Scaling 4-20Ma rslogix5000

sparkytex · Mar 5, 2019

Hey guys,

Normally I have an add in instruction that does this for me but the version of RSLogix I have (borrowed laptop) doesn’t have AOI’s so I have to do it old school. I need to scale a 4-20 mA input to 0-3173. I have the analog input selected as Eng Units so it’s changed the raw data to 4000-20000.

I can’t for the life of me find my old books that show me the formula on how to scales 4 mA to 0 and 20 mA to 3173 using the 4000-20000 Eu. Can someone help me remember the formula using the CPT function to achieve this please?

Thanks in advance.

- Tex

JZerb · Mar 5, 2019

just a guess but i think its

y = mx + b

but i havent actually done it that way so im not entirely sure.

mikeav · Mar 5, 2019

Hello!

It is easy!
You can use linear function with two points.
For Example "X" - is your input (current 4-20ma).
"Y" - is your output.

sparkytex · Mar 5, 2019

Now I remember, thanks guys. I got it going!

mikeav · Mar 5, 2019

sparkytex said:
Now I remember, thanks guys. I got it going!

For 4..20ma to 0...3173

(X-4)/(20-4) = (Y-0)/(3173-0)

Ken Moore · Mar 5, 2019

CPT "Scaled Value" "(Input-4000/16000)* 3173"

fredbear · Mar 5, 2019

Why wouldn't you just do the scaling on the card?

Ken Moore · Mar 5, 2019

I assumed it was Flex IO.

harryting · Mar 5, 2019

fredbear said:
Why wouldn't you just do the scaling on the card?

I had the same question but I guess some IO doesn't do scaling like Ken pointed out.

rdrast · Mar 6, 2019

fredbear said:
Why wouldn't you just do the scaling on the card?

I don't here either, I scale the card to actual current/voltage if possible, but I handle the scaling to the real units separately; that way I can put the scaling values on the HMI screen for calibration purposes.

Ron Brown · Mar 6, 2019

I have used this formula a few times to calculate the scaling. Just thought it might help someone out there.

(Input Raw - Input Min) * ((Scaled Max - Scaled Min) / (Input Max - Input Min) + Scaled Min) = Output Value

blessdstry · Apr 26, 2021

I found this didn't work

(Input Raw - Input Min) * ((Scaled Max - Scaled Min) / (Input Max - Input Min) + Scaled Min) = Output Value

Changed to-

Code:

 (Input Raw - Input Min) * (Scaled Max - Scaled Min) / ((Input Max - Input Min) + Scaled Min) = Output Value

alive15 · Apr 26, 2021

Missing parentheses have been known to cause pain and suffering, especially when you make a C on a test in Middle school and your parents find out.

drbitboy · Apr 26, 2021

blessdstry said:
I found this didn't work

Changed to-
(Input Raw - Input Min) * (Scaled Max - Scaled Min) / ((Input Max - Input Min) + Scaled Min) = Output Value

I am pretty sure that is not quite correct e.g. say

InputMin,InputMax = 4,20
ScaledMin,ScaledMax = 4,20
i.e. OutputValue should always equal InputRaw
So for InputRaw = 12 (half-scale):
- (12 - 4) * (20 - 4) / ((20 - 4) + 4) = OutputValue
- (8) * (16) / ((16) + 4) = OutputValue
- (8 * 16) / (20) = OutputValue
- (128) / (20) = OutputValue
- (6.4) = OutputValue, which is not equal to the InputRaw (12)

That said, it will give correct results if [Scaled Min] is zero, and believable results of [Scaled Min] is a small fraction (1% or so) of [Scaled Max].

The correct formula is

(Input Raw - Input Min) * (Scaled Max - Scaled Min) / (Input Max - Input Min) + Scaled Min = Output

with "conventional" precedence rules (mult and div before adding; left-to-right or right-to-left for equal operator precedence), or

((Input Raw - Input Min) * (Scaled Max - Scaled Min) / (Input Max - Input Min)) + Scaled Min = Output

to eliminate more operator precedence rules as a possible issue other than parenthetical statements evaluated first and inside to outside.

I used to be amazed at the frequency at which scaling questions comes up on this forum. But then I watched a quiz competition on a twitch channel where one adult thought named "Van Golf" painted the Mona Lisa, another said the product of 19 and 11 was 201, and a third said 209 but needed the entire thirty seconds to get there.

Bonemarrow · Apr 28, 2021

Conversion Formula

Conversion Formula
( E – A ) / (B – A ) X (D – C ) + C
THE EQUATION :Conversion Formula
• E : THE KNOWN VARIBLE
• A : 1ST LOWEST SCALE VARIBLE
• B : 1ST HIGHEST SCALE VARIBLE
• C : 2ND LOWEST SCALE VARIBLE
• D : 2ND HIGHEST SCALE VARIBLE

Conversion Formula
Example
E : THE KNOWN VARIBLE = 9 PSI
A : 1ST LOWEST SCALE VARIBLE = 3 PSI
B : 1ST HIGHEST SCALE VARIBLE = 15 PSI
C : 2ND LOWEST SCALE VARIBLE = 4 MA
D : 2ND HIGHEST SCALE VARIBLE = 20MA
[ ( 9 – 3 ) / (15 – 3 ) X (20 – 4 ) + 4 ]
= 12 MA

Scaling 4-20Ma rslogix5000

sparkytex

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