Sensor Sense: Fiber-optic sensors

Nov. 3, 2005
Industrial sensors using fiber-optic cables often have a role sensing small targets or working under conditions that would hamper other types of sensors.

come in both thrubeam and diffuse-beam types. In thrubeam, light from the transmitter (Tx) fiber strand aims directly into the receiver (Rx) input fiber strand. The sensed object breaks the beam indicating its presence. The diffuse-beam sensor still has two fiber strands, but both Tx and Rx fibers are combined into a single bifurcated cable. The sensed object reflects light from the Tx fiber into the Rx fiber, triggering a response. Only one fiber is visible in the thru-beam cylinder, while the diffuse-beam cylinder holds two fiber ends. The fiber strand lays inside the protective PVC sleeve which is wrapped in turn by the outer protective sheath. The inner PVC sleeve lets the fiber strand slide inside the tube when the cable flexes. Otherwise internal bending stresses may snap the fiber. The outer sheath protects the PVC sleeve and fiber strand from external damage. The sheath can be metal, silicone, or PVC.

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 ( provided information for this article.

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