Cascade Control
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.
Next Page: Cascade Control Modes, Windup >>>
