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M91 & PT100 issue


Donovan

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Hello!

I have come across an interesting issue and am hoping someone can clarify what's going on:

I have an M91-2-RA22 with 2 PT100 probes connected to it.  What was happening was the input values from the probes were not accurate (ie: the indicated value in degress C would go off by a lot).  One input would fluctuate based on the input of the other (ie: if one probe got hotter, the other would respond as if it got slightly hotter as well).  I suspected a ground loop problem but eliminated that possibility (by further testing as indicated).  I suspected a bad sensor but eliminated that as well (by replacing them)  I removed one probe from the input channel of the PLC (leaving only one probe online) just to find that it did not matter: the watched value shown in U90Ladder (and displayed by the HMI) was simply wrong.  I also confirmed the resistance values from all the probes were accurate, and that an independant PT100 monitor was having no problems reading correct values from them.

Realizing This really had nothing to do with the probes or wiring, I decided to take another approach:  I switched the PLC HW Configuration. I changed the Analog Inputs from PT100 (in Degrees C) to Resistance (in Ohms).  Immediately I had accurate and stable (resistance) readings every time, from both inputs.  From there I did a linear conversion in the ladder logic and got my readings in degrees C with no more fluctuations.

So can anyone explain why the PT100 HW configuration would not be stable?  I was hoping to get the most accuracy by using this feature, since PT100 probes are not perfectly linear across their full temperature range, but it seems I am getting the exact opposite.  Is there a solution?  Is there a good linearization formula for PT100 probes that will get the highest accuracy?

 

Thanks.

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Hi Donovan, nowhere in your description have you mentioned your jumper settings.  Perhaps your reference to "PLC HW configuration" implies this, but I read it as you have only done things via U90 access.

Check that your jumpers are properly placed, including the CM one.

cheers, Aus

RA22.jpg.d1e0f5ba4b6b77d25cd23eec5bc01551.jpg

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Hi guys, thanks for the replies.  Answers as follows:

- Yes, while I did not mention the jumper setting in my original post, they are set correctly.  I just triple-checked them today since I was on site anyways. Photo attached... As you can see, all of them, including the CM jumper, are set correctly. 

- Yes, the Analog input is set to PT100 385 (I am in North America, this is the standard PT100 probe profile here, althouth I tried 392 and it made no difference in the behavior of the controller)

- Temperature range being measured is 0C - 100C, nothing out of range or even out of the ordinary, especially for a 3 wire PT100 probe.

 

Thanks.

20211221_140811_HDR[1].jpg

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Hmmmm.  At this stage all I can offer is  the following:

1).   Ensure that the firmware matches what your U90 version (and the PLC itself) wants to work with.

2).  If that is all up to scratch, load an empty program into the plc, reset, clear MB & MI, then load in a very simple program that is ONLY set to read the PTs and do all the same resets then see how it goes.  Even just setting the hardware up to read it directly online will do...in theory no ladder work needed at all.

cheers, Aus

 

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Thanks.  The firmware was confirmed yesterday while I was onsite (U90 Ladder indicated it was 'up to date'), and it was the exact same laptop / software version that performed the initial update to this PLC.  The version of U90 Ladder being used was downloaded from Unitronics a month ago.

This unit is installed on site and is working well enough with the HW configuration I mentioned previously (Ohms Resistance instead of PT100-385).  I have another unit here that is to be the exact same configuration, so I will be doing some in-shop testing as you describe to see if/when the new unit behaves the same way.

I wonder if this is an issue regarding the two PT100 inputs for each Analong channel:  Changing HW config to Ohms Resistance would then monitor only one of the inputs per channel, correct?  Maybe this is a defective unit?

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I suspect the Ground Loop Gremlins.  Are you getting these errors with the probes installed in the system? When you tested it with only one probe, was it hanging in the air or was it installed?

Is the 0V terminal of your 24V power supply grounded?

In Ohms mode the A/D converter is only considering two of the terminals.  RTD mode pumps current through the third wire and throws the voltage drop across it into the mix.  That error is coming from somewhere.  If the Ohms mode is working the input is OK.

I would test it with a couple of 100 ohm resistors and a jumper wire to simulate the sensor.  The exact value of the resistor is not critical.

Do you have a friendly distributor locally that can help you?

Joe T.

 

P. S. -  RTD trivia - 

PT100-392 is the original American Standard.  Another name for those is an NIST 392 RTD.  A few years back those crafty Europeans snuck in with the DIN 385 standard, and it's caught on as the global standard.  I don't know if one is actually better than the other, but anymore just about all RTDs are 385s.  

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On 12/22/2021 at 1:47 PM, kratmel said:

One more tip.

You PLC is installed on site for some time.  Some times i found problem with green connector pins corrosion  (I think 11pin is defective) .

As test i recommend replace connector to new one or replace (if it possible) Input -output connectors.

 

 

Interesting, and good to know, however this is a brand new install, and the enclosure the PLC is in has no moisture ingress.  There is zero corrosion.

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On 12/22/2021 at 10:29 AM, Joe Tauser said:

I suspect the Ground Loop Gremlins.  Are you getting these errors with the probes installed in the system? When you tested it with only one probe, was it hanging in the air or was it installed?

Is the 0V terminal of your 24V power supply grounded?

In Ohms mode the A/D converter is only considering two of the terminals.  RTD mode pumps current through the third wire and throws the voltage drop across it into the mix.  That error is coming from somewhere.  If the Ohms mode is working the input is OK.

I would test it with a couple of 100 ohm resistors and a jumper wire to simulate the sensor.  The exact value of the resistor is not critical.

Do you have a friendly distributor locally that can help you?

Joe T.

 

P. S. -  RTD trivia - 

PT100-392 is the original American Standard.  Another name for those is an NIST 392 RTD.  A few years back those crafty Europeans snuck in with the DIN 385 standard, and it's caught on as the global standard.  I don't know if one is actually better than the other, but anymore just about all RTDs are 385s.  

I initially thought it was a ground loop issue as well... I am familiar with ground loop issues, and have ensured proper grounding of shielding, ground wires and 0V wires.  There are too many other factors pointing away from that as the problem:

- The power supply is properly grounded, and I even tried various grounding combinations just to see if the behavior would change: it did not.

- There is a remote (independant) temperature monitor that monitors a second set of locally installed PT100 probes, and it has no ground loop errors whatsoever, even when I connect the same sensors the M91 uses to it.

- I have tested two probes installed, one probe installed, and two different (single) probes 'hanging freely' from the M91: all give me temperature values that are not correct.

- When I switch the HW config to 'Ohms Resistance', the values are solid:  No fluctuation, and 100% accurate readings.  If I do testing as mentioned in the above points, everything remains stable and accurate using this configuration.  If there was a ground loop issue, the resistance readings would still fluctuate or be incorrect.

Given the last point above in particular, it would seem that whatever algorithm the M91 is using to convert the inputted resistance values to temperature value is not functioning correctly.

(I knew 385 was the most common, but now I know why, thanks.  From what I do know, the only difference between the two is a small variance in the resistance to temperature conversion chart, both are equally accurate.)

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Sounds like you've done your troubleshooting.  If you have a spare M91 to try that would be the next step.  There may indeed be something wrong with the M91 I/O board.

On 12/21/2021 at 1:01 PM, Donovan said:

Realizing This really had nothing to do with the probes or wiring, I decided to take another approach:  I switched the PLC HW Configuration. I changed the Analog Inputs from PT100 (in Degrees C) to Resistance (in Ohms).  Immediately I had accurate and stable (resistance) readings every time, from both inputs.  From there I did a linear conversion in the ladder logic and got my readings in degrees C with no more fluctuations.

I would seriously consider leaving this solution in place if it works and you don't have a spare.  I am a US Unitronics distributor and lead time on almost everything now is at least six weeks.  I don't stock M91's any more.  

Joe T.

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Confirmed: It was the PLC.  I switched it out with another (new) unit, and the PT100 HW configuration worked without any fluctuation or deviation.  It'll be monitored closely for the next few days in any case.

I have a feeling the regulated supply voltage for the analog inputs is faulty or there's a poor solder junction somewhere.  It seems to be related to the differential reading between the two wires and the two input channels (one channel affected the other).

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20 hours ago, Donovan said:

regulated supply voltage for the analog inputs is faulty or there's a poor solder junction somewhere

Good news it's fixed. 

It would be interesting to see if the theory holds up with judicious meter testing of the PCB areas involved.  Might be an easy fix that would be worthy of note.  Without one of them in hand I don't know how hard this would be in practice. 

Kratmel loves/isreallygoodat this sort of thing!   🕵️‍♀️

cheers, Aus

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3 hours ago, Ausman said:

Kratmel loves/isreallygoodat this sort of thing!   🕵️‍♀️

Yes, my favorite hobby is finding the cause of inappropriate behavior in various industrial animals.

Unfortunately, I do not have such a PLC on the table - I would draw a diagram to solve this puzzle.

Despite the fact that there are many other different Unitronics PLC in my cave.

 

I am especially pleased when I manage to accurately repeat the behavior of the problem device by changing something in a working normal device.

Although it seems strange to someone else.....  :)

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  • 4 weeks later...

Yes Kratmel, similar to your hobby, finding the cause of errant behavior in process and production machinery is part of my job description, so once I had solved the issue for my customer (by replacing the PLC), I had to get to the bottom of this issue.  Besides, I really don't want to have to bench this unit. 😬

It is now SOLVED. 😎

I opened up the controller and had a very close look at the circuit boards.  Under 5x magnification, I found the problem:  Flux residue.  Apparently this unit managed to get past quality checks and shipped with a bunch of the stuff all over the I/O board (the other board was pristine).  Given the proximity of the semiconductors on the board, it doesn't take much to cause issues with analog circuitry.  I cleaned the board using a special flux cleaner for circuit boards and the unit is now reading both PT100 channels with no more instability issues.

I do notice the internal PLC PT100 temperature table is a bit off (on both PLC's, this model in general it seems)... The probes will have a resistance of 105.3/105.4 ohms and the resulting value on the controller is 15.5C.  Per the PT100 charts I find online, 105.46 ohms equates to 14C, which means at that specific resistance, the unit is off a degree or two. I will probably change back to resistance input just to confirm the value read by the PLC is the same as the probe value.  It only takes one ohm of added resistance to throw off the value 1-2 degrees C (and even more in F), which seems to be the case here.  Once I have determined that and do a bit of range testing, I may do a linear conversion of my own if I want it to be more accurate than that.

At least I have rock solid stable readings now.

I assume there is no way to modify the PT100 table in the PLC?

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1 hour ago, Donovan said:

At least I have rock solid stable readings now.

I assume there is no way to modify the PT100 table in the PLC?

In general, I think that experiments with measuring the temperature with a PLC should only check the reference points and determine whether the behavior of the sensor resembles the curve of its calibration.

If so, as a result, I create a variable to correct the temperature and can shift it in the right direction - up or down.

In most cases, I place a metrologically verified temperature sensor on the machine next to the PLC sensor and adjust the readings according to the operating point. That's enough to make the system work. The main thing is the stability of the readings and the expected results of the behavior of the machine during temperature changes.

Only once did I have to deal with temperature control with an accuracy of 0.001C. Three independent temperature stabilization systems of three zones built into each other.

Therefore, I understand that PLCs are not designed for this.

So it's good that you found the cause of the problem and got a stable result. I think that it would be enough to shift the measurement result by adding or subtracting a certain value. But to make a manual correction for all the characteristics of the sensor for a particular PLC does not make sense because with time and heating of the components, it will still shift.

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5 hours ago, kratmel said:

I create a variable to correct the temperature and can shift it in the right direction - up or down.

In most cases, I place a metrologically verified temperature sensor on the machine next to the PLC sensor and adjust the readings according to the operating point. That's enough to make the system work. The main thing is the stability of the readings and the expected results of the behavior of the machine during temperature changes.

Agreed.  I did the same, and currently have (the other PLC) setup to shift the value. Right now the shifted value is accurate to +/- 0.5C across the required temperature range, which is more than adequate, but I will most likely implement a calibration feature to allow the user to shift it as needed if the value deviates too much over time.

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