Enhanced Control Strategies
7.1 Cascade Control
Cascade control uses an additional internal feedback loop to reject disturbances more effectively. The typical structure of a cascade control is shown in Figure 7.1a. It is composed of a primary (master) controller Gc1(s) and a secondary (slave) controller Gc2(s). Gp1(s) and Gp2(s) are the primary process and the secondary process respectively. The output y2(s) of the secondary process Gp2(s) is measurable. u1(s) and u2(s) are the control outputs of the primary controller and the secondary controller respectively. ys1(s) and ys2(s) are the setpoints of the primary controller and the secondary controller respectively. Note that the control output of the primary controller is the setpoint of the secondary controller; that is, u1(s) = ys2(s).
In Figure 7.1a, the disturbance d(t) directly affects y2(t). Meanwhile, d(t) indirectly affects y1(t) because it goes through the dynamics of Gp1(s). The cascade control system in Figure 7.1a can detect the disturbance more quickly by measuring y2(t), compared with the conventional control system in Figure 7.1b. If the dynamics of Gp2(s) are fast, then the internal feedback loop (the secondary controller) can reject the disturbance quickly before the disturbance affects y1(t), resulting in an improved disturbance rejection performance. Meanwhile, the conventional control system inevitably shows a slow response to the disturbance because it measures only y1(t). The following three conditions should ...