Process Control: Modeling, Design, and Simulation

12.2. Ratio Control

Ratio control is similar to feed-forward control, since both typically involve the measurement of a stream flow rate. Ratio control is often used in component blending problems. Consider the control strategies shown in Figure 12-1, where stream A is a “wild stream” (disturbance) and it is desirable to maintain the flow rate of stream B at a constant ratio to stream A. In Figure 12-1a the F_B/F_A ratio is calculated and sent to a ratio controller. In Figure 12-1b the flow rate of stream A is measured and multiplied by desired ratio to determine the setpoint for the stream B flow rate.

Figure 12-1. Ratio control. Comparison of two alternatives.

Alternative 12–1b is favored over 12–1a since it results in a linear input-output relationship. ...