What is the detection principle of the optical fiber sensor?

Last Update Time: 2021-02-27 10:46:10

For fiber-optic sensors, if a through-beam fiber is used, it is a through-beam detection mode; if a direct-inversion fiber is used, it is a proximity detection mode. The ultrasonic sensor is divided into two types of detection modes: through-beam and proximity.

Reflective plate

In the reflector detection mode, a sensor has both a transmitter and a receiver. The transmitter emits light onto a reflecting plate, and the reflected light returns to the receiver. When the object blocks the light beam, the measured object is detected.

The detection distance of the reflective plate sensor is the distance from the sensor to the reflective plate. Its effective beam is usually tapered, from the edge of the lens to the edge of the reflector. This is different in special cases, such as: when the sensor is too close to the reflector, the beam cannot cover the entire reflector; or when the emitted light is a laser beam. In these cases, the size of the effective beam does not extend to the entire area of the reflector.

The reflector is usually a matrix of multiple geometric prisms. Each prism has three mutually perpendicular planes and an inclined plane. The light beam enters from the oblique plane, and is reflected by the other three planes, and returns in parallel from this oblique plane. The reflector then reflects the incident light back to the receiver.

Most prismatic reflectors are die-cast from transparent acrylic plastic and come in a variety of sizes and shapes. Prisms or reflectors are often used as automobile safety reflectors. When the car's headlights shine on the reflectors, the reflectors reflect a strong light so that drivers can observe them in time.

Warning signs on highways can be made with reflective tape, the surface of which is coated with a thin layer of reflective material with geometric prisms or fine glass sand. (Smooth glass surfaces can also reflect light back, but the surface of glass-coated sand has a lower reflectance than the surface with geometric prisms.)

Very bright surfaces can also be used as reflectors, but incident light will be reflected back in opposite directions at the same angle. In order for the sensor to receive reflected light, the emitted light must be perpendicular to the mirror surface. However, for a reflector, it can reflect incident light back at an angle of up to 20 ° from the vertical line, which makes alignment very easy.

The reflectivity of a good reflector is 3,000 times that of a piece of white paper, so the reflector sensor can easily receive the light reflected from the reflector. However, for a test object with a strong reflectance, when the light beam is blocked, it can also reflect the strong light back to the sensor and make it mistakenly assume that the test object does not appear. We also have corresponding solutions to this problem.

If an object with a very flat and bright surface always passes the detection area in a fixed direction, then we can install the sensor and the reflective plate obliquely so that the light reflected from the surface of the measured object does not return to the sensor. The tilt angle is usually 10 ~ 15.

But if the surface of the bright test object is round or the test object enters the detection area at an uncertain angle, the problem will be more complicated. At this time, we can rotate the sensor and the reflector both horizontally and vertically at a certain angle, which can usually solve the problem. If the problem is still not resolved, consider using a polarizing reflector or through-beam detection mode.

Polarization

The combination of polarizing lens and visible light plate sensor can solve the problem of detecting bright objects well. Two polarizing lenses are mounted in front of the transmitter and receiver lenses, with the polarization directions perpendicular to each other.

The transmitted light is polarized by the vertical polarizing lens of the transmitter and becomes a vertically vibrating light wave. This light wave is reflected (depolarized) by a reflective plate and becomes a horizontally vibrating light wave. This light wave can be filtered by the horizontal polarizing lens of the receiver. Although the polarizing lens solves the problem of malfunction when the sensor detects bright objects, it also greatly reduces the energy of the effective beam. This is especially important for applications where the environment is dusty or requires a longer detection distance. Polarizing reflector sensors can only be used with reflectors with geometric prisms.

Beam Alignment-Reflective Plate

In recent years, with the continuous improvement of LED technology, the number of transmitters using visible light sources has gradually increased. When using a visible light source, alignment of the reflector sensor is easy. When we see visible light on the reflector, the light path is basically aligned.

 

If you want to know more, our website has product specifications for fiber optic sensors you can go to ALLICDATA ELECTRONICS LIMITED to get more information