Cascade Control in The Loop

Cascade Control uses the output of the primary controller to manipulate the setpoint of the secondary controller as if it were the final control element.
Reasons for cascade control:

• Allow faster secondary controller to handle disturbances in the secondary loop.
• Allow secondary controller to handle non-linear valve and other final control element problems.
• Allow operator to directly control secondary loop during certain modes of operation (such as startup).

Requirements for cascade control:

• Secondary loop process dynamics must be at least four times as fast as primary loop process dynamics.
• Secondary loop must have influence over the primary loop.
• Secondary loop must be measured and controllable.

Reasons not to use cascade:

• Cost of measurement of secondary variable (assuming it is not measured for other reasons).
• Additional complexity.

Examples of cascade control:

Control of heat exchanger outlet temperature using steam flow as secondary loop.

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.