The design of controllers for nonlinear, nonminimum-phase processes is one of the most difficult control problems currently faced. Current available control algorithms for nonlinear processes rely implicitly or explicitly on an inverse of the process. Linear control methods for nonminimum-phase processes are based on a decomposition of the process into a minimum-phase and a nonminimum-phase part. Such a decomposition is an open problem for nonlinear systems.
In this work, a control structure called the minimum-phase output predictor for nonlinear, nonminimum-phase processes is developed. The structure is based on the notion of statically equivalent outputs; a minimum-phase, statically equivalent output is estimated on-line and then an available nonlinear control algorithm is used to control it to set point. The advantage of the proposed formulation is that it is based on the calculation of an output function, not on a decomposition of the process dynamics. The proposed control methodology is applied and its performance is evaluated for a chemical engineering example.