View Full Version : Adjusting analogue inputs
May 15th, 2002, 10:26 PM
How can I adjust my RTD readings into an A-B plc i/o card?
If I test the RTD by inserting into a temperature bath, how can I adjust A-B reading to reflect the temperature bath reading? Is there a publication that covers this? I do not have a specific model number for the plc card. I went to the a-b.com web site and I was overwhelmed
with info. What I have in mind is to test Resistance Temperature Devices with a known temperature standard (0 deg F - 100 deg F.). I would then "scale" the corresponding integer 0-4096? Am I on the right track? Thanks.
May 15th, 2002, 10:57 PM
This has been done to death recently.
May 16th, 2002, 12:42 AM
Is the temperature displayed on some sort of screen?
May 16th, 2002, 09:05 AM
You are conceptually correct, but A-B has their own way of dealing with these things. I am going to assume you have aSLC-500, and that you are using the 1746-NR4 or -NR8 cards. You want to download Publication 1746-UM003A-EN-P from ab.com and print it out. I learned a long time ago that I cannot intuitively determine how A-B does things, and I try to work with their manuals in hand. This particular manual is a pain in the rectal area to work with, but it does have the info. (Why has Allen Bradley never figured out the value of including a few sample rungs of ladder logic in their manuals for analog cards?)
You can just get the temperature reading directly, with a fixed range in either °C or °F. Or you can get data in two different formats, and scale it yourself. In either case the resolution isn't improved by narrowing the "calibration" range. You can set the configuration from your ladder logic.
May 16th, 2002, 04:45 PM
I like to scale to the correct value in the ladder program. That way, I don't care about how the darn card converts the number from the analog device, or calibration values in the card, or anything else about the card--it just gives me a number--then I deal with it..
You can see what the card sends the cpu as a reading, so just use the math commands in your ladder to arrive at a usable, accurate number. If you think the analog card may tell you something different later, or drift, put in a "tunable" number that may be easily adjusted later, or 2--perhaps an offset and a span . If you have a touchscreen in the system, make these numbers accessible by you and you only--so you don't have to drag out tha programmer every time you adjust.
It's easier for me to do these "home made" calibration schemes because I understand how they work cause I wrote them--no manual necessary.
May 16th, 2002, 06:22 PM
Another sourse of information you might check is omega.com Click on technical reference then Temperature Related and you'll find everything you ever wanted to know about RTD's, thermocouples etc.