Process Identification and PID Control

7

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 G_c1(s) and a secondary (slave) controller G_c2(s). G_p1(s) and G_p2(s) are the primary process and the secondary process respectively. The output y₂(s) of the secondary process G_p2(s) is measurable. u₁(s) and u₂(s) are the control outputs of the primary controller and the secondary controller respectively. y_s1(s) and y_s2(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) = y_s2(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 G_p1(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 G_p2(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 ...