Rescaling 4-20mA Input with Separate Multiplier Adjustment

I am a beginner in the world of PLC, but my Google and Bing searches have been fruitless to this point so I am turning to this forum.

I am trying to take a 0-20mA input that corresponds to a specific speed range (0-5.9ft/min), multiply this value by an adjustable ratio (ie, 1.5x, so a value of 3ft/min becomes 4.5ft/min), then scale this input so that it matches a 0-20mA range of 0-300ft/min for input.

Any and all suggestions are appreciated, perhaps I am missing a simple solution?

If we knew the Brand and Model of the PLC you are using that would help.

The AB line of PLC's can scale the input to any units you want. This can be done either by instructions or via a module configurations.

Welcome to the forum. You've come to the right place.

I suggest you start with this thread:
http://www.plctalk.net/qanda/showthread.php?t=15069. It is hands down the best explanation of scaling ever posted on the Internet. And I'm not trying to be obsequious, I just haven't seen it explained better anywhere. Its kind of long and Ron has two groupings of posts and answers a lot of questions. Even though it was geared around an AB PLC, the thread teaches what you need to understand in order to scale an analog value. Once you understand the principles of scaling as Ron explains it then you can do it on any PLC.

If you're using an Allen-Bradley PLC, the SCP instruction is what you're looking for.

thanks

thank you, TConnolly, for the kind compliments ...

if anyone is interested, there's a free PDF version available for download on my website ... many people have found it easier to read - and to print - than the original forum thread ...

you'll find the links near the bottom of the "Sample Lessons and Videos" page ... I had to divide it into two sections since it was so long ... many people had trouble downloading it all at one time ...

Thank you Ron, TConnolly and Bullzi for the input, links and discussion, this was all extremely helpful!

After reviewing the information, I understand the y=mx+b and how to scale the input. Could someone provide a hardware recommendation based on the following - low-cost options preferred.

A. Accepts 0-10V input (standard)
B. Scale function (standard)
C. Adjustable Multiplier Input - Operator select-able through interface would be easiest as this is a rather stand-alone operation. This has been the hard function for me to find, perhaps I am not searching for the right term? The issue is that at certain points in the process, this value may need to be changed to correctly compensate for a manual hardware change that does not have any output associated with it.
D. 4-20mA output

Once again, thank you everyone for your input it is very helpful! I was looking at the Micro800 series - am I heading down the right path?

S7 - 1200? It has 2 built-in 0-10V analog input, and has ScaleX and NormX function which is what you need.

aaaa, my bad. it does not have a built-in analog output 4-20mA.

The micro800 might be OK. I'm not familiar with it. Another option is the MicroLogix1100. Software for the ML1100 is free and you can add analog expansion IO modules. It interfaces easily with a RedLion G3 HMI, for which the software is also free.

One thing to consider is what kind of resolution you need for your signal. It looks like the Micro800 has 12 bit and some limited selection of 14 bit analog IO modules. A 12 bit module gives you one part in 4096 resolution. A 14 bit module gives you one part in 16348 resolution. The ML1100 has two 10 bit analog inputs on board and expansion IO is 14 bit.

If you require higher resolutions you'll have to choose a different PLC family, which will also require a lot more money, both for hardware and for software.