scaling analog inputs

[wetback]

This is generic , so I won’t go into the hardware unless necessary, range is simplified 0-20Ma in, 0-20 scale out.

I am having problems with scaling in Lookout, my input is 0-20Ma, input conversion is done in BCD for a unipolar FS output of 16533.

My scaling in Lookout is; raw 0-16533 /  eng units 0-20

The readings are like this;

Input   -  output

0Ma.  - 0

4Ma. – 2.52

12Ma. – 11.27

20Ma. – 20

So I tried converting to BIN  unipolar FS output of 4095

My scaling in Lookout is raw 0-4095 /  eng units 0-20

The readings are like this;

Input   -  output

0Ma.  - 0

4Ma. – 1.65

12Ma. – 12

20Ma. – 20

What am I missing?

[jbesselm]

Create a display to show the raw or unscaled units at each of the mA steps. This will probably give a clue. What is your source transmitter? Is it linear or is it some other function. 

[Mike@Forshock]

Like Jim posted, what is the RAW unscaled reading at each analog step.

What is generating your test measurements (0,4,12,20mA)?

[wetback]

The source is an Altek calibrator that I checked against another meter. The raw values are;

ma.      read    calculated

0           0            0

5        4135      4133.25

10      8273      8266.5

15     12405    12399.75

20     16533    16533

Looks like a nonlinear a/d converter, but I didn't expect the scaled values to be that far off.

[jbesselm]

Your numbers look quite linear to me, only a few counts off what is expected. The values given scale just fine with my testing. The puzzle is fun but I think it's time for you to share more about your hardware and what driver object you are using. The only hardware I have handy that uses BCD datatypes are automation direct PLCs. Every thing scales just as it should. i.e 2084 BCD =2.52 scaled, 3306 BCD scales to 3.99, likewise 3306 Dec scales to 3.99  and 2084 Dec scales to 2.52.

When you did your test what where the scaled values at each step?

%maReadScaled

000??

2554135??

751512405??

1002016533??

[jbesselm]

Sorry my formating was messed up in the previous response.

%    ma  read    scaled

0      0     0         ??

25    5     4135    ??

75    15   12405   ??

100  20   16533   ??