A design procedure for feedforward filters is proposed to improve the tracking performance of existing closed-loop systems. A parametric model is not required because it directly utilizes the frequency response of the closed-loop system, which can be measured accurately. This frequency-shaping approach enables the tradeoff between the quality of model-matching and the size of control efforts by adjusting weighting filters. Time domain specifications can be specified indirectly by approximating them in the frequency domain. The procedure is intuitively simple for SISO systems. Generalization for MEMO systems is possible by formulating convex optimization problems. A feedforward filter is designed and implemented on a nanopositioning system for semiconductor manufacturing, to improve its transient response.
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