Valve positioner and electrical converter

A valve positioner is a key accessory for control valves; it uses the signal of the valve stem displacement as an input feedback measurement signal, and the output signal from the controller as the setpoint signal. By comparing these two signals, any discrepancy is addressed by adjusting the output signal sent to the actuator, thereby causing the actuator to move. This establishes a one-to-one relationship between the valve stem displacement signal and the controller’s output signal. Therefore, the valve positioner forms a feedback control system in which the stem displacement serves as the measurement signal, while the controller’s output acts as the setpoint signal. Valve positioners are classified into pneumatic valve positioners and electrical valve positioners based on the input signal. The input signal for pneumatic valve positioners is a standard gas signal, such as 20–100 kPa gas pressure, while the input signal for electrical positioners is a standard current or voltage signal, such as 4–20 mA or 1–5 V. Inside the positioner, the electrical signal is converted into electromagnetic force, which is then used to generate a gas signal that is sent to the pneumatic control valve. The electro-pneumatic converter is a type of conversion unit in electric actuator combined instrument systems. It receives the DC signal generated by electric control instruments and converts it proportionally into a pneumatic signal ranging from 20 to 100 Kpa, which is then used as a pneumatic control signal for pneumatic diaphragm control valves and pneumatic valve positioners. It can also serve as a power source for pneumatic instruments; in essence, it functions as a signal converter between electric instruments and pneumatic instruments. Now, some people install it on pneumatic valves to replace electrical potentiometers, believing it is more convenient to use as it requires no feedback rod. This is worth exploring, but in theory it is not advisable; precisely because there is no feedback mechanism, it cannot function as a closed-loop control system, so the control accuracy will definitely be a serious issue. Generally, an electro-pneumatic converter must be used in conjunction with a pneumatic positioner in order to achieve accurate positioning of the valve.

Valve positioners include pneumatic valve positioners and electrical valve positioners. Pneumatic valve positioners are now rarely used, as pneumatic control instruments have been largely phased out. Electrical valve positioners are the main accessories for pneumatic control valves today; they convert electrical signals from control instruments into pneumatic signals in order to control the control valves, improve their dynamic characteristics, eliminate friction in the valve stem, reduce the action lag of the control valves, and enhance their positioning accuracy. An electrical converter converts electrical signals into pneumatic signals; it has only the function of signal conversion and lacks the other functions of a positioner, and is now rarely used in control valves.

Valve positioners ensure the precise positioning of control valves; they are divided into electric valve positioners and pneumatic valve positioners. Electrical converters are used to convert electrical signals into pneumatic signals, acting as the electrical conversion component within electric valve positioners

I think, simply put, it is that the valve positioner has feedback while the electrical converter does not

Valve positioners come in two types: electric/pneumatic positioners and pneumatic/pneumatic positioners. Electrical/pneumatic positioners typically receive a 4–20mA electrical signal to control the output gas pressure, thereby controlling the actuator to adjust the valve opening degree. Pneumatic/pneumatic positioners typically receive a pneumatic signal of 20–100 kgf/cm2 to control the output gas pressure, thereby controlling the actuator to adjust the valve opening. Electrical/gas converters are usually used together with gas/gas positioners. The electrical signal of 4–20 mA is converted into a pneumatic signal of 20–100 mA, which is then fed to the pneumatic/pneumatic positioner; this positioner is used to determine the valve opening degree. The difference between an electric/gas converter and a valve positioner lies not in precision, but in function. Since electric/pneumatic converters are cheaper than valve positioners, some people use them directly to control small valves, which results in lower precision.

When manual operators, PID regulators, DCSs, or PLCs use pneumatic control valves for process control, it is necessary to convert electrical signals into pneumatic signals in order to control the operation of the pneumatic actuators. Devices that fulfill this function include electrical converters and electrical valve positioners; however, since the structure and functions of these two types of devices are different, there are also differences in their usage. http://yunrun.com.cn/upload/201605/30/201605301455581990.jpg Changhui Instruments: yunrun.com.cn Electrical converters – The input current signal of an electrical converter is proportional to the output pressure signal. That is, as the input signal changes from 4-20mA, the output pressure of the electrical converter also changes from 20-100kPa, thereby converting the current signal into a pneumatic pressure signal. An electrical converter is essentially a 1:1 amplifier, except that it receives electrical signals. Since there is no mechanical connection between the electrical converter and the control valve, it has advantages such as lower cost, as well as easier installation, commissioning, and maintenance compared to electrical valve positioners; therefore, under the same conditions, the electrical converter should be given priority. The electrical converter is installed directly on the pneumatic control valve for use; no feedback rod is required. However, due to the absence of a feedback mechanism, it cannot function as a closed-loop control system, and its control accuracy is very poor, which is why it is rarely used on its own! Typically, an electrical converter must be used in conjunction with a pneumatic positioner in order to achieve accurate positioning of the valve. Electrical valve positioner: An electrical valve positioner is essentially a combination of the functions of an electrical converter and a valve positioner. Thus, the functions and roles of electrical valve positioners have been further expanded; for example, they can be used to improve the linearity of the valve position ； Since it can overcome the friction of the valve stem and eliminate the effects of unbalanced forces in control valves, it is very suitable for use in applications with high-pressure media and large pressure differences ； Applied to large-diameter control valves ； Applied to high and low temperature medium control valves ； It can also be used in situations requiring rapid adjustment, or where it is necessary to improve the flow characteristics of control valves. Valve positioner: A valve positioner is a key accessory for pneumatic control valves. It uses the signal of the valve stem’s displacement as an input feedback measurement signal, and the output signal from the controller as the setpoint signal; by comparing these two signals, any discrepancy leads to changes in the output signal sent from the valve positioner to the pneumatic actuator, thereby causing the actuator to move. This establishes a one-to-one relationship between the valve stem’s displacement signal and the controller’s output signal, creating a feedback control system in which the valve stem’s displacement serves as the measurement signal while the controller’s output serves as the setpoint signal. Uses of valve positioners: A valve positioner is an accessory for pneumatic actuators; it is used in conjunction with these actuators. The functions of a valve positioner include the following: 1. It improves the linearity of the valve stem’s position, overcomes the frictional forces acting on the valve stem, and eliminates the effects of changes in the pressure of the medium being controlled as well as high pressure differences on the valve’s position, thereby ensuring that the valve can be positioned accurately according to the control signals. 2. Valve positioners can increase the actuation speed of the actuator and improve the dynamic characteristics of the control system. 3. It is possible to operate pneumatic actuators with non-standard signal pressures of 40–200 kPa using standard signal pressures of 20–100 kPa. 4. Valve positioners enable range control, allowing one control instrument to operate two control valves. When a signal pressure of 20–60 kPa is applied to the positioner of the first pneumatic control valve, the valve moves through its full stroke; when a signal pressure of 60–100 kPa is applied to the positioner of the second pneumatic control valve, that valve also moves through its full stroke. 5. The valve positioner can achieve a counteraction action. 6. The valve positioner can correct the flow characteristics of the control valve. 7. Valve positioners can convert the on-off operation of piston actuators and long-stroke actuators into proportional operation. 8. After installing an electrical valve positioner, a 4-20mA signal can be used to operate the pneumatic actuator; one electrical valve positioner serves both as an electrical converter and as a pneumatic valve positioner. http://yunrun.com.cn/upload/201605/30/201605301456295531.jpg Original address: yunrun.com.cn/tech/563.html Siemens electrical valve positioners; Classification of valve positioners: Valve positioners can be divided into pneumatic valve positioners and electrical valve positioners based on their input signals, and there are significant differences between the two: 1. The input signal for pneumatic valve positioners is a standard air pressure signal of 20-100 kPa. 2. The input signal for electrical valve positioners is a standard current or voltage signal (such as 4-20mA or 1-5V); inside the electrical valve positioner, this electrical signal is converted into electromagnetic force, which is then used to generate a pneumatic signal that is sent to the pneumatic control valve. 3. Pneumatic valve positioners can be used in conjunction with pneumatic diaphragm control valves and pneumatic piston control valves. They receive signals of 20–100 kPa from pneumatic control instruments in order to control the stroke of these valves. Through a feedback system, they ensure that the position of the valve element is accurately adjusted according to the pneumatic signals from the control instruments, thereby achieving proper positioning of the valve element. 4. Electrical valve positioners are used in conjunction with pneumatic control valves to form a closed-loop control circuit. Convert the DC current signal provided by the control system into a pneumatic signal to drive the control valve and regulate its operation. At the same time, feedback is provided based on the opening degree of the control valve, enabling the valve position to be accurately positioned in accordance with the control signal generated by the system. By adding a valve positioner, a secondary loop is formed with the valve stem displacement as the secondary controlled variable; this secondary loop, together with the original single-loop control system, constitutes a cascade control system. The controlled variable of the original control system becomes the primary controlled variable in the cascade control system. Therefore, the addition of a valve positioner can improve the performance of the control system. Since a cam is used as the feedback element, changing the shape of the cam can effectively adjust the gain of the secondary loop, thereby compensating for the nonlinear characteristics of the controlled object. For valves with only a fixed flow characteristic, such as butterfly valves, the positioner can use a customized cam to improve the corrected flow characteristic. Valve positioners have evolved from pneumatic-to-pneumatic valve positioners and electrical valve positioners to the digital valve positioners and fieldbus valve positioners that exist today, but their basic principles and main functions remain unchanged.