The Sensor That Read 0.05 mm Vibration: Analog Filtering, Not a Loose Sensor
A pressure transducer (4-20 mA output) measuring air system pressure. The PLC reading fluctuated ±0.2 bar even though the actual pressure was steady. The transducer was new. The tubing was firm. The fluctuation was electrical noise on the 4-20 mA loop. The fix wasn’t a better transducer — it was a filter.
The noise source
A 4-20 mA signal is a current loop — it’s inherently more immune to noise than a voltage signal. But if the cable runs near a VFD or a motor cable, the noise couples into the loop. The PLC reads the current as a varying pressure. The actual pressure is steady. The reading bounces.
We measured the actual pressure with a reference gauge. It was 6.0 bar, steady. The PLC read 5.8 to 6.2 bar. The ±0.2 bar variation was noise, not pressure fluctuation.
The filtering options
| Method | What it does | Best for |
|---|---|---|
| PLC analog filter (averaging) | Averages readings over time | Slow processes, general |
| Hardware low-pass filter | RC filter on the signal line | High-frequency noise |
| Shielded cable | Blocks external noise | Near VFDs and motor cables |
| Isolator (signal isolator) | Breaks ground loop | Ground potential differences |
What we did
1. PLC averaging. We set the PLC’s analog input filter to average 16 samples over 100 ms. The ±0.2 bar noise dropped to ±0.02 bar. The reading was stable. The 100 ms delay is irrelevant for a pressure signal (which changes slowly).
2. Shielded cable. The original cable was unshielded. We replaced it with a shielded twisted pair. The shield grounded at the PLC end. The noise dropped further. The reading now fluctuates ±0.01 bar.
3. Move the cable. We rerouted the signal cable away from the VFD output cable. They crossed at 90 degrees instead of running parallel. The remaining noise was negligible.
The filter time constant
For a pressure signal that changes over seconds, a 100 ms filter is fine. For a fast process (e.g., a force measurement that must react in 10 ms), you can’t filter heavily — you need to eliminate the noise at the source (shielding, separation). The filter time constant should be about 5-10x faster than the process you’re measuring. If the process changes in 1 second, filter at 50-100 ms. If it changes in 10 ms, filter at 1-2 ms.
The damping on the transducer
Many pressure transducers have an internal damping adjustment (a needle valve or a digital setting). We set the transducer’s damping to 1 second. The transducer itself smooths the pressure reading. This is better than a PLC filter because the transducer does it before the signal leaves the sensor.
The filtering rule: If an analog reading fluctuates but the actual process is steady, it’s noise. Use shielded cable, separate from power cables. Add a PLC filter (10-100 ms). Set the transducer’s internal damping. Don’t replace the sensor — the noise is in the wiring or the loop, not the sensor.
