Control of Complex Processes with a Cascade Scheme

March 25, 2024

This type of control system requires two sensors. The primary sensor measures the main technological parameter that needs to be regulated. The additional sensor measures a parameter that directly affects the main one but also responds quickly and sensitively to changes in the control action. Such systems are called “cascade systems” and are applicable in two cases:

1. Sluggish Systems – These are systems where the control action, after a relatively large delay, causes an unacceptable change in the output variable. Attempts to achieve a satisfactory response speed using a simple single-loop controller often result in large oscillations of the controlled variable.

2. Systems Affected by Disturbances – These are systems where disturbances alter the calculated control action. An example of such a system is using a valve to control the flow rate of saturated steam to regulate temperature in a heat exchanger. The specific position of the actuator (valve) does not guarantee a precise steam pressure after the valve (and thus the temperature), because the steam pressure before the valve can vary widely.

This is also the case with the system illustrated above. In this system, the steam pressure entering the heat exchanger is maintained by a separate control loop. The setpoint for this loop (the steam pressure) is generated as a control action from the outer loop that encompasses the entire heat exchanger and controls the milk’s output temperature.
Any changes in steam pressure caused by disturbances in the steam supply line will be detected early and corrected promptly by the inner loop before it affects the milk temperature, which is the final parameter to be regulated.
In this case, either two cascaded controllers or a controller with two inputs and the appropriate internal cascade structure can be used. The controller in the outer loop should be PI or PID, while the controller in the inner loop can even be ON/OFF.