How to achieve forward and reverse operation of valve positioners with each other

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An intelligent valve positioner is an intelligent process control instrument based on microprocessor technology, which uses digital techniques for data processing, decision-making, and two-way communication. It does not require manual calibration; it can automatically detect the zero point, full scale, and friction coefficient of the control valve it is connected to, and automatically set the control parameters. The electrical series of valve positioners is commonly used in our production and daily life. They operate on the principle of force balance, and their working mechanism is that of a current signal-controlled valve positioner developed on the basis of pneumatic valve positioners. The operation of valve positioners can be divided into direct and reverse action; below we will explain the working principle in the case of direct action. When the current signal from the electric regulator is applied to the coils of the torque motor assembly, a magnetic field is generated in the air gap of the torque motor. This magnetic field, together with that produced by the permanent magnets, exerts a force on the armature in the left direction. The main lever (armature) 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 causes the feedback rod to rotate around its pivot point. The feedback cam, which is connected to the same axis, rotates counterclockwise. Through the rollers, the lever is caused to rotate around its pivot point. When the pulling force of the feedback spring on the main lever equals the torque exerted by the torque motor on the main lever, the lever system reaches a state of equilibrium. At this point, a certain signal current corresponds to a certain valve position. The spring is used to adjust the zero position. To change the mode of operation, simply flip the cam; switching from direction A to direction B will achieve the conversion between forward and reverse action. So how can one distinguish between positive and negative action valve positioners? In a reverse-acting valve positioner, as the signal current increases, the output pressure decreases; whereas in a direct-acting positioner, as the signal current increases, the output pressure also increases. A direct-acting actuator can perform the functions of a reverse-acting actuator once a reverse-acting positioner is installed; similarly, a reverse-acting actuator can perform the functions of a direct-acting actuator once a reverse-acting positioner is installed.