JPS6217173B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a liquid level detection sensor that optically detects a liquid level inside a tank or the like containing water or oil.

Conventionally, a U-shaped glass sensor as shown in Figure 1 has been used as a sensor for optically detecting the position of water or oil surfaces.

This sensor consists of a glass rod 1 with a diameter of several millimeters bent in the middle into a U shape, through which light propagates as shown by arrow A, and when it is in air or in liquid. By measuring the difference in optical transmission loss between the sensor and the sensor, it is determined whether the liquid level is above or below the sensor position.

In other words, inside this glass rod 1, the light is directed at arrow A.
According to the refractive index of the surrounding material, the light component with a large incident angle θ ₁ at the curved portion 2 jumps out of the glass rod 1, increasing optical transmission loss.

By the way, in such a sensor, the incident angle θ ₁
In order to select such that the optical transmission loss is small,
The bending radius of the glass rod must be set with high precision, and considerable strength is required to maintain the U-shape, making it difficult to miniaturize.

Furthermore, in a sensor with such a structure,
The light component that is reflected back at the curved portion 2 is relatively small compared to the total injected light, and the component that contributes to detection is even smaller than this, so there was also the problem that the detection sensitivity was insufficient.

Furthermore, as the size of the sensor increases, if it comes out on the liquid surface, a large amount of the substance to be detected will adhere to it and remain, which increases the amount of light leaking above the liquid surface and causes further detection. Another problem was that the sensitivity decreased.

The present invention has been made to solve these conventional problems, and is made by aligning the tips of two or more optical fibers, arranging them in substantially the same direction, and fusing the tips to form a spherical part. This provides a highly sensitive and reliable liquid level detection sensor.

The details of the present invention will be explained below with reference to the illustrated embodiments.

FIG. 1 is a vertical sectional view showing the main part of the liquid level detection sensor of the present invention, in which the ends of two parallel optical fibers 3 and 4 are fused and integrated, and the fused part A spherical portion 5 is formed. This optical fiber 3,
4 consists of a core 6 and a cladding 7 covering the core 6, and the optical refractive index of the core 6 is larger than that of the cladding 7.

A sensor with such a configuration consists of aligning the ends of two optical fibers, for example optical fibers made of multi-component glass, and arranging them in approximately the same direction, heating these ends and fusing the ends together. As the heating continues, the fused end gradually deforms into a spherical shape due to its surface tension.

At this time, the heating location is adjusted so that most of the bulb is covered by the cladding, and the cut end surface of the optical fiber before fusion is made oblique to its axis to adjust the fusion state.

In the sensor configured as described above, light injected from one end face is propagated through the following three routes. First, as shown by arrow B ₁ in Figure 2,
There is a route in which the light that has passed through the optical fiber 4 leaks into the cladding 7 at the spherical part 5 and, after being repeatedly reflected on the surface of the spherical part 5, enters the core 6 of the optical fiber 3. Next, as shown by arrow _B2 , there is a route in which the light that has passed through the core of the optical fiber 4 undergoes total internal reflection at the boundary between the core and the cladding in the sphere 5, and then enters the core of the optical fiber 3 as it is. Furthermore, as shown by arrow _B3 , there is a route through which the light propagating through the core of the optical fiber 4 leaks directly to the outside at the spherical portion 5. The light passing through the arrow B ₂ route is the optical power that returns to the output side regardless of external conditions, and the light passing through the arrow B ₁ route determines the sensitivity (light loss amount) of this sensor.

3 and 4 each show a longitudinal sectional view of another embodiment of the invention.

In the embodiment shown in FIG. 3, the two optical fibers 3 and 4 are brought into contact with their tips at an acute angle θ ₂ . At this time, the structure is such that the light (indicated by arrow B ₄ ) that has traveled through the core 6 of the optical fiber 3 in parallel to the axis of the optical fiber 3 is incident on the cladding 7 at exactly the spherical portion 5 . In the embodiment shown in FIG. 4, optical fibers 3 and 4 are arranged closely in parallel so that the distance l between them in FIG. 2 is 0, and their tips are fused in this state. When the optical fibers 3 and 4 are cut along a plane perpendicular to the axes of the optical fibers 3 and 4 at the position of the X-X line connecting the upper end of the spherical part 5, the maximum angle of the spread of the emitted light, that is, the output angle θ ₃ to ball part 5
The spherical portion 5 is formed with a radius of curvature such that the tangent lines of the upper end of the spherical portion 5 are parallel to each other. Further, in the embodiment shown in FIG. 5, the output beam parallel to the axis of the optical fiber is
It is configured to be parallel to a tangent to the line XX connecting the upper ends of.

At the boundary between the spherical portion 5 and the optical fiber 4, the light coming from the optical fiber 4 side passes through three types of routes as described above. Among them, the arrow mark B ₁ shown in Figure 2
The more light there is, the better the detection sensitivity becomes.
The more light equivalent to B ₃ , the more the loss will increase.

Therefore, it can be seen that the radius of curvature of the spherical portion is desirably in a range smaller than that shown in FIG. 4 and larger than that shown in FIG. 5.

According to experiments, when the radius of curvature of the spherical part is smaller than the diameter of the optical fiber, the sensitivity improves but the loss increases, and when the radius of curvature of the spherical part becomes larger than the radius of curvature in the embodiment shown in FIG. The sensitivity was lower and the loss was lower than that of the example shown in FIG.

The liquid level detection sensor of the present invention described above transmits the detection light using an optical fiber and has no connection points, so the transmission loss in the middle is extremely small. Therefore, it is possible to put it into a large tank and perform optical-to-electrical conversion outside the tank. In addition, since it is extremely thin and highly flexible as a whole and can be made compact, it is easy to select the installation location.

In addition, since the spherical part, that is, the detection part, can be made approximately twice the outer diameter of the optical fiber (several 100 μm), it is significantly smaller than conventional ones, and the substance to be detected is affected by surface tension in this part. Difficult to adhere due to twisting. Therefore, the reliability of detection is also improved.

When we actually manufactured this sensor and measured its characteristics, we found that when 300 μW of light is input from one side of the optical fiber, the output when the sensor is in the air is -6.5 dB. −
Output decreased to 25.5dB.

It has become clear that approximately 20% of the total input light is involved in detection.

In the above embodiments, the optical fiber portion is not coated, but the optical fiber portion may be provided with a protective coating as a well-known single-core or multi-core optical fiber cable structure, or may be coated as necessary. Align the tension member accordingly,
From the viewpoint of mechanical strength, it is desirable to have a structure in which the optical fiber is exposed only in the detection section. Additionally, if the cladding of the optical fiber is coated with a silicone resin that has a refractive index lower than that of the cladding,
It is possible to reduce the loss caused by cladding mode leakage in areas other than the sphere.

[Brief explanation of the drawing]

Figure 1 is a vertical cross-sectional view of a conventional liquid level detection sensor.
2 to 5 are longitudinal cross-sectional views of embodiments of the present invention, respectively. 3, 4... Optical fiber, 5... Sphere, 6... Core, 7
…Cratsud.

Claims (1)

[Scope of Claims] 1. A plurality of optical fibers each having a core and a cladding are arranged in substantially the same direction with their ends aligned, and the respective ends are separated and integrated to form a spherical part. A liquid level detection sensor featuring: 2. The liquid level detection sensor according to claim 1, wherein most of the spherical portion is covered with a cladding.