Compensation for repeatable tracking errors in hard drives using discrete-time repetitive controllers

Presents the repeatable runout (RRO) compensation in hard disk drives (HDDs). To cancel the RRO, we applied repetitive controllers to the tracking system. Through experiments, we obtained a high reduction ratio of from 10 to 45 dB for up to 10th harmonics component of the RRO. Furthermore, by switching filter parameters between the learning state and the steady state, we obtained fast learning of less than two disk rotations.