Control valve position feedback

russrmartin

Member
Join Date
Aug 2002
Location
Eastman, Wisconsin
Posts
744
Hi all. I've got a new requirement I've never run into before. I'm being required to read control valve position feedback back the PLC. In the past, on control valves I've used, we have included discrete position feedback on the open and closed positions, but not an analog 4-20mA type of signal to monitor valve position. Has anyone done this? Can one specify a valve to come with analog feedback, or am I going to need some sort of external device? Thanks in advance.

Russ
 
The valve positioner is the device you need to focus on. You need to find out if there is an analog out on the positioner giving feedback. At our plant we have hundreds of fieldvue positioners on valves and dampers. They all have a device hooked parallel across the output from the DCS/PLC to the valve. We uses 2 different brands. A Rosemount Tri-loop or a Moore Industries HIM. I prefer the HIM because it has an LCD display on the front. They both do the same thing. Take a 4-20 from the valve and send a 4-20 to an analog input giving position in percent. They both have some software where you can configure the module and it's output channels. It's a very simple solution. Just have to buy a module, run a little wire, and do some simple programming.
 
The items Russ mentioned use the Hart protocol to communicate the valve position and other info by imposing the info onto the analog output signal cable for the valve.

Russ's fieldvue positioners perform that function. Back at the PLC end, the communicated Hart info is read off the cable and broken out into one or several 4-20mA loops for you to use as inputs. That's where the Tri-Loop and HIM do their job.

If you want to keep the 4-20mA output signal and the analog position feedback separate, run another analog cable and find a positioner or limit switch assembly that suits your application.

Westlock is another manufacturer of positioners and limit switch assemblies w/ 4-20mA or Hart position feedback.
http://www.westlockcontrols.com/home.asp

I use the AccuTrak 2000 Series for open/close limit w/ or w/o 4-20mA position feedback for our hazardous location controls, and will be installing the ICOT 5200/5300 series positioners in the near future.
 
If this is an electric operator or pneumatic operator the 4-20 mA feedback should be a standard option. Contact the supplier of the operator.
 
Thanks guys. Sounds like I can either use HART or a wired analog feedback, and both are standard options I've just not utilized to date. I'll talk with the supplier.

Thanks again.
 
Yeah, but . . . .

4-20 feedback is always an option, and on many positioners it's a card that field installs so it can be added, if needed.

4-20 is easy, everyone understands it, but it requires home run wiring another twisted pair and an analog input for each feedback signal.

HART is either part of the positioner or not. I don't think I've ever seen a HART upgrade. If the positioners are smart positioners, they probably have HART. But getting data from HART is not a walk in the park.

To get a HART data value via a PLC's analog input, the AI card needs to be HART compliant. Most PLCs don't bother. I know Spectrum makes a card for A-B CLX and CompactLogix, and there may be others, but I doubt it is 'common'.

As Chadhoot78 mentioned, there are breakout boxes, one needed per positioner.

Doing feedback for numerous valves will be a project, money-wise.
 
Last edited:
if you use a tri loop or a HIM, the only wire you will have to run can be in the local cabinet. all you need is to parallel the output signal to the valve and run an in put from the breakout box to an analog input to the PLC. Be sure to turn burst mode on in the positioner with a Hart communicator, or it will not broadcast it's feedback.
 
Hi All I am familiar with the Fisher DVC function of putting the unit into Burst control to transmit the position on the analog loop and stripping it off in the marshaling panel. Is this the only positioner that has this function? Are there any other manufacturers that include this function?
Thanks
 
Hart is a master/slave protocol. My understanding is that the Moore Industries HIM acts as a HART master to poll the HART slave positioner to get the HART position data, which it then converts to an analog signal.

Or so the Moore vendor told me.

This came up because I was told the Triloop requires HART burst mode operation and the particular multivariable I had did not operate in Burst mode. But it worked with a HIM module.
 
I've utilized this many times. I find that this seems to be a fairly standard thing on most motorized valves positioners. Less likely on electropneumatic (ie 4-20MA and air signal to the positioner). It is very common with HVAC type valve/damper acutators (Think Belimo, Johnson Controls, etc).

Here is an example of a butterfly valve and positoner that I just put into service at a customers site last summer.
https://www.valworx.com/product/electric-actuated-butterfly-valve-6-lug-epdm-120-vac-eps-positioner

The position feedback is used to alarm if it is more than 5% different than command for more than1min.
 

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