Allen-Bradley 1756-IT6I2 (Thermocouple Module)

Has anybody else experienced random and intermittent spiking with the subject modules. It seems all our troubleshooting always eliminates everything else as the culprit and it always seems to lead to the module... probe (good) wiring (good)

it doesn't spike on all the channels, typically just one channel thats always the problem child. I've trended the channel and have seen the momentary spike. Just wondering if anybody else has seen this.

great day and rest of the week to all...

Ed

Is this an old module ? Make sure it has the most recent firmware from A-B.

There was a problem with the original 1756-IT6I modules that arose from noise on the cold junction input that resulted in wildly out-of-range calculated values. That's one of the reasons the IT6I2 has better noise rejection and a new board layout.

But the IT6I2 isn't totally immune to the same sorts of problems. I chose to implement a "discard values more than X degrees different than the last value" logic to prevent inadvertent alarming on temperature trends.

Ken,

Thanks for the reply. I will check to see if it has the latest firmware. I love the discard values more than X degrees idea. I think its better than my current timer workaround.

What was happening with us was that the input channel was jumping to some totally out of range value; it was like 18 million degrees F for just one A/D conversion cycle, then back to freezer temperature.

This happened every couple of minutes, but the historian only happened to sample one of those events every few days.

So it was very easy to just discard those very-out-of-range values and substitute the previous scan's value in the equations and data tables I was running off the input module.

Thermocouple signals are so low (Seebeck voltages are in the millivolts, after all) that it's awfully easy to have noise and spikes enter the data stream.

Ken Roach said:

So it was very easy to just discard those very-out-of-range values and substitute the previous scan's value in the equations and data tables I was running off the input module.

Thermocouple signals are so low (Seebeck voltages are in the millivolts, after all) that it's awfully easy to have noise and spikes enter the data stream.

Click to expand...

Do you not think this "can't be right" filtering should be built into the module ??

daba,

I looked up filtering and your statement does not seem far fetched. This is what it says.... I may try it.


Filter
A device that controls the bandwidth of the signal that is being transmitted. It is generally used to eliminate high frequency oscillations or spikes from input or output signals. By removing the high frequency noise, the application may be able to accurately operate on true variation of the input or output.
Note: Use caution when applying filtering since, by definition, it makes the system more sluggish and less responsive.

This is a typical set up on all our modules

People get confused by the "Notch Filter".

This on-board filter is a frequency dependant filter used to filter out one of the biggest sources of electrical "noise", that is mains-borne interference.

Its default value of 60Hz suits you guys in the USA without modification. Here in the UK we use 50Hz. I make sure my students are aware that the modules are manufactured by an American company, and the defaults need to be changed for maximum noise immunity.

One of the selections is 10Hz !! I've always been curious why ?