Gain-scheduling trajectory control of a continuous stirred tank reactor

The control of continuous stirred tank reactors is often a challenging problem because of the strong pronounced nonlinearity of the process dynamics. Exact feedback linearization and gain-scheduling are two well-known approaches to the design of nonlinear process control systems. The basic idea in this paper is to combine these techniques to obtain a control structure which preserves the advantages and overcomes some of the problems of the two concepts. In a first step, a nonlinear state feedback controller is computed by exact linearization of the process model to shape the nominal closed-loop system. The required unmeasurable state variables are obtained by simulation of the process model. This part of the controller thus is a pure nonlinear feedforward compensator for the nominal plant. To act against disturbances and model uncertainty, a nonlinear gain-scheduled controller is designed by approximately linearizing the process model not for a number of operating points as in the standard gain-scheduling approach but around the nominal trajectory generated by the nonlinear feedforward controller. The design approach is applied to a non-trivial concentration control problem in a continuous stirred tank reactor with nonminimum phase behaviour, unmeasurable states, and model uncertainties as well as unknown disturbances. The nonlinear control structure is compared to a linear controller and to a pure gain-scheduling controller and shows excellent performance even for worst case disturbances and model uncertainties. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.