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Structure of pneumatic valve positioners: Main functions of valve pneumatic positioners: The positioners for pneumatic valves, together with pneumatic actuators, form an automatic control unit that is connected to various control valves; after adjustment and installation, they constitute a pneumatic control valve. The positioner of a pneumatic valve, together with the pneumatic actuator, forms an automatic control unit that is connected to various control valves; after adjustment and installation, they combine to form a pneumatic control valve. It is used in various fields of industrial automation process control. Used in conjunction with pneumatic systems: It can be applied to single-acting (spring-return type) and double-acting pneumatic actuators. Working principle of pneumatic positioners: Pneumatic valve positioners operate on the principle of torque balance. When the signal pressure P1 acting on the diaphragm 2 increases, it causes the main lever 3 to rotate around its pivot point, bringing the nozzle flap 9 closer to the nozzle. The back pressure at the nozzle is amplified by the one-way amplifier 8, which in turn increases the pressure in the actuator’s diaphragm chamber, causing the valve stem to move downward. This causes the feedback rod to rotate around the pivot, and the feedback cam also rotates in a counter-clockwise direction. The roller enables the secondary lever 4 to rotate around the pivot, thereby stretching the feedback spring. When the pulling force of the spring on the primary lever 3 equals the force exerted by the signal pressure on the bellows, the instrument reaches a state of equilibrium. The valve position of the actuator is maintained at a certain opening; a specific signal pressure corresponds to a specific valve opening degree. The above mode of operation is positive action; to change the mode of operation, simply flip the cam, changing from direction A to direction B, etc. A positive-action positioner is one in which an increase in signal pressure results in an increase in output pressure as well ； A reaction locator is one in which an increase in signal pressure results in a decrease in output pressure. A direct-acting actuator can achieve the operation of a reverse-acting actuator simply by installing a reverse-acting positioner ； On the contrary, a reaction actuator can achieve the same functionality as a direct-acting actuator simply by being equipped with a reaction positioner. Structural principle of valve pneumatic actuator: http://www.jdfam.com/uploads/allimg/160706/1-160F6093R9631.jpg A pneumatic valve positioner is a device that receives weak electrical signals such as 4–20mA from a controller or control system, and sends air signals to the pneumatic actuator in order to control the valve’s position. It is used in conjunction with pneumatic control valves to form a closed-loop control circuit. Convert the DC current signal provided by the control system into a pneumatic signal to drive the control valve and regulate its operation. At the same time, feedback is provided based on the opening degree of the control valve, enabling the valve position to be accurately positioned in accordance with the control signal generated by the system. Performance of pneumatic valve assemblies: 1. The rated output force or torque of the pneumatic unit shall comply with the provisions of GB/T12222 and GB/T12223. 2. When testing under the maximum operating pressure used for seal testing, the amount of air leaking from each side under backpressure shall not exceed (3+0.15D) cm3/min (under standard conditions) ； The amount of air leaking from the end caps and output shaft shall not exceed (3+0.15d) cm3/min. 3. At an air pressure of 0.6 MPa, the output torque or thrust of the pneumatic device in both the opening and closing directions shall be no less than the value indicated on its label; moreover, its operation shall be smooth, with no permanent deformation or other abnormal phenomena occurring in any part of it. 4. Under no-load conditions, when the specified air pressure from Table 2 is applied to the cylinder, its operation should be smooth, without any jamming or crawling phenomena. 5. For strength testing, the test is conducted at 1.5 times the maximum operating pressure; after maintaining this pressure for 3 minutes, there shall be no leaks or structural deformation at the cylinder end caps and static sealing areas. 6. Number of operation cycles: The pneumatic actuator simulates the operation of a pneumatic valve; while maintaining the capability to generate output torque or thrust in both directions, the number of opening and closing operations should be no less than 50,000 times (one opening-closing cycle counts as one operation). 7. Pneumatic devices with a buffering mechanism shall not experience shock when the piston reaches the end of its stroke.