10.2. Introduction to Cascade Control
Cascade control involves the use of multiple measurements and a single manipulated input. As a motivating example, consider the temperature-control problem shown in Figure 10-1, where a fired furnace is used to heat a process fluid stream. The outlet temperature is controlled by manipulating the valve position of the fuel gas control valve. Clearly, disturbances in the fuel gas header pressure (upstream of the valve) will end up changing the fuel gas flow rate, and, therefore, the outlet temperature. Also, any problems with the control valve, such as stiction or hysteresis (see Module 15), will affect the fuel gas flow rate.