Feed Forward Control Loops


#1

A feedback control loop is reactive in nature and represents a response to the effect of a load change or upset. A feedforward control loop, on the other hand, responds directly to load changes and thus provides improved control.

In feedforward control, a sensor is used to detect process load changes or disturbances as they enter the system. A block diagram of a typical feed- forward control loop is shown in the following Figure. Sensors measure the values of the load variables, and a computer calculates the correct control signal for the existing load conditions and process set point…

Feed%20Forward%20Control%20Loops

Feedforward control poses some significant problems. Its configuration assumes that the disturbances are known in advance, that they will have sensors associated with them, and that no important undetected disturbances will occur…

Therefore, feedforward control system is more complicated and more expensive, and it requires operator to have a better understanding of the process than does a standard feedback control loop. .

So, feedforward control is generally reserved for well-understood and critical applications.


#2
  • Feed forward is an open loop control design in which disturbance is measured first then mathematical relation between disturbance and manipulated variable is derived based on statistical data, & feed to controller.
  • This acts in anticipatory manner and hence provide output to desired setpoint.
  • Important draw back is understanding affect of disturbance on controller.

#3

@mehta2155

feedforward is an closed loop, as it have an input and an output, so loop closing.


#4

@john sorry to say but in feedforward we are not measuring output by means of feedback loop feeding error signal to controller, whereas in feedback we do that.
The measuring variable is external disturbance in case of feedforward controller.
Feedforward compensates for disturbances.


#5

@mehta2155

yes, still its a closed loop !!!

consider feedforward example for a simple heatexchanger or any similar process where we installed temperature sensor for measuring the heat loss and the same was compensated using feedforward. so its a closed loop even with sensing using primary variable or a distrubance variable.


#6

@John

  • Closed loop system basically posses feedback loop.
  • In feedforward system the output is not dependent on previous output,rather output is dependent on current error.

#7

@mehta2155

Yes, FeedForward is open loop only. Thanks for clarification