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Old February 9th, 2010, 05:08 AM   #1
rigicon
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Twido analog input calcualtion

Hi all,
I have an analogue input on a Schnieder Twido PLC.
The sensor measures distance and has a range of 100mm
It seems to only start working at 55mm away and reaches full range at approx 155mm.
So from 55mm I get 0(4mA) and at 155mm I get 4095(20mA)

My simple formula was z =(measured value 0-4095)
(z x 100) / 4095 this gives me a 0 to 100 value call it Y.
Y is its inverse so I have 100 - Y.

This seems to work but on different sizes i need to recalibrate and end up 80-Y or 85 - Y.
The formula is not right somehow although it seems logical to me - i have definitely missed the plot I wish the Twido had scaling.
Any mathematicians available ???
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Old February 9th, 2010, 01:51 PM   #2
Lare
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Hi!


Basic scaling methos is to use formula y=kX+b which you already have used, but take a look to thread http://www.plctalk.net/qanda/showthr...2&highlight=EU and read Tom Jenkins reply to threat.

It's is also maybe better if you reconfigure twido analog module to use raw data values from 0 to 10000 (4
mA =0%, 20mA=100% ) instead of 0-4095. (You can use different raw data values to analog modules if you right mouse click above analog module and select configure => Range = custom...)

It's more easier to understand raw data values 0-10000 than 0-4095, when you use math in twido and you can see better were the error to scaling happens...
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Old February 9th, 2010, 02:27 PM   #3
rigicon
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Thanks Lare
Just after the post i found out about the custom scaling I was going to use 0 to 100. But I think you are right 0 to 10000 is better.
This gives me a nice resolution to 2 decimal places.
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