Joint design method based on coprime factorization of 2DOF control system

In many cases, control system synthesis is formulated as minimization of prescribed closed loop performance reflecting control requirements. Since the closed loop performance is a function of a controlled plant and a controller, a model identification and a controller design must interact with each other. This motivates us to consider the model identification and the controller design simultaneously. However, most of the previous joint design methods are not applicable to unstable plants because the identified plant model is usually used as the design parameter. On the other hand, we have analyzed the internal structure of 2DOF control system using a coprime factorization on RH_∞ and shown that two free parameters, K and Q RH_∞, specify the tracking performance and the feedback performance, respectively. Also Tay et.al have proposed a parameterization of the plant dynamics by switching the role of the controlled plant and the controller, and introduced a free parameter R RH_∞. In this paper, we propose a new joint design strategy based on the identification of R and the design of Q. Since the identified plant parameter R is always stable, the proposed joint design strategy can be applied to a wider class than the conventional joint design method.