Disturbance Rejection Through an External Model for Nonminimum Phase Systems

This paper considers the use of an external model for the cancellation of disturbances in a nonminimum phase system where disturbances can be regarded as the output of a stable (in the sense of Lyapunov) autonomous system. It is assumed that the parameters in the nonminimum phase system are unknown, but the orders of the numerator and denominator polynomials, and the delay steps are known. External model method is a kind of adaptive feedforward controller that the control input is the function of estimated external disturbance. In order to cancel the effect of disturbances, we use two consecutive recursive parameter identification algorithms to estimate external disturbances. The stability of the closed-loop system is proved.