Can feedforward control work without any feedback at all?

Technically yes, but it is then a form of open-loop control with a smarter input — it has no way to detect or correct for an unmeasured disturbance, sensor drift, or its own model error, since it never checks the actual process output. Virtually all real feedforward implementations in SCADA/DCS systems run alongside a feedback loop specifically to cover that gap.

What happens if the feedforward model is inaccurate?

The feedforward correction will be wrong by whatever margin the model is off, producing a residual error at the process output — either an overcorrection or an undercorrection. This residual is exactly what the feedback loop, still running underneath, is there to trim out; it is not something feedforward itself can detect or fix.

Does feedforward need its own measured disturbance, separate from the process variable?

Yes. Feedforward specifically measures the disturbance variable (cold inlet flow rate, in the heat exchanger example) rather than the controlled variable (outlet temperature). If the disturbance can't be measured directly with a sensor, feedforward simply cannot be applied to it — there is nothing else it can act on.

Is feedforward the same thing as ratio control or cascade control?

No, though they're often used together. Ratio control maintains a fixed proportion between two measured flows; cascade control nests one feedback loop's output as another loop's setpoint. Feedforward is distinct from both: it applies a correction computed directly from a measured disturbance and a process model, independent of any setpoint cascade or flow ratio.

Why not just make the feedback loop faster instead of adding feedforward?

A feedback loop can only be made so fast before it fights the process dynamics and destabilizes — there is a hard limit set by the process's own dead time and lag, no matter how the PID is tuned. Feedforward sidesteps that limit entirely for measurable disturbances because it doesn't wait for the output to move at all; it acts on the disturbance measurement the instant it changes, which is fundamentally faster than any feedback loop reacting to the consequence of that same disturbance.

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Concept Explainer · SCADA

Feedforward vs. Feedback Control

Reacting to a disturbance after it has already hit the process, versus not waiting for it to become an error at all.

Feedback control, by definition, can only correct an error it has already measured — which means the disturbance has already reached the process output before anything happens. Feedforward control takes a different approach entirely: instead of waiting for the disturbance to show up as error, it measures the disturbance directly, the moment it occurs, and uses a model of how that disturbance affects the process to apply a correction immediately — before the output ever significantly moves. The catch is that feedforward only works for disturbances you can actually measure and reasonably predict the effect of. That's exactly why real control systems almost never use one instead of the other — they run both together.