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A final control element (or control device or actuator ) is used in technical systems when an output variable of the system is to be influenced. It is located in a control or a control system . A process is controlled by setting or adjusting it .

In the case of pure controls , the actuator is often adjusted by hand, such as the steering wheel of a vehicle. In control systems, the input and output variables are often not constantly changing, but only have binary values ​​(on and off or high and low ). The corresponding actuators are just switches .

In closed-loop control , the actuator and the controlled system are part of a closed circuit ( control circuit ). His adjustment is from the controller specified in, and causes the control system controlling the flow of mass, energy or information. In particular, the adjustment to change a mass flow requires energy that can sometimes and with simple controls be taken from the accompanying energy flow. In most cases, an external energy source is used to adjust the actuator ( auxiliary energy ). Are used electric , pneumatic or hydraulic actuators .

Example valve and thermostatic valve

Radiators for space heating used to be equipped with simple valves that were used to more or less throttle the flow of hot water by hand. Such valves are simple actuators. They are adjusted by hand. The room temperature was controlled (if the person making the adjustment did not act depending on the difference between the desired and actual room temperature; otherwise there was a "manual" control (manual control)).

The thermostatic valves common today regulate the room temperature automatically. The rotary handle that remains on them is used to set the temperature setpoint . An expansion element acts as the drive for the valve actuator , which simultaneously measures the room temperature (actual value of the controlled variable ) and acts as a controller. The drive energy is withdrawn from the hot water.