On design issues of servo system for hard disk drives

The position error signal (PES) in hard disk drive (HDD) servo system contains several distinct components and is sometimes not well understood. The traditional track following control objective of minimizing the PES variance may not necessarily lead to the desired drive performance. For example, suppose that one can achieve continuous servo sampling rate and ultra high servo bandwidth such that the PES variance is virtually zero. Does this mean that, from the track following stand point, one can support ultra high TPI? Not necessarily because of the PES noise, and that high bandwidth system tracks both NRRO and RRO. The latter leads to a trade-off between WRTMR (write-to-read track misregistration) and WWTMR (write-to-write track misregistration). Thus, this paper explores this behavior in further detail