Valve position feedback signal of the control valve
Thread Content
Could some expert please explain whether the 4–20mA signal for valve position feedback of a control valve needs to be routed separately from the actuator to the DCS? Is it still sharing the same signal line with the DCS for the AO signal of the locator? Does the smart locator have this function?
With separate wiring, it is intelligent and can transmit valve position information via HART communication, provided that the system cards and software support HART.
Single wire; I’ve never seen a shared one.
Those intelligent locators that incorporate a HART signal can be read using a dedicated HART module, after which signals such as DI and AI are outputted to the DCS; otherwise, a separate wiring setup is required. . .
Separately, I think a DCS power supply is also required, right?
The real-time position signal feedback of the control valve’s valve position is in the 4–20 mADC range; this signal must be sent separately from the actuator to the DCS. Any intelligent actuator that meets this requirement will suffice, which means that the actuator needs to be equipped with a corresponding feedback module or related hardware. At the same time, while the DCS shows that the AO output for this valve is 50.0%, the actual valve position AI could be 50.1% or 49.9%, or it might also be 50.0%. There is no such thing as AI and AO sharing a single cable. If the valve position AI signal displayed on the DCS relies directly on the AO signal, it’s nothing but nonsense.
4-20mA valve position feedback for control valves: If it is a separate feedback device (i.e., a valve position transmitter), then two wires are necessary and the signal must be active. In other words, the DCS generates its own 24V DC, as the feedback itself is just a variable resistor. Depending on the valve position, the resistance value of the potentiometer changes, which in turn results in different current signals. If a mechanical valve positioner comes with built-in valve position feedback, it has four wires: two control signal wires and two valve position feedback wires. The two sets of wires are independent of each other; the control signals are output as 4-20mA standard signals directly from the DCS or control instruments. Valve position feedback, similar to the separate valve position transmitter mentioned above, also requires 24V.DC power supply. The feedback-enabled valve positioner of Xin Chang Instruments has a non-polarized output, as it features an adaptive program. So there’s no need to worry about the positive and negative poles being reversed. However, positioners without a feedback circuit board need to be distinguished. Another aspect is the feedback from intelligent valve positioners; in principle, these also require four wires. However, since intelligent positioners need to continuously monitor the valve’s position signal during control, and use an internal microcontroller processor along with appropriate algorithms to regulate the valve’s opening degree, different intelligent valve positioners require either 4 wires, 3 wires, or 6 wires for connection. For details, please refer to the manufacturer’s instructions. (I’m very sorry for being so busy and not being able to reply in time.)