Perfect tracking digital motion control based on two-degree-of-freedom multirate feedforward control

In this paper a novel perfect tracking control method based on the multirate sampling control is proposed in which the concepts of the two-degree-of-freedom control and coprime factorization are employed. In the proposed method it is assumed that the usual single-rate robust feedback controller such as the disturbance observer or the H_∞ controller already exists, and the feedforward controller is obtained by using the multirate sampling control. The advantages of the proposed method are that (1) the controller can be designed without considering the unstable zeros of the discrete-time plant; (2) the states of the plant match the desired trajectories at every sampling point; (3) high robust performance is assured by the robust feedback controller. An illustrative example of position control using a DC servo is presented, and the advantages of this approach are demonstrated both in the time and frequency domain