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Old May 26th, 2010, 12:53 AM   #1
Andreik
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Analog (4-20mA) Reading Problem

Hi good day to all,

I'm Andrei and came from the Philippines. I had a project which deals with an Auma Actuator and Omron PLC (Analog Input Devicenet). The Auma Actuator outputs the Motor position 0%(Close) to 100%(Open) through 4-20mA. I've had success the first time reading the Analog Signal.

After few Months, I was called and the case is that the reading went OFF. When I checked the current (I) reading through Digital Multimeter (DMM), I can see 4-20mA clearly. But when I try to look in the PLC program, it can't read 4-20mA. I've tried to use other 4-20mA sensor to check if the Analog Terminal is the problem but I encountered no problem at all.

In our application, we have used two (2) Auma Actuator. I have no problem with the other one.

What can I possibly do to make the PLC read the 4-20mA Analog Signal from the Actuator? I'm already out of options and blanked right now. Hope you can help me. Thank you very much Automation experts and I'll so much appreciate any help I can get.
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Old May 26th, 2010, 02:58 AM   #2
BryanG
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So just to confirm the problem:
If you connect another 4-20mA signal to the PLC - works
If you connect a meter to your existing 4-20mA signal - you get the right reading.
If you connect the existing 4-20mA source to the PLC - problem.

Have you tried connecting the second actuauot output to the first input on the PLC, if that works then the problem is in the actuator. Even though you get a right reading on the meter there may be a problem with something earthing where it shouldn't.

You may be able to 'fix 't things by adding a 4-20mA signal isolator, something like this:
http://uk.farnell.com/status/sem1020...1?Ntt=489-4881
You will need to make the PLC give a supply voltage for it's part of the loop, or choose an isolator with it's own supply terminals.

If the problem is in the actuator then the correct thing to do is to replace it rather than 'fix' things. The symptom may signal the start of a failure in the actuator.

Bryan
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Old May 28th, 2010, 01:04 PM   #3
Roy Matson
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As BryanG pointed out you probably have a grounding problem, something is now grounded that wasn't before. It might be un-related to the Auma actuator.
A simple test you can do is disconnect the actuator mA signal and see if you get mA from just one terminal with the other probe of your meter connected to ground.
You should always try to ensure that all field device signals are isolated from ground.

Last edited by Roy Matson; May 28th, 2010 at 01:12 PM.
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Old May 28th, 2010, 01:11 PM   #4
shooter
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if below 4 mA it will stop reading so be sure it is above, is ground connected?
is program rerading correct value?
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Old June 7th, 2010, 06:22 AM   #5
Andreik
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Thank you all so much for the help. The problem was resolved and it was indeed a grounding problem. But I'm really wondering how the Ammeter was still able to read the I output from the actuator since there has been a grounding problem. I've already encountered countless tricky situations in automation. Applications is indeed more challenging than being a theorist.
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