Cascade Control Modes

In most applications, the control loop is not functioning as a cascade loop all the time. The operator (in the case of batch control, the batch control program) has the ability to change modes. Following is the typical selection of modes of operation available for a cascade control loop. Manual and Auto are usually used during startup while cascade is used for normal operation.

Tracking:

In A, Manual mode, the set point of the flow controller tracks the actual flow variable.

In B, Auto mode, the output of the temperature controller tracks the setpoint of the flow controller.

In C, Cascade mode, the temperature controller manipulates the setpoint of the flow controller.

Therefore, when switching from one mode to another there is no sudden change in the output to the valve.

Windup

If the secondary controller cannot deliver enough flow, even with its valve wide open, to bring the primary measured variable to its set point, the primary controller will "windup" and continue to increase the flow setpoint above the maximum flow. Later, when the flow is sufficient to bring the primary measurement to its set point, the primary controller must take the time to "wind down" the secondary set point to the actual flow before the valve begins to close.

Use of external feedback to limit windup:

By connecting the secondary measurement to the external feedback of the primary, we can eliminate the windup problem.

Block Diagram of external feedback.

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