Cancellation of Discrete Time Unstable Zeros by Feedforward Control

This paper presents a design methodology for the cancellation of unstable zeros in linear discrete time systems with tracking control objectives. Unstable zeros are defined to be those zeros of the rational transfer function that occur outside the unit circle. Unstable zeros cannot be canceled by feedback without compromising stability. In light of this fact, a feedforward scheme is used. Future desired trajectory information is required because the feedforward scheme is noncausal. The amount of future desired trajectory information that is required depends upon the zero locations and design specifications. It is shown that for a known plant with no zeros on the unit circle one can obtain a frequency response arbitrarily close to 1. Robustness issues and simulation results are discussed.