An open-loop control system operates without using feedback about its output, whereas a closed-loop control system monitors its output and uses feedback to adjust its operation. Closed-loop systems can therefore respond to disturbances or differences between the desired and actual results.
In an open-loop system, the controller receives an input and produces a predetermined output. It assumes that the chosen action will achieve the desired result, but it does not check whether this has happened. For example, a garden sprinkler controlled only by a timer runs for a fixed period regardless of how wet the soil becomes.
A closed-loop system includes a sensor that measures the system's actual output. This measurement is returned as feedback to the controller, which compares it with a desired value called the set point. If there is an error, the controller changes the system's operation.
For example, a thermostat-controlled heating system measures room temperature. If the temperature is below the set point, the controller activates the heater; when the set point is reached, it switches the heater off or reduces its output.
| Feature | Open-loop system | Closed-loop system |
|---|---|---|
| Feedback | No feedback is used | Output is fed back to the controller |
| Sensors | Not required to measure output | Required to monitor output |
| Response to disturbances | Cannot automatically compensate | Can adjust its operation |
| Complexity and cost | Usually simpler and cheaper | Usually more complex and expensive |
| Accuracy | Depends on correct initial settings | Can reduce differences between desired and actual output |
| Example | Timer-controlled sprinkler | Thermostat-controlled heating |
A common misconception is that every automated system is closed-loop. Automation alone does not make a system closed-loop; it must measure its output and use that information to influence later control actions.
Exam technique: For a command term such as distinguish or compare, identify feedback as the central difference, then explain the role of the sensor and controller. Apply both systems to the same scenario and state a relevant advantage or limitation rather than merely listing examples.
