Disk drive calibrating bias signal for touchdown sensor

1. A disk drive comprising:

a disk;

a head actuated over the disk, the head comprising a touchdown sensor comprising a resistance;

a fly height actuator; and

control circuitry operable to:

apply a fly height control signal to the fly height actuator to control a fly height of the head;

configure the fly height control signal to substantially zero and apply a bias signal at a reference value to the touchdown sensor at a reference temperature and measure a corresponding reference resistance of the touchdown sensor;

estimate an operating resistance of the touchdown sensor at a predetermined operating temperature of the touchdown sensor based on the reference resistance, the reference temperature, and a thermal coefficient of resistance (TCR) of the touchdown sensor, wherein the TCR specifies a change in temperature of the touchdown sensor relative to a change in the resistance of the touchdown sensor; and

configure the fly height control signal to achieve a target fly height of the head and generate an operating value for the bias signal by adjusting the bias signal until a measured interim resistance of the touchdown sensor substantially equals the operating resistance.

2. The disk drive as recited in claim 1 , wherein the resistance of the touchdown sensor changes relative to an amplitude of the bias signal.

3. The disk drive as recited in claim 1 , wherein the reference value for the bias signal comprises a minimum value out of a plurality of values.

4. The disk drive as recited in claim 1 , wherein the reference temperature comprises an ambient temperature of the disk drive.

5. The disk drive as recited in claim 1 , wherein the operating value for the bias signal depends on a geometry of the touchdown sensor.

6. The disk drive as recited in claim 1 , wherein the control circuitry is operable to estimate the operating resistance of the touchdown sensor according to:

R — Op=R — Ref *(1+( T — Op−T — Ref )* TCR )

where:

R_Ref is the reference resistance of the touchdown sensor;

T_Ref is the reference temperature of the touchdown sensor;

R_Op is the operating resistance of the touchdown sensor; and

T_Op is the predetermined operating temperature of the touchdown sensor.

7. The disk drive as recited in claim 1 , wherein the control circuitry is operable to generate the operating value for the bias signal by:

applying the bias signal at an interim value to the touchdown sensor and measuring the corresponding interim resistance of the touchdown sensor;

scaling the interim value based on a ratio of the operating resistance and the interim resistance; and

applying the bias signal at the scaled interim value to the touchdown sensor.

8. The disk drive as recited in claim 1 , wherein the operating value for the bias signal depends on the fly height control signal that achieves the target fly height.

9. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to apply the bias signal at the operating value to the touchdown sensor and detect a thermal asperity on the disk in response to the touchdown sensor.

10. A method of operating a disk drive, the disk drive comprising a head actuated over a disk, the head comprising a touchdown sensor comprising a resistance, the method comprising:

applying a fly height control signal to a fly height actuator to control a fly height of the head;

configuring the fly height control signal to substantially zero and applying a bias signal at a reference value to the touchdown sensor at a reference temperature and measuring a corresponding reference resistance of the touchdown sensor; and

estimating an operating resistance of the touchdown sensor at a predetermined operating temperature of the touchdown sensor based on the reference resistance, the reference temperature, and a thermal coefficient of resistance (TCR) of the touchdown sensor, wherein the TCR specifies a change in temperature of the touchdown sensor relative to a change in the resistance of the touchdown sensor; and

configuring the fly height control signal to achieve a target fly height of the head and generating an operating value for the bias signal by adjusting the bias signal until a measured interim resistance of the touchdown sensor substantially equals the operating resistance.

11. The method as recited in claim 10 , wherein the resistance of the touchdown sensor changes relative to an amplitude of the bias signal.

12. The method as recited in claim 10 , wherein the reference value for the bias signal comprises a minimum value out of a plurality of values.

13. The method as recited in claim 10 , wherein the reference temperature comprises an ambient temperature of the disk drive.

14. The method as recited in claim 10 , wherein the operating value for the bias signal depends on a geometry of the touchdown sensor.

15. The method as recited in claim 10 , further comprising estimating the operating resistance of the touchdown sensor according to:

R — Op=R — Ref *(1+( T — Op−T — Ref )* TCR )

where:

R_Ref is the reference resistance of the touchdown sensor;

T_Ref is the reference temperature of the touchdown sensor;

R_Op is the operating resistance of the touchdown sensor; and

T_Op is the predetermined operating temperature of the touchdown sensor.

16. The method as recited in claim 10 , further comprising generating the operating value for the bias signal by:

applying the bias signal at an interim value to the touchdown sensor and measuring the corresponding interim resistance of the touchdown sensor;

scaling the interim value based on a ratio of the operating resistance and the interim resistance; and

applying the bias signal at the scaled interim value to the touchdown sensor.

17. The method as recited in claim 10 , wherein the operating value for the bias signal depends on the fly height control signal that achieves the target fly height.

18. The method as recited in claim 10 , further comprising applying the bias signal at the operating value to the touchdown sensor and detect a thermal asperity on the disk in response to the touchdown sensor.