Control systems are primarily classified as open-loop and closed-loop or feedback control systems.

The elements of an open-loop system are the controller and the controlled process. An input signal is applied to the controller, which controls the process so that the controlled variable performs according to prescribed standards.

Open-loop systems operate independently of the output. A practical example is a washing machine that operates on a time basis without measuring the clothes' cleanliness.

They are advantageous when the output is difficult to quantify or precise measurement is not economically feasible.

Closed-loop systems include one or more feedback links from the output to the input for more accurate control.

In a closed-loop idle-speed control system, any difference between the desired idling speed and the actual speed is sensed and corrected by adjusting the throttle angle. So, due to the use of feedback, the system quickly recovers after a disturbance to the preset value.

However, they are more complex to build and are generally higher in cost and power due to the use of more components.