Fiber-optic cables contain thin strands of lightconductive glass or plastic. Each strand reflects and refracts light along its length, effectively piping light from one location to another. PVC jackets encased in a protective sheath individually protect two fiberoptic strands, one for the light source and the other for the receiver. The sheath material is application specific and is typically stainless steel, silicone, or PVC. Thru-beam sensors require two individual cables.
Light from the source travels along the Tx fiber-optic strand to the open end. The Rx strand conducts light back to the receiver for detection. The way by which light transfers from one fiber-optic strand to the other defines the type of sensor. In a thru-beam system, the light from one fiber aims directly at the other. An object passing through the beam interrupts the light path, triggering the sensor. In a diffusebeam system, light is reflected from the sensed object back into the receiver strand.
Fiber-optic cable strands made of glass operate over longer distances than plastic strands. This is because glass strands are slightly larger and optically purer than plastic strands and thus have less optical loss. Glass strands can also withstand more heat than plastic, operating as high as 900°F. Plastic cables only have temperature ratings up to 220°F. But the small-diameter plastic strands create sharper light beams for easy detection of small parts like the pins on an IC. Technicians can cut plastic cables to length in the field so exact-length specifications aren't necessary at time of order. Custom cutting plastic fibers to the shortest possible length reduces inherent light loss inside the cable.
Pepperl+Fuchs (am.pepperl-fuchs.com) provided information for this article.