A pneumatic actuator converts the energy of compressed air (or another gas) into mechanical energy, which can be in the form of rotary or linear movement, depending on the type of actuator.[1]​ They are usually used to regulate control valves in piping systems, to operate pneumatic brakes,[2]​ in the access doors of public transport vehicles,[3]​ or in some types of clutches.[4]​.

A pneumatic actuator mainly consists of a piston or a diaphragm "Diaphragm (mechanism)") that develops a driving force. It keeps air at the top of the cylinder, allowing air pressure to force the diaphragm or piston to move a valve stem or rotate its control element.[5]​.

The valves require little pressure to operate and typically double or "triple" the input force. The larger the piston size, the higher the output pressure. Having a larger piston can also be good if the air supply is low, allowing for higher forces with lower input energy. With an input of 100 kPa, a basic small pneumatic valve could lift a small car (over 1000 pounds (450 kg)) easily. However, the resulting forces required on the stem would be too great and would cause the actuator to fail.

This pressure is transferred to the valve stem, which is connected to the valve plug (see plug valve or butterfly valve). Higher forces are required in high-pressure or high-flow pipelines to allow the valve to overcome these forces and allow the moving parts of the valves to move to control the passage of fluid flowing through a pipeline.

The valve input is the "control signal", which can come from different measuring devices, and each different pressure is a different set point for a valve. A typical standard signal is 20 to 100 kPa. For example, a valve could control the pressure in a vessel that has a constant outflow and a variable inflow (regulated by the actuator and valve). A pressure transmitter will monitor the pressure in the vessel and transmit a signal of 20 to 100 kPa: 20 kPa means no pressure, 100 kPa means maximum pressure is recorded (which can be varied depending on the calibration points of the transmitter). As the pressure in the vessel increases, the output of the transmitter increases, this increase in pressure is sent to the valve, causing the valve to begin to close, decreasing the flow into the vessel, reducing the pressure in the vessel as the excess pressure is evacuated through the outlet flow. This is called the direct action process.[6]​.

Some types of pneumatic actuators include:.