10.11 Summary

Disturbance rejection provides the motivation for cascade and feed-forward control. Cascade control uses a secondary process measurement to detect and correct for a disturbance before it affects the primary process output. Feed-forward control uses a direct measurement of the disturbance to change the manipulated input before the disturbance affects the output. It is common to combine feed-forward/feedback with cascade control, as shown in the furnace control block diagram, Figure 10-17.

Although some texts refer to cascade and feed-forward control as "advanced control," these techniques are commonly used in industry. Almost every control system that has a flow rate as a manipulated variable will have a flow controller as an inner loop in a cascade strategy.