Modified Bode Plots for Head-Positioning Control in Hard Disk Drives with Structured and Unstructured Uncertainties

This paper presents a loop-shaping method for robust control design that can account for structured uncertainties using modifications of the classical Bode plot. By using this method, the robust performance criterion is explicitly translated into allowable and forbidden regions on Bode plot of a compensated SISO system. There is no need for transfer function-based models of the plant, the sensitivity function performance, or the uncertainty. As a result, the designed controllers are able to avoid excessive conservativeness that may lead to high-order controllers while improving the control performance. This paper shows utility of the proposed method by applying it to a head-positioning control system in a hard disk drive.