The Sensor That Cut Out After a Month: LED Aging and Dust Buildup
A photoelectric sensor that worked fine for a month, then started failing intermittently. The LED light output had degraded. The lens had a thin film of oil mist from the machine’s cutting fluid. The sensor was slowly dimming. This is a preventive maintenance issue, not a sensor failure.
What ages a photoelectric sensor
The LED in a photoelectric sensor loses about 10-20% of its output over 5 years of continuous operation. That’s normal. But in a dusty, oily environment, the lens gets a film that blocks more light. After a month, the lens transmittance drops from 95% to 70%. Combined with the LED aging, the received light is 50% of what it was. The sensor’s margin shrinks. Small variations (part angle, surface reflectivity) push it below threshold. It starts missing.
The preventive maintenance
| Environment | Lens cleaning interval | Sensor replacement |
|---|---|---|
| Clean (assembly, packaging) | Every 6 months | 5+ years |
| Dusty (wood, plastic dust) | Every month | 3-5 years |
| Oily (machining, cutting fluid) | Every 2 weeks | 2-3 years |
| Washdown (food, chemical) | Weekly | 1-2 years (IP rating matters) |
What we did
We cleaned the lens with isopropyl alcohol. The transmittance returned to 95%. The sensor worked. We added lens cleaning to the PM checklist (every 2 weeks for this machine). We also added a protective air blast — a small nozzle that blows a puff of air across the lens at each cycle. The air keeps the dust off. The cleaning interval dropped from 2 weeks to 2 months.
The sensor output margin
A new sensor has a 10x margin between “no target” and “target present.” After a year of lens fouling and LED aging, the margin drops to 2x. That’s when it starts failing. Re-teaching the sensor (with the dirty lens) sets the threshold to the degraded level. It works for a while, then degrades further. The proper fix is to clean the lens first, then re-teach.
The sealed sensor option
For oily environments, use a sensor with a stainless steel face and IP67 rating. The sealed housing prevents oil from getting inside. The lens is sapphire (scratch-resistant) instead of plastic. These cost 2-3x more but last 3x longer. For a machine that needs to run 24/7, the cost is justified by the reduced downtime.
The sensor PM rule: Clean the lens on a schedule matched to the environment. Re-teach after cleaning. Use an air blast to keep lenses clean in dusty areas. A sensor that fails after a month in an oily environment wasn’t defective — it wasn’t maintained. Budget for lens cleaning and eventual replacement.
