What are the main classifications of common valve positioners?

The valve positioner is classified by structure into: pneumatic valve positioner, electrical valve positioner, and intelligent valve positioner. It is a main accessory of the regulating valve, usually used in conjunction with pneumatic regulating valves. It receives the output signal from the regulator and uses its output signal to control the pneumatic regulating valve. When the regulating valve operates, the displacement of the valve stem is fed back to the valve positioner through a mechanical device, and the valve position status is transmitted to the upper-level system via an electrical signal.
Valve positioners can be divided into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners based on their structural form and working principle.
The valve positioner can increase the output power of the regulating valve, reduce the occurrence of transmission lag of the regulating signal, accelerate the movement speed of the valve stem, improve the linearity of the valve, overcome the friction of the valve stem, and eliminate the impact of unbalanced forces, thereby ensuring the correct positioning of the regulating valve.
Classification of valve positioners:
1. Valve positioners are divided into pneumatic valve positioners, electrical valve positioners, and intelligent valve positioners based on input signals.
(1) The input signal of the pneumatic valve positioner is a standard air signal, for example, a 20~100kPa air signal, and its output signal is also a standard air signal.
(2) The input signal of the electrical valve positioner is a standard current or voltage signal, for example, a 4~20mA current signal or a 1~5V voltage signal. Inside the electrical valve positioner, the electrical signal is converted into electromagnetic force, which then outputs an air signal to actuate the control valve.
(3) The intelligent electrical valve positioner converts the current signal output from the control room into an air signal that drives the regulating valve. It compensates for the unbalanced forces caused by the friction of the valve stem during the operation of the regulating valve and the pressure fluctuations of the medium, ensuring that the valve opening corresponds to the current signal output from the control room. It can also perform intelligent configuration to set corresponding parameters to improve the performance of the control valve.
2. Based on the direction of action, they can be divided into unidirectional valve positioners and bidirectional valve positioners. Unidirectional valve positioners are used with piston-type actuators, where the valve positioner only acts in one direction, while bidirectional valve positioners act on both sides of the piston-type actuator cylinder, functioning in both directions.
3. Based on the gain sign of the output and input signals of the valve positioner, they can be divided into direct-acting valve positioners and reverse-acting valve positioners. When the input signal of a direct-acting valve positioner increases, the output signal also increases, thus the gain is positive. When the input signal of a reverse-acting valve positioner increases, the output signal decreases, thus the gain is negative.
4. Based on whether the input signal of the valve positioner is an analog signal or a digital signal, they can be divided into ordinary valve positioners and fieldbus electrical valve positioners. The input signal of ordinary valve positioners is an analog air pressure or current/voltage signal, while the input signal of fieldbus electrical valve positioners is a digital signal from the fieldbus.
5. Based on whether the valve positioner has a CPU, they can be divided into ordinary electrical valve positioners and intelligent electrical valve positioners. Ordinary electrical valve positioners do not have a CPU, thus they are not intelligent and cannot perform related intelligent calculations. Intelligent electrical valve positioners have a CPU and can perform related intelligent calculations, such as nonlinear compensation for the forward channel. Fieldbus electrical valve positioners can also include PID and other functional modules to achieve corresponding calculations.
6. The valve positioners can also be classified based on the method of detecting feedback signals. For example, valve positioners that detect valve position signals using mechanical linkage, valve positioners that detect valve stem displacement using the Hall effect, and valve positioners that detect valve stem displacement using electromagnetic induction.