What are the main classifications of common valve positioners?

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　　Valve positioners can be classified by structure into pneumatic valve positioners, electrical valve positioners, and intelligent valve positioners. They are essential accessories for control valves, usually used in conjunction with pneumatic control valves. These positioners receive the output signal from the controller and use it to control the pneumatic control valve. When the control valve moves, the displacement of the valve stem is fed back to the valve positioner through mechanical mechanisms, and the valve’s position is transmitted to the higher-level system via electrical signals. 　　Based on their structural design and working principles, valve positioners can be divided into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners. 　　Valve positioners can increase the output capacity of control valves, reduce delays in the transmission of control signals, speed up the movement of the valve stem, improve the linearity of the valve, overcome frictional forces on the valve stem and eliminate the effects of unbalanced forces, thereby ensuring the accurate positioning of the control valve. 　　Classification of valve positioners: 1. Valve positioners can be classified into pneumatic valve positioners, electrical valve positioners, and intelligent valve positioners based on their input signals. 　　(1) The input signal of a pneumatic valve positioner is a standard pneumatic signal, such as a 20–100 kPa pneumatic signal, and its output signal is also a standard pneumatic signal. 　　(2) The input signal for electrical valve positioners is a standard current or voltage signal, such as a 4–20 mA current signal or a 1–5 V voltage signal. Inside the electrical valve positioner, this electrical signal is converted into electromagnetic force, which is then used to generate a pneumatic signal that operates the control valve. 　　(3) The intelligent electrical valve positioner converts the current signal output from the control room into a pneumatic signal used to drive the control valve. It compensates for the unbalanced forces resulting from fluctuations in medium pressure, due to the frictional force on the valve stem during operation, so that the valve opening corresponds to the current signal sent from the control room. Furthermore, intelligent configuration can be used to set the corresponding parameters, thereby improving the performance of the control valve. 　　2. Based on the direction of movement, they can be divided into single-direction valve positioners and double-direction valve positioners. When a one-way valve positioner is used with a piston actuator, the valve positioner operates in only one direction; whereas a two-way valve positioner acts on both sides of the cylinder of the piston actuator, allowing it to function in two directions. 　　3. Valve positioners are classified into positive-action valve positioners and reverse-action valve positioners based on the sign of the gain of their output and input signals. As the input signal to a positive-action valve positioner increases, the output signal also increases; therefore, the gain is positive. As the input signal to the counteraction valve positioner increases, the output signal decreases; therefore, the gain is negative. 　　4. Depending on whether the input signal to the valve positioner is an analog signal or a digital signal, they can be divided into conventional valve positioners and fieldbus-based electrical valve positioners. The input signal for a conventional valve positioner is an analog pressure or current/voltage signal, while the input signal for a fieldbus-based electrical valve positioner is a digital signal from the fieldbus. 　　5. Based on whether the valve positioner is equipped with a CPU, they can be divided into conventional electrical valve positioners and intelligent electrical valve positioners. Ordinary electrical valve positioners do not have a CPU; therefore, they lack intelligence and are unable to perform any intelligent calculations. Intelligent electrical valve positioners come equipped with a CPU that can handle various intelligent calculations; for example, they can perform nonlinear compensation for the forward channel. Fieldbus electrical valve positioners may also include functional modules such as PID to carry out corresponding calculations. 　　6. It can also be classified according to the detection method of the feedback signal. For example, valve positioners that use mechanical linkages to detect the valve position signal; valve positioners that use the Hall effect to measure the displacement of the valve stem; valve positioners that use electromagnetic induction to detect the displacement of the valve stem, and so on.