Dan Truitt

Frozen Analog Input on Compact Logix

11 posts in this topic

This is a bizarre one...

CompactLogix, 1769-IF8 with 1492-AIFM8-F-5 wiring system.

My client has reported several instances where one of his flow rate displays is frozen at its maximum value.  Each time, I have gone to the site, found that the value in the analog input tag is at full scale, and measured more than 20mA at the input - as much as 28mA!  I measure it using a clamp-on meter and I measure it at the input module (1769-IF8) and the external, pre-wired terminal block (1492-AIFM8-F-5).  I cycle power to the PLC.  No change.  I cycle power on the transmitting device.  No change.  I disconnect and reconnect the input (signal wire at the module or the terminal block) and everything goes back to normal.  This has happened on multiple channels on multiple input modules and with two different types of transmitters.

I didn't install this system, but it seems to have been done properly and it had been running fine for many years.  These problems just started happening out of the blue.

Has anyone out there had any similar experience?  This is a new one for me after more than 25 years in the business.

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Did you disconnect the signal wire at the transmitter and measure the output signal there?

Double check the configuration of the IF8?

Verify the scaling either in the block or in the code?

Any new equipment been installed near the transmitter?  Check for proper shielding of the signal cable?

Edited by pcmccartney1

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sounds like the v+ line dropped

 

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Thanks for the ideas! 

I did check the scaling and configuration of the IF8.  It's scaled for percentage and when the problem occurs, the channel goes to 105%+.  But that matches the current going into the module.  So, I'm pretty sure that this is a hardware issue. 

This has happened with multiple transmitters (not at the same time) at different locations (literally opposite ends of the building).  So, I don't think EMF from another (new) device is to blame and I'm not aware of anything new. 

As soon as we disconnect the wire at any point, the signal drops from 22mA+ to zero and everything goes back to normal, but I will try to get my clamp-on meter onto the wire at the transmitter next time.

I don't recall seeing the shield wires at all, but I wasn't looking for them.  I'll check that next time as well.

I'm not sure how this would result from the V+ dropping, but I'll think that through and try to find a means to detect if it does drop out momentarily.

Everything is working fine today, but we expect that at some point over the next month, it will happen again.  There are about 16 flow meters of two different types and only one will fail.  It could be one of either type.  It could be one from any part of the building.  It could be one attached to one of three different analog input modules.  The only common denominator is the symptom - actual signal current going way above 20mA and freezing there until we physically break the loop.

When it happens again, I'll check some of these ideas.

Thanks again!

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It does not appear to be a fault in the AI because you can measure the 20mA+ (28mA is some cases), so the AI is jsut reflecting the loop current it 'sees'.

The flow meter's AO should regulate the current in the loop.  That's what 4-20mA outputs do.  But the flow meter can not prevent currents from ground loops adding to or subtracting from the loop current.   It would be unusual if ground loops developed over time.  Which makes me wonder, how many of the 16 flow meters exhibit this over-current phenomenon?

Is it the same flow meter's output going high or does this occur over different flow meters?

Are the loops 2 wire loop powered (external power supplied) or active, powered by the flowmeter?

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12 hours ago, DanW said:

It does not appear to be a fault in the AI because you can measure the 20mA+ (28mA is some cases), so the AI is jsut reflecting the loop current it 'sees'.

The flow meter's AO should regulate the current in the loop.  That's what 4-20mA outputs do.  But the flow meter can not prevent currents from ground loops adding to or subtracting from the loop current.   It would be unusual if ground loops developed over time.  Which makes me wonder, how many of the 16 flow meters exhibit this over-current phenomenon?

Is it the same flow meter's output going high or does this occur over different flow meters?

Are the loops 2 wire loop powered (external power supplied) or active, powered by the flowmeter?

I'm with Dan as I'd think its the flow meter possibly failing high? What brand and model of flowmeter? (Had some quirky thing with a toshiba lately)

I'd also suggest a 4-20 simulation if you suspect the card with a long runtime. Still not thinking its the card though.

John

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I would be interested to know the model and manufacturer of the flow meter.  It is not unusual for 4-20mA devices to transmit outside of the 4-20mA boundaries if there is a problem with the sensor.  This is built-in to some models so your controller can tell if there is a problem with the sensor.  On some ultrasonic distance sensors we once used, you could select if the signal would go less than 4mA or higher than 20mA if the sensor failed its diagnostics.

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Thanks to everyone for the additional input.

So far, it has happened with three different transmitters.  Two Grundfos units that I believe are powering their own loops and one other simple, generic device that is getting it's power from the control panel.  I didn't see a nameplate on it to get the mfr/part number.  Like many of you, I had reached the conclusion that the Grundfos flow meter controllers were failing, but when a third device that is completely different failed with the exact same symptoms.  I thought that it was time to reach out to some other experts and see what you guys might have seen in the field.

It's just so weird that we can get everything back to normal by simply disconnecting and reconnecting the signal wire.  Of course, that's not a practical solution for the long haul.  Especially since it is at an unmanned facility.

Next time it happens, I'm going to take a harder look at the grounding and gather additional information like the exact make an model of the flow transmitters.

Thanks again to everyone.  I promise to try to pay it forward!

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transmitters output stage used opeamp. depending on transmitter brand, different opamps could be used. some opamps tend to latch up if output voltage gets very close or beyond  rail voltage. if the same happens with different brand products, i would say it is not choice of the OpAmp in the product (in fact i would imagine all of them are rail-to-rail versions), it is something external that is a problem. most likely cable or some interferrence (EMI, ground loop, inadequate shielding, intermittent short to another circuit, floating ground etc.).

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The symptoms are excessive common mode.   Some source of common  mode exceeding the ±10V limitation is forcing the input above the rail voltage saturating the input, just as Panic Mode describes. 

The A-B manual says:

common-mode-limitation-of-10-Vdc.jpg

multiple-power-supplies-commons-must-be-

Chances are the AO's on the flowmeters are active outputs and you can't connect the commons of the power supplies because you don't have access to them.

I'd put a 4-20mA isolator/repeater on the one that seems to fail first and hopefully that'll solve it without isolating the other inputs.

 

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The typical story one hears when the problem is excessive common mode sounds something like this,

"I had x number of analog inputs connected and they were all fine.  Then I added one more and all of them went crazy.   What happened?"

An AI card can work within its common mode limitations but adding just one AI can drive the common mode past its limits.

If you can establish that X number of AI's worked OK for some period of time and then when an additional AI was added this overrange situation started, then the last one added is the one to that needs the isolator.  An isolator will decouple the common mode and allow the other inputs to function OK. 

 

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