Chapter 8. Internal Model Control

In Chapter 6 we presented several methods for tuning PID controllers and developed a model-based procedure (direct synthesis) to synthesize a controller that yields a desired closed-loop response trajectory. In this chapter, we first develop an "open-loop control" design procedure that then leads to the development of an internal model control (IMC) structure. There are a number of advantages to the IMC structure (and controller design procedure), compared with the classical feedback control structure. One is that it becomes very clear how process characteristics such as time delays and RHP zeros affect the inherent controllability of the process. IMCs are much easier to tune than are controllers in a standard feedback control structure.

After studying this chapter, the reader should be able to do the following:

• Design internal model controllers for stable processes (either minimum or nonminimum phase)

• Sketch the closed-loop response if the model is perfect

• Derive the closed-loop transfer functions for IMC

• Design IMC for improved disturbance rejection

The major sections of this chapter are as follows:

8.1 Introduction to Model-Based Control

8.2 Practical Open-Loop Controller Design

8.3 Generalization of the Open-Loop Control Design Procedure

8.4 Model Uncertainty and Disturbances

8.5 Development of the IMC Structure

8.6 IMC Background

8.7 The IMC Structure

8.8 The IMC Design Procedure

8.9 Effect of Model Uncertainty and Disturbances

8.10 Improving Disturbance Rejection Design

8.11 Manipulated Variable Saturation

8.12 Summary