Nonlinear Process Control by a Combination of Exact Linearization and Gain Scheduling Trajectory Control

Abstract

Exact linearization and gain scheduling are two different well-known approaches to the design of nonlinear process control systems. The basic idea of the design approach presented in this paper is to combine these techniques for open loop stable nonlinear systems to get a resulting control structure which makes use of the advantages and overcomes some of the disadvantages of both concepts. In a first step, a nominal nonlinear state feedback controller is computed to linearize the input/output map of the process model and to specify a nominal closed loop trajectory. A simulation of the process model is used to calculate the required state variables. Subsequently we design a nonlinear gain scheduling trajectory controller 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 state feedback controller. The linear approximation of the trajectory control loop meets the specifications of the respective linear design. The gain scheduling trajectory controller forces the process output to keep up with the nominal trajectory in case of modelling errors and disturbances. A neutralization process in a stirred tank is used as an example to demonstrate the performance of the resulting nonlinear control structure.