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cascade controller is feed back or feed forward or both
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Answer Posted / rajkumar
its a feed back 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.
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