Is there communication between the control valve and the DCS system, and what protocol is used?
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Among the transmitters I have encountered, many use the HART protocol and are capable of half-duplex communication. It seems that control valves don’t have a similar protocol. So, is the control valve nothing more than a component that sends a 4–20mA current signal to it, with the control valve not sending any feedback signal of its own back to the component in the control room? Thinking about it this way, it’s impossible anyway. Is it a protocol created by the manufacturer itself? If so, what multiplexing method is used: time-division? Frequency division?
It seems that none of the control valves I’ve seen have feedback·
The 2# fowind control valve can also have valve position feedback, but this is achieved through the module of an electrical valve positioner. The valve position output signal is 4-20MA. The protocol for valve positioners is mostly HART.
I don’t understand what the original poster means The positioner only receives 4–20 mA signals and does not output any signals; the valve position signal that we see is actually the 4–20 mA signal generated by a valve position sensor installed within the valve positioner.
Hehe, I was thinking about whether a valve can feedback its status signal to the DCS system. I considered this scenario: for example, if a valve on site is damaged and we send a signal to set its opening degree to 50%, but it actually opens to 70%. Without such feedback, we only know that a command was sent, but we have no idea how it was executed. Additionally, I would like to ask whether the valve position sensor and the valve actuator share the same circuit; if they do, communication issues will arise. Thank you!
This post was last edited by flyboot on 2010-1-4 16:08. 6# njutzdh13 I understand! But is that unlikely? It depends on what kind of system you have; if it’s a DCS, I think it’s impossible. These two signals are not the same thing.
For 6# njutzdh13, the valve position feedback signal is transmitted via a separate instrument signal cable; it shares no wires with the input signal. The valve position output is in the 4-20MA format, and it can be connected to the DCS card through a safety barrier, thereby enabling the display of the valve position. Of course, by installing proximity switches on the valves on-site, it is also possible to display whether the valve is open or closed (this signal is digital).
This feature is optional on the ABB TDZ positioner. Take a look at the selection samples.
Valve positioners don’t just accept 4-20mA signals; they can also work with the HART protocol. However, position feedback requires separate wiring, which isn’t necessary, is it?
The intelligent valve positioner with Hart functionality allows the valve position feedback signal to be obtained from the Hart signal, provided that the DCS card is capable of reading the instrument’s Hart signal. In the valve positioner Hart signal, there are a current input signal and a valve feedback signal. Emerson, Yokogawa, and Honeywell systems are all acceptable
The locator itself has closed-loop control; it’s sufficient to check whether it’s necessary to display the position signal – those are the issues to investigate if they arise