Chapter 13. Control-Loop Interaction

Most process unit operations have a number of manipulated inputs, each of which affects some measured output, and it is not always clear which input should be "paired" with which output for control-system design. This general pairing problem is known as control structure selection and is often more important than the actual controller design and tuning method used. After studying this chapter, the reader should be able to do the following:

• Use the relative gain array (RGA) to determine the proper input-output pairings for multiple single input–single output controllers

• Understand that the RGA gives insight about process sensitivity

• Discuss the implications of the RGA for failure sensitivity

The major sections of this chapter are as follows:

13.1 Introduction

13.2 Motivation

13.3 The General Pairing Problem

13.4 The Relative Gain Array

13.5 Properties and Application of the RGA

13.6 Return to the Motivating Example

13.7 RGA and Sensitivity

13.8 Using the RGA to Determine Variable Pairings

13.9 MATLAB RGA Function File

13.10 Summary