Robust Compensation of Periodic Disturbances by Multirate Control

We propose an adaptive feedforward cancellation (AFC) scheme based on multirate discretization for compensation of periodic disturbance in hard disk drive servo systems. By discretizing the plant by a multirate scheme, we obtain the inverse of the closed-loop transfer function. Our scheme cancels selected repeatable runout (RRO) components more rapidly even when the disturbance signal is time-varying in both frequency and amplitude. We also show by simulation and experiment that the scheme can attenuate selected RRO harmonics effectively in a wide frequency range without amplifying other frequency harmonics, which cannot be achieved by conventional AFC. Our scheme is robust against time-varying periodic disturbances and actuator resonance frequency shifting.