What is feedforward control?
Feedforward control is a control technique that measures a disturbance and adjusts the control signal immediately, before the disturbance causes the process variable to deviate. Where feedback only responds once a deviation already exists, feedforward anticipates a known cause. Pure feedforward is a form of open-loop control; in practice it is almost always combined with a PID controller that removes the remaining deviation.
🧠 How does feedforward work?
Example: heat exchanger heating process water with steam
| Element | Role |
|---|---|
| Disturbance | The flow of cold process water varies strongly |
| Feedforward measurement | A flow transmitter measures the water flow |
| Feedforward model | Calculates how much extra steam is needed for more water |
| Feedback controller | PID controller on the outlet water temperature corrects model errors |
| Control signal | Sum of the feedforward and feedback parts goes to the steam control valve |
If the water flow increases, the steam valve opens immediately, before the temperature drops. The PID controller only needs to correct small deviations.
🔄 What is the difference between feedforward and feedback?
| Feedforward | Feedback (closed-loop) | |
|---|---|---|
| Responds to | Measured disturbance | Measured deviation from the setpoint |
| Moment of action | Before the deviation | After the deviation |
| Corrects unknown disturbances | No | Yes |
| Accuracy | Depends on the process model | High, independent of a model |
| Risk of instability | None | Possible with poor tuning |
That is why the combination is so powerful: feedforward handles the large, known disturbances quickly, feedback cleans up the rest.
🏭 Where is feedforward applied?
- Heat exchangers and furnaces — compensating for varying product flow or inlet temperature
- Boiler control — the three-element level control of a steam boiler uses steam flow as feedforward
- Dosing and blending — additive dosing follows the main flow directly (ratio control)
- Motion control — velocity and acceleration feedforward for precise positioning of servomotors
- Building automation — heating responds to the outdoor temperature before it gets colder indoors
🛠️ How do you design feedforward control?
- Identify the main disturbance — which measurable variable causes the largest deviations?
- Check measurability — is there a reliable, fast measurement of that disturbance?
- Determine the process model — how strongly and with what delay does the disturbance affect the process variable?
- Implement statically — start with a simple gain factor
- Add dynamics — with a lead-lag block if disturbance and control signal act at different speeds
- Combine with feedback and test performance against real disturbances
⚖️ What is ratio control?
Ratio control is the most widely used form of feedforward in the process industry. A second stream automatically follows a main stream in a fixed ratio:
| Application | Main stream (measured) | Controlled stream | Ratio |
|---|---|---|---|
| Combustion | Fuel | Combustion air | E.g. 10–15 kg of air per kg of gas |
| Water treatment | Water to be treated | Chlorine or coagulant dosing | E.g. mg per litre |
| Mixing | Base product | Additive or colourant | Fixed percentage |
| Blending | Main fuel component | Blend component | According to recipe |
The main stream is the measured disturbance, the ratio is the feedforward model, and the flow control of the controlled stream ensures the requested quantity is actually delivered. An analytical measurement, such as oxygen in flue gas or residual chlorine, often corrects the ratio via a feedback controller.
❓ Frequently asked questions
What is the advantage of feedforward?
The advantage of feedforward is that the control system acts before a deviation arises. In processes with long dead times or delays, such as heat exchangers or furnaces, feedforward prevents large swings that a feedback controller could only correct after the fact.
Why is feedforward almost always combined with feedback?
Feedforward only corrects the disturbances that are measured, and only as well as the process model is accurate. Unknown disturbances, model errors and wear go unnoticed. A feedback controller measures the actual result and corrects those remaining deviations, making the combination both accurate and fast.
What is three-element boiler control?
Three-element boiler control regulates the water level in a steam boiler using three measurements: the level itself, the steam flow as feedforward and the feedwater flow as the inner loop of a cascade. This keeps the level stable during sudden changes in steam demand, despite the swell and shrink effect in the boiler.
Is feedforward the same as open-loop control?
Pure feedforward is a form of open-loop control, because the control signal is calculated without measuring the result. In practice, however, feedforward is almost always added to a closed-loop control system, so that the system as a whole still has feedback.
What is a lead-lag block in feedforward?
A lead-lag block adjusts the timing of the feedforward signal so that the correction arrives in the process at the right moment. If the disturbance acts faster or slower than the control signal, the block advances (lead) or delays (lag) the correction. Without lead-lag, feedforward can act too early or too late and cause a deviation itself.
Can feedforward be programmed in a PLC?
Yes, feedforward can easily be programmed in a PLC or DCS. The measured disturbance signal is multiplied by a gain factor, optionally through a lead-lag block, and added to the output of the PID controller. Many PID function blocks have a dedicated feedforward input for this, so that the sum is limited within the block.
📌 In summary
Feedforward control measures a disturbance and adjusts the control signal before the process deviates, while a feedback controller corrects the remaining deviation. The combination is particularly valuable in slow processes with large, measurable disturbances, such as heat exchangers, boilers and furnaces.