Cascade Control System

Cascade control is a sophisticated multi-loop control structure commonly utilized in industrial applications, facilitating the implementation of advanced controllers. This method integrates two feedback loops, where the output of the primary controller sets the set-point for the secondary controller (master and slave).

Essentially, the feedback loop of one controller is nested within the other, enabling an enhanced response to disturbances.

The accompanying figure demonstrates the application of cascade control in regulating the liquid level in a container (Steam drum level). In a single-loop control system, the level sensor provides feedback, generating an error signal for the controller to adjust the fluid inflow rate (Feedwater control valve). Rapid changes in flow can lead to significant delays before the liquid level changes sufficiently to prompt a correction. In contrast, a cascade control system utilizes the level sensor’s feedback for the outer loop controller, which then adjusts the set-point for the secondary controller responsible for regulating the liquid flow rate (feedwater).

This arrangement allows the flow level loop to respond swiftly to allow disturbances, significantly minimizing level fluctuations compared to a single-loop control system.

One of the most common applications of cascade control is in managing the steam drum level of a boiler vs the feedwater control valve.