A plant with unstable zeros is known as difficult to control because of initial undershoot of step response and unstable poles of its inversion system. There are two reasons why a plant has unstable zeros in a discrete time domain: 1) non collocation of actuators and sensors and 2) discretization by zero-order hold. Problem 2) has been solved using the perfect tracking control (PTC) method based on multirate feedforward control proposed by our research group. However, the conventional PTC method cannot achieve the perfect tracking for a plant with continuous-time unstable zeros because of the divergence of the desired state trajectories. This paper proposes a preactuation perfect tracking control (PPTC) method to solve problem 1) through the state trajectory generation based on a time axis reversal. The validity of the proposed method is demonstrated through simulations in comparison with three single-rate feedforward control methods.
