Answer / jai

feed back controller is slow response ,feed forward is fast
response loop,becuae we are measuring the process value
before the control element.we can use wher we required fast
response.actually both loop prsent in the case cad control.

Answer / vaishnavi

In the case of feedfwd controller we need to know the complete mathematical model and all the load variables acting on the process which is not possible and this takes anticipatory action b4 the disturbances can affect the process.
weras feedback control takes corrective action by measuring the error once the process is affected by load variables.and in this case we need not know the load var acting on the sys and no need for mathematical model.
hence usually feedfwd cant be used alone and is used in combination with feedback.

Answer / muslim

feedback controller: to monitor the controlled variables
(pressure, temperature,..) and feedback the measured value
to the controller to be compared with set point within the
controller , and finally to give corrective signal used to
stabilize the control loop.

feedforward controller: to monitor all loads that influence
the process.
theoritically we have to monitor all loads, but practically
it's impossible .. as there's unmeasured disturbances (like
pipe fitting leaks, composition change ,..) that can't be
predicted.
so if we are to make a complete feedforward control so we
have no feedback mechanism and so we will have a non
disturbed process, which is impracticale.

practically we have feedforward in conjunction with
feedback control , which enhance the loop stability.

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