Predicting track misregistration (TMR) from disk vibration of alternate substrate materials

Disk vibration is a significant contributor to track mis-registration (TMR) in todays high performance drives with tracks per inch (TPI) growing at the rate of more than 60% per year and disk rotating speeds of 10000 rpm. Aluminum disk substrates result in significantly large TMR that can limit the TPI that can be achieved and hence the drive industry is being forced to consider alternate disk substrate materials. In this paper we identified a quantitative measure called "displacement metric" that predicted closely the TMR due to vibration of disk substrate materials. The displacement metric depends on substrate material properties of Young's Modulus, damping and Poisson's ratio and is a better predictor of TMR than traditional measures like specific stiffness. Many substrate vendors are now using this metric to improve the performance of new alternate substrate materials. Laser doppler vibrometer (LDV) measurements in a 10000-RPM disk drive with aluminum, glass, glass-ceramic, alumina, silicon carbide and zirconia disks showed that the cumulative 3-sigma TMR metric due to disk vibration was proportional to the reciprocal of the "displacement metric". Furthermore, we developed a methodology to predict the closed loop TMR due to disk vibration. The closed-loop TMR due to disk vibration was compared for a range of servo bandwidths (800-1600 Hz) for aluminum and alternate substrate materials. This modeling work has proven to be useful in the development of new disk substrate materials and predicting TMR due to disk vibration for higher performance disk drives.