Determining head non-linearity in a disk drive

1. A method to determine head non-linearity in response to a dynamic fly height (DFH) control signal for a disk drive, the method comprising:

estimating a first and second set of head media spacing (HMS) actuation data for the head for a first and second data frequency by applying power to the head across a pre-defined power range;

determining a first and second HMS value based upon the first and second sets of HMS actuation data;

calculating a difference value between the first and second HMS values; and

determining that the difference value exceeds a threshold.

2. The method of claim 1 , further comprising setting a fly height value for the head to a default value.

3. The method of claim 1 , wherein the difference value is a means square difference value.

4. The method of claim 1 , wherein the first and second HMS values are a first and second power value for a given fly height.

5. The method of claim 1 , wherein the first and second HMS values are a first and second fly height for a given power value.

6. The method of claim 1 , wherein the HMS actuation data is estimated based upon a Wallace spacing loss equation.

7. The method of claim 1 , wherein the HMS actuation data is estimated based upon the head reading a fixed pattern from a disk having a fixed pattern density.

8. The method of claim 7 , wherein the first and second data frequencies are based upon harmonics of the fixed pattern.

9. The method of claim 7 , wherein the first and second data frequencies correspond to at least one of a first and a third harmonic of the fixed pattern.

10. The method of claim 1 , wherein the HMS actuation data is estimated based upon the head reading a first pattern having a first pattern density and a second pattern having a second pattern density that is different from the first pattern density.

11. The method of claim 10 , wherein the first and second frequencies are based upon the first and second pattern densities.

12. A disk drive to determine head non-linearity comprising:

a disk;

a dynamic fly height (DFH) head to write and read data to and from the disk, wherein a fly height of the DFH head is controlled responsive to a DFH control signal;

a processor for controlling operations to determine DFH head non-linearity, the processor under the control of a program to:

estimate a first and second set of head media spacing (HMS) actuation data for the head for a first and second data frequency by applying power to the DFH head across a pre-defined power range;

determine a first and second HMS value based upon the first and second sets of HMS actuation data;

calculate a difference value between the first and second HMS values; and

determine that the difference value exceeds a threshold.

13. The disk drive of claim 12 , wherein the processor further sets a fly height value for the DFH head to a default value.

14. The disk drive of claim 12 , wherein the difference value is a means square difference value.

15. The disk drive of claim 12 , wherein the first and second HMS values are a first and second power value for a given fly height.

16. The disk drive of claim 12 , wherein the first and second HMS values are a first and second fly height for a given power value.

17. The disk drive of claim 12 , wherein the processor estimates the HMS actuation data based upon a Wallace spacing loss equation.

18. The disk drive of claim 12 , wherein the processor estimates the HMS actuation data based upon the DFH head reading a fixed pattern from the disk having a fixed pattern density.

19. The disk drive of claim 18 , wherein the first and second data frequencies are based upon harmonics of the fixed pattern.

20. The disk drive of claim 18 , wherein the first and second data frequencies correspond to at least one of a first and a third harmonic of the fixed pattern.

21. The disk drive of claim 12 , wherein the processor estimates the HMS actuation data based upon the DFH head reading a first pattern having a first pattern density and a second pattern having a second pattern density that is different from the first pattern density and, wherein, the first and second frequencies are based upon the first and second pattern densities.

22. A disk drive to determine head non-linearity comprising:

means for controlling a fly height of a head responsive to a dynamic fly height (DFH) control signal;

means for estimating a first and second set of head media spacing (HMS) actuation data for the head for a first and second data frequency including means for applying power to the head across a pre-defined power range;

means for determining a first and second HMS value based upon the first and second sets of HMS actuation data;

means for calculating a difference value between the first and second HMS values; and

means for determining that the difference value exceeds a threshold.

23. The disk drive of claim 22 , further comprising means for setting a fly height value for the head to a default value.

24. The disk drive of claim 22 , wherein the difference value is a means square difference value.

25. The disk drive of claim 22 , wherein the means for estimating the HMS actuation data includes means for the head to read a fixed pattern from a disk having a fixed pattern density.