A pneumatic control valve

actuator An actuator is a machine element, component of a machine that produces force, torque, or Displacement (geometry), displacement, when an electrical, Pneumatics, pneumatic or Hydraulic fluid, hydraulic input is supplied to it in a system (called an ...

converts energy (typically in the form of

compressed air Compressed air is air kept under a pressure that is greater than atmospheric pressure. Compressed air in vehicle tires and shock absorbers are commonly used for improved traction and reduced vibration. Compressed air is an important medium for t ...

) into

mechanical Mechanical may refer to: Machine * Machine (mechanical), a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement * Mechanical calculator, a device used to perform the basic operations o ...

motion. The motion can be rotary or

linear In mathematics, the term ''linear'' is used in two distinct senses for two different properties: * linearity of a '' function'' (or '' mapping''); * linearity of a '' polynomial''. An example of a linear function is the function defined by f(x) ...

, depending on the type of actuator.

Principle of operation

A pneumatic actuator mainly consists of a piston or a diaphragm which develops the motive power. It keeps the air in the upper portion of the cylinder, allowing air pressure to force the diaphragm or piston to move the valve stem or rotate the valve control element. Valves require little pressure to operate and usually double or ''triple'' the input force. The larger the size of the piston, the larger the output pressure can be. Having a larger piston can also be good if the air supply is low, allowing the same forces with less input. These pressures are large enough to crush objects in the pipe. On 100 kPa input, you could lift a small car (upwards of 1,000 lbs) easily, and this is only a basic, small pneumatic valve. However, the resulting forces required of the stem would be too great and cause the

valve stem A valve stem is a self-contained valve that opens to admit gas to a chamber (such as air to inflate a tire), and is then automatically closed and kept sealed by the pressure in the chamber, or a spring (device), spring, or both, to prevent the g ...

to fail. This pressure is transferred to the valve stem, which is connected to either the valve plug (see plug valve), butterfly valve etc. Larger forces are required in high pressure or high flow pipelines to allow the valve to overcome these forces, and allow it to move the valves moving parts to control the material flowing inside. The valve's input is the "control signal." This can come from a variety of measuring devices, and each different pressure is a different set point for a valve. A typical standard signal is 20–100 kPa. For example, a valve could be controlling the pressure in a vessel that has a constant out-flow, and a varied in-flow (varied by the actuator and valve). A pressure transmitter will monitor the pressure in the vessel and transmit a signal from 20–100 kPa. 20 kPa means there is no pressure, 100 kPa means there is full range pressure (can be varied by the transmitters calibration points). As the pressure rises in the vessel, the output of the transmitter rises, this increase in pressure is sent to the valve, which causes the valve to stroke downward, and start closing the valve, decreasing flow into the vessel, reducing the pressure in the vessel as excess pressure is evacuated through the outflow. This is called a direct-acting process.

Types

Some types of pneumatic actuators include: *Tie-rod cylinders *

Rotary actuator A rotary actuator is an actuator that produces a rotation, rotary motion or torque. The simplest actuator is purely mechanical, where linear motion in one direction gives rise to rotation. The most common actuators are electrically powered; other ...

s *Grippers *Rodless actuators with a magnetic linkage or rotary cylinders *Rodless actuators with mechanical linkage * Pneumatic artificial muscles *Vane Motors *

Pneumatic motor A pneumatic motor (air motor), or compressed-air engine, is a type of motor which does mechanical work by expanding compressed air. Pneumatic motors generally convert the compressed-air energy to mechanical work through either linear or rotary ...

s *Speciality actuators that combine rotary and linear motion—frequently used for clamping operations * Vacuum generators

Principle of operation

Types

See also