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It should be noted that: 1. The valve is neither single-acting nor double-acting, while the actuator is only piston-acting or double-acting. 2. Valve positioners are also divided into two functions: one output (OUT1) and two outputs (OUT1& ； OUT2)。 3. Single-acting valve positioners are generally used with single-acting actuators, including diaphragm actuators, single-cylinder single-acting cylinders, double-cylinder single-acting cylinders, etc. Double-acting valve positioners are generally used with double-acting actuators, such as single-cylinder double-acting actuators and double-cylinder double-acting actuators. _When used in unconventional ways, some pneumatic components can be used to mix the above. Pneumatic actuators are divided into two-functional and one-functional types. A single-acting pneumatic actuator (which contains a spring inside; in the absence of air supply, the spring will automatically reset, providing the force needed to bring the ball valve back to its initial open or closed state). If a double-acting pneumatic actuator is chosen, it will function even in the absence of air supply. The actuator loses power, and the valve remains in the position it was in at the moment the gas was lost. Therefore, if automatic valve reset under degassing conditions is required, choose a single-acting pneumatic actuator; otherwise, choose a double-acting pneumatic actuator. It’s difficult to distinguish between double-acting pneumatic actuators and single-acting pneumatic actuators based on their appearance! It depends on the nameplate on the valve: if there is FO, FC is single-acting. If not, then it’s a dual effect. The above pneumatic systems are mainly designed for piston cylinders. A membrane actuator has only one function. A pneumatic valve positioner is a device that converts electrical signals into pressure signals. It uses compressed air or nitrogen as the working gas source to control the opening of the control valves in industrial furnaces. It is widely used for the continuous control of pneumatic valve actuators in industrial furnace temperature control systems. Pneumatic valve positioners are designed based on the principle of force balance. As shown in the figure, as the signal pressure acting on the bellows increases, lever 2 rotates around its pivot point, causing the baffle to move closer to the nozzle. The back pressure at the nozzle is amplified by an amplifier and then sent to the air chamber of the diaphragm actuator, which causes the valve stem to move downward. This in turn drives the feedback rod (rocking rod) to rotate around its pivot point, and the feedback cam (eccentric cam) connected to the same axis also rotates as a result. It rotates counterclockwise, and rotates lever 1 around the pivot through a roller. The instrument reaches an equilibrium state when the forces on the bellows are balanced by the tension of the feedback spring, the tension of the spring on lever 2, and the signal pressure. At this time, a certain signal pressure corresponds to a certain valve position. The aforementioned approach is positive. If you want to change the motion pattern, simply turn the cam around, change direction A to direction B, and so on. The so-called positive positioner means that as the signal pressure increases, the output pressure increases as well. A reaction positioner refers to one in which the output pressure decreases as the signal pressure increases. As long as the positive actuator is equipped with a reactive positioner, the function of the reactive actuator can be achieved ； On the contrary, as long as the negative actuator is equipped with a reaction locator, the function of the positive actuator can be achieved.