The Proximity Switch That Chattered: Snubber and Hysteresis, Not the Sensor
A proximity sensor detecting a metal target. The PLC input flickered (on-off-on-off) as the target approached. The customer thought the sensor was failing. The sensor was fine. It was switching on and off rapidly at the edge of its detection range. This is about sensor hysteresis and load switching, not about the sensor being defective.
Why sensors chatter
A proximity sensor has a detection range (e.g., 8 mm nominal). When the target approaches within 8 mm, the sensor turns ON. When it backs away beyond 8 mm, it turns OFF. But at the exact edge (7.9-8.1 mm), small vibrations or target movement cause the sensor to oscillate between ON and OFF. The PLC sees chatter. The sensor doesn’t know the difference between “approaching” and “leaving.”
The fix: hysteresis
A sensor with hysteresis turns ON at one distance and OFF at a slightly different distance. For example: ON at 8 mm, OFF at 6 mm. The gap between ON and OFF is the hysteresis (2 mm). The target must move 2 mm past the ON point before the sensor switches OFF. Vibration at the edge doesn’t cause chatter because the hysteresis band absorbs it.
The solutions
| Solution | How it works |
|---|---|
| Use a sensor with hysteresis | Most modern sensors have built-in hysteresis (10-20% of range) |
| Relocate the sensor | Move the sensor closer to the target (within the stable range, not at the edge) |
| Add a snubber (RC network) | For inductive loads (relays, solenoids), an RC snubber absorbs switching spikes |
| PLC input filter | Set the PLC input filter to 10-20 ms. Ignores short pulses. |
| Debounce in software | The PLC program waits 50 ms after a state change before trusting it. |
What we did
1. Relocated the sensor. The sensor was mounted 8 mm from the target (at the edge). We moved it to 5 mm (well within the 8 mm range). The sensor was solidly ON when the target was present. No chatter.
2. Set the PLC input filter. We set the input filter to 10 ms. Any pulse shorter than 10 ms is ignored. The remaining vibration (now 1 mm at 5 mm distance) doesn’t cause chatter.
3. Added an RC snubber. The sensor was switching a relay (inductive load). The switching spike (from the relay coil) caused the sensor to see a false state. We added an RC snubber across the relay coil. The spike was absorbed. No more chatter.
The sensor chatter rule: If a proximity sensor flickers at the edge of its range, relocate it closer to the target. Set a PLC input filter. Add an RC snubber for inductive loads. The sensor isn’t failing — it’s switching at the edge of its range. Hysteresis and filtering solve it.
