Recent years have witnessed a tremendous growth of hard disk storage capacity while the cost and physical size of hard disk drives (HDD) continue to shrink. The areal data density of HDDs has increased from 1 GB per square inch in 1996 to 100 GB per square inch in 2006. Today, HDD tracks per inch (TPI) has already been pushed beyond 200K. In order to read/write (R/W) data reliably, servo-positioning system must keep the R/W head with precision less than ± 10 nm under electronic noise, actuator resonance, internal airflow disturbance and external shock and vibration. It is estimated that the areal density of HDDs will increase at the rate of 40% every year. This poses great challenges for HDD servo system to achieve ultra-accurate positioning control.
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