Feedforward Feedback Control Standard
Feedforward Feedback Control Standard
(OP)
I am a civil engineering PE working on a Construction project that includes a control system. The control system is intended to regulate the position of a control valve via feedforward-feedback control scheme in response to 4-20 mA signals from two sensors in comparison to a known setpoint. Is there a recognized industry standard that prescribes how this type of control strategy is normally mathmatically implemented? I would be grateful for any feedback. Thank you.





RE: Feedforward Feedback Control Standard
If you only care about regulating the position of a control valve then you shouldn't need feed forwards UNLESS you need to precisely control the rate of how that valve changes position.
Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
RE: Feedforward Feedback Control Standard
For disturbance rejection in process control, i.e., regulating a variable to a constant setpoint, feedforward is used differently. Given a known output disturbance transfer function Gd(s) that relates the disturbance input to the controlled output variable, and a known plant transfer function Gp(s), the feedforward controller transfer function CFF(s) for disturbance rejection would be:
CFF(s) = -Gd(s)/Gp(s)
The measured disturbance is passed through this transfer function and the resulting signal is added to the output of the feedback controller to make the total control signal sent to the plant.
There are two methods typically used to find the CFF, both deriving from the above equation. If first-order transfer function approximations (without dead time) are known for the disturbance and the plant, the resulting controller is a lead-lag controller:
CFF(s) = [-Kd(τps+1)]/[-Kp(τds+1)]
The lead-lag function is available in most commercial controllers like PLCs or DCSs. However, for many people it's too complicated to find both transfer function models, so an approximation is used that just requires the steady-state gains to be known for the disturbance and plant functions:
CFF = -Kd/-Kp
This works fine for most applications, as the feedback controller will "trim" any remaining errors that the feedforward controller doesn't remove from disturbances.
xnuke
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