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Problem with COM on FP-AI-110

I am having an issue reading a 4-20 mAmp signal from an oxygen sensor. I am using a FP-A110 to read the signal but the values in Measurement and Automation Explorer are not valid. I know the sensor and signal box are working correctly because I observe the correct amperage with my amp meter.

 

My setup (see attachment) has an incoming 120 VAC power supply. I am driving an OXY-SEN control box and a SOLA AC->DC converter. The SOLA is then powering a FP1000 and FP-AI110. The sensor is a "loop powered" signal so I am using the I_in and COM terminals on the AI-110. I've read some stuff here(http://forums.ni.com/t5/FieldPoint-Family/4-20-mA-current-source-not-reading-properly-on-AI-110/td-p...) about multiple connections between FP's and measurement instruments causing problems. So, I checked and I have continuity (dotted line in image) between the ground on my machine (i.e. ground of the OXY-SEN) and the COM terminal on my AI-110. Is this OK?

 

Any help would be appreciated.

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Message 1 of 8
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tir38,

 

It sounds (and looks) like everything is wired up correctly. You may want to take a look at the field wiring guide http://zone.ni.com/devzone/cda/tut/p/id/3344 for some further tips on reading analogu signals. Have you also checked the current transducer? I think the best thing to do is divide and conquer the system. 🙂

 

 

 

 

Sam S
Applications Engineer
National Instruments
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I still don't have this figured out so, to clarify a few things:

 

My sensor is definitely working correctly. When I probe the sensor 4-20 mA output terminals with my amp meter, I read a legitimate value (17 mA when reading 21 on a 0-25 scale).

 

However, I think I have a ground loop problem. The COM terminal on my AI-110 should be at the same potential as the building system ground, right? Well if I probe between the COM terminal and ground, I read 3.5 mA. Obviously current is flowing through this connection which mean they aren't at the same potential.(see 1st attachment)

 

I am trying to use a signal conditioner (http://www.automationdirect.com/adc/Shopping/Catalog/Process_Controls/Signal_Conditioners/FC-11) but it doesn't seem to be helping. When I hook up the sensor 4-20 to the signal conditioner, I read zero amps on my amp meter from the "output" side. (see 2nd attachment)

 

Any suggestions on what I'm doing wrong?

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Message 3 of 8
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Doesn't he FC-11 needs a separate 24V supply on wired on its output side? See pg 2 of the sheet:

 

http://www.automationdirect.com/static/manuals/scdatasheet/fc_11.pdf

 

Don't use the same 24V supply you already have or you will be defating the purpose of isolation.

 

-AK2DM

 

 

~~~~~~~~~~~~~~~~~~~~~~~~~~
"It’s the questions that drive us.”
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Message 4 of 8
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Why do you have the common of the analog input module connected to building ground? 

 

Wiring an ammeter between common and ground isn't going to tell you much.  Wire in a voltmeter.  The ammeter is going to have a near zero resistance.  So 3.3 or 3.5 mA through a near zero resistance could be a very small voltage difference as well.

 

You haven't said what the reading is on your AI-110 when you have the oxygen sensor readout connected to the Iin and Com terminals of your AI-110.

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"Why do you have the common of the analog input module connected to building ground? " Are you refering to the picture in the intial post? I was saying that I had continuity between those two points, unknowingly. That is not the case now.

 

Regarding the additional powersupply: It seems like I'm isolating the power supply and not the signal. I understand how this will work but something seems "off" to me. Do people really have to buy an isolator and power supply every time this happens? Is there a way to isolate the entire power supply or to run multiple isolators off the same supply? The only reason I ask is that I have two sensors with the same problem (i'm actually reading from two O2 sensors).

 

Also, if I have two power supplies but the same ground (see attachment) won't I still have the same ground loop?

 

 

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@tir38 wrote:

"Why do you have the common of the analog input module connected to building ground? " Are you refering to the picture in the intial post? I was saying that I had continuity between those two points, unknowingly. That is not the case now.

 


I was thinking more about the first picture in message #3.  I guess it is not so much that they are directly connected, but your ammeter is connecting those two.

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Right, I just wanted to see if the COM and building system ground were at the same potential. I will put a voltmeter in there instead and get a reading.

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