Whats the purpose of feed forward s

[Herzt]

Whats the purpose of feed forward signal to control level in boiler? feed forward signal shall be scaled at the controller inlet to a value between -50% and 50%, to get a BIAS=0 when the feed forward signal is 50%.

 

[georgeverghese]

Feedforward control is used to effect a change in the primary controller's output to the final control device before the effect of a disturbance is sensed by the sensing transmitter of the primary controller. It therefore helps to keep a tighter control on the variable being controlled.
This is described very well in Perry Chem Engg Handbook in the chapter on process control. Feedforward control is useful, and easier to implement in some cases. But in other cases, it can cause control instability, mainly because the model used to effect the output of the feedforward controller, is not correct or accurate enough. I have implemented FF control on heat exchangers and it worked. But in another case, a FF control scheme I configured turned out to be a disaster, and operators were NOT pleased.
It looks like in your case, the correction from the FF controller is acting on the INPUT to the primary loop level controller, and not on the output of the primary loop controller. Does it work in your plant ?

 

[pierreick]

Hi,
Consider these resources:

https://controlguru.com/cascade-feed-forward-and-boiler-level-control/

https://control.com/textbook/basic-process-control-strategies/feedforward-control/

Pierre