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.