Open Loop and Closed Loop Systems

Automatic control systems may operate under open or closed loop control. The input of an open loop system is not influenced by its output. This is in contrast with a closed loop system that monitors its output and then manipulates the input, to ensure that the output meets set conditions.

An example of an open loop system is a bullet fired from a rifle and an example of a closed loop systems is a cruiser missile, fired from a ship, continuously monitoring and adjusting its trajectory.

Open loop control strategies may perform well in situations where the behaviour of the system is very well defined and modelled. For example a room temperature controller set to a certain on/off heater schedule, will produce the desired room temperature.

Open loop system: The input to the system is not determined by the output.
Closed loop system: The input to the system is determined by the output.