Method to balance spindles in a plurality of disk drives

1. A method to balance spindles in a plurality of disk drives, the method comprising:

a) rotating the spindle of one of the plurality of disk drives;

b) while the spindle of that one of the plurality of disk drives is rotating, measuring an initial amplitude and an initial phase of an electrical signal that corresponds to an initial imbalance for that one of the plurality of disk drives;

c) determining the initial imbalance based on the initial amplitude and an amplitude calibration;

d) selecting one of a plurality of balancing masses, the selection being based on the initial imbalance;

e) attaching the selected balancing mass to the spindle of that one of the plurality of disk drives, with the selected balancing mass at an angular position selected based upon the initial phase and a phase calibration;

f) again rotating the spindle of that one of the plurality of disk drives;

g) while the spindle of that one of the plurality of disk drives is again rotating, measuring a final amplitude and a final phase of an electrical signal that corresponds to a final imbalance for that one of the plurality of disk drives;

h) determining the final imbalance based on the final amplitude and an amplitude calibration;

i) repeating steps a through h for each of the plurality of disk drives, recording the final imbalance and the initial imbalance for each of the plurality of disk drives;

j) adjusting the amplitude calibration based on a comparison of a measured distribution and a theoretical distribution of the final imbalance versus the initial imbalance.

2. The method of claim 1 wherein the theoretical distribution of the final imbalance versus the initial imbalance is a distribution expected for a fictitious population of disk drives having randomly distributed imbalances.

3. The method of claim 1 wherein each of the plurality of balancing masses comprises a C-shaped wire ring having a ring center and a circumferential gap that defines a circumferential gap length.

4. The method of claim 3 wherein a wire cross-sectional diameter of each of the plurality of balancing masses is the same.

5. The method of claim 1 wherein each of the plurality of balancing masses has a known imbalance that differs from another of the balancing masses by a first imbalance difference.

6. The method of claim 5 wherein each of the plurality of balancing masses comprises a C-shaped ring having a ring center and a circumferential gap that defines a circumferential gap length, the circumferential gap length determining the known imbalance.

7. The method of claim 6 wherein each of the plurality of balancing masses defines a ring radius measured between the ring center and the circumferential gap, and wherein the ring radius for each of the plurality of balancing masses is the same.

8. The method of claim 5 wherein theoretical distribution of the final imbalance versus the initial imbalance is a periodic sawtooth distribution having a theoretical period that is equal to the first imbalance difference, and having a theoretical amplitude that is equal to half of the first imbalance difference.

9. The method of claim 1 further comprising adjusting the phase calibration based on a comparison of the measured distribution and the theoretical distribution.

10. The method of claim 9 wherein the phase calibration is adjusted to reduce the amplitude of minima in the measured distribution of final imbalance versus initial imbalance.

11. The method of claim 1 wherein the amplitude calibration is adjusted based on an asymmetry of peaks in the measured distribution where peaks in the theoretical distribution are expected to be symmetric.

12. The method of claim 1 wherein the amplitude calibration is adjusted to reduce the amplitude of maxima in the measured distribution of final imbalance versus initial imbalance.

13. The method of claim 1 wherein the electrical signal that corresponds to the initial imbalance, and the electrical signal that corresponds to the final imbalance, originate from a transducer coupled to said one of the plurality of disk drives.

14. The method of claim 1 further comprising adjusting the phase calibration based on a comparison of the initial phase and the final phase for any one of the plurality of disk drives.

15. The method of claim 1 wherein the measured distribution of the final imbalance versus the initial imbalance for the plurality of disk drives is determined in the memory of a computer by a computer processor without a resulting plot being displayed on any display device.

16. The method of claim 1 wherein the theoretical distribution of the final imbalance versus the initial imbalance is determined by a computer without a resulting plot being displayed on any display device.

17. The method of claim 1 wherein the measured distribution of the final imbalance versus the initial imbalance for the plurality of disk drives is determined in the memory of a computer by a computer processor with a resulting plot being displayed on a display device.

18. The method of claim 1 wherein comparison of the measured distribution and the theoretical distribution is accomplished by the processor of a computer without the comparison being displayed.

19. The method of claim 1 wherein at least steps b through e, g, and h are performed by a first automated balancing apparatus.

20. The method of claim 1 further comprising temporarily stopping the spindle rotation after step c.