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Delay compensation for servo systems using state prediction control and disturbance observer, and its application to HDD head servo control system

A new delay compensation method is derived for servo systems using state prediction control and disturbance observer. The delay in the closed loop deteriorates the stability margin because of the phase delay in high frequency region. The proposed method consists of three steps, (1) state estimation with state observer, (2) state prediction, and (3) state feedback of predicted state. For SISO(single input, single output) systems, the proposed method can be rewritten in the scheme of conventional disturbance observer design by using Youla parameterization. This result can be drawn by the analysis of the closed loop system poles and by applying the separation theorem. Improvement of the stability margin is confirmed by simulation in the track following mode of a hard disk drive with the synchronous servo scheme.