This example shows how to design a cascade control loop with two PI controllers using the pidtune command.
Cascade control is mainly used to achieve fast rejection of disturbance before it propagates to the other parts of the plant. The simplest cascade control system involves two control loops (inner and outer) as shown in the block diagram below.
Controller C1 in the outer loop is the primary controller that regulates the primary controlled variable y1 by setting the set-point of the inner loop. Controller C2 in the inner loop is the secondary controller that rejects disturbance d2 locally before it propagates to P1. For a cascade control system to function properly, the inner loop must respond much faster than the outer loop.
In this example, you will design a single loop control system with a PI controller and a cascade control system with two PI controllers. The responses of the two control systems are compared for both reference tracking and disturbance rejection.
In this example, the inner loop plant P2 is
P 2 ( s ) = 3 s + 2
The outer loop plant P1 is
P 1 ( s ) = 1 0 ( s + 1 ) 3
