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Scale an Analog Output
This example shows the scaling of analog output values to
engineering units for monitoring or controlling purposes.
We are making these assumptions.
• The FIO4I module is located in slot 2 of an SLC 500 system.
• An actuator of a flow control valve is wired to output channel 0.
• The actuator accepts a 4...20 mA signal for a 0...100% of valve
opening.
• The actuator can not receive a signal out of the 4...20 mA range.
• The percentage of valve opening is manually input to the SLC
processor.
This graph displays the linear relationship.
20mA = 31208
(scaled max.)
Scaled
Value
4mA = 6242
(scaled min.)
0%
(input min.)
Calculate the Linear Relationship
Use these equations to compute the scaled output value:
Scaled value = (input value x slope) + offset
Slope = (scaled range) / (input range)
= (scaled max – scaled min) / (input max – input min)
= (31208 – 6242) / (100 – 0) = 24966/100
The slope is greater than 3.2767 so you cannot use SCL instruction.
Offset = scaled min – (input min x slope)
= 6242 – [0 x (24966 / 100)] = 6242
Scaled value = [input value x 24966 / 100] + 6242
100%
(input max.)
Input Value
(from data table)
Publication 1746-UM009B-EN-P - September 2007
Write Ladder Logic
47
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