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HARD DISC Applications

Hard Disc Wobble

The Requirement

A customer inquired whether or not we could detect the axial runout (wobble) of a hard disc drive to establish a pass/fail criteria at ±20 μm pk-pk.

The Test

The picture to the right shows a good hard disc drive with the cover removed. A Philtec model
RC20 sensor is mounted above the disc at a gap of 0.5 mm. It has a sensitivity of 6.3 mv/μm.


A live trace capture at full speed, ~7,000 rpm, shows a 60 millivolt pk-pk signal (CH1), about 10 microns pk-pk, with excellent signal-to-noise. Therefore, the model RC20 sensor is a very good candidate for this test requirement.