ANALOG · SCALING
How to Scale a 4–20mA Load Cell Signal in a PLC
A load-cell transmitter does not give you kilograms directly. Convert raw counts to engineering units first.
4–20mALoad CellMUL / DDIV
Signal chain
Hopper Weight→Load Cell mV/V→Transmitter 4–20mA→PLC AI→D300 Raw→D310 kg
Lab Case P-001 uses a training range of D300 = 0–4000 and D310 = 0–5000, where 5000 represents 500.0 kg with one implied decimal place.
Scaling
D310 = (D300 × 5000) / 4000
MUL D300 K5000 D320
DDIV D320 K4000 D310| D300 raw | D310 | Weight |
|---|---|---|
| 0 | 0 | 0.0 kg |
| 800 | 1000 | 100.0 kg |
| 2000 | 2500 | 250.0 kg |
| 3200 | 4000 | 400.0 kg |
| 4000 | 5000 | 500.0 kg |
Why feed-rate calculation is more sensitive
At 500 kg over 4000 counts, one count equals 0.125 kg. With a simple two-second difference, a one-count change corresponds to 225 kg/h, so quantization and noise can be magnified.
A short-window difference is not enough by itself. The averaging window, filter, analog update period and post-refill settling time all matter.
The analog module is not final
FX3U-3A-ADP was a compact/cost-focused candidate, not a final approved module. Required dosing accuracy, noise, update rate and AO accuracy still need validation before purchase.
Wiring and filtering checks
- Shielded twisted pair
- Single-point grounding
- Separate analog wiring from VFD power wiring
- Check transmitter/module digital filtering
- Verify analog update period vs calculation period
- Define a post-refill settling interval