Feedforward control corrects for a disturbance the moment you measure it, before it ever moves the variable you care about. Where ordinary feedback waits for an error to appear and then reacts, feedforward anticipates. It is one of the most powerful ways to tighten a slow or hard-to-control process, and it appears throughout oil and gas heating, blending, and flow control. This guide explains how feedforward works, why it is almost always paired with feedback, and where it fits.
Feedforward Control in one line: Feedforward control measures a disturbance directly and takes corrective action before that disturbance affects the process variable - rather than waiting for an error to develop the way feedback does. It is a predictive, model-based correction added on top of a normal feedback loop.
A feedback controller only acts after the fact: the process variable must first deviate from setpoint before the controller sees an error and responds. On a slow process with long dead time - say the temperature of oil leaving a heater treater - that delay means the upset has already done its damage before feedback catches up.
Feedforward flips the timing. It measures the disturbance itself - the inlet oil flow rate or the feed temperature - and computes the change in fuel or valve position needed to cancel it out, applying that correction immediately. The result is that the primary variable barely moves. The trade-off is that feedforward relies on a model of how the disturbance affects the process, and it is blind to anything you are not measuring.
No feedforward model is perfect. Sensor calibration drifts, the process gain changes with throughput, and there are always unmeasured disturbances. If feedforward ran alone, those errors would accumulate and the process variable would slowly wander off setpoint with nothing to correct it.
For that reason feedforward is almost always combined with feedback in a feedforward-plus-feedback scheme. Feedforward does the heavy lifting - knocking out the large, measurable disturbance fast - while a feedback (usually PID) loop trims the residual error and handles everything the model missed. This pairing gives both the speed of prediction and the accuracy of correction.
Classic examples include heater and heater-treater temperature control, where inlet flow rate is fed forward to preset the fuel-gas valve before the extra cold oil can drag the outlet temperature down. In blending and injection, a change in the main stream flow is fed forward to scale the additive or chemical injection rate so the ratio stays constant. Compressor and pump control also use feedforward on suction conditions.
In each case the disturbance is measurable, its effect on the process is reasonably well understood, and it enters faster than a feedback loop could catch it. Those three conditions are the practical test for whether feedforward is worth the added engineering.
Feedback reacts to an error after the process variable has already deviated from setpoint. Feedforward measures a disturbance directly and corrects for it before it affects the process variable. Feedforward is predictive; feedback is corrective. They are usually used together.
In practice, no. Feedforward relies on a model of the process and only acts on disturbances you measure, so errors and unmeasured upsets accumulate with nothing to correct them. It is almost always paired with a feedback loop that trims the residual error.
Common examples are heater and heater-treater temperature control with inlet flow fed forward to the fuel valve, chemical injection and blending where main-stream flow scales the additive rate, and compressor or pump control that anticipates suction conditions.
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