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Principle and classification of pneumatic actuators: Pneumatic actuators are devices that use air pressure to open, close, or regulate valves; they are also known as pneumatic actuating mechanisms or pneumatic devices, though they are commonly referred to as pneumatic heads. Pneumatic actuators are sometimes also equipped with certain auxiliary devices. Commonly used ones include valve positioners and handwheel mechanisms. The function of a valve positioner is to utilize the principle of feedback to improve the performance of the actuator, enabling it to achieve accurate positioning in response to the control signals from the controller. The function of the handwheel mechanism is to allow direct operation of the control valve in cases where the control system fails due to power outages, gas supply interruptions, no output from the controller, or malfunction of the actuator, thereby ensuring the normal continuation of production. Pneumatic actuators are classified as follows: diaphragm type https://image.uc.cn/o/wemedia/s/upload/2020/a9cc9344af32fea412af55514e4ab447.jpg;,4,jpegx;3,700x.jpg. The stroke of the diaphragm type is relatively small, and it can only directly drive the valve stem. A pneumatic diaphragm actuator is one that uses an elastic diaphragm to convert the input air pressure into a thrust on the push rod; this thrust causes the valve core to move accordingly, thereby changing the degree of opening of the valve. Pneumatic piston actuators generate thrust through the piston inside the cylinder; since cylinders can withstand high pressures, a large thrust can be achieved, and it is easy to create actuators with long strokes. A typical pneumatic diaphragm actuator mainly consists of an elastic diaphragm, a compression spring, and a push rod. Piston type: https://image.uc.cn/o/wemedia/s/upload/2020/86276ce141c7ea523b3b9b366a1af095.png;,4,png;3,700x.png. The piston type has a long stroke and is suitable for applications that require high thrust. A pneumatic piston actuator is an actuator designed to make full use of the pressure from the air supply in order to increase its output force and reduce its mass and size. Piston actuators are generally limited to high-thrust applications and are used in few situations. This is because the air supply pressure for previous positioners was 140–250 kPa, whereas positioners with an air supply pressure of 700 kPa have lower reliability. Current positioners can be used in applications ranging from 140 to 250 kPa, as well as in applications at 700 kPa. Fork-type https://image.uc.cn/o/wemedia/s/upload/2020/gMZFF21eef9jstb/ce66066761eed94f120a055e93516d07.png;,4,png;3,700x.png Fork-type pneumatic actuators feature high torque, compact size, and a torque curve that better matches that of valves, but they are not very aesthetically pleasing. A Scotch Yoke Pneumatic Actuator is a piston-type actuator used for opening, closing, or regulating valves with 90° rotation angles, such as ball valves, butterfly valves, and plug valves. A fork-type pneumatic actuator generally refers to a pneumatic actuator with a structure of \"single cylinder, dual pistons – variable torque fork type\". Gear-rack type: https://image.uc.cn/o/wemedia/s/upload/2020/14136398fc7a8debc0492e401d14a361.jpg;,4,jpegx;3,700x.jpg. Gear-rack type pneumatic actuators have advantages such as a simple structure, stable and reliable operation, as well as safety and explosion-proof properties. It is widely used in production processes with high safety requirements, such as power plants, the chemical industry, and oil refining.