Time-Varying Compensation for Mid-Frequency Repeatable Runout in Hard Disk Drives Via a Linear Feedback Scheme

Conventional add-on feedback filters for mid-frequency (mid-f) repeatable runout (RRO) compensation in a hard disk drive (HDD) servo system either have a long filter transient or constitute a large sensitivity hump and poor stability margins. This paper presents a novel linear time-varying (LTV) group filtering scheme for the compensation of a few mid-f RRO harmonics. While having a short filter transient that ensures a fast disturbance attenuation, the proposed group filter does not cause any substantial unnecessary sensitivity function gain amplification at the steady state. Implementation results show the effectiveness of the proposed scheme used to compensate two mid-f RRO harmonics simultaneously.