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Principles of particular optical sensing systems - Photoelectric Sensors

Principles of particular optical sensing systems

 

Fiber cables

Principle of optical fiber

An optical fiber comprises of a core and a cladding, which have different refractive indexes.
When light is incident on the core, it propagates in the core by being totally reflected at the boundary between the core and the cladding. After traveling through the fiber, light spreads at an angle of approx. 60° at the cable end and is directed on the sensing object.

Types of fiber cables and their features

Type Features
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 cable structure

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
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.

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Retroreflective type sensor with polarizing filters

Principle

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.
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.

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Adjustable range reflective type photoelectric sensor

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.
It contains an emitting lens and a receiving lens. The beam from the emitting lens falls on the sensing object and, after being reflected, is guided by the receiving lens onto a 2-segment diode. Here, the sensing object distance is determined by taking the position at which the upper and lower segments of the 2-segment photodiode generate equal output voltages as the reference.
This method, besides being suitable for long distance, is also good for high accuracy position alignment. Further, the equal output voltages are obtained by adjusting the position of the receiving lens.

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.

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Digital mark sensor / LX-100 series

When the mark mode is set

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.

When the color mode is set

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.

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Color detection fiber sensor / FZ-10 series

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.

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Liquid level detection sensor (Pipe-mountable type)

Thru-beam type

When liquid is present, the lens focuses as per the liquid lens effect and the beam is received.

<Filled pipe>

The lens focuses as per the liquid lens effect and the beam is received.

<Empty pipe>

The beam is scattered and not received.

Reflective type

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.

<Empty pipe>

The beam reflected from the inner surface of the pipe wall returns to the beam-receiving part.

<Filled pipe>

The beam passes through the wall into the liquid.

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Cross-beam scanning (NA1-11)

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.

NA1-11

General purpose area sensor

Liquid level detection fiber (Contact type)

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.

In the air

In liquid

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Leak liquid detection (Leak detection fiber sensor / Leak detection sensor)

The unique effect of capillarity enables reliable detection of small leaks and viscous liquids.

Capillarity effect

New type of detection method

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.

<When leakage occurs>

The beam from the beam-emitting part scatters through the leaked liquid and is not transmitted to the beam-receiving part.

<When there is no leakage>

The beam from the beam-emitting part reflects off of the surface of the sensor and is transmitted to the beam-receiving part.

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