Flying height measurement while seeking in hard disk drives

Abstract

A new technique has been developed to measure flying height changes during a high velocity seek in a fully operational hard disk drive. During a seek, a change in the flying height of the read-write head results from the change in direction and speed of air flow under the air bearing surface (ABS). The application of the Wallace spacing loss formula to peak amplitudes of the magnetic recording signal acquired as the head crosses tracks during the seek forms the basis of the method. The method also requires a means to precisely measure the location along the track at which each track crossing occurs. This additional information is used to remove the variation in the amplitude of the magnetic readback signal which is due to surface roughness and magnetic inhomogeneity of the disk surface and the fact that each seek operation crosses the track in a different circumferential location. Plotting the peak amplitude versus the track crossing location for several thousand seeks provides a profile of the roughness variations of the disk surface over a short length of track. Except for a fixed amplitude offset, the track profile acquired when crossing a particular track from the inner diameter (ID) to the outer diameter (OD) of the disk was the same as that acquired during OD to ID direction. This amplitude offset allows calculation of the difference in fly-height between the three cases of ID/spl rarr/OD seek, OD/spl rarr/ID seek, and track following. This measurement technique can reliably resolve dynamic fly-height changes of 0.05 /spl mu/in. Comparison of the measured data to numerical air bearing simulations will be presented.