Analysis of unobservable oscillation in repeatable run-out for head-positioning system of hard disk drive

In a head-positioning system of hard disk drives (HDDs), unobservable oscillations in a repeatable run-out (RRO) may lead to destruction of user data. Therefore, this paper presents an estimation method for the unobservable oscillation in the RRO for the head-positioning control systems. This study is based on a relationship between an oscillation frequency and a sampling frequency of the head-positioning system. To estimate amounts of the unobservable oscillations in RROs, unobservable magnitudes are defined by using its maximum and root-mean-square values. These magnitudes show that a sampling frequency is an important parameter which designers must choose carefully so as not to destroy the user's data by the unobservable RROs. To show a usage of the proposed method, unobservable magnitudes of RROs were analyzed for the head-positioning control system in a 7200-rpm HDD.