JPH09152308A - Displacement sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a displacement sensor which is not affected by a vibration from the outside and which can detect the feeble displacement of an optical fiber. SOLUTION: In a sensor, a laser light source 2 is installed at one end of an optical filber 1 in which the diameter of a core part is at ▵a (mm), and a light-receiving body 3 comprising a light-receiving part 3A whose diameter is at 10Lλ/▵a (mm) or lower [where λ represents the wavelength (nm) of the laser light source] is installed in a position at a distance of L (mm) from the other end. At this time, the optical fiber 1 is used as a multimode optical fiber, and it is preferable that a single-mode optical fiber 5 is interposed between the laser light source 2 and the optical fiber 1 and/or that an optical fiber 6 is interposed between the light-receiving part 3A and the light-receiving body 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a displacement sensor that can be used as a safety device for an industrial machine, a seismograph, a hydrophone, an acceleration sensor, or the like.

[0002]

2. Description of the Related Art Conventionally, a light source is provided at one end of an optical fiber,
An optical fiber type displacement sensor having a light receiving body at the other end is known.

This method detects the presence or absence of displacement of the optical fiber by utilizing the fact that the light transmittance changes due to the bending of the optical fiber due to an external force.

However, in this case, there is a problem in sensitivity that the presence or absence of the displacement cannot be detected unless the optical fiber is bent considerably, and the minute displacement cannot be detected.

Further, if the optical fiber is bent too much, the optical fiber may be damaged.

[0006]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art and to provide a displacement sensor capable of detecting a weak displacement of an optical fiber.

[0007]

As a result of intensive studies to solve the above problems, the present inventor considered applying a speckle pattern by a laser light source to an optical fiber, and completed the present invention. I arrived.

That is, according to the present invention, (1) a laser light source is provided at one end of an optical fiber having a core portion and a clad portion, and the diameter of the core portion is Δa (mm), and L is provided from the other end of the optical fiber. (Mm) at a distance of 10 Lλ / Δa
(Mm) (λ indicates the wavelength (nm) of the laser light source) Displacement sensor having a light receiving portion having the following length, (2) A multimode optical fiber as the optical fiber, and a laser light source A single mode optical fiber is interposed between the multimode optical fibers, and (3) the displacement sensor according to the above (1), and (3) the optical fiber is interposed between the light receiving portion and the light receiving body. The displacement sensor according to the above (1) or (2) is characterized.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1 is a schematic side view showing an example of the displacement sensor of the present invention.

In FIG. 1, 1 is an optical fiber, 2 is a laser light source, 3 is a light receiving body such as a photodiode or phototransistor, 3A is a light receiving portion of the light receiving body 3, and 4 is a light shielding plate.

The present invention utilizes development in which the speckle pattern due to laser light changes in response to fluctuations in the optical fiber. Here, the speckle pattern means a speckled pattern generated as a result of superposition of wavefronts having random phases due to scattering of laser light.

Regarding the speckle pattern by laser light, see J. C. Danity: Laser S
peckle and Related Phenom
en, a Second Edition (Sprin
ger Verlay, Berlin, 1984).

The core diameter Δa of the optical fiber 1 is the wavelength λ of the light source.
It is known that when the size is sufficiently larger, the size of the spot becomes approximately Lλ / Δa, where L is the distance from the optical fiber end face (outgoing light side) to the light receiver 3.

Here, when the optical fiber 1 is displaced, the optical path of the light transmitted through the optical fiber 1 changes, so that the speckle pattern changes.

Therefore, the size of the light-receiving portion 3A of the light-receiving body 3 is set to be 10 times or less the size of the speckle pattern spots, so that the fluctuation of the optical fiber can be detected.

In the present invention, the light receiving portion 3A of the light receiving body 3
It is necessary that the diameter of 10 Lλ / Δa (mm) or less. Here, Δa (mm) is the diameter of the core part of the optical fiber 1, L (mm) is the distance from the end of the optical fiber 1 on the side opposite to the laser light source 2 installation part, and λ (n
m) indicates the wavelength of the laser light source.

The diameter of the light receiving portion 3A is 10 Lλ / Δa (m
When it exceeds m), the displacement of the optical fiber cannot be detected. Further, the sensitivity gradually decreases as the diameter of the light receiving portion 3A becomes smaller, and measurement becomes impossible at λ (mm) or less.

As for the diameter of the light receiving portion 3A, the light receiving surface of the light receiving body 3 itself may be 10 Lλ / Δa (mm) or less.
As shown in, a light shielding plate 4 or a mask having an opening with a diameter of 10 Lλ / Δa (mm) or less may be provided.

The optical fiber 1 is composed of a core and a clad having a refractive index lower than that of the core. The material of the core and the clad is not particularly limited as long as it has elasticity enough to be displaced with respect to an external force, but in order to appropriately bend the optical fiber when an external force is applied to the optical fiber, It is preferable that the core is made of a silicone resin or an acrylic resin, and the clad is made of a silicone resin, a fluororesin, or an elastomer having a refractive index lower than that of the core material. The outer diameter of the optical fiber is 0.5 (m
m) to 5 (mm) is suitable.

The laser light source 2 is, for example, HeN.
An e laser, a semiconductor laser, or the like can be used, the output is about 1 (mW) to 100 (mW), and the wavelength is 100 to
About 2000 (nm) is suitable.

In the above displacement sensor, vibrations and displacements of the laser light source 2 and the light receiving body 3 which are received from the outside affect the optical fiber 1 and the light receiving portion 3A, and are likely to cause disturbance.

Therefore, as shown in FIG.
Is a multi-mode optical fiber, and a single-mode optical fiber 5 is interposed between the laser light source 2 and the multi-mode optical fiber 1, so that vibrations and displacements received from the outside at the laser light source 2 portion are caused by the multi-mode optical fiber 1.
Can be prevented.

Here, the single mode optical fiber is
A fiber that has only one propagation mode that can be transmitted at a used wavelength, and a multimode optical fiber is a fiber that can propagate a plurality of modes at a used wavelength.

That is, in the single mode optical fiber 5, only light of a certain one phase is transmitted, so that the laser light source 2 is connected to the single mode optical fiber 5, for example.
Since the speckle pattern does not change even when vibration is applied, the optical fiber 1 is not affected by the vibration.

Further, since the optical fiber 1 is composed of a multimode optical fiber, the speckle pattern changes even with a slight displacement, and the change can be detected. That is, the combination of the single-mode optical fiber 5 and the multi-mode optical fiber 1 makes it possible to detect vibrations and minute displacements.

Further, as shown in FIG. 2, by interposing the optical fiber 6 between the light receiving portion 3A and the light receiving body 3, the vibration and displacement received from the outside at the light receiving portion 3 are received by the light receiving portion 3A.
Can be prevented.

Here, the optical fiber 6 may be either a single mode optical fiber or a multimode optical fiber, and when the single mode optical fiber is used, the light shielding plate 4 becomes unnecessary.

In FIG. 2, numeral 7 is a dark box, which is unnecessary when the laser light is modulated. Both the single mode optical fiber 5 and the optical fiber 6 are preferably used, but only one of them may be used.

[0029]

Embodiment (First Embodiment) In FIG. 1, a semiconductor laser is used as a laser light source 2, an output is 15 (mW), and a wavelength is 780 (nm).
It was used. Further, as the optical fiber 1, an optical fiber whose core has a polymethylmethacrylate (PMMA) clad and whose outer diameter is 1.0 (mm) and which is made of fluororesin is used. Further, the position of the light shielding plate 4 having the light receiving portion 3A having a diameter of 0.3 mm is installed at a position 5 (cm) away from the output of the optical fiber 1, and a silicone photodiode is installed as a light receiving member 3 just behind this, A displacement sensor was created.

When the optical fiber 1 was displaced using this displacement sensor, an optical output signal as shown in FIG. 2A was obtained. Further, as a result of binarizing this signal by a known signal processing circuit such as a comparison circuit, a binarized signal shown in FIG. 2B was obtained. The displacement amount can be obtained by counting the number of pulses of this binarized signal.

Further, if the binarized signal is processed by an appropriate signal processing circuit, velocity and acceleration can also be obtained. Therefore, the present invention can be applied to an acceleration sensor as shown in the third embodiment below or a seismograph as shown in the fourth embodiment.

(Second Embodiment) Next, a second embodiment according to the present invention will be described with reference to FIG. In FIG. 3, a HeNe laser, an output of 10 (mW), and a wavelength of 780 (nm) was used as the laser light source 2. Further, as the optical fiber 1, an optical fiber whose core has a polymethacrylate (PMMA) clad and whose outer diameter is 0.5 (mm) and which is made of fluororesin is used.

Further, a light shielding plate 4 having a light receiving portion 3A having a diameter of 30 μm is installed at a position 5 (cm) away from the output of the optical fiber 1, a silicone photodiode is installed as the light receiving body 3, and a single mode optical fiber 5 is installed. A displacement sensor was prepared using a silica glass fiber having an outer diameter of 0.5 (mm) and a plastic optical fiber having an outer diameter of 0.5 (mm) as the optical fiber 6.

(Third Embodiment) This embodiment is an embodiment as an acceleration sensor, as shown in FIG. 4, and in addition to the embodiment of FIG. 1, an amplifier by an operational amplifier, a comparator, a counter, an arithmetic circuit, A display circuit is added. The operation of the present embodiment will be briefly described. First, the amplifier amplifies the optical output signal of the photoreceptor 3, and the next comparator (comparator) outputs the optical output signal amplified by the amplifier (shown in FIG. 2A). It is converted into a binarized signal (shown in FIG. 2B).

Next, the number of pulses of this binarized signal (digital signal) is counted by a counter for a fixed period to obtain a displacement. Furthermore, the acceleration is obtained by processing the output (displacement value) of the counter with a computing unit.

(Fourth Embodiment) This embodiment is shown in FIGS.
As shown in B, it is an example as a seismograph. In the case of FIG. 5A, the optical fiber is attached on a stretched film (cloth or the like) so as to be movable in only one direction, and in the case of FIG. It was wrapped and installed so that it could be linked to the weight. Here, as the optical fiber 1, a plastic fiber having an outer diameter of 1 (mm) was used.

[0037]

According to the displacement sensor of the present invention, a weak displacement of an optical fiber can be detected by a simple method.

[Brief description of the drawings]

FIG. 1 is a schematic side view showing an example of a displacement sensor of the present invention.

FIG. 2A is a graph showing an optical output signal in the displacement sensor of the present invention.

FIG. 2B is a graph showing a binarized signal in the displacement sensor of the present invention.

FIG. 3 is a schematic side view showing a second embodiment of the displacement sensor of the present invention.

FIG. 4 is a schematic view showing a third embodiment of the displacement sensor of the present invention.

FIG. 5A is a schematic view showing a fourth embodiment of the displacement sensor of the present invention.

FIG. 5B is a schematic view showing a fourth embodiment of the displacement sensor of the present invention.

[Explanation of symbols]

 1 optical fiber 2 laser light source 3 light receiving body 3A light receiving section 4 light-shielding plate

Claims (3)

[Claims] 1. A laser light source is provided at one end of an optical fiber having a core portion and a clad portion, the diameter of the core portion being Δa (mm), and the laser light source is provided at a position L (mm) away from the other end of the optical fiber. A displacement sensor comprising a light-receiving body having a light-receiving portion with a diameter of 10 Lλ / Δa (mm) (λ represents a wavelength (nm) of a laser light source) or less. 2. The displacement sensor according to claim 1, wherein the optical fiber is a multimode optical fiber, and a single mode optical fiber is interposed between the laser light source and the multimode optical fiber. 3. The displacement sensor according to claim 1, wherein an optical fiber is interposed between the light receiving portion and the light receiving body.