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.

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