TABLE OF CONTENTS When one steps beyond basic feedback control, cascade and ratio control are probably the first of the so-called advanced regulatory control techniques one encounters. In a cascade control system, one feedback controller adjusts the set point of another feedback controller. The upper-level controller is called the "primary," while the lower level is called the "secondary." A typical application of cascade control is a temperature controller cascaded to a flow controller. Figure 9-1 depicts a cascade control system, using both ISA (Ref. [9-1]) and SAMA symbols (Ref [9-2]).
Before investigating the finer points of cascade control systems, let us first consider a situation that illustrates the motivation for using cascade control. In Figure 9-2a the process outlet temperature of a heat exchanger is sensed. The temperature controller then adjusts the set point of the steam-flow controller to maintain the outlet temperature at set point. An alternative control strategy would be for the temperature controller to directly manipulate the control valve, as shown in Figure 9-2b. To compare these two schemes, we need to consider the disturbances to the process. We will consider several sources of disturbance, both with and without the cascade control system.
First, suppose there is no cascade, as shown in Figure 9-2b, and suppose also that the process flow rate increases. This increased load on the heat exchanger will cause the outlet temperature...
