Acoustic Emissions Signal-Analysis

The read/write heads of a disk drive fly at a very minimum height (0.2 [µm]) on an aerodynamic air cushion over the disk. Transient processes such as the spinning down/up of the spindle occasionally cause the head to hit against the disk. This process depending on the impact momentum leads to head crashes.

Based on the fixation of a single disk in a test stand, head crash testers have the significant disadvantage that they do not completely reflect the actual mechanical boundary conditions of a disk drive.

The lifting and lowering process of the heads over the disk in a disk drive environment as an inspection test can be investigated with the method described below. The vibrations of the structure-borne noise coming from the disk drive housing are recorded by a sensor (B&K 8314) and evaluated up to the 100 [kHz] range as an Acoustics Emission Signal. The lifting and lowering process of the heads can be precisely detected.

By applying the maximum entropy method (I-DEAS), the evaluation of the time signals provides a time variant spectrum analysis of these transient processes. The results are time dependent spectra, as illustrated in the center figure. This clearly shows that the dynamic energy during the lowering process of the heads (80 [kHz] range) is higher than during the lifting process. This is also true for the 37 [kHz] and 47 [kHz] range (bottom figure).

The accompanying FEA shows that the occurring frequencies represent the inherent vibrations of the read/write heads as pitching and vertical lifting.

Therefore, it could be determined that mainly particles with a diameter of approximately 80% - 120% of the head’s flight elevation cause head crashes.