Modified Bode Plots for Robust Performance in SISO Systems With Structured and Unstructured Uncertainties

We have developed a loop-shaping methodology for robust control design in single-input single-output (SISO) systems with structured and unstructured uncertainties. This design method employs visualization tools that are modifications of the classical Bode plot. Using the proposed method control engineers can easily consider the effects of the structured and unstructured uncertainties without the need for transfer-function models of the plant, the sensitivity function performance, or the uncertainty. The method simultaneously can avoid excessive conservativeness and excessively high order controllers while improving controller performance and robustness. We show utility of the proposed method by applying it to a head-positioning control system in a hard disk drive.