Disk drive detecting when head is parked on ramp

1. A disk drive comprising:

a disk;

a head coupled to a distal end of an actuator arm;

a voice coil motor (VCM) operable to rotate the actuator arm about a pivot in order to actuate the head radially over the disk;

a ramp located near an outer edge of the disk for parking the head; and

control circuitry operable to:

measure a first back electromotive force (BEMF) voltage of the VCM;

apply a first current to the VCM for a first interval in order to move the actuator arm in a first radial direction;

after the first interval, measure a second BEMF voltage of the VCM; and

first detect that the head is un-parked and free to move when a difference between the first and second BEMF voltages exceeds a threshold.

2. The disk drive as recited in claim 1 , wherein the first radial direction is toward an outer diameter of the disk.

3. The disk drive as recited in claim 1 , wherein when the control circuitry first detects that the head is parked, after the first interval the control circuitry is further operable to:

measure a third BEMF voltage of the VCM;

apply a second current to the VCM for a second interval in order to move the actuator arm in a second radial direction opposite the first radial direction;

after the second interval, measure a fourth BEMF voltage of the VCM; and

second detect whether the head is un-parked and free to move in response to the third and fourth BEMF voltages.

4. The disk drive as recited in claim 3 , wherein the first radial direction is toward an outer diameter of the disk and the second radial direction is toward an inner diameter of the disk.

5. The disk drive as recited in claim 3 , wherein the second interval is greater than the first interval.

6. The disk drive as recited in claim 3 , wherein a magnitude of the second current is greater than a magnitude of the first current.

7. The disk drive as recited in claim 3 , wherein the control circuitry first detects that the head is un-parked and free to move when a first difference between the first and second BEMF voltages exceeds a first threshold, and second detects that the head is un-parked and free to move when a second difference between the third and fourth BEMF voltages exceeds a second threshold.

8. The disk drive as recited in claim 7 , wherein the second threshold is greater than the first threshold.

9. The disk drive as recited in claim 3 , wherein when the control circuitry second detects that the head is parked, after the second interval the control circuitry is further operable to:

measure a fifth BEMF voltage of the VCM;

apply a third current to the VCM for a third interval in order to move the actuator arm in the first radial direction;

after the third interval, measure a sixth BEMF voltage of the VCM; and

third detect whether the head is un-parked and free to move in response to the fifth and sixth BEMF voltages.

10. The disk drive as recited in claim 9 , wherein the third current substantially equals the first current.

11. The disk drive as recited in claim 9 , wherein the third interval substantially equals the first interval.

12. A method of operating a disk drive, the disk comprising, a disk, a head coupled to a distal end of an actuator arm, a voice coil motor (VCM) operable to rotate the actuator arm about a pivot in order to actuate the head radially over the disk, and a ramp located near an outer edge of the disk for parking the head, the method comprising:

measuring a first back electromotive force (BEMF) voltage of the VCM;

applying a first current to the VCM for a first interval in order to move the actuator arm in a first radial direction;

after the first interval, measuring a second BEMF voltage of the VCM; and

first detecting that the head is un-parked and free to move when a difference between the first and second BEMF voltages exceeds a threshold.

13. The method as recited in claim 12 , wherein the first radial direction is toward an outer diameter of the disk.

14. The method as recited in claim 12 , wherein when the first detecting detects that the head is parked, after the first interval further comprising:

measuring a third BEMF voltage of the VCM;

applying a second current to the VCM for a second interval in order to move the actuator arm in a second radial direction opposite the first radial direction;

after the second interval, measuring a fourth BEMF voltage of the VCM; and

second detecting whether the head is un-parked and free to move in response to the third and fourth BEMF voltages.

15. The method as recited in claim 14 , wherein the first radial direction is toward an outer diameter of the disk and the second radial direction is toward an inner diameter of the disk.

16. The method as recited in claim 14 , wherein the second interval is greater than the first interval.

17. The method as recited in claim 14 , wherein a magnitude of the second current is greater than a magnitude of the first current.

18. The method as recited in claim 14 , wherein the first detecting detects that the head is un-parked and free to move when a first difference between the first and second BEMF voltages exceeds a first threshold, and the second detecting detects that the head is un-parked and free to move when a second difference between the third and fourth BEMF voltages exceeds a second threshold.

19. The method as recited in claim 18 , wherein the second threshold is greater than the first threshold.

20. The method as recited in claim 14 , wherein when the second detecting detects that the head is parked, after the second interval further comprising:

measuring a fifth BEMF voltage of the VCM;

applying a third current to the VCM for a third interval in order to move the actuator arm in the first radial direction;

after the third interval, measuring a sixth BEMF voltage of the VCM; and

third detecting whether the head is un-parked and free to move in response to the fifth and sixth BEMF voltages.

21. The method as recited in claim 20 , wherein the third current substantially equals the first current.

22. The method as recited in claim 20 , wherein the third interval substantially equals the first interval.