Frequency-shaping optimal compensation of low fly-height vibration in high density HDD servo systems

In this work, a novel robust observer-based frequency-shaping optimal feedback (OBFSOF) controller is designed to actively compensate low fly height vibration (LFHV) in hard disk drive (HDD) servo systems. LFHV is a new vibration source due to continuous contact of the read/write head with the media in high density HDDs. The main characteristics of LFHV are the time-varying phase and magnitude, and the wideband frequency spectrum. Differing from many existing non-repeatable runout (NRRO) compensation schemes that focus on a finite number of dominant frequency components, the proposed OBFSOF scheme can suppress infinitely many frequency components concurrently, meanwhile keep robust stability to model uncertainties and disturbances. The effectiveness of the proposed OBFSOF scheme is verified by theoretical analysis and numerical simulations on a real HDD model. By replacing the linear part of PTOS controller with the proposed OBFSOF controller, the maximal LFHV magnitude is decreased by 62.8% comparing with the PTOS controller during the track following stage.