Disk drive detecting disk warping by detecting negative correlation between read signal amplitudes from top and bottom disk surfaces

1. A disk drive comprising:

(a) a disk comprising a top surface and a bottom surface;

(b) a top head actuated over the top surface of the disk;

(c) a bottom head actuated over the bottom surface of the disk; and

(d) control circuitry operable to detect a warping of the disk by:

writing a first test pattern to the top surface;

reading the first test pattern to generate a first read signal;

monitoring a first read signal value proportional to an amplitude of the first read signal;

writing a second test pattern to the bottom surface;

reading the second test pattern to generate a second read signal;

monitoring a second read signal value proportional to an amplitude of the second read signal; and

processing the first and second read signal values to detect a negative correlation between the amplitudes of the first and second read signals.

2. The disk drive as recited in claim 1 , wherein the first test pattern is the same as the second test pattern.

3. The disk drive as recited in claim 1 , wherein the first test pattern is different than the second test pattern.

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

(a) the control circuitry comprises a variable gain amplifier (VGA) for amplifying the first read signal and the second read signal; and

(b) the first and second read signal values are used to adjust a gain of the VGA.

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

(a) the control circuitry further comprises a gain control operable to compute a gain error relative to a difference between a target amplified read signal amplitude and a measured amplified read signal amplitude;

(b) the gain control is further operable to adjust the gain of the VGA in response to the gain error; and

(c) the first and second read signal values comprise the gain error.

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

(a) monitor a first plurality of read signal values used to adjust the gain of the VGA while reading the first test pattern, and compute a first average value in response to the first plurality of read signal values;

(b) compute a deviation of each of the first plurality of read signal values from the first average value to generate a first plurality of deviation values;

(c) monitor a second plurality of read signal values used to adjust the gain of the VGA while reading the second test pattern, and compute a second average value in response to the second plurality of read signal values;

(d) compute a deviation of each of the second plurality of read signal values from the second average value to generate a second plurality of deviation values; and

(e) process the first and second plurality of deviation values.

7. The disk drive as recited in claim 6 , wherein the control circuitry is further operable to compute a running average of the first and second plurality of deviation values.

8. The disk drive as recited in claim 6 , wherein the control circuitry is further operable to compare the first and second plurality of deviation values to a threshold, wherein:

the warping of the disk may be detected if n out of m deviation values exceed the threshold; and

n and m are predetermined integers where n is less than or equal to m.

9. The disk drive as recited in claim 6 , wherein the control circuitry is further operable to:

(a) compute a first peak in the first plurality of deviation values;

(b) compute a second peak in the second plurality of deviation values; and

(c) compare a location of the first peak to a location of the second peak.

10. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to compare the negative correlation to a threshold.

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

(a) the control circuitry is further operable to detect a first bit sequence in response to the first read signal, and detect a second bit sequence in response to the second read signal; and

(b) the first read signal value comprises at least one bit in the first bit sequence, and the second read signal value comprises at least one bit in the second bit sequence.

12. A method of detecting disk warping in a disk drive, the disk drive comprising a disk having a top surface and a bottom surface, a top head actuated over the top surface and a bottom head actuated over the bottom surface, the method comprising the steps

(a) writing a first test pattern to the top surface;

(b) reading the first test pattern to generate a first read signal;

(c) monitoring a first read signal value proportional to an amplitude of the first read signal;

(d) writing a second test pattern to the bottom surface;

(e) reading the second test pattern to generate a second read signal;

(f) monitoring a second read signal value proportional to an amplitude of the second read signal; and

(g) processing the first and second read signal values to detect a negative correlation between the amplitudes of the first and second read signals.

13. The method as recited in claim 12 , wherein the first test pattern is the same as the second test pattern.

14. The method as recited in claim 12 , wherein the first test pattern is different than the second test pattern.

15. The method as recited in claim 12 , further comprising the steps of:

amplifying the first read signal in response to the first read signal value; and

amplifying the second read signal in response to the second read signal value.

16. The method as recited in claim 15 , further comprising the step of computing a gain error relative to a difference between a target amplified read signal amplitude and a measured amplified read signal amplitude, wherein the first and second read signal values comprise the gain error.

17. The method as recited in claim 15 , further comprising the steps of:

monitoring a first plurality of read signal values used to adjust the amplification of the first read signal while reading the first test pattern;

computing a first average value in response to the first plurality of read signal values;

computing a deviation of each of the first plurality of read signal values from the first average value to generate a first plurality of deviation values;

monitoring a second plurality of read signal values used to adjust the amplification of the second read signal while reading the second test pattern;

computing a second average value in response to the second plurality of read signal values;

computing a deviation of each of the second plurality of read signal values from the second average value to generate a second plurality of deviation values; and

processing the first and second plurality of deviation values.

18. The method as recited in claim 17 , further comprising the step of computing a running average of the first and second plurality of deviation values.

19. The method as recited in claim 17 , further comprising the step of comparing the first and second plurality of deviation values to a threshold, wherein:

the warping of the disk may be detected if n out of m deviation values exceed the threshold; and

n and m are predetermined integers where n is less than or equal to m.

20. The method as recited in claim 17 , further comprising the steps of computing a first peak in the first plurality of deviation values;

computing a second peak in the second plurality of deviation values; and

comparing a location of the first peak to a location of the second peak.

21. The method as recited in claim 12 , further comprising the step of comparing the negative correlation to a threshold.

22. The method as recited in claim 12 , wherein further comprising the steps of detecting a first bit sequence in response to the first read signal, and detecting a second bit sequence in response to the second read signal, wherein the first read signal value comprises at least one bit in the first bit sequence, and the second read signal value comprises at least one bit in the second bit sequence.