Disk drive calibrating touchdown sensor

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

• applying a bias signal at a first bias value to the touchdown sensor and measure a corresponding first resistance of the touchdown sensor;
• applying the bias signal at a second bias value to the touchdown sensor and measure a corresponding second resistance of the touchdown sensor;
• estimating a reference resistance of the touchdown sensor based on the first and second bias values and the first and second resistances;
• and
• generating an operating bias value for the bias signal based on a predetermined operating temperature of the touchdown sensor, the reference resistance, 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,
• wherein the bias signal comprises a voltage and the operating bias value for the bias signal is generated according to:
• Vop
• =
• (
• Rhot
• -
• Rref
• )
• ·
• Rhot
• TCR
• where:
• Vop is the operating bias value;
• Rhot is a resistance of the touchdown sensor at the operating temperature of the touchdown sensor;
• and
• Rref is the reference resistance of the touchdown sensor.

A disk drive comprising:

• a disk;
• a head actuated over the disk, the head comprising a touchdown sensor comprising a resistance;
• and
• control circuitry operable to:
• apply a bias signal at a first bias value to the touchdown sensor and measure a corresponding first resistance of the touchdown sensor;
• apply the bias signal at a second bias value to the touchdown sensor and measure a corresponding second resistance of the touchdown sensor;
• estimate a reference resistance of the touchdown sensor based on the first and second bias values and the first and second resistances;
• generate an operating bias value for the bias signal based on a predetermined operating temperature of the touchdown sensor, the reference resistance, 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;
• apply the bias signal at the operating bias value to the touchdown sensor and measure a corresponding third resistance of the touchdown sensor;
• update the reference resistance of the touchdown sensor based on the first, second, and operating bias values and the first, second, and third resistances;
• and
• update the operating bias value for the bias signal based on the updated reference resistance.