Micrologix 1200 analog

I have a 4-20ma signal coming from a scale unit into one of my analog cards that has some horrible fluctuation on it. I am using the scp instruction to scale the data to 0-5000 lbs and on the output side with no weight on the scale it fluctuates as much as +/-15 lbs. The scale sources it's own current to the card so the problem I am seeing could possibly be some kind of grounding problem. Unfortunatly, I am not at work so I cannot try that to see if it is the problem. Any other idea's?

What I am looking for is a simple analog software filter to smooth the raw incoming data. I have searched the archives and haven't found what I was looking for. Also the analog card that I have does not use the internal software filter that most AB cards do, so that is out. Is there some kind of hardware filter that I could use, such as a capacitor and how would I go about sizing it?

If someone could give me a hand, I would very much appreciate it. If you possibly have an example in rslogix 500 that would be great to.

Years ago on a PLC5 we did an averaging routine to eliminate noise fluctuations....we had vibration issues that made the scales fluctuate. Take a specific number of values over a specified period of time, you need to determine how often, total that number then divide by the number of values obtained....ie a value take every 5 seconds will give 12 values a minute, total that then divide by 12, that will give you an average over a 1 minute period. This can be done as frequently as needed based on scan time...ie you can take a value every second for 10 seconds then divide the total by 10...etc etc.

http://www.plctalk.net/qanda/showthread.php?s=&threadid=9614

This thread had a nice technique on averaging values coming in.
I had a simlilar problem but mine was with fluctuations when filling.
I had to average the values and play around with a breakpoint.
If you average the last 10 values then it would only fluctuatate +- 1.5.

Have you tried shielding it on only one end. What about the conduit run from the scale back. Is it isolated or near some higher voltage.

Some time ago I posted a MicroLogix digital filtering routine. If you can't find it, I'll post it next week - I'm trying to finish the start-up from hell here in Texas.

I found it Tom

Thanks guys, I found the one Tom posted.

Worked like a charm.

Second half to that is the major part of the noise that I was seeing was because I did not have the analog card grounded correctly.

Note to students and also myself: RTFM