Simple causal fixed-structure feedforward control law for general continuous-time LTI SISO systems

Feedforward control design for time-varying reference signals becomes a non-trivial problem when the plant dynamics is nonminimum-phase, since the inverse of a plant model is unstable or non-causal. A possible approach in this case is to use a stable approximate inverse of a plant model. Several solutions of this kind, devised in a frequency domain, have been proposed in the literature. An alternative approach to feedforward control design proposed in this article utilizes simple interpretation of an internal plant-model structure in a time domain. Following this approach a simple causal fixed-structure feedforward controller is proposed in the form of a linear combination of a reference trajectory and its time-derivatives. By providing explicit formulas for weighting parameters of the combination, it is shown how one can compensate for the detrimental influence of a time delay and right-half-plane zeros on the resultant tracking performance. The concept has been examined by simulation examples.