A quick and reliable active flying height calibration is critical in today’s hard disc drive short product development and high volume production cycles. In addition to the servo signals, passive HDI acoustic and proximity monitoring techniques are tracking equilibrium of the head gimble assembly and air bearing modes. Passive acoustic HDI monitoring had been proven to be capable in active head protrusion detection applications [1] where derived passive and active Air Bearing flying profiles help to set active flying clearance. With enhanced sensitivity, passive acoustic techniques such as AE can detect embedded particles [2]. The drawback of this technique is that noisy HDD environment causes loss of detectability of the useful AE signal [3]. Special attention has to be paid for extracting desirable HDI information. On the other hand, recording head based proximity probes are immune to torsional HDI modes and operate on pitch mode induced modulation. Advanced HDD today uses a combination of recording head based proximity sensors that are designed to sense HDI modulation at close proximity to the disc. It has to be pointed out that interpretation of proximity probe contact signatures always has been challenging.

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