.IndustrialControlSystems.Applications.ControlProblems.CascadeControl

Information

Description

In this examples are compared two control schemes. The aim of this example is to show how classic control strategy can be implemented with the models contained within the ControlLibrary.
The system to be controlled is composed by two dynamics: a fast one and a slow one. The trasfer functions follow:
Fast

           Y(s)           1
   P1(s) = ----  =  -------------
           U(s)       (1+0.05s)

Slow

           Y(s)           1
   P2(s) = ----  =  -------------
           U(s)       (1+2s+s^2)

Two different control scheme (both using discrete dime controllers) are compared. In the first one there is one controller that acts
directly on the fast process P1 and measures the output of the second process P2.
In the second case, there are two controllers that act respectively on the fast and slow dynamics. The second approach is called cascade control.

The Step responce of the two scheme are listed below.

Despite for a step variation of the Set Point, the two responces (classic scheme red line, and cascade the green one) are
practically the same, this is not true when disturbances occur. It is evident that the cascade control scheme
performs better than the classic one. Below are reported the control signal of the controllers

where is evidenced the fast action of the inner control loop.

Since the controller are discrete time ones, it is important to choose the right sampling time for each loop
(external and internal one). In this case have been choosen a sampling time of 0.5 s for the outer loop, while a
sampling time of 0.01 s for the inner one.

Revisions

Industrial Control Systems (v 1.0.0) : April-May 2012