EFFICIENT IMPLEMENTATION OF REPEATABLE RUNOUT COMPENSATION IN A HARD-DISK DRIVE

Abstract

In this thesis, an advanced repeatable runout (RRO) compensation technique referred to as Modified Adaptive Feedforward Cancellation (MAFC) is formulated, designed, and implemented. There are important advantages to the modified AFC scheme over the normal AFC scheme. The normal AFC is designed based on the frequency response information at a particular desired frequency. Thus, it may amplify the adjacent harmonics and high-frequency noise resulting in increased sensitivity and decreased stability margin of the overall system while cancelling a single harmonic of RRO. Further more, if the plant is not a strictly positive real (SPR) system, the adaptation gains are limited, such that the convergence rate of the algorithm is slow. On the other hand, the MAFC approach uses a precompensator based on the parametric model of the plant to shape overall model to a near zero-phase system. This allows all low frequencies and the adjacent higher harmonics to be attenuated while canceling a single harmonic of RRO. Another important advantage of the MAFC approach is that it reduces the limitation in selecting a higher adaptation gain because of a non-SPR plant. This ensures rapid convergence. Furthermore the MAFC algorithm can use digital filter technology to attenuate the high-frequency noise to minimize any increase in the sensitivity and decrease in the stability margin of the total system. An analytical method for determining the upper bound on the adaptation gains for the stability and the disturbance rejection capabilities are provided. The averaging analysis is used to predict the convergence of the adaptive closed-loop system by estimating the pole locations. The design of this adaptive periodic disturbance rejection controller is quite straightforward. An effective design would cancel the largest harmonics directly and take advantage of additional attenuation at other harmonics. This can be easily achieved through the MAFC technique. This thesis also explores the issues involved in the implementation of both AFC and MAFC techniques in a commercial magnetic disk drive. The simulation and experimental results match very closely. It proves the capabilities of MAFC as a more effective compensator of RRO.