Choosing between diffuse, retro-reflective and through-beam photoelectric sensors and inductive proximity sensors, with the mounting and shielding rules that prevent false triggers.
Most sensor faults on a packaging line are specified, not manufactured: the wrong sensing principle for the material, or the right sensor mounted where it was never going to work.
| Target | Best principle | Why |
|---|---|---|
| Cardboard, paper, plastic film, glass | Photoelectric | Non-metallic; optical detection does not need metal |
| Transparent film, clear bottles | Through-beam photoelectric | Interruption detection is insensitive to surface reflection |
| Metal cans, caps, machine parts | Inductive | No optics to foul with dust; unaffected by colour |
| Liquid level through a wall | Capacitive | Detects the medium, not the container |
| Label presence, print mark | Contrast / colour-mark sensor | Compares reflected intensity between mark and background |
| Position of a moving carriage | Inductive with a metal flag | Repeatable to a fraction of a millimetre |
Through-beam (separate emitter and receiver) gives the longest range and the highest excess gain, so it keeps working when the lens gets dusty. It costs more because both ends need wiring. Retro-reflective uses one head and a reflector, roughly half the wiring and nearly the same reliability — but a shiny target can be mistaken for the reflector. Diffuse uses one head and relies on light returning from the target; it is the cheapest and easiest to mount, and the first to fail when the target is dark, angled or far away.
The key number is excess gain, not nominal range. A sensor specified at 300 mm with low excess gain and a dusty lens behaves worse than one specified at 200 mm with high excess gain. In a flour-dust or sawdust environment, specify through-beam and add air purge.
European and most modern PLC inputs are PNP (sourcing): the sensor switches the positive supply to the input. Many Asian controllers and older equipment are NPN (sinking). Buying the wrong one is a common and entirely avoidable delay. Also confirm whether the input needs a two-wire, three-wire or four-wire (NO + NC, or IO-Link) connection, and whether the sensor is rated for 10–30 V DC.
ELEHPD supplies photoelectric and colour-mark sensors, inductive proximity sensors, magnetic and cylinder sensors in M12/M18/M30 bodies with PNP and NPN outputs, plus the mounting brackets and connectors.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.