Open Wire Detection - FlexIO/PointIO analog input cards

[Paullys50]

Guys, looking for some help at a customer site...

I need to add open wire detention to a system using 1794-IE8 and 1734-IE4C modules.

For the 1794-IE8, it's configured as 4-20mA mode. (Under-range does not apply to 0-20mA mode).

Originally, I thought piece-o-cake. Just look at the under-range bits and trigger errors off of that. Well, shame on me for thinking life can be so easy. Turns out that the under-range bit is triggered when the signal is AT or below 4mA. Well, I have a few pressure transmitters which are loop powered. 4mA = 0 PSI, and at an "IDLE" state the transmitter is putting out 4mA and the under-range bit is being set. This is clearly not a broken wire situation, therefore I cannot use this as a broken wire trigger.

I have not looked too deep into the 1734-IE4C cards yet, but I assume a similar situation may occur.

Open to suggestions on how to detect an open wire w/o false triggers, basically trying to reliably detect if I have a bad 4-20mA analog transmitter.

 

[Donnchadh]

Hi

How about if you set it up as a 0 - 20 then change your scale to suit let's say 0 ma = - 20 and 20 ma = your max. You will have to change the scale to shut your range.
Now when you get a- value you know you have a wire break.

Donnchadh

 

[Paullys50]

Looks like that is what I will have to do...just made my days a little longer. But at least I learned something today!

 

[Donnchadh]

Hi
You could set up a scaling aoi and add the you need in so at least you only write it once and it might make the days a little shorter


Donnchadh

 

[Paullys50]

We have AOI's that handle all the scaling, it's just that I have to do this while the plant is operating, so I have to be selective and take my time with it.

Actually went a little quicker than I thought, it also affected my PID loops to so I had to make sure it was ported across everything.

Good times, hopefully that's my biggest hurdle of this trip.

 

[Donnchadh]

Hi

Great hope all goes well

Donnchadh

 

[shooter]

calibrate the sensors a little higher like 4.4 mA
and at the top use 19.6 mA.
this way the broken wire detection will be good. and in the PLC you will have to make a conversion to get the numbers correct again with a span and zero.

 

[Paullys50]

calibrate the sensors a little higher like 4.4 mA
and at the top use 19.6 mA.
this way the broken wire detection will be good. and in the PLC you will have to make a conversion to get the numbers correct again with a span and zero.

Thought about that for a minute and quickly dismissed it. They have a calibration crew come in every 3-6months to calibrate all their sensors. Last thing I need to do is provide documentation and reasoning for such an odd zero and span.

Reconfiguring the IO ensures and replacement devices will just "work" regardless of the zero/span settings (well, assuming they configure it 4-20mA).