The Sensor That Stopped Seeing Parts
We used a photoelectric sensor (through-beam, article 108) to detect parts on a conveyor. The sensor worked at start-up. After a month, it started to miss parts (the sensor didn’t trigger). We checked the alignment (it was fine). The problem: the lens was dirty. The sensor’s emitter (and receiver) had a lens. Over a month, dust (from the parts, or the ambient air) coated the lens. The beam (emitted light) was attenuated (dimmer). The receiver didn’t see it (or saw it weakly). We cleaned the lens (with a soft cloth and lens cleaner). The sensor saw the parts again. The mistake was not cleaning the lens (it’s a maintenance item). We scheduled monthly lens cleaning.
Photoelectric lens maintenance keeps the beam strong. This article covers the cleaning.
Why the Lens Gets Dirty
A photoelectric sensor emits light (through the lens) and receives it (through the lens). Dust, oil, coolant mist, or part debris coats the lens. The light passing through the dirty lens is attenuated (weaker). For a through-beam sensor (article 108), the emitter’s beam hits the receiver’s lens — both lenses must be clean. A dirty lens on either side attenuates the signal.
Step 1: Symptoms of a Dirty Lens
A dirty lens causes:
- The sensor misses parts (doesn’t trigger when it should).
- The sensor triggers randomly (when the beam flickers through the dust).
- The stability indicator (LED) on the sensor is weak (or flashing).
- Cleaning fixes it (temporarily).
If the alignment is fine (article 108) but the sensor still misses, suspect the lens.
The lens cleaning rule: If a photoelectric sensor misses parts (after the alignment is checked), clean the lens. The sensor that stopped seeing had a dusty lens. Use a soft cloth and lens cleaner (not a harsh solvent). Schedule monthly cleaning (sooner in a dusty environment). Don’t touch the lens with fingers (oil from the skin adds to the dust).
Step 2: How to Clean the Lens
- Use a soft cloth: A microfiber cloth (like for glasses). Don’t use a paper towel (it scratches the lens).
- Lens cleaner (or isopropyl alcohol): Spray a little on the cloth (not directly on the sensor). Wipe the lens gently.
- Don’t use harsh solvents: Acetone, thinner, or gasoline can damage the lens (or the housing). Use isopropyl alcohol (70%).
- Dry with a clean cloth: Don’t leave streaks.
Step 3: Protect the Lens
To reduce the cleaning frequency:
- Shield the sensor: Mount it out of the dust path (article 109). A cover (a hood) blocks the dust.
- Air purge: For a very dusty environment, add a small air purge (a stream of clean air over the lens). The air blows the dust away. Some sensors have an air-purge port.
- Stainless steel housing: For washdown (food/pharma), use a stainless sensor (IP69K, article 77) that can be hosed.
Step 4: Check the Stability Indicator
Most photoelectric sensors have a stability LED (or a margin indicator). It shows how strong the received signal is. A strong signal (green LED) means clean lenses, good alignment. A weak signal (yellow, or flashing) means dirty lenses (or misalignment). Check the LED during maintenance. If it’s weak, clean the lens (or re-align).
| Symptom | Likely Cause |
|---|---|
| Misses parts (alignment OK) | Dirty lens |
| Random triggers | Dirty lens (flicker) |
| Weak stability LED | Dirty lens or misalignment |
| Cleaning fixes it | Dirty lens (confirmed) |
| No signal at all | Broken emitter/receiver (not cleaning) |
A Lens Maintenance Checklist
- Does the sensor miss parts? (After alignment check?)
- Is the stability LED weak? (Check?)
- Clean the lens? (Microfiber + isopropyl?)
- Did cleaning fix it? (Confirms dirty lens?)
- Is the sensor shielded? (Out of dust?)
- Is an air purge needed? (Dusty environment?)
- Is the cleaning scheduled? (Monthly?)
- Are spare sensors in stock? (Article 100?)
- Is the lens scratched? (Replace if scratched?)
- Is the sensor IP-rated? (Article 77?)
The Bottom Line
Photoelectric lens maintenance cleans the dust off the lens. The sensor that stopped seeing had a dusty lens. Clean with a microfiber cloth and isopropyl alcohol (not a harsh solvent). Check the stability LED. Shield the sensor (or add an air purge) to reduce the cleaning frequency. The sensor that saw every part wasn’t the most sensitive one — it had a clean lens.
