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Categories: Formulas
PLC Analog Input Scaling
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An Allen-Bradley SLC500 programmable logic controller (PLC) uses a 16-bit analog-to-digital
converter in its model 1746-NI4 analog input card to convert 4-20 mA signals into digital number
values ranging from 3277 (at 4 mA) to 16384 (at 20 mA). However, these raw numbers from the
PLC’s analog card must be mathematically scaled inside the PLC to represent real-world units
of measurement, in this case 0 to 700 GPM of flow. Formulate a scaling equation to program
into the PLC so that 4 mA of current registers as 0 GPM, and 20 mA of current registers as 700
GPM.
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We are already given the raw number values from the analog card’s analog-to-digital converter
(ADC) circuit for 4 mA and 20 mA: 3277 and 16384, respectively. These values define the
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domain of our linear graph:
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Instrumentation Tools
Learn Instrumentation and Control Engineering. Inst Tools covers topics of Industrial
Instrumentation, PLC, DCS, SCADA, Field Instruments, Analyzers and so on.
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Calculating and substituting the slope (m) value for this equation, using the full rise-over-run of
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the linear function:
Also Read : PLC Raw Count Calculation
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Instrumentation Tools
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Instrumentation, PLC, DCS, SCADA, Field Instruments, Analyzers and so on.
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This type of scaling calculation is so common in PLC applications that Allen-Bradley has
provided a special SCL (“scale”) instruction just for this purpose. Instead of “slope” (m) and
“intercept” (b), the instruction prompts the human programmer to enter “rate” and “offset”
values, respectively. Furthermore, the rate in Allen-Bradley’s SCL instruction is expressed as
the numerator of a fraction where the denominator is fixed at 10000, allowing fractional (less
than one) slope values to be specified using integer numbers. Aside from these details, the
concept is exactly the same. Expressing our slope of 700/13107 as a fraction with 10000 as the
denominator is a simple matter of solving for the numerator using cross-multiplication and
division:
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Thus, the SCL instruction would be configured as follows
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Also Read : 4-20mA Conversion Formulas
Credits : by Tony R. Kuphaldt – Creative Commons Attribution 4.0 License
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