Process Engineering Division – Instrumentation and Automation Section – Daily Topics – Topic No. 2021-01-10

Valve positioners are classified 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 gas 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. The pneumatic valve positioner can be used in conjunction with pneumatic diaphragm control valves and pneumatic piston control valves. It receives a signal of 20–100 kPa from the pneumatic control instrument, which is used to control the stroke of the pneumatic control valve. Through the feedback system, it ensures that the position of the valve stem is accurately adjusted according to the pneumatic signal from the control instrument, thereby achieving proper positioning of the valve stem. 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.

Valve positioners are classified 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 gas 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. The pneumatic valve positioner can be used in conjunction with pneumatic diaphragm control valves and pneumatic piston control valves. It receives a signal of 20–100 kPa from the pneumatic control instrument, which is used to control the stroke of the pneumatic control valve. Through the feedback system, it ensures that the position of the valve stem is accurately adjusted according to the pneumatic signal from the control instrument, thereby achieving proper positioning of the valve stem. 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.

Both electrical converters and electrical valve positioners perform the conversion of electrical signals to pneumatic signals, with most of them converting 4–20 mA to 20–100 kPa. The electrical converter is a separate unit, while the electrical valve positioner is an accessory for pneumatic control valves that can be installed directly on them; the electrical converter cannot be installed in that way. An electrical converter is used to convert electrical signals into pneumatic signals, while an electrical valve positioner provides feedback on the displacement of the valve stem, and it is through this feedback that positioning is achieved.

The input current signal of the electrical converter is converted in a proportional relationship with the output pressure signal； An electrical valve positioner is essentially a combination of an electrical converter and the functions of a valve positioner.

Valve positioners are classified 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 gas 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. The pneumatic valve positioner can be used in conjunction with pneumatic diaphragm control valves and pneumatic piston control valves. It receives a signal of 20–100 kPa from the pneumatic control instrument, which is used to control the stroke of the pneumatic control valve. Through the feedback system, it ensures that the position of the valve stem is accurately adjusted according to the pneumatic signal from the control instrument, thereby achieving proper positioning of the valve stem. 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.

Difference between electrical converters and electrical valve positioners: The input current signal of an electrical converter is in a proportional relationship with 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. A valve positioner is a key accessory for pneumatic control valves. It uses the signal of the valve stem displacement as the 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 displacement signal and the controller’s output signal, creating a feedback control system in which the valve stem displacement serves as the measurement signal while the controller’s output serves as the setpoint signal. An electrical valve positioner is essentially a combination of an electrical converter and the functions of a valve positioner. As a result, 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 when it is necessary to improve the flow characteristics of control valves. Therefore, the electrical valve positioner not only possesses all the functions of an electrical converter but also has additional valve positioning functions beyond those of an electrical converter.

Valve positioners are classified 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 gas 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. The pneumatic valve positioner can be used in conjunction with pneumatic diaphragm control valves and pneumatic piston control valves. It receives a signal of 20–100 kPa from the pneumatic control instrument, which is used to control the stroke of the pneumatic control valve. Through the feedback system, it ensures that the position of the valve stem is accurately adjusted according to the pneumatic signal from the control instrument, thereby achieving proper positioning of the valve stem. 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 available today, but their basic principles and main functions remain unchanged

The input current signal of the electrical converter is converted in a proportional relationship with the output pressure signal； An electrical valve positioner is essentially a combination of an electrical converter and the functions of a valve positioner.

Understand it simply. An electrical converter simply converts an electrical current signal into a gas signal of fixed pressure in a linear proportion. The specific ratios have been clearly explained by those above; for example, an electrical signal of 4 mA results in an air pressure signal of 20 KPa, and this is fixed. The main function of a valve positioner is still positioning. In other words, the electrical signal is converted into a corresponding degree of valve opening using a specific method; air serves merely as a tool in this process and has little to do with the electrical signal. If an electrical signal of 4 mA is required to achieve a 10% valve opening, then the air pressure needed for this purpose can be set to whatever value is necessary, as long as it remains within the range of the available air pressure – the goal is to reach the desired valve position. This is the locator. The above are merely rough opinions expressed by an individual after working for some time; they are only useful for discussion and not intended as guidance. Everyone can take a look at them. -,-