Disk drive estimating fly height using a PLL tuned by a fly height capacitance
View Patent ↗A disk drive is disclosed comprising a disk, a head actuated over the disk, and control circuitry operable to estimate a fly height of the head by tuning a phase locked loop (PLL) in response to a fly height capacitance. In one embodiment, the PLL comprises a resistive component having a second terminal coupled to the fly height capacitance, a variable oscillator operable to generate a first oscillating signal applied to a first terminal of the resistive component to generate a second oscillating signal at the second end of the resistive component, and a phase detector operable to generate a control signal by comparing a phase of the first oscillating signal to a phase of the second oscillating signal. The control signal is applied to the variable oscillator to adjust a frequency of the first oscillating signal.
1. A disk drive comprising:
a disk;
a head actuated over the disk; and
control circuitry operable to estimate a fly height of the head by tuning a phase locked loop (PLL) in response to a fly height capacitance.
2. The disk drive as recited in claim 1 , wherein the PLL comprises:
a resistive component comprising a first terminal and a second terminal, wherein the second terminal is coupled to the fly height capacitance;
a variable oscillator operable to generate a first oscillating signal applied to the first terminal of the resistive component to generate a second oscillating signal at the second end of the resistive component; and
a phase detector operable to generate a control signal by comparing a phase of the first oscillating signal to a phase of the second oscillating signal, wherein the control signal is applied to the variable oscillator to adjust a frequency of the first oscillating signal.
3. The disk drive as recited in claim 2 , wherein the control signal adjusts the frequency of the first oscillating signal to maintain a target phase difference between the phase of the first oscillating signal and the phase of the second oscillating signal.
4. The disk drive as recited in claim 3 , wherein the target phase difference is selected from the group consisting of zero degrees and 180 degrees.
5. The disk drive as recited in claim 2 , wherein the control circuitry is further operable to convert the frequency of the oscillator signal into the estimate of the fly height.
6. The disk drive as recited in claim 2 , wherein the control circuitry is further operable to convert the control signal into the estimate of the fly height.
7. The disk drive as recited in claim 2 , further comprising a first transmission line coupling the second terminal of the resistive component to a surface of the head.
8. The disk drive as recited in claim 7 , further comprising a second transmission line coupling the surface of the head to a ground.
9. The disk drive as recited in claim 7 , further comprising a filter inserted between the second terminal and the surface of the head along a transmission path of the transmission line.
10. The disk drive as recited in claim 9 , wherein the filter is implemented by varying a geometry of the first transmission line.
11. The disk drive as recited in claim 2 , wherein:
the head comprises a first and second capacitor plates; and
the fly height capacitance comprises a first capacitance formed between the first capacitor plate and the disk and a second capacitance formed between the second capacitor place and the disk.
12. The disk drive as recited in claim 11 , further comprising:
a first transmission line coupling the second terminal of the resistive component to the first capacitor plate; and
a second transmission line coupling a third terminal of the resistive component to the second capacitor plate.
13. The disk drive as recited in claim 1 , wherein the fly height capacitance comprises a capacitance formed between a surface of the head and a surface of the disk.
14. The disk drive as recited in claim 1 , further comprising:
an actuator operable to adjust the fly height of the head;
a comparator operable to compare the estimate of the fly height to a target fly height to generate a fly height error; and
a fly height controller operable to generate a control signal applied to the actuator in response to the fly height error.
15. A method of estimating a fly height of a head over a disk in a disk drive, the method comprising tuning a phase locked loop (PLL) in response to a fly height capacitance.
16. The method as recited in claim 15 , wherein the PLL comprises a resistive component comprising a first terminal and a second terminal, wherein the second terminal is coupled to the fly height capacitance, the method further comprising:
generating a first oscillating signal applied to the first terminal of the resistive component to generate a second oscillating signal at the second end of the resistive component;
generating a control signal by comparing a phase of the first oscillating signal to a phase of the second oscillating signal; and
adjusting a frequency of the first oscillating signal in response to the control signal.
17. The method as recited in claim 16 , wherein the frequency of the first oscillating signal is adjusted to maintain a target phase difference between the phase of the first oscillating signal and the phase of the second oscillating signal.
18. The method as recited in claim 17 , wherein the target phase difference is selected from the group consisting of zero degrees and 180 degrees.
19. The method as recited in claim 16 , further comprising converting the frequency of the oscillator signal into the estimate of the fly height.
20. The method as recited in claim 16 , further comprising converting the control signal into the estimate of the fly height.
21. The method as recited in claim 16 , wherein a first transmission line couples the second terminal of the resistive component to a surface of the head.
22. The method as recited in claim 21 , wherein a second transmission line couples the surface of the head to a ground.
23. The method as recited in claim 21 , further comprising filtering the second oscillating signal to reduce a sensitivity of the fly height estimate to impedance variations in the first transmission line.
24. The method as recited in claim 23 , wherein the filter is implemented by varying a geometry of the first transmission line.
25. The method as recited in claim 16 , wherein:
the head comprises a first and second capacitor plates; and
the fly height capacitance comprises a first capacitance formed between the first capacitor plate and the disk and a second capacitance formed between the second capacitor place and the disk.
26. The method as recited in claim 25 , wherein:
a first transmission line couples the second terminal of the resistive component to the first capacitor plate; and
a second transmission line couples a third terminal of the resistive component to the second capacitor plate.
27. The method as recited in claim 15 , wherein the fly height capacitance comprises a capacitance formed between a surface of the head and a surface of the disk.
28. The method as recited in claim 15 , further comprising:
comparing the estimate of the fly height to a target fly height to generate a fly height error; and
adjusting the fly height of the head in response to the fly height error.