Pneumatic actuatorPneumatic actuator or pneumatic actuator is the actuator of opening and closing or regulating valve driven by pneumatic force, but it is commonly called pneumatic head. The actuating mechanism and regulating mechanism of pneumatic actuator are unified as a whole. The actuating mechanism of pneumatic actuator includes film type, piston type, fork type and gear rack type.

　　Membrane actuator

　　Membrane actuator is the most commonly used. It can be used as a driving device of general control valves to form a pneumatic membrane actuator. The signal pressure P of the pneumatic membrane actuator acts on the diaphragm to deform it, drive the push rod on the diaphragm to move, and cause the displacement of the valve core, thus changing the opening of the valve. It has the advantages of simple structure, low price, convenient maintenance and wide application.

　　The pneumatic film actuator has two forms: positive and negative.

　　When the signal pressure from the controller or valve positioner increases, the downward action of the stem is called a positive actuator; when the signal pressure increases, the upward action of the stem is called a reaction actuator. The signal pressure of the positive actuator is to enter the film chamber above the corrugated diaphragm, while the signal pressure of the reaction actuator is to enter the film chamber below the corrugated diaphragm. By replacing individual parts, the two can be altered to each other.

　　Piston actuator

　　Pneumatic piston actuator makes the piston move in the cylinder to generate thrust. Obviously, the output force of the piston type is much greater than that of the film type. Therefore, the membrane type is suitable for small output and high precision occasions; the piston type is suitable for large output occasions, such as large caliber, high pressure drop control or butterfly valve driving device. Besides membrane type and piston type, there is also a long travel actuator, which has long travel and large torque, and is suitable for output angular displacement and large torque occasions.

　　The signal standard received by pneumatic actuator is 0.02 to 0.1 MPa.

　　The main components of pneumatic piston actuator are cylinder, piston and push rod. The piston in the cylinder moves with the change of the pressure difference between the two sides of the cylinder. According to the characteristics, it can be divided into two types: proportional type and two-way type. According to the operating pressure on both sides of the input piston, the piston is pushed from the high pressure side to the low pressure side. Proportional type is based on two-way valve positioner, so that the push rod displacement and signal pressure proportional relationship.

　　Rack-and-pinion actuator

　　Gear rack type (double piston rack type) pneumatic actuator has the characteristics of compact structure, beautiful appearance, quick response, stable operation and long service life. All fittings adopt the most advanced anti-corrosion treatment technology, which can adapt to all kinds of adverse working conditions. Its high and low temperature and various special travel actuators have good performance in various application fields.

The piston stroke is long and suitable for situations requiring greater thrust, while the membrane stroke is small and can only drive the valve stem directly. Pneumatic fork actuator has the characteristics of large torque, small space, torque curve more in line with the torque curve of the valve, but it is not very beautiful; it is commonly used in large torque valves. The rack-and-pinion pneumatic actuator has the advantages of simple structure, stable and reliable operation, and safety and explosion-proof. It has been widely used in power plants, chemical industry, oil refining and other production processes with high safety requirements.