The Proximity Switch That Chattered (No Snubber)
We used a proximity switch (article 108) on a moving slide (the slide approaches a stop). The slide hit the stop (hard). The proximity switch (mounted near the stop) detected the slide. But the switch chattered (it turned on/off rapidly) — the slide bounced (it vibrated at the stop). The PLC (article 181) saw multiple triggers. The problem: no snubber (the slide bounced). We added a snubber (a shock absorber, or a urethane bumper) at the stop. The slide stopped (no bounce). The proximity switch was stable. The mistake was not snubbing (the bounce triggered the switch multiple times).
Proximity switch snubber stops the slide (no bounce). This article covers the setup.
The Bounce
A moving slide (a carriage, a cylinder, article 85) hits a hard stop. It bounces (it rebounds, a few mm). The bounce (the slide moves back and forth) makes the proximity switch (article 108) chatter (it triggers on/off). The PLC (article 181) sees multiple pulses (it thinks the slide moved).
Step 1: The Snubber (Shock Absorber)
A snubber (an industrial shock absorber, a pneumatic damper) absorbs the impact. The slide hits the snubber (it decelerates, no bounce). The snubber (a self-compensating shock absorber) stops the slide (smooth). No bounce, no chatter.
The snubber rule: Add a snubber (shock absorber) at the stop. The switch that chattered had a hard stop (the slide bounced). Add a snubber (urethane, or shock absorber). The slide stops (no bounce). The switch is stable.
Step 2: Urethane Bumper (Light Load)
For a light load (a small slide), a urethane bumper (a soft pad) absorbs the impact. Cheap. For a heavy load (a big carriage), use a shock absorber (a hydraulic damper, rated for the energy).
Step 3: The Switch Delay (Debounce)
If you can’t add a snubber, debounce the switch (in the PLC, article 181). The PLC waits (20 ms) after the trigger (it ignores the chatter). The switch is read (stable). But this doesn’t fix the bounce (the slide still bounces, it’s just ignored). The snubber is better (it stops the bounce).
Step 4: The Sensor Location
Mount the proximity switch (article 108) away from the impact (not at the stop). The switch detects the slide (when it’s near the stop, not when it hits). The switch doesn’t take the impact (it’s mounted away).
| Stop | Bounce |
|---|---|
| Hard stop (steel) | Bounce (chatter) |
| Urethane bumper | Less (light load) |
| Shock absorber | No (heavy load) |
| Debounce (PLC) | Ignores (but bounces) |
A Snubber Checklist
- Does the slide bounce? (Chatter?)
- Is there a snubber? (Shock absorber?)
- Is it sized? (Energy? Load?)
- Is it urethane (light) or shock (heavy)?
- Is the switch debounced? (PLC delay?)
- Is the switch away from impact? (Mounted?)
- Does the PLC see one trigger? (Not multiple?)
- Is the cycle consistent? (Repeats?)
- Is the snubber maintained? (Worn?)
- Is the stop adjustable? (Position?)
The Bottom Line
Proximity switch snubber stops the slide (no bounce). The switch that chattered had a hard stop. Add a snubber (shock absorber). Debounce (if needed, article 181). The switch that was stable wasn’t the best sensor — it had a snubber.
