Analog input scaling

In PLC, analog input scaling is the converting process of raw electrical signal (voltage, current) into a engineering value (temperature, pressure, level).

This is used as PLCs can't “understand” real-world units directly, but instead read analog signals as digital numbers.

E.g., A sensor outputs 4–20 mA, which is converted by the PLC into a raw value like 0–32767.

Scaling translates the raw value into something useful values that may be called scaled value or controller tag value:

• 0–100°C

• 0–10 bar

• 0–1000 liters

• Scaled value = resulting value (often measured ampere or volt)

• Raw or Vin = input voltage used to convert

• Rawmax or Signalmax = max signal range

• Rawmin or Signalmin = min signal range

• Engmax = max engineering value

• Engmin = min engineering value 

E.g., Input signal: 4–20 mA, PLC raw units: 0–32767 (converted from 4-20 mA), engineering units: 0–100°C. If the PLC reads 16384 (about halfway), scaling gives: scaled value = [(16384 - 0)/(32767 - 0)]*(100 - 0) + 0 ≈ 50°C.

But suppose the sensor outputs 12 mA.

We map 4–20 mA to 0–32767: scaled value = [(12-4)/(20-4)] *(100 - 0) + 0 ≈ 50°C.

References

• [1] PLC Analog Input Scaling

Related materials

• [1] ELE8932 Week 10: High Speed Analog (Canva)