Useful Tips: Why are valve positioners widely used in control loops?

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This post was last edited by lilyqiu on 2019-4-29 23:10. Valve positioners are classified into electrical valve positioners and pneumatic valve positioners based on the input signal. An electrical valve positioner is one in which the input is standard signals of 0–10 mmA DC or 4–20 mmA DC, and the output is a standard signal of 20–100 kPa. A pneumatic valve positioner is one in which the input and output are both standard signals of 20–100 kPa. A valve positioner is an important accessory for 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, for comparison. When there is a discrepancy between the two, as originally published on the WeChat official account \"Self-Control Academy\", the output signal sent to the actuator is modified to cause the actuator to move, thereby establishing a one-to-one relationship between the valve stem displacement and the controller’s output signal. Therefore, the valve positioner forms a feedback control system that uses the stem displacement as the measurement signal and the controller output as the setpoint signal. Due to its six key functions, it has become an important component in control systems: 1) The valve positioner can increase the output power of the control valve, reduce the delay in the transmission of control signals, and improve the movement speed of the valve stem. 2) When the pressure difference across the control valve exceeds 1 MPa, the valve positioner can increase the output pressure to overcome the unbalanced forces. 3) Valve positioners can improve the dynamic characteristics of control valves and reduce the transmission lag of control signals. 4) Another important function of the valve positioner is that it can be used to eliminate the instability of the actuator diaphragm as well as the effects of dry friction in various moving parts, thereby improving the precise positioning and reliability of the control valve. 5) The valve positioner can modify the flow characteristics of the valve and change the forward and reverse action of the actuator. 6) The use of a valve positioner allows the valve to move through its full stroke across different ranges of control signals, enabling step control. For example, if a regulator outputs to control two valves, two valve positioners can be used to receive the lower-range input signal (4–12 mmA DC) and the higher-range input signal (12–20 mmA DC), respectively; the valve positioners then output signals that command full-stroke operation of the valves, thereby enabling step-controlled operation. The regulator output signal can also be divided into several segments (two or more), with each segment corresponding to a valve, thereby enabling step-controlled operation of multiple valves.