Formulas

I know that this question is kind of vague but it is just a general question with no specific type of PLC in mind.

Are there any standardized formulas that should be used when calculating temperature, weight, speed, etc... that should be used in programs that rely on 4-20ma analog input/output signals?

I will throw some general information out there to see if it helps.

The first thing with any plc is to understand the resolution of the A/D converter...ie if 10 bit resolution then your range would be 0-1024. If 12 bit then 4096. You obtain the range by taking 2 to the power of the resolution of your A/D converter. 2^10=1024....2^12=4096.

Now you know the range you will work with...lets use a 12 bit A/D for an example. 4ma would equal 0 and 20ma would equal 4096. Take 4096/16...subtract the 4 from 20 to get actual numerical input range...this gives you 256 divisions per MA.

This is a chapter of a book by Hugh Jack that is based on AB PLC5 but the information is generic enough to apply in many cases. http://www.patchn.com/plc_analog.pdf
The math cant change, just how the PLC allows you to do the math.

When it comes to speed, depending on situation, a high speed counter would be used more often than analog.

Maybe others will clarify or add more. Maybe you could ask a more specific question?

Most PLCs have a very good scaling function available. This is exactly what the function is designed for. Just put in your data and the function does it for you.

Mike,

actually what (I think) you’re asking about is often referred to in general terms as “scaling” ...

in addition to the method which Ron Doran pointed out, there is another way which “makes sense” to most people who are familiar with basic algebra ... it is based on the mathematical formula “y=mx+b” ... mathematicians refer to this as the “formula for a straight line” ... this is the theory behind the automatic “scaling” features which BobB mentioned ... you can do a Google search on this formula and get about 32,000 hits ... unfortunately most of these just approach the subject as an exercise in raw mathematics ... if you’d like to see how it actually applies to PLC’s and to the field of instrumentation, just post again and ask ... I’ll be pretty tied up next week so it might take me a few days to put together one of my detailed “overkill” descriptions of the procedures but I won’t mind doing it if you’re really interested ...

and another thing ... if you’re interested enough to ask for more, please tell me whether or not you have a Texas Instruments model TI-36 calculator handy ... these are about $19 at Wal-Mart and if you have one, I can show you an easy shortcut method to solve the math involved in using the “straight-line” formula ...

finally, this same mathematical approach (without the shortcut) is used in the:

SLC-500 Analog I/O Modules User Manual

I recommend that you start by reading all of Chapter 6 if you’re interested in seeing some examples of its use ... note: like most subjects, once you understand it, it's not as complicated as it looks ...