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:
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).
- 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.
- 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.
Examples of Cascade Control :
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