Minimization of unobservable oscillation in repeatable run-out for magnetic-head positioning system of hard disk drives

A positioning control system for a magnetic head in hard disk drives (HDDs) is a sampled-data control system that has a sampler and a hold. As a result, the control system has unobservable oscillations in a repeatable run-out (RROs) whose frequencies are above the Nyquist frequency. These unobservable oscillations may lead to destruction of user data in HDDs. To solve this problem, this paper presents a minimization method for the unobservable oscillation in the RROs. This minimization method focuses on a relationship between RRO frequencies and a sampling frequency of the sampled-data control system. To estimate amounts of the unobservable RROs beyond the Nyquist frequency, this study employs two types of unobservable magnitudes: a maximum value based magnitude, and a root-mean-square value based magnitude. By using these unobservable magnitudes, we can choose the best suited sampling frequency to minimize the unobservable oscillations in RROs of HDDs.