Platform for Light Guide Photointerrupters
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- Published on Tuesday, 21 November 2017 18:31
Light Guide Photointerrupters
The design of an Optoelectric interrupter rests on two functions:
1- The electro-optical function
2- The transport and delivery of the light
The electro-optical is common to a lot of photoelectric interrupters:
- Emitting light with proper characteristics in terms of wavelength and optical power. That is achieved with a solid state Light Emitting Diode.
- Receiving light and convert it into a usable electrical signal. This is obtained with a silicon photodetector, either a photodiode, a phototransistor, a photodarlington or a photo-IC.
That electro-optical function can be embedded into the core of the system electronics.
The light transport and delivery is determined by the configuration of the Photointerrupter within the application, and for this reason must be customized.
This customization can use materials that are designed for that purpose, Plastic Optical Fibers or hard Light guides. The POF (Plastic Optical Fiber) option is ideal for configurations where the light must be transported over long distances or when moving parts are involved (think of a robotic arm). The hard light guide is more appropriate for small size applications and offer the benefit of stronger signals.
Light in Motion’s platform provides the core electro-optical function and the interface between the active electro-optical structure and the light guide.
It is made of a substrate that holds the two active parts (LED and Photodetector). Those two active parts are designed to optimize the amount of light injected in the light guide and to reduce the cross talk between the emitter and the detector.
A housing covers the substrate and provide the access for light guides. This housing can be customized to accommodate various types of light guides.
Studs are added underneath to allow a robust attachment to the PCB. The contacts are made of four metallized pads (2 for the emitter and 2 for the detector).
The footprint is 7mm x 4mm.
On the basis of that platform, various types of photointerrupters can be designed. Just to name a few:
1- Transmissive photointerrupters, also called limit switches, where the response of the system is resulting from an object blocking the transmission of light between an emitter arm and a receiver arm in direct view of each other.
2- Reflective photointerrupters: the arms are placed side by side, so that a reflective object moving in front of them will redirect a fraction of the light from the emitter arm toward the receiver arm. 
3- Retroreflective photointerrupters. In that case, the emitter arm and the receiver arm are optically connected via a reflective mirror. An object moving in the space between the two arms and the mirror will block the travel of the light and be detected.
4- Encoders: they are extensions of transmissive or reflective photointerrupters in their principle, but with specific arrangements to allow an accurate counting of the encoder wheel spokes and possibly a quadrature output.
Typical measurement readings:
1. Transmissive mode
Light guide: 2mm Plastic Optical Fiber, 5 feet long
Measurement taken at IF = 10 mA (LED driving current)
a. Distance tip to tip: 1.5mm
Light path blocked: Ic = 0.03 mA
Light path unblocked: Ic = 47.0 mA
b. Distance tip to tip: 5mm
Light path blocked: Ic = 0.03 mA
Light path unblocked: Ic = 1.0 mA
2. Reflective mode
Light guide: 2mm Plastic Optical Fiber, 5 feet long
Measurement taken at IF = 10 mA (LED driving current)
Reflector: metal blade
Angle of incidence 45 deg
Ic = 0.5 mA
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