An open-loop electronic system controls an output without checking the result, whereas a closed-loop electronic system uses feedback to monitor the output and adjust its operation. The essential difference is therefore the presence of a feedback pathway.
How the Systems Work
Both systems can be represented using input, process, and output stages. For example, a user may provide an input through a switch, an electronic controller processes the signal, and an actuator produces the output.
In an open-loop system, information travels in one direction: input → process → output. The system assumes that its programmed action will produce the required result. A timed toaster, for example, heats bread for a selected duration but does not measure how brown the bread becomes.
In a closed-loop system, a sensor measures the actual output and returns this information as feedback. A controller compares the measured output with the desired value, or set point. If there is a difference, the system adjusts the output to reduce the error. A thermostat-controlled heating system measures room temperature and switches the heater on or off to maintain the selected temperature.
| Feature | Open-loop system | Closed-loop system |
|---|---|---|
| Feedback | No feedback pathway | Uses feedback from a sensor |
| Response to disturbance | Cannot automatically correct the output | Adjusts the output to reduce error |
| Complexity and cost | Usually simpler and cheaper | Usually more complex and expensive |
| Accuracy | Depends on calibration and predictable conditions | Usually maintains the required output more accurately |
| Example | Timer-controlled toaster | Thermostat-controlled heating |
A common misconception is that every automatic system is closed-loop. A system can operate automatically but still be open-loop if it follows a fixed program without sensing the result.
Exam Technique
For IB Design Technology HL A3.4, identify the components and trace the flow of information. To distinguish the systems, explicitly state whether output feedback is measured and used to modify the process; do not merely say that one system is more advanced.
