Business > Industrial Devices > Automation Controls Top > Service & Support > FA Technical Support > Technical Guide (FA Sensors) > Photoelectric Sensors > Principles of particular optical sensing systems
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An optical fiber comprises of a core and a cladding, which have different refractive indexes. |
Type | Features |
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Plastic | The fiber is made of acrylic. The core is made up of one or several ø0.125 to ø1.5 mm ø0.005 to ø0.059 in acrylic resin fibers. It is widely used because of its low price. The sharp bending fiber is made up of several hundred ø0.075 mm ø0.003 in acrylic resin fibers bound together into a single multi-core fiber, so that it can be bend at right angles without causing a decrease in light intensity or breaking. |
Glass | The fiber is made of glass that provides better heat-resistance and chemical-resistance than plastic. The cable consists of multiple fiber strands of ø0.05 mm ø0.002 in. It is used mainly for special applications because of its high price. |
Fiber sensors are classified broadly into two groups thrubeam type and reflective type.
The thru-beam type has two fiber cables: the emitting cable and the receiving cable. The reflective type has one fiber cable that contains, both, the emitting part and the receiving part.
The cable can be classified into parallel, coaxial or partition types, depending on the structural arrangement of the fiber strands.
Cable structure | Description |
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Parallel |
Generally used for plastic fiber cables. |
Coaxial |
The center fiber is for beam emission, and the surrounding fibers are for receiving the beam. This structure is suitable for high accuracy measurements since the sensing position does not change with the travel direction of the sensing object. |
Partition |
Generally used for glass fiber cable. It comprises of a number of glass fiber strands of ø0.05 mm ø0.002 in, and is divided into the emitting part and the receiving part. |
Opposite types of polarizing filters are placed in front of the emitting and receiving elements. A horizontal polarizing filter placed in front of the emitting element passes only horizontally polarized light and a vertical polarizing filter placed in front of the receiver ensures that only vertically polarized light is received.
Using this configuration, even specular objects can be reliably detected.
1 | : | Normal unpolarized beam emitted from the LED oscillates in a random manner. As it passes through the horizontal polarizing filter, the oscillation is aligned horizontally and the beam is horizontally polarized. |
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2 | : | When the polarized beam falls on the reflector, its polarization is destroyed and the reflected beam oscillates in a random manner. So, the reflected beam can pass through the vertical polarizing filter and reach the receiving element. However, a specular object does not destroy the polarization. The reflected beam oscillates horizontally, as before, and cannot pass through the vertical polarizing filter. |
Employing the optical triangulation method, it reliably senses an object at a given distance, irrespective of its reflectivity, by measuring the angle of the received beam. |
We also have the MQ-W series that uses two PSDs (Position Sensitive Detector) on the receiving element for one emitting element in order to improve reliability.
The optimal light source is automatically selected from the 3 colors of the R, G, B LEDs so that the contrast between the mark and base becomes the largest. This makes detection more stable.
The color mode utilizes all the R, G, B LEDs and detects the reflected light by calculating the R, G, B ratio. Thus, high precision detection is possible by sensing only the mark color that teaching was performed on. |
Three LEDs, red, green and blue, are used as the emitting elements. Each of them emit in turn to illuminate the sensing object and the color components of the reflected beam are processed to determine the sensing object color. |
When liquid is present, the lens focuses as per the liquid lens effect and the beam is received.
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When the pipe is empty, the beam is reflected from the inner surface of the pipe wall and returns to the beam-receiving part since the difference in the refractive indexes of the pipe and air is large.
When there is liquid in the pipe, the beam enters the liquid through the wall and does not return to the beam-receiving part as the difference in the refractive indexes of the pipe and the liquid is small.
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In a conventional area sensor, slim objects cannot be detected since the emitting and the receiving elements are scanned, synchronously, as a set. In contrast, in NA1-11, only the elements (1) to (11) of the emitter are scanned to obtain emission. The elements of the receiver are not scanned, so that when element (1) of the emitter emits light, all the elements of the receiver receive light. Hence, even if there is one element on the receiver which does not receive light, it results in light interrupted operation. With this technique, detection of slim objects is possible.
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When the fiber tip is in the air, as there is a large difference between the air and the tube refractive indexes, the tube boundary reflects the emitted beam back to the receiver. On the other hand, when the fiber tip is immersed in a liquid, the emitted beam scatters from the fiber into the liquid because of the small difference in the liquid and the tube refractive indexes.
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The unique effect of capillarity enables reliable detection of small leaks and viscous liquids.
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When a leak occurs, the beam from the beam-emitting part scatters through the leaked liquid and is not transmitted to the beam-receiving part.
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