setting in response to the count.

3. The disk drive as recited in claim 2 , wherein the control circuitry is further operable to generate the actual next point by adjusting at least one of the TD threshold and the DFH setting until the count reaches a target value.

4. The disk drive as recited in claim 2 , wherein the control circuitry is further operable to generate an initial TD threshold based on a plurality of TD thresholds measured for an initial DFH setting that causes a high fly height for the head.

5. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to select the prediction threshold based on the plurality of TD thresholds measured for the initial DFH setting.

6. The disk drive as recited in claim 5 , wherein the control circuitry is further operable to select the prediction threshold based on a standard deviation of the plurality of TD thresholds measured for the initial DFH setting.

7. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to initialize the estimated curve based on the initial TD threshold.

8. The disk drive as recited in claim 7 , wherein the initial TD threshold comprises a mean of the plurality of TD thresholds measured for the initial DFH setting.

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

initialize the estimated curve by initializing a window of points based on the initial TD threshold and the initial DFH setting; and

replace points in the window over time with the actual next points.

10. The disk drive as recited in claim 9 , wherein the control circuitry is further operable to update the estimated curve based on the replaced points in the window.

11. A method of operating a disk drive, the disk drive comprising a head actuated over a disk, a dynamic fly height (DFH) actuator operable to control a fly height of the head over the disk in response to a DFH setting, and a touchdown (TD) sensor operable to generate an analog TD signal, the method comprising:

comparing the analog TD signal to a TD threshold in order to generate a calibrated TD threshold for a plurality of different DFH settings;

curve fitting the calibrated TD thresholds and corresponding DFH settings to an estimated curve;

adjusting at least one of the TD threshold and the DFH setting and predict a corresponding next point in the estimated curve;

determining an actual next point corresponding to the adjustment;

generating a prediction error based on a difference between the predicted next point and the actual next point; and

detecting a touchdown of the head on the disk when the prediction error exceeds a prediction threshold.

12. The method as recited in claim 11 , further comprising generating the calibrated TD threshold for a corresponding DFH setting by:

counting a number of times the analog TD signal exceeds the TD threshold over a predetermined window; and

determining the calibrated TD threshold for the corresponding DFH setting in response to the count.

13. The method as recited in claim 12 , further comprising generating the actual next point by adjusting at least one of the TD threshold and the DFH setting until the count reaches a target value.

14. The method as recited in claim 12 , further comprising generating an initial TD threshold based on a plurality of TD thresholds measured for an initial DFH setting that causes a high fly height for the head.

15. The method as recited in claim 14 , further comprising selecting the prediction threshold based on the plurality of TD thresholds measured for the initial DFH setting.

16. The method as recited in claim 15 , further comprising selecting the prediction threshold based on a standard deviation of the plurality of TD thresholds measured for the initial DFH setting.

17. The method as recited in claim 14 , further comprising initializing the estimated curve based on the initial TD threshold.

18. The method as recited in claim 17 , wherein the initial TD threshold comprises a mean of the plurality of TD thresholds measured for the initial DFH setting.

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

initializing the estimated curve by initializing a window of points based on the initial TD threshold and the initial DFH setting; and

replacing points in the window over time with the actual next points.

20. The method as recited in claim 19 , further comprising updating the estimated curve based on the replaced points in the window.