Fixed-Structure Feedforward Control Law for Minimum- and Nonminimum-Phase LTI SISO Systems

Fundamental limitation of the model-inverse feedforward control results from instability or noncausality of the inverse of nonminimum-phase dynamics which cannot be applied in practice. To overcome this limitation, the approximate-inverse methods have been proposed in the literature. Structures of feedforward controllers proposed so far highly depend on a plant model structure. Therefore, in general, their implementation may be inconvenient or troublesome in industrial applications. In this context, a simple fixed-structure feedforward control law is proposed in this paper in the form of a weighted linear combination of a reference trajectory and its time derivatives. Design rules for selection of the weights are derived and provided in an explicit (analytical) form. The proposed control law can be applied to both nonminimum- and minimum-phase linear time invariant single-input-single-output systems. The new method has been compared with classical feedforward controllers known from the literature, revealing its advantages and limitations. The results of numerical examples and experimental validation tests obtained for an electronic plant have been reported.