The Proximity Switch That Never Triggered
We used an inductive proximity switch to detect a metal part arriving at a station. The switch was rated for 2 mm sensing distance. On the bench, it detected a test piece. On the floor, the part (a thin steel bracket) passed under the switch. The switch didn’t trigger. The problem: the part’s sensing face was only 1.5 mm thick (the switch sees the edge, not a full face), and the mounting bracket had flexed 1 mm out of position. The gap was 3 mm — over the 2 mm rating. We moved the switch closer (gap 1 mm) and stiffened the bracket. The switch triggered. The mistake was mounting the switch at its rated sensing distance on a flimsy bracket — no margin, and the bracket flexed.
Limit switch and sensor actuation is about the switch’s operating point, the over-travel, and the mounting rigidity. This article covers the design.
Switch Types and Operating Force
Mechanical Limit Switch
A physical lever (or plunger) is pushed by the part. The switch clicks at a certain force. Operating force: 1–10 N (standard), 0.5–2 N (light duty). Over-travel: the lever can be pushed past the operating point without damage (1–5 mm).
Inductive Proximity Switch
No physical contact. Detects metal at a distance (1–15 mm). No operating force (the part just passes near). But the sensing distance depends on the metal type and the face size. And it needs a rigid mount (no flex).
Photoelectric Sensor
Beam of light (through-beam, retro-reflective, or diffuse). Detects any object. Sensing distance: meters (through-beam) to mm (diffuse). Needs alignment and a clean lens.
| Switch Type | Operating Force | Sensing Distance | Best For |
|---|---|---|---|
| Mechanical limit | 1–10 N | Contact (lever) | Hard stops, repeatable position |
| Inductive proximity | 0 (non-contact) | 1–15 mm (metal only) | Part present detection |
| Photoelectric | 0 (non-contact) | mm to meters | Any material, long distance |
Step 1: Operating Point and Over-Travel
A mechanical limit switch doesn’t just “click” at one point. It has a range:
- Operating point (OP): The position where the switch changes state.
- Release point (RP): Where it changes back (usually slightly before OP — differential).
- Over-travel (OT): How much farther the actuator can move past OP without damage.
The part must push the switch past OP (into OT) so that even if the part stops slightly short, the switch still triggers. If the part stops exactly at OP, vibration or wear might cause it to chatter. Design for the part to push 1–2 mm into over-travel.
Step 2: Mounting Rigidity
The switch mount must be rigid. A flimsy bracket flexes under the operating force, changing the gap. The proximity switch that didn’t trigger was on a bent sheet-metal bracket. The bracket flexed 1 mm, pushing the gap over the rating.
Mount the switch on a rigid bracket (aluminum extrusion, thick plate). Adjust the position with a slot (so you can fine-tune), then lock it. Don’t rely on a thin bracket.
The switch rule: Mount the switch on a rigid bracket. Set the gap at 50–70% of the rated sensing distance (not at 100%). The switch that never triggered was at 100% on a flexing bracket. Move it to 50% and stiffen the mount. For mechanical switches, design the part to push into over-travel.
Step 3: Inductive Switch Derating
An inductive proximity switch’s rated sensing distance (Sn) is for a standard mild steel test piece. Real parts differ:
- Stainless steel: 0.7× Sn (less sensitive).
- Aluminum or copper: 0.4× Sn (much less sensitive).
- Small part (smaller than the switch face): Reduced distance.
- Thin part (edge, not full face): Reduced distance.
If the switch is rated for 2 mm (on mild steel), and the part is thin stainless, the effective distance is 2 × 0.7 × 0.7 (thin) = ~1 mm. Set the gap to 0.5 mm (50%). Don’t rely on the rated number — derate for the actual part.
Step 4: Bounce and Chatter
A mechanical switch chatters (bounces) for a few milliseconds when it actuates. The PLC input might see multiple pulses. Use a debounce filter (in the PLC input, 10–50 ms) or a proximity switch (no bounce). For high-speed counting, a mechanical switch won’t keep up — use a proximity or photoelectric.
Step 5: Actuator Style
For mechanical limit switches, the actuator (the part that the pushrod contacts) matters:
- Plunger (straight): The part pushes straight in. Good for stops.
- Roller lever: The part hits a roller on a lever. Tolerates side misalignment. Standard for conveyor stops.
- Wobble stick: A flexible rod that triggers from any direction. For safety interlocks (trip wires).
A Switch Selection Checklist
- What is the part? (Metal, plastic, size?)
- Contact or non-contact? (Mechanical vs proximity?)
- What sensing distance is needed?
- Is the mount rigid? (No flex?)
- Gap set at 50–70% of rated distance?
- For inductive: derated for material and part size?
- For mechanical: is over-travel designed in?
- Is the actuator style right? (Roller, plunger?)
- Is there PLC debounce? (For mechanical switches?)
- Is the switch adjustable? (Slot for fine-tuning?)
- Is the IP rating right? (Dust, washdown?)
- Is it accessible for cleaning/cleaning the lens?
The Bottom Line
Limit switch actuation is margin and rigidity. The switch that never triggered was at its rated distance on a flexing bracket. Mount on a rigid bracket, set the gap at 50–70% of the rated distance, and derate inductive switches for the actual material. For mechanical switches, design the part to push into over-travel. The switch that triggers every time wasn’t the closest one to the part — it had margin and a solid mount.
