JPH08114424A - Strain detecting method and strain sensor - Google Patents

Abstract

PURPOSE: To provide a strain detecting method and a strain sensor that is compact and excellent in reliability without being affected by electric noises and radiating noises to the periphery. CONSTITUTION: Light wave guide elements 4a and 4b in which light wave guides 2a and 2b are respectively placed on substrates 1a and 1b are stuck on an object wherein its strain is to be measured so as to generate strain in the guide 2, and the change in refractive index that is generated by the photoelastic effect due to the strain is detected on the basis of the change in light wave passing through the guide 2, so that the strain in the object to be measured can be detected. In addition, there are two types of operation for this purpose; a light wave guide is divided into two, a strain receptor and a reference body, and a light wave is made incident into two light wave guides from the end part of one and the combined light waves are exited from the other end part so as to detect a strain. On the other hand, only one strain receptor as a light wave guide is prepared, and a light wave is made incident from one end part through a semi-transmitting film and it is totally reflected by a total reflection film on the other end part, then it is exited from the end part so as to detect a strain.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the detection of distortion of an object, and more particularly to a method for optically detecting the distortion of an object, a distortion sensor for implementing the method, a distortion detecting device and sound pressure using the same. Regarding sensors.

[0002]

2. Description of the Related Art A mechanical, electrical or optical method has been used to detect the distortion of an object. In particular, the strain gauge method is convenient and widely used because it can accurately detect the strain of a target portion at a distant place. Further, there is also known a means using a so-called fiber sensor for detecting the strain by the phenomenon that the refractive index of the optical fiber changes due to the photoelastic effect due to the strain. This method is characterized by not using direct electrical means.

[0003]

However, the electrical means such as the strain gauge is susceptible to electrical noise,
There is a problem that not only spatial noise is taken in through the conductor cable, but also noise is emitted into the space. on the other hand,
For example, when measuring the strain of a crystal having an electro-optical effect used in a strain sensor, the electrical detection means causes a change in the crystal that is an object to be measured due to the electrical signal, noise, etc., which makes measurement difficult. Met. As described above, the electrical detection means may not be preferable in the usage environment, and there is a technical problem that an amplification means is required to complement the attenuation of the signal in the remote measurement. Further, in the case of strain detection using an optical fiber, there is a problem in downsizing because the sensor is composed of an optical fiber coil.

In view of these problems, the technical problem of the present invention is to provide a compact and highly reliable strain detection method and strain sensor which are not affected by electrical noise and emit noise to the surroundings. The purpose is to provide a sound pressure sensor method to which this is applied.

[0005]

According to the present invention, a substrate provided with an optical waveguide is attached to an object to be strained to cause a strain in the optical waveguide, and this strain causes a change in refractive index caused by a photoelastic effect. There is provided a strain detecting method characterized in that a change is detected by a change in light passing through the optical waveguide to detect the strain of the strained object.

Further, according to the present invention, the central part of the optical fiber whose one end is connected to the light source and the other end is connected to the photoelectric converter is divided into two parts, and at least a part of one of the optical fibers is provided with the optical waveguide on the substrate. By replacing the formed optical waveguide element with the optical waveguide element as a strain receptor and the optical waveguide element replaced with the other optical fiber or a part thereof as a reference body, the light passing through the strain receptor and the reference body A branch interference type strain sensor is obtained which is configured so as to combine with the light that has passed therethrough and send it to the photoelectric converter.

Further, according to the present invention, two optical waveguides, which are respectively coupled at both ends in the length direction, are provided on one of the main surfaces of a single substrate having a step in the lateral direction, and are coupled. Further, one end of each of the two optical waveguides is connected to an optical fiber leading to a light source, and the other end is connected to an optical fiber leading to a photoelectric converter, which is formed on the thin side of the substrate among the two optical waveguides. A branch interference type strain sensor is obtained in which the formed optical waveguide is the optical waveguide of the strain receptor and the optical waveguide formed on the thicker side of the substrate is the optical waveguide of the reference body.

Further, according to the present invention, the main surface of the substrate is provided with two optical waveguides which are respectively coupled at both ends in the longitudinal direction, and one end of the coupled optical waveguides is connected to the light source which leads to the light source. The other end is connected to an optical fiber leading to a photoelectric converter, an elastic plate is installed on at least one optical waveguide of the two optical waveguide devices, and the other optical waveguide is installed on the other optical waveguide. , An elastic plate having a thickness different from that of the elastic plate on the one optical waveguide is installed, the optical waveguide on the one side serves as an optical waveguide of a reference body, and the optical waveguide on the other side forms an optical waveguide of a strain receptor. A branch interference type strain sensor having a waveguide is obtained. Here, in the present invention, it is preferable that the elastic plate is made of a non-metallic material, such as an elastic resin, that absorbs less guided light.

Further, according to the present invention, two optical waveguides which are respectively coupled at both ends in the length direction are provided on the main surface of the substrate, and one outer end of the coupled portion of the optical waveguides is provided. An optical fiber leading to a light source is connected, and an optical fiber leading to a photoelectric converter is connected to the other end of the substrate, and thus, in the substrate, an intermediate portion of the two optical waveguides is connected to one of the optical waveguides. It is a substrate that is engraved or removed to the extent that it does not substantially affect the other optical waveguide side even if strain is applied, and any optical waveguide can be used as the optical waveguide of the strain receptor. A branch interference type strain sensor is obtained.

Further, according to the present invention, an optical waveguide having one end in contact with the total reflection film and the other end connected to the optical fiber through the semitransparent film is provided on the substrate, and the optical waveguide is connected to the strain receptor. By changing the intensity of the light incident on the semi-transparent film, reflected by the total reflection film, and emitted again to the semi-transparent film, the strain generated in the optical waveguide is used as an optical waveguide and the substrate is attached to the strain measurement target. As a result, a strain sensor characterized by being configured to detect

Further, according to the present invention, the strain sensor, the light source, the photoelectric converter, the light from the light source is incident on the optical fiber, and the light from the optical fiber is emitted to the photoelectric converter. A strain detection device having a circulator and a measuring device for measuring the output of the photoelectric converter is obtained.

Further, there is provided a sound pressure sensor characterized in that a vibration plate is mechanically coupled to the strain receptor portion of any one of the strain sensors of the present invention. Here, in the present invention, the substrate is preferably a transparent crystal substrate that absorbs little light.

[0013]

The present invention is based on the phenomenon that the refractive index changes due to the photoelastic effect due to the strain generated in the optical waveguide. An interferometer is composed of a branching interference type optical waveguide that combines optical waveguides constructed on individual substrates with optical fibers, or branches into two optical waveguides on the same substrate and then combines them again.
This is called a strain sensor. In such a configuration, when one optical waveguide receives stress and is distorted, the refractive index locally changes and a phase difference occurs in the transmitted light of the two waveguides.
Perceived as a change in light intensity.

In this method, distortion is converted into a change in light intensity, which is detected and transmitted to an optical / electrical converter by an optical fiber, where it is first transmitted as an electric signal to a measuring instrument.
When strain is applied to one waveguide after branching, the strain is transmitted to the substrate and affects the other waveguide as well, causing a problem in sensitivity as a strain sensor. Here, by providing a gap between the waveguides after branching, the strain applied to one side is transmitted through the substrate and hardly transmitted to the other side waveguide, so that it operates as a strain sensor with high sensitivity. Therefore, noise is not transmitted or received in this transmission line system that uses an optical fiber, and it is not affected by the magnetic field, which enables highly reliable detection. The principle of the strain gap can be applied to detection of sound pressure in air or liquid.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing a schematic configuration of a strain inspection apparatus including a strain sensor according to an embodiment of the present invention. As shown in FIG. 1, optical fibers 10 and 11 are connected to two optical waveguide elements 4a and 4b, respectively, on which optical waveguides 2a and 2b are respectively constructed on substrates 1a and 1b, to construct an interferometer. And One of the optical waveguide elements 4a, which serves as a strain receptor, is attached to a measurement target (not shown), and the other optical waveguide element 4a is attached.
b was placed in an environment that was not subjected to strain, and this was used as a reference body. The light wave that is incident on the strain sensor 6 from the light source 12 through the optical fiber 10 is divided into two, and the optical waveguide 2 on the side of one strain receptor is
The light wave entering a passes through with a light intensity corresponding to the strain received by the sensor 6. The other light wave is the optical waveguide 2 on the reference body side.
The light wave entering b passes without changing the intensity. The two light waves are combined in the strain sensor, enter the optical converter 14 via the transmission side optical fiber 11, are measured by the measuring device 16, and are recorded in the recorder 17.

The optical waveguide element 4 serving as a reference body here
b may be replaced with a fiber for practical purposes. At this time, the fiber is branched by a coupler and branched so that the phases are equal and the intensity is 1/2. On the other hand, the fiber coupling is similarly constructed so that upon combining, the strength is twice the strength of the respective inputs.

FIG. 2 is a diagram showing the configuration of the second embodiment, in which the thickness of a glass substrate 1 on which two optical waveguides forming a branching interference type optical waveguide are constructed is roughly divided into two and one is One is thick and the other is thin. The optical waveguides 2a and 2b are the substrate 1
Is formed on the flat side of the substrate, and the entire surface or at least the part with a small thickness is attached to the object to be measured, the optical waveguide of the part with a small thickness is used as a strain receptor, and the part with a large thickness of the substrate is subject to strain. It is difficult to use this as the reference body.

FIG. 3 is a diagram for explaining the principle of the strain sensor using the optical waveguide of FIG. When the incident light 31 before branching as shown in FIG. 3A is adjusted to have an intensity Po and a phase shift of 0 as shown in FIG.
As shown in (a) and (a) of FIG. 3C, the two incident lights 32 and 33 have an intensity Po / 2 with a phase shift of 0.
It becomes the light of. One of the waveguides is distorted, so that the output light 34 is shifted by the phase difference τ corresponding to the applied distortion as shown in (c) of FIG. Becomes Further, the other of the waveguides is not distorted, and the output light does not change its intensity and phase, as shown in (d) of FIG. 3 (d).

Therefore, the output light 36 which is a composite wave of (c) and (d) in FIG. 3 (d) is as shown in FIG. 3 (e).
A phase difference occurs, and an intensity Pe smaller than the intensity Po is detected by a detector (not shown). This intensity is proportional to the magnitude of strain, and therefore the magnitude of strain can be detected from the intensity Pe.

The strain sensor 6 shown in FIG. 4 is similar to the strain sensor shown in FIG. 2 except that the thickness of the substrate 1 made of a translucent resin applied to the two optical waveguides 2a and 2b is different.
The optical waveguides 2a and 2 are formed in different steps due to the difference in the thickness of the substrate 1, and when the entire back surface (or at least the small thickness portion) is attached to the measurement object, the thickness of the substrate is increased. Distortion is less likely to occur in the area where is large, and this serves as a reference body to detect distortion.

A strain sensor 6 shown in FIG. 5 is a branch interference type strain sensor in which optical waveguides 2a and 2b are constructed on a substrate 1 made of a transparent resin having a uniform thickness. The strain sensor 6 has resin films 7a and 7b having different thicknesses added on the two optical waveguides 2 and detects a difference in strain generated between the two optical waveguides. The resin film 7b may be omitted. In this case, it is not particularly limited which side is used as the strain receptor or which side the resin film is omitted.

The strain sensor 6 shown in FIG. 6 has optical waveguides 2a, 2 on a substrate 1 made of lithium niobate single crystal having a uniform thickness.
b is constructed to construct a branch interference type strain sensor. In this strain sensor 6, the middle part of the two optical waveguides of the substrate 1 is removed, one optical waveguide 2a serves as a strain receptor, and the strain generated in this one optical waveguide 2a serves as a reference body. It has a structure that does not directly affect the waveguide 2b.
The back surface of the strain receptor side, which is separated from the portion where the substrate is removed, is attached to the measurement target to detect the strain. Needless to say, which side is the strain receptor is free.

FIG. 7 is a view showing the schematic arrangement of an apparatus of another embodiment of the strain detecting method according to the present invention. As shown in FIG. 7, the strain sensor 6 is a reflective type in this embodiment. The structure of the strain sensor is such that a semi-transmissive film 8 is attached to the incident end face of the optical waveguide 2 constructed on the glass substrate 1 to partially reflect the incident light from the optical fiber 11 and A total reflection film 9 is attached to the end face for total reflection, and the two reflected lights propagate again from the incident end face through the same optical fiber 11. By attaching the strain sensor 6 to the measurement target, the intensity of the reflected light changes depending on the magnitude of strain generated in the optical waveguide. The device uses a light source 12, a polarization-maintaining fiber as an optical fiber 18 in which incident light and reflected light propagate in the same line, and receives a reflected light through a circulator 15, and a photoelectric converter 14
It is composed of a measuring device 16 for measuring and displaying an electric signal from the recording device 17 and a recorder 17 for recording the electric signal.

Based on the configuration of the apparatus shown in FIG. 7, a strain sensor is attached to the outer wall of the high-pressure reaction vessel to detect a minute strain of the reaction vessel due to a high-pressure chemical reaction in an isolated place.
Confirmed that it can be monitored.

FIG. 8 shows a sound pressure sensor 24 constructed by applying a strain sensor to the sound pressure detection in air or in liquid. A cantilever 22 interlocked with the diaphragm 21 is mechanically coupled to the strain sensor 6. The vibration of the diaphragm 21 is transmitted to the strain sensor 6, and information is converted as the intensity of transmitted light or reflected light. Needless to say, a method in which a strain sensor is attached to the diaphragm for detection is also possible.

In the above embodiment, an example in which the lengths of the branched optical paths from the branching to the combining are equal has been described.
It may be composed of different branched optical paths,
In that case, since phase modulation occurs at the time of coupling, the output light intensity is apparently reduced, and bias fluctuation can be introduced.

[0028]

In contrast to the conventional method for detecting the strain of an object by the electrical method, the strain detecting method according to the present invention forms an optical waveguide on a substrate and uses the optical wave as the strain generated in the optical waveguide. Since it is measured by means of electrical measurement, there is no induction of electrical noise through the conductor cable and no noise is emitted to the surroundings. Moreover, it is not affected by the magnetic field. Therefore, this method is combined with the realization of a small sensor that enables safe measurements in flammable or explosive environments, for example.
It is useful as a distortion detection method that can obtain highly reliable results.

[Brief description of drawings]

FIG. 1 is a configuration of an apparatus showing a distortion detection method according to the present invention.

FIG. 2 is a perspective view of a strain sensor including a branch interference type waveguide device.

3A to 3E are diagrams for explaining the measurement principle of the strain sensor of FIG.

FIG. 4 is a perspective view showing another example of the strain sensor using the branch interference type optical waveguide device of the present invention.

FIG. 5 is a perspective view showing still another example of the strain sensor using the branch interference type optical waveguide device of the present invention.

FIG. 6 is a perspective view showing another example of a strain sensor using the branch interference type optical waveguide device of the present invention.

FIG. 7 is a configuration diagram of an apparatus showing a distortion detection method according to the present invention.

FIG. 8 is a sectional view of a sensor according to a sound pressure detecting method.

[Explanation of symbols]

 1, 1a, 1b Substrate 2 Optical waveguide 2a Optical waveguide (strain receptor) 2b Optical waveguide (reference body) 4a Optical waveguide element (strain receptor) 4b Optical waveguide element (reference body) 6 Strain sensor 7a, 7b Resin film 8 Semi-transmissive film 9 Total reflection film 10, 11 Optical fiber 12 Light source 13 Lens 14 Photoelectric converter 15 Circulator 16 Measuring instrument 17 Recorder 21 Vibration plate 22 Cantilever 23 Frame 24 Sound pressure sensor

Claims (8)

[Claims] 1. A substrate provided with an optical waveguide is attached to an object to be strained to generate a strain in the optical waveguide, and a change in the refractive index caused by a photoelastic effect due to the strain is caused by a change in the light passing through the optical waveguide. A strain detection method comprising detecting the strain of the strained object by detecting the change. 2. An optical waveguide element in which an optical fiber whose one end is connected to a light source and the other end is connected to a photoelectric converter is divided into two parts, and at least one of the optical fibers is formed with an optical waveguide on a substrate. By replacing the optical waveguide element as a strain receptor and the optical waveguide element replaced by the other optical fiber or a part thereof as a reference body, the light passing through the strain receptor and the light passing through the reference body are combined. A branch interference type strain sensor, characterized in that the strain sensor is combined and sent to the photoelectric converter. 3. A single substrate having a step in the lateral direction is provided with two optical waveguides which are respectively coupled at both ends in the longitudinal direction on one main surface of the single substrate, and the two optical waveguides are coupled. An optical fiber leading to a light source is connected to one end, an optical fiber leading to a photoelectric converter is connected to the other end, and the optical waveguide formed on the thinner side of the substrate of the two optical waveguides is distorted. A branch interference type strain sensor characterized in that it is used as an optical waveguide of a receptor, and the optical waveguide formed on the thicker side of the substrate is used as a reference optical waveguide. 4. A main surface of a substrate is provided with two optical waveguides which are respectively coupled at both ends in a length direction, one end of the coupled optical waveguides is connected to an optical fiber leading to a light source, and the other is connected to the other. Is connected to an optical fiber leading to a photoelectric converter, an elastic plate is installed on at least one optical waveguide of the two optical waveguide elements, and on the other optical waveguide,
An elastic plate having a thickness different from that of the elastic plate on the one optical waveguide is installed, the optical waveguide on the one side is used as a reference optical waveguide, and the optical waveguide on the other side is used as a strain receiving optical waveguide. A branch interference type strain sensor characterized in that 5. The main surface of the substrate is provided with two optical waveguides that are coupled at both ends in the length direction, and one outer end of the coupled portion of the optical waveguides is provided with an optical fiber leading to a light source. An optical fiber leading to a photoelectric converter is connected to the other end of the substrate, and thus, in the substrate, even if an intermediate portion of the two optical waveguides applies a strain to one of the optical waveguides. It is a substrate that is engraved or removed to the extent that it does not substantially affect the other optical waveguide side, and any optical waveguide can be used as the optical waveguide of the strain receptor. Strain sensor. 6. An optical waveguide, one end of which is in contact with a total reflection film and the other end of which is connected to an optical fiber through a semitransparent film, is provided on the substrate, and the optical waveguide is used as a strain receptor optical waveguide. Is attached to the strain measurement object, and the strain generated in the optical waveguide is detected as a change in the intensity of light that is incident from the semitransparent film, reflected by the total reflection film, and emitted again to the semitransparent film. A strain sensor characterized in that 7. The strain sensor according to claim 7, a light source,
A photoelectric converter, a circulator that allows light from the light source to enter an optical fiber and emits light from the optical fiber to the photoelectric converter, and a measuring device that measures the output of the photoelectric converter. Characteristic strain detection device. 8. A sound pressure sensor comprising a strain sensor according to claim 2, wherein a vibration plate is mechanically coupled to the strain receptor portion.