What is open-loop control?
Open-loop control is a form of control in which a control signal is sent to a process without the actual result being measured and corrected. The controller assumes that a given command always produces the desired effect. A toaster that heats for two minutes regardless of how brown the bread is works open-loop. The opposite is closed-loop control, which continuously measures and adjusts through feedback.
🧠 How does open-loop control work?
Open-loop control has three elements:
- Input signal — the desired value or command, such as “run the pump at 50%”
- Controller — converts the command into a control signal, for example a PLC or timer relay
- Process and actuator — carries out the command, such as a motor via a variable frequency drive
There is no sensor reporting the outcome. If a disturbance occurs, such as a higher load or a blocked pipe, the controller does not notice it and the deviation is not corrected.
🔄 What is the difference between open-loop and closed-loop control?
| Open-loop | Closed-loop | |
|---|---|---|
| Feedback | No | Yes, via a measurement (process variable) |
| Response to disturbances | None | Corrects automatically |
| Accuracy | Depends on calibration and stable conditions | High, even under changing conditions |
| Complexity and cost | Low | Higher: sensor, controller, tuning |
| Risk of instability | None | Possible with poor tuning |
| Example | Conveyor at a fixed speed | Temperature control with a PID controller |
🏭 Where is open-loop control used?
- Time-based processes — filling machines, washing programmes, traffic lights with fixed cycles
- Stepper motors — a stepper motor turns a fixed number of steps without position feedback
- Dosing by volume or time — a dosing pump delivering a fixed quantity per stroke
- Start-up and manual mode — a control loop in manual mode temporarily operates open-loop
- Feedforward — feedforward control is an open-loop component often combined with closed-loop control
🛠️ When do you choose open-loop control?
Open-loop control is a good choice when:
- The process is predictable and stable, with few disturbances
- A small deviation has no consequences for quality or safety
- A measurement is expensive, unreliable or impossible
- Simplicity matters more than accuracy
As soon as disturbances affect quality or safety, closed-loop control with a measurement is needed. In practice many installations combine both: an open-loop part that responds quickly to known changes and a closed-loop part that removes the remaining deviation.
🔐 Why is open-loop control relevant for OT security?
With open-loop control there is no measurement showing whether the process is doing what the controller assumes. That makes manipulation harder to detect. If an attacker changes the control signal or parameters of an open-loop control, for example the dosing time of a chemical pump, no measurement raises an alarm.
| Situation | Risk under manipulation | Measure |
|---|---|---|
| Timed dosing pump (open-loop) | Over- or underdosing goes unnoticed | Independent concentration measurement, limits in the PLC |
| Control loop with measurement (closed-loop) | Deviation becomes visible in the process variable | Alarming on deviations from the setpoint |
| Safety function | Must always work, even under manipulation | Independent SIS with its own sensors |
Safe designs therefore combine open-loop parts with independent check measurements and hard limits in the controller. The water treatment incident in Oldsmar (Florida, 2021), where the sodium hydroxide dosing was raised remotely, shows why dosed quantities must also be verified.
❓ Frequently asked questions
What is an example of an open-loop system?
A washing machine that runs a programme on a timer is a classic example of an open-loop system: it does not measure whether the laundry is clean. In industry, a conveyor at a fixed speed, a stepper motor without an encoder and a timed dosing pump are common examples of open-loop control.
Is open-loop worse than closed-loop?
Open-loop is not inherently worse than closed-loop, but suits different situations. Open-loop is simpler, cheaper and cannot become unstable, but does not correct disturbances. Closed-loop is more accurate under changing conditions, but requires a measurement and a well-tuned controller.
What is the difference between open-loop and closed-loop in simple terms?
In open-loop control, the controller gives a command without checking the result. In closed-loop control, the controller measures the result, compares it with the desired value (setpoint) and adjusts. This distinction between control without and with feedback is fundamental in control engineering.
Does a PLC work open-loop or closed-loop?
A PLC can do both. Much of the logic in a PLC, such as starting a motor when a push button is pressed, is open-loop. A PLC also performs closed-loop control, for example with a PID function block that controls a temperature or pressure based on a measured value.
What is an open-loop stepper motor?
An open-loop stepper motor turns a fixed number of steps per control pulse, without an encoder measuring the actual position. As long as the load stays within limits, the motor reaches the correct position. Under overload the motor can miss steps without the controller noticing; critical applications therefore add an encoder and operate closed-loop.
Can open-loop control become unstable?
Pure open-loop control cannot become unstable through feedback, because there is no feedback. The process itself may be unstable, however, and open-loop control will not keep it under control. Instability caused by poorly tuned controllers, such as oscillation, only occurs in closed-loop control.
📌 In summary
Open-loop control drives a process without measuring and correcting the result; it is simple and stable, but does not respond to disturbances. For predictable processes open-loop is sufficient; where quality or safety depends on the result, closed-loop control is needed.