There are two types of valve
positioners, pneumatic and electric. They are the main accessories of pneumatic actuators and are used in conjunction with pneumatic control valves. The valve positioner receives the regulator output signal, and then outputs the regulator's output signal to the actuator proportionally. When the valve stem moves, its displacement acts on the valve positioner through the negative feedback of the mechanical device, so it forms a closed loop system with the actuator. The valve positioner can increase the discovery power of the actuator, reduce the transmission lag of the control signal, and accelerate the movement speed of the valve stem. Improve the linearity between the signal and the valve position, overcome the friction of the valve stem, and eliminate the influence of unbalanced force, thereby ensuring the correct positioning of the regulating valve and improving the performance of the regulating valve. At present, most electric regulators are used. Here is an introduction to the electric valve positioner. The structure and working principle of valve positioner adopt electric-pneumatic valve positioner, 0-10mA or 4-20mA DC current signal output by electric regulator can be used to control pneumatic actuator. An electric-pneumatic valve positioner has the dual functions of an electric-pneumatic converter and a pneumatic valve positioner. Figure 9-18 is the action principle diagram and physical diagram of an electro-pneumatic valve positioner equipped with a pneumatic membrane actuator. The permanent magnet 1, the permeable magnet 2, the coil, the armature (ie the main lever 3) and the working air gap constitute a torque motor assembly, which is a conversion element that converts current into force (torque). The magnet and armature are made of permalloy with high permeability. The permanent magnet is U-shaped, and its end N and S poles are covered on the magnet. When the signal current passes through the coil, due to the interaction of the electromagnetic field and the permanent magnet, the main lever 3 receives a leftward force, so it deflects around the fulcrum 16, making the baffle 14 close to the nozzle 15, and the nozzle back pressure is amplified by the amplifier 17. After that, the film actuator 9 is sent to move the valve rod downward, and drives the feedback rod 10 to rotate around the fulcrum 5. The feedback cam 6 connected on the same shaft also rotates counterclockwise, and the secondary lever 7 is rotated around the fulcrum 8 through the roller 11 to stretch the feedback spring 12. When the pulling force of the pop-like 12 on the main lever and the force exerted by the torque motor on the main lever are balanced, the instrument reaches a balanced state. At this time, a certain signal current is converted into a certain air pressure signal, and has a precise corresponding relationship with the valve position. The spring 13 is used to adjust the zero position. Changing the shape of the cam 6 can change the corresponding relationship between the input current signal and the output valve stem displacement.
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