In brief
A photoelectric sensor is an optical sensor that detects objects as soon as they interrupt or reflect a light beam between emitter and receiver.
Contents
What is Photoelectric sensor?
Photoelectric sensors belong to the optoelectronic sensors and are used in automation for counting, positioning and presence detection, for example on conveyor lines, in packaging plants or on storage and retrieval machines. They usually work with red light, infrared or laser light. They should not be confused with safety light curtains, which are designed as protective devices for people.
In a through-beam sensor, emitter and receiver face each other; it achieves long ranges. The retro-reflective sensor uses a reflector, often with a polarisation filter for shiny objects. The diffuse sensor evaluates the light reflected back by the object, and variants with background suppression ignore objects beyond a set distance. Key parameters are range, light type, output type, light-on or dark-on switching, response time, degree of protection and design. Laser light allows a very small light spot for detecting small parts.
Faults are mostly caused by dirty optics, misaligned brackets, damaged reflectors or ambient light. Regular cleaning and stable mounting prevent many failures. Many devices use a stability indicator to show when the optics should be cleaned or realigned.
In practice: maintenance and procurement
For a spare parts enquiry, the manufacturer part number, operating principle, range, light type, output type, light-on or dark-on switching, voltage and connector are needed. For through-beam sensors, always state whether the emitter, the receiver or both are to be replaced.
Synonyms and abbreviations: photo eye · light barrier · through-beam sensor · retro-reflective sensor · diffuse sensor
Frequently asked questions
What is the difference between a light barrier and a diffuse sensor?
A light barrier works with a separate receiver or reflector; the object is detected when it interrupts the beam. A diffuse sensor emits light and detects the object by the light reflected back, so no counterpart is needed. Diffuse sensors are easier to mount but usually have shorter ranges.
What do light-on and dark-on switching mean?
With light-on switching, the output is active when light falls on the receiver. With dark-on switching, it is active when the light path is interrupted. Many devices can be switched between the two. When replacing, the switching logic must match, otherwise the controller works with inverted signals.
May a standard photoelectric sensor be used to protect people?
No. To protect people at danger points, safety light barriers or safety light curtains are required, which are designed as safety components and monitor themselves. A standard photoelectric sensor can fail without this being detected and is therefore only suitable for process tasks.