12.1 Motivation

In Chapter 10 we discussed feed-forward and cascade-control strategies. The purpose of these control strategies was to take advantage of additional measurements to improve the disturbance rejection capability. In this chapter, we also develop strategies that use more than one measurement and, in addition, manipulate more than one input. Ratio control, presented in Section 12.2, is very similar to feed-forward control. In Section 12.3, we discuss selective and override control, which involves the use of "selectors" to decide the proper control action to take. In Section 12.4, split-range control is presented as a way to use more than one valve to regulate a process.