Instrumentation

Cascaded PID Control

In this lesson, we will study the cascaded PID control.

In single-loop control, the controller’s set point is set by an operator, and its output drives a final control element. For example, a level controller driving a control valve to keep the level at its set point.

When one process has affected the other process then cascaded PID is used.

In a cascade control arrangement, there are two controllers of which one controller’s output drives the set point of another controller.

For example, as shown in the below figure a level controller driving the set point of a flow controller to keep the level at its set point.

The flow controller, in turn, drives a control valve to match the flow with the set point the level controller is requesting.

cascaded pid
                                                                       cascaded PID

The controller driving the setpoint (the level controller in the example above) is called the primary.

an outer, or master controller. The controller receiving the setpoint (flow controller in the example) is called the secondary, inner or slave controller.

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