Advanced Direct Synthesis Controller Design

Abstract

Within the IMC framework, synthesis results for several advanced control methodologies, such as feedforward control and constrained control (anti-windup control), have been proposed for linear systems. The notion of incorporating constraints into nonlinear IMC (NLIMC) has been explored by a number of investigators. Li and Biegler (1988) developed a single-step method that incorporated state and input constraints into the NLIMC framework. The control action was calculated using a specialized sequential quadratic programming problem and assumed that the model was perfect (plant = model). In that case, the incorporation of constraints into NLIMC resulted in a controller that preserved the structure of the partitioned nonlinear inverse (a linear dynamic controller with an additive nonlinear correction); however, it did not yield a direct synthesis nonlinear controller. In addition, compensation for measured disturbances, standard for linear IMC approaches (LIMC), was not considered. Wassick and Camp (1988) applied NLIMC to an industrial extruder and detailed several important considerations for application of nonlinear control in industry. Advanced issues such as nonlinear feedforward control and manipulated variable constraints were addressed in an effective, but sub-optimal, manner.