Extended Precision Tracking Control of Discrete-Time Nonminimum Phase Systems

The need for an inverse of a closed loop system dynamic model arises naturally in many discrete-time control system design applications. If the system to be inverted is nonminimum phase, an approximate inverse is needed. A procedure to design an approximate inverse of a nonminimum phase plant with real and/or complex nonminimum phase zeros is described here. As compared to other available methods for the same purpose, the proposed method allows for greater control of the gain characteristic of the compensated system, namely, the tracking bandwidth and a measure of the flatness of the compensated gain characteristic can be specified. The proposed method is applied to a number of example systems and its capabilities demonstrated. The reasons for nonminimum phase zeros in discrete-time systems are explored, with particular reference to complex nonminimum phase zeros. The robustness of the compensated gain characteristic to errors in the nonminimum phase zero locations is also assessed.