Analogue Output Failure - Why?

[Colin Carpenter]

A few weeks ago, I was called out to have a look at at a steam valve fitted with a Samson positioner ... the sort that has an LCD screen that is powered up by the milliamps of the analogue output and displays various things about the valve position etc.

The PLC is a Q04-UDE with several 8 way current analogue output cards fitted and this has been in operation for around eight years with no problems.

The Samson screen was blank and it rapidly became clear that the analogue output must have failed as zero milliamps were coming out.

It was during the recent heatwave and I assumed that it was probably a combination of heat and age, so reprogrammed the valve to work using a spare analogue output, changed the wires and all was good again .... for about a week, before that output failed after a few days / week or so, and after having to use at least two more outputs that both failed after varying times, the time had come to do something about it.

So, new 8 way analogue output card, new valve positioner head, new twin core screened cable from the output to the positioner  and a milliamp to milliamp isolator has it all running again and it's been fine for the last few days .......

My question is ..... what could have caused those 4 channels to fail in such a short space of time?

I'm used to the odd one failing, but this was a pattern and all related to the one positioner. All other outputs on the PLC that use the same power supply have been fine, and as far as I'm aware, the channels are short circuit protected and if the resistance of the positioner is high, it will cause an issue but is unlikely to actually destroy the inner works of the channel.

Any ideas please? 

[pturmel]

In my experience, ground loops are the most common culprit.  When an isolated device's insulation starts to fail, it can (intermittently, perhaps) impose unusual DC voltages.  Analog outputs aren't often protected from backfeeds or random reverse spikes.  Or not well protected.

Less likely would be repeated shorts that break down the AO's short circuit protection.

Even less likely would be the loop-powered device's internal power tap's isolation/filtering failing, putting spikes on the loop that, again, wear down the AO's protection.

I'm sure there are other failure modes, but these are the generic ones that come to mind.

[Colin Carpenter]

Thanks for that.

So, it sounds like it could be an issue with the circuitry within the positioner that is causing unusual voltages to potentially cause problems in the internal circuitry of the analogue output?

 

[pturmel]

Ground loops could be surrounding circuits and metal housings--anything conductive.  Otherwise within the positioner.

[panic mode]

normally the only reason any output fails is because its specs are exceeded. so either something is connected incorrectly or there is an intermittent problem in the cable or target device. to me this looks like an intermittent problem that causes some short (not necessarily GND, could be also the 24VDC etc.). if the root problem is not found, switching to another output will just lead to inevitable - again and again. the analog output card are not cheap.... adding an isolator should protect the PLC analog output but the isolator itself may still be damaged if problem is not fixed. but since you have already replaced both cable and the positioner, this is most likely fixed - hopefully permanently.

i would still be inspecting the entire length of the removed cable and each and every termination as well as the old positioner. if this does not yield results, there is a good chance the same thing will happen again. and if that is he case i would probably consider adding in some extra protection... 

About protection:

the datasheet for Q68DAIN shows that load can be 0-600 Ohm. so zero Ohm load is ok. it also stated that short circuit protection is available. So short to DC common is not a reason that outputs were failing. It must be something else. And that something else could be another circuit...

Normally analog IO are using shielded cables and they tend to be done with single cable from PLC to the target device, with shield connected to ground and DC common in one place (at the PLC cabinet). This is pretty much the practice we used on numerous projects, some with thousands of analog IO points and very long cable runs, never had an issue (used to do controls for chemical plants). 

But when that is not the case, the best bet is to scrutinize the wiring methods. Any split (multiple cable sections, shared terminal strips, shared multiconductor cables, routing near power cables or sources of EMI etc) is potential problem and therefore a red flag that requires closer look. 

good luck... 

 

[Colin Carpenter]

Thanks for the information and your obvious knowledge and experience.

I also have installed probably hundreds of analogue 4-20 ma sensors and outputs and have never come across an issue like this one.

In general, we use multi core cable to go from the PLC to remote junction blocks and then split out from those to the individual local items. The multi cores can have up to 50 cores and are generally 0.5 mm squared cores to give good cable strength. The multi cores will only ever carry the 24 VDC Io signals and there will never be any AC voltage used in the cores.

We've never had a problem before and this particular output had been running happily for more than ten years. The individual two core cable from the junction block to the valve was changed at the time but the failures kept happening.  The multi core cable was still used, and once the isolator was installed the failures stopped. For extra security, the valve has now been wired all the way back to the PLC in a dedicated two core, but the isolator had solved the problem already.

The only issue I've ever had on analogues before, was a PRT probe that had the 4-20 converter head installed remotely to the resistance head, and that was very susceptible to interference when inverters started up. Changing to a probe with the ma converter on the probe cured the problem.

[edisson.75]

On 11/6/2022 at 8:00 PM, panic mode said:

normally the only reason any output fails is because its specs are exceeded. so either something is connected incorrectly or there is an intermittent problem in the cable or target device. to me this looks like an intermittent problem that causes some short (not necessarily GND, could be also the 24VDC etc.). if the root problem is not found, switching to another output will just lead to inevitable - again and again. the analog output card are not cheap.... adding an isolator should protect the PLC analog output but the isolator itself may still be damaged if problem is not fixed. but since you have already replaced both cable and the positioner, this is most likely fixed - hopefully permanently.

i would still be inspecting the entire length of the removed cable and each and every termination as well as the old positioner. if this does not yield results, there is a good chance the same thing will happen again. and if that is he case i would probably consider adding in some extra protection... 

About protection:

the datasheet for Q68DAIN shows that load can be 0-600 Ohm. so zero Ohm load is ok. it also stated that short circuit protection is available. So short to DC common is not a reason that outputs were failing. It must be something else. And that something else could be another circuit...

Normally analog IO are using shielded cables and they tend to be done with single cable from PLC to the target device, with shield connected to ground and DC common in one place (at the PLC cabinet). This is pretty much the practice we used on numerous projects, some with thousands of analog IO points and very long cable runs, never had an issue (used to do controls for chemical plants). 

But when that is not the case, the best bet is to scrutinize the wiring methods. Any split (multiple cable sections, shared terminal strips, shared multiconductor cables, routing near power cables or sources of EMI etc) is potential problem and therefore a red flag that requires closer look. 

good luck... 

 

Hi Mr. Panic. 

 

I stumbled upon your post while searching on Google. Like Colin, we are getting damaged analog current outputs in a Flex I/O 1794-IE8XOE4 module. The output drives a Baker Hughes Masonelian SVI II AP digital positioner. This device is passive, so it is powered from the current loop. The positioner works differently from the generally used positioners, where when you increase the current output, the voltage drop through the device also increases. For the Masonelian, the compliance voltage expected is 11 VDC at 4 mA, and 9 VDC at 20 mA, so, as you can see, the voltage is inversely proportional to current. The device achieves this by varying its internal DC resistance from 450 Ohms to 2750 Ohms in proportion to the current applied. I think this is done because the internal electronics must be kept over 9 volts to ensure the operation.  Initially, we thought this was the problem because the specs for the IE8XOE4 module say the expected range of resistive load is from 0 to 750 Ohms @24VDC, and at 4 mA, you are getting 2750 Ohms in the Masonelia, but at 11 VDC, not 24 VDC. Later, I simulated the current output circuitry for the Flex I/O module and learned that the only problem when you have a higher load is that you will not be able to source the desired current through the loop if the voltage drop through the load goes far from the compliance voltage.  

In your experience, have you seen a current output damaged by a higher load at the output than the module's specs? I am now turning to believe the same as you explained in your post about cabling problems, like ground loops or other equipment's voltage leaks filtering into the loop. 

I would like it if you could please give me your opinion about the case where the module's current loop has a higher resistive load than the maximum expected.

 

Thanks a lot for your help and have a nice day!

 

NOTE:  Analog output simulation on Falstad.com

https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWcMBMcUHYMGZIA4UA2ATmIxAUgoqoQFMBaMMAKADcQU88RsVPuIYoSiiALFTSiqMBCwBOAnsKXgMI2pBYAzTijGcJq-VWzZw0JDEgoWAd1Vh1x-VAXGEIiZM+iwWgEMQPEkjMGJ+E2DwPEtwBGgDf3g4cGJYeCcubAQxHH5wmnh7YMlXELUNEq4eJxEKut4WABMQb05fMTEeFF9+ZrptAIBXABsAF3cEZxqKZ3aqDEoS6a8+cDX+LQdZ9bARde2N3gLNtwBzY7E9r0IDGRYs0o760ylFtM4qQpk4gDUAPYTALnOglLo9XyrF5uBzQ3oieGCbAreapaHXLYlcKRIwoVxRFFAA

 

 

Edited April 22 by edisson.75