The effect of nonminimum-phase zero locations on the performance of feedforward model-inverse control techniques in discrete-time systems

Noncollocated sensors and actuators, and/or fast sample rates with plants having high relative degree, can lead to nonminimum-phase (NMP) discrete-time zero dynamics that complicate the control system design. In this paper, we examine three stable approximate model-inverse feedforward control techniques, the nonmimimum-phase zeros ignore (NPZ-Ignore), the zero-phase-error tracking controller (ZPETC) and the zero-magnitude-error tracking controller (ZMETC), which have frequently been used for NMP systems. We analyze how the discrete-time NMP zero locations in the z-plane affect the success of the NPZ-Ignore, ZPETC, and ZMETC model-inverse techniques. We also provide simulation examples using plants based on the system identification of an atomic force microscope and a hard disk drive, showing the tradeoffs in performance relative to NMP zero locations in these different application systems.