JP2003014561A - Strain sensor and strain sensing unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low cost strain sensor capable of directly sensing the position of a strain measuring point without necessity of temperature correction, and to provide a strain sensing unit. SOLUTION: The strain sensor is provided with an optical fiber in which grating is written in a core, and a sensing section for including the grating. The sensing section is formed so as to transmit an external force applied thereto to the grating. The sensor changes its reflecting light amount when the force is applied to the grating. The strain sensing unit displays distance-reflecting light amount characteristics including a reflecting waveform in each grading on an OTDR by connecting the sensor having a plurality of the gratings having different reflecting beam wavelengths to the OTDR having a wide band light source including the reflecting peak wavelength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a strain sensor and a strain sensing device using an optical fiber grating, and more particularly to a strain sensor and a strain sensing device which do not require temperature compensation or an expensive measuring device.

[0002]

2. Description of the Related Art An optical fiber grating reflects light of a specific wavelength determined by its lattice spacing. Therefore,
When this lattice spacing changes due to stretching strain, the specific wavelength (hereinafter referred to as "reflection peak wavelength") also changes.
Heretofore, various strain sensors and strain detection devices have been proposed which utilize this property of an optical fiber grating to detect an external force or a strain generated by the external force.

For example, in Japanese Patent Laid-Open No. 2000-39309, as shown in FIG. 6A, a built-in grating 41 is provided, to which both ends 43 and 43 are fixed in a state in which a measured object 40 is stretched to a certain extent. A sensor optical fiber 42 and a light source 44 connected to one end thereof through an optical fiber 46.
And a deformation inspection device (strain detection device) including a spectrum analyzer 45 connected to the other end through an optical fiber 47. From the light source 44 to the continuous wavelength range λ ₀ ~
The light of λ ₁ is sent through the optical fiber 46, but since the light of the wavelength (reflection peak wavelength) determined by the grating spacing d _g is reflected by the fiber grating 41, the light passing through the sensor optical fiber 42 is converted into the optical fiber. When guided to the spectrum analyzer 45 via 47, the absorption spectrum of the reflected wavelength component is obtained.

If the object 40 to be measured is expanded, the sensor optical fiber 42 is also expanded, the grating spacing d _g of the grating 41 incorporated therein also increases, and the absorption spectrum becomes long as shown in FIG. 6 (b). Shift to the wavelength side. On the other hand, when the measured object 40 contracts, the amount of expansion of the sensor optical fiber 42 decreases and the grating spacing d _{g of the} grating incorporated therein also decreases, so the absorption spectrum is
As shown in FIG. 6C, the wavelength shifts to the short wavelength side. Therefore, if these wavelength shift amounts are measured by the spectrum analyzer 45, the expansion / contraction strain of the measured object 40 can be known.

However, the highly accurate spectrum analyzer used in the above deformation inspection apparatus is very expensive.
In addition, the degree of extension also changes with changes in temperature, so
The reflection peak wavelength shown by the absorption spectrum obtained from the spectrum analyzer is a composite of the change in the degree of expansion of the DUT and the change in the extension due to temperature changes. I couldn't know the exact change of things.

The above temperature correction can be performed by placing a thermometer near the measurement point to measure the temperature and using the coefficient of linear expansion of the object to be measured and the coefficient of linear expansion of the optical fiber, but it is troublesome in itself. Besides, if there are multiple measurement points,
There is a problem in that it is necessary to identify the position of the measurement point and measure the temperature at each point, which is more troublesome.

For example, in Japanese Unexamined Patent Publication No. 2000-258190, an optical fiber core wire having a plurality of fiber gratings having different reflection peak wavelengths is used, the position of the inserted fiber grating is identified from the reflection peak wavelength, and measurement reflection is performed. A method for detecting the strain of the fiber grating from the deviation of the wavelength from the reflection peak wavelength of the fiber grating is disclosed. However, in this method, it is necessary to refer to a comparison table of the reflection peak wavelength assigned to each fiber grating at the beginning and the insertion position (for example, the distance from the start end of the optical fiber), and it is possible to know the position directly. There was a problem that I could not.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems, and it is possible to directly know the position of a strain measuring point without requiring temperature correction, and at a low cost. An object is to provide a strain sensor and a strain detection device.

[0009]

In order to solve the above-mentioned problems, a strain sensor according to a first aspect of the invention is an optical fiber in which a core is covered with a clad and a grating is written in the core, and the optical fiber. And a detection section including the grating section therein, the detection section being formed so as to transmit an external force applied thereto to the grating section.

With this configuration, the external force generated by the strain, deformation, or other causes of the object to be measured is transmitted to the grating part via the detection part, and the strain generated in the grating part by the external force is applied to the grating part. Since the amount of reflected light at is changed, it can be detected by an inexpensive measuring device. Also, this reflected light amount is not affected by temperature,
There is no need to perform temperature correction.

According to a second aspect of the present invention, in the strain sensor according to the first aspect, an optical fiber core wire having a coating layer and an outer coating layer on an outer peripheral surface of the cladding of the optical fiber, and a detecting section are provided. The detecting unit removes the outer coating layer and the coating layer on the grating portion of the optical fiber core, and the outer periphery thereof is covered with bridges that are coated so as to bridge the outer coating layers on both sides that are not removed or the coating layers. It is characterized by having a tangling member.

Here, "covering the outer coating layers on both sides or coating layers so as to bridge them" means that both ends of the bridging member are fixed to the outer coating layers on both sides or the outer periphery of the coating layer by adhesion or the like. The other parts are usually coated so as not to contact the optical fiber.

With this structure, the detecting portion for transmitting the external force to the grating portion can be easily formed.

According to a third aspect of the present invention, in the strain sensor according to the second aspect, the detecting portion includes the bridging member and an assisting member fixed to a substantially central portion of the bridging member. And

With this configuration, the external force is more concentrated and applied to the portion where the assisting member is provided than the other portions, so that the sensitivity is improved.

According to a fourth aspect of the present invention, in the strain sensor according to the third aspect, the bridging member is made of an elastic soft member, and the assisting member is made of an elastic ring-shaped member.

Here, the "elastic / soft member" is deformed to the extent that it contacts the optical fiber when an external force is applied thereto,
It means a member that has elasticity to return to its original state when an external force is removed, and that has a softness that relaxes the local concentration of the external force to the extent that the optical fiber is not damaged when it comes into contact with the optical fiber.

With this arrangement, the grating portion is not damaged and can be repeatedly used, and the sensitivity is improved regardless of the radial direction of the optical fiber core wire to which an external force is applied.

According to a fifth aspect of the present invention, in the strain sensor according to any one of the first to fourth aspects, the grating portion and the detection portion of the optical fiber core wire are arranged in a length direction of the optical fiber core wire. Therefore, it is characterized in that it is provided at a plurality of locations, and the reflection peak wavelengths of the grating portions provided at the plurality of locations are different from each other.

In this way, there is no risk of multiple reflection at each grating, and it becomes easy to identify the grating that has received an external force.

The invention according to claim 6 comprises the strain sensor according to claim 5 and an OTDR having a broadband light source including reflection peak wavelengths of a plurality of grating portions of the strain sensor, and an optical fiber core of the strain sensor. Connect one end of the line to the OT
By connecting to the DR and measuring a distance-reflected light amount characteristic of the strain sensor, a strain due to an external force applied to the grating unit via the detection unit is detected.

In this way, the amount of reflected light in the fiber grating can be measured with an inexpensive OTDR, and the strain caused by an external force can be detected. Further, a grating section and a detecting section are provided at a plurality of locations along the length direction of the optical fiber core, and the strain sensor and the plurality of grating sections in which the reflection peak wavelengths of the grating sections respectively provided are different from each other Since the OTDR having a broadband light source including the reflection peak wavelength of is used, the OTDR displays the distance-reflected light amount characteristic including the reflection waveform at each grating portion, and the position of the grating to which the external force is applied is displayed at a glance from the display. Can be detected.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described based on illustrated examples. FIG. 1 is a longitudinal sectional view showing an example of the strain sensor of the present invention, FIG. 2 is a plan view of the same, FIG. 3 is an explanatory view of its operating principle, and FIG. 4 is an example of the strain sensing device of the present invention and its use. FIG. 5 is a block diagram showing a state, and FIG. 5 is an explanatory diagram of the operation principle.

In FIGS. 1 and 2, the strain sensor 1 is composed of an optical fiber core wire 2 and a detector 10. The optical fiber core wire 2 is provided with a coating layer 6 made of an ultraviolet curable resin or the like on an optical fiber 5 made up of a core 3 and a clad 4 surrounding the core to form an optical fiber element wire, and further coated with nylon, PVC. An outer coating layer 7 such as a jacket is provided, and a grating G is formed on the core 3.

As the optical fiber, for example, a single mode fiber for communication is used, and the grating G is formed as follows. The outer coating layer 7 and the coating layer 6 including the portion where the grating of the core 3 of the optical fiber core wire 2 is to be written are removed to expose the optical fiber 5. The exposed optical fiber 5 is irradiated with ultraviolet rays having a wavelength of about 240 nm through a phase mask (not shown). The ultraviolet rays diffracted by the phase mask generate interference fringes, pass through the clad 4, and irradiate the core 3 doped with GeO ₂ or the like. The irradiated portion of the core 3 has a strong refractive index due to a strong interference fringe portion. Therefore, core 3
The fringes of the high refractive index portion are formed along the length direction of, and the fiber grating G is formed. When the coating layer 6 is made of a UV transmissive UV curable resin, only the outer coating layer 7 is removed and the optical fiber strand having the coating layer 6 made of the UV transmissive UV curable resin is irradiated with UV rays. The fiber grating G can be formed in the core through the coating layer 5 and the cladding layer 4.

The detecting section 10 covers the outer coating layer / coating layer removing section 9 of the optical fiber core wire 2 (hereinafter referred to as "outer coating layer removing section") 9 and bridges the outer coating layers that have not been removed. Thus, the bridging member 11 is formed by coating.
Although the detection unit 10 is formed of at least the outer coating layer removal unit 9 and the bridging member 11, it is preferable to provide the assisting member 12 to concentrate the external force locally because the sensitivity is improved.

If the bridging member 11 is an elastic soft member having elasticity, it is soft and does not damage the grating portion 8 of the optical fiber. Since it has elasticity, it is in the original state when removed even when an external force is applied. Since it returns to, it can be used repeatedly. The assisting member 12 is provided so as to sandwich the central portion of the bridging member 11. The outer periphery of the end of the outer coating layer 7, the inner surface of the end of the bridging member 11, the central portion of the bridging member 11 and the assisting member 12 are fixed by an adhesive or another appropriate method.

The material of the elastic and soft member 11 is preferably polyethylene, but not limited to polyethylene, a plastic material such as soft polypropylene can be applied. The shape can be appropriately selected such as a tube shape or a sheet shape.

The assisting member 12 is a bridging member (elastic soft member) made of a metal elongated columnar body such as a wire having elasticity.
It is fixed to the central portion of the bridging member (elastic / soft member) 11 such that it is wound around the central portion of 11 in a ring shape or sandwiched by two arc-shaped metal columnar bodies. As a method of fixing the assisting member 12, the bridging member (elastic / soft member) 11 is sandwiched by the elastic force of the assisting member 12, or a groove is provided on the outer periphery of the bridging member (elastic / soft member) 11 to support the assisting member 12. It is appropriately selected whether to fit it or to bond the assisting member 12 to the bridging member (elastic / soft member) 11. The material is not limited to metal, and may be plastic, and the shape is not limited to a ring shape or an arc shape, and any shape may be used as long as it concentrates an external force locally. Further, the cross-section of the columnar body is not limited to a circular shape, and a semicircular shape, a triangular shape, or the like can be appropriately selected. If it is not necessary to use the bridge member repeatedly, a bridging member having no elasticity or an assisting member may be used.

In the above example, the outer coating layer removing portion 9 of the detecting portion 10 is formed by removing the outer coating layer 7 and the coating layer 6 by the same length, but the outer coating layer 7 is removed longer. Then, the bridging member 11 may be bridged between the coating layers. In that case, the bridge member 1 with the optical fiber 5
Since the distance from 1 is small, even a smaller external force can be detected. Further, although the bridging member 11 and the assisting member 12 are separate members, they may be integrated so that the external force easily concentrates locally.

Next, the operation of the strain sensor will be described with reference to FIG. FIG. 3A shows, for convenience, only a portion of the optical fiber 5 in which the grating G of the strain sensor 1 is written, and is illustrated.
External force P to the grating portion 8 of the optical fiber 5 via
Shows the state where is added. Then, in that state, when the light Li is incident, the reflected light L is reflected by the grating G.
r is generated. FIG. 3B shows an example of the waveforms of the reflected light Lr ₀ before the external force P is applied and the reflected light Lr ₁ in the state where the external force P is applied. The latter has a lower peak value than the former, and the amount of reflected light corresponding to the area of the waveform also decreases.

If the external force P is applied to the grating portion 8 and some kind of distortion occurs, the amount of reflected light is reduced. Therefore, by monitoring the reflection characteristic of the strain sensor 1 and detecting the decrease in the amount of reflected light, the external force is generated. It is possible to detect that distortion has occurred.

FIG. 4 is a block diagram showing a strain sensing device of the present invention, a strain sensor used for the strain sensing device, and an example of use thereof. In FIG. 4, the strain detection device 30 includes a strain sensor 20 and an OTDR (Optical Time Domain Reflectometer) 31. The strain sensor 20 includes a plurality of grating portions 22 (22a, 22b, 22c) and a detecting portion 23 (23a, 23b, 23c) provided on an optical fiber 21. Then, the grating portions 22a, 22b, 22
c has different reflection peak wavelengths, for example, the reflection peak wavelengths are λa, λb, and λc, respectively.

The OTDR 31 comprises a broadband light source 32, a light receiver 33, a directional coupler 34, a display means 35, and a synchronizing pulse generator 36. As the broadband light source 32, for example, a super luminescence diode or the like is used, and as the light receiver, for example, an avalanche photo diode can be used.

The configuration of the strain measuring device 30 will be further described along with its usage example. The strain sensor 20 is embedded in the road R, which is the measurement target, and the OTDR 31 is installed in the monitoring center. OTDR31 pulse generator 36
When the pulse signal is transmitted from the broadband light source 32 to the broadband light source 32, the pulsed laser light Li (having the wavelength of the grating portion 22
a, 22b, 22c reflection peak wavelengths λa, λb, λc
Is generated and is incident on the optical fiber core wire 21 through the directional coupler 34. First, the incident light Li is reflected light L having a reflection peak wavelength λa in the grating portion 22a.
ra is reflected. Of the light of the band component transmitted through the grating portion 22a, the reflected light Lrb having the reflection peak wavelength λb is reflected by the grating portion 22b. Further, of the light of the band component that has also transmitted through the grating portion 22b, the reflected light Lrc having the reflection peak wavelength λc is reflected by the grating portion 22c. Reflection peak wavelength λ
Since a, λb, and λc are different from each other, there is no possibility of causing multiple reflection.

The reflected lights Lra, Lrb, Lrc are received by the light receiver 33 through the directional coupler 34. Light receiver 33
Includes a light-receiving element and an amplifier. The light received by the light receiver 33 is sent to the display means 35. Display means 35
Is, for example, an A / D converter, an averaging processing circuit, a display circuit,
The A / D converter includes a display and the like, and operates in synchronization with the pulse signal sent from the pulse generator 36. Then, the light receiving signal is sampled and converted into digital data at a constant period after a lapse of a constant delay time from the pulse signal. First the pulse generator 3
6, a pulse signal is sent to the broadband light source 32, and the delay time from when the pulsed laser light Li is incident on the optical fiber core wire 21 until each reflected light Lra, Lrb, Lrc is received by the light receiver 33. Corresponds to the distance from the starting end of the optical fiber core 21 to each of the grating portions 22a, 22b, 22c. Therefore, the sampling data for each delay time corresponding to each distance is averaged by the averaging processing circuit and displayed on the display via the display circuit.

FIG. 5 is a characteristic diagram showing the relationship between the distance displayed on the display means 35 of the strain sensing device 30 and the amount of reflected light. FIG. 5A is a distance-reflected light amount characteristic diagram when no external force is applied to the road R that is the measurement target and there is no distortion, and FIG. 5B is as shown in FIG. FIG. 6 is a distance-reflected light amount characteristic diagram in a state where the truck T is traveling I on the road R and an external force P is applied on the grating portion 22b.

The optical fiber can be used in places other than the grating part
Since there is a loss due to Rayleigh scattered light in the Iba core,
Even in the case of deviation, the amount of reflected light increases as the distance from the start end increases.
It tends to decrease. In addition, the grating portions 22a, 22
At b and 22c, the amount of reflected light is naturally large. Figure
In FIG. 5 (a), the reflection waveform La is generated in any grating part.
₀, Lb₀, Lc₀The heights of the
Then, the external force P is applied to the grating portion 22b.
Therefore, the reflection waveform Lb₁The height of the reflected waveform Lb ₀compared to
You can see that it is getting smaller. No external force is applied
Reflection waveform La of the grating₁, Lc₁The height of
The reflection waveform La of FIG.₀, Lc₀Same as
is there.

As described above, the strain detector 30 can measure the amount of reflected light in the fiber grating portion 22 with the inexpensive OTDR 31, and can detect the external force P and the strain generated in the grating portion by the external force P. Further, the grating section 22 and the detecting section 23 are provided at a plurality of positions along the length direction of the optical fiber core wire 21, and the fiber gratings of the grating sections 22a, 22b, and 22c respectively provided are light reflection peak wavelengths. λa, λb, λ
Since the OTDR 31 having the strain sensor 20 and the broadband light source 32 having different c values is used,
A distance-reflected light amount characteristic including a reflection waveform at each of the grating portions 22a, 22b, and 22c is displayed at 31, and the position of the grating portion to which an external force is applied can be detected at a glance from the display.

[0040]

As described above, according to the first aspect of the present invention, the detecting portion is formed so as to include the grating portion therein and to transmit the external force applied thereto to the grating portion. The strain generated in the grating part by the external force transmitted to the grating part through the detecting part due to distortion, deformation, etc. changes the reflected light amount in the grating part, so it can be detected with an inexpensive measuring device that can measure the reflected light amount. it can. Further, since the reflected light amount is not affected by the temperature, it is not necessary to correct the temperature.

According to the second aspect of the invention, in addition to the effect of the first aspect of the invention, the detecting section removes the outer coating layer and the coating layer on the grating section of the optical fiber core wire, and the outer periphery thereof. In addition, since it is formed by the bridging member that covers the outer coating layers on both sides which are not removed or the coating layers are bridged, it can be easily formed.

According to the invention described in claim 3, in addition to the effect of the invention described in claim 2, since the detecting portion has a bridging member and an assisting member fixed to a substantially central portion of the bridging member, the assisting member is assisted. Since the external force is more concentrated and applied to the place where the member is provided than the other parts, the sensitivity is improved.

According to the invention of claim 4, in addition to the effect of the invention of claim 3, since the bridging member is made of an elastic soft member and the assisting member is made of an elastic ring member,
There is no risk of damaging the grating portion, it can be used repeatedly, and even if an external force is applied from any direction in the radial direction of the optical fiber core wire, it can be detected with uniform sensitivity.

According to the invention described in claim 5, in addition to the effect of the invention described in any one of claims 1 to 4, the grating portion and the detection portion of the optical fiber core wire have a length of the optical fiber core wire. It is made up of multiple locations along the direction, and the reflection peak wavelengths of the grating sections provided at the multiple locations are different, so there is no risk of multiple reflection at each grating section, and it is easy to identify the grating that has received external force. .

According to the invention described in claim 6, an OTDR having a broadband light source including reflection peak wavelengths of the strain sensor of claim 5 and a plurality of grating portions of the strain sensor.
The external force applied to the grating portion can be detected by measuring the distance-reflected light amount characteristic of the strain sensor with an inexpensive OTDR. Further, a grating section and a detecting section are provided at a plurality of locations along the length direction of the optical fiber core, and the strain sensor and the plurality of grating sections in which the reflection peak wavelengths of the grating sections respectively provided are different from each other Since an OTDR having a broadband light source including the reflection peak wavelength of is used, the OTDR displays the distance-reflected light amount characteristic including the reflection waveform at each grating part, and the position of the grating part to which an external force is applied is displayed at a glance from the display. Can be detected.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing an example of a strain sensor of the present invention.

FIG. 2 is a plan view showing an example of a strain sensor of the present invention.

FIG. 3 is an explanatory diagram of an operation principle of the strain sensor of the present invention.

FIG. 4 is a block diagram showing an example of a strain sensing device of the present invention and a usage state thereof.

FIG. 5 is an explanatory diagram of the operation principle of the strain sensing device of the present invention.

FIG. 6 is an explanatory diagram showing an example of a conventional strain sensing device.

[Explanation of symbols]

1,20 strain sensor 2,21 Optical fiber core 3 core 4 clad 5 optical fiber 6 coating layers 7 External coating layer 8,22 Grating part 9 Cover layer removal section 10,23 Detector 11 Bridge member 12 Assisting member 30 strain detector 31 OTDR G fiber grating

Claims (6)

[Claims] 1. An optical fiber having a core coated with a clad and having a grating written in the core, and a detection unit including the grating unit of the optical fiber, wherein the detection unit detects an external force applied thereto. A strain sensor formed so as to be transmitted to the grating portion. 2. The strain sensor according to claim 1, further comprising: an optical fiber core wire provided with a coating layer and an outer coating layer on an outer peripheral surface of the clad of the optical fiber; and a detection unit, wherein the detection unit is The outer coating layer and the coating layer on the grating portion of the optical fiber core are removed, and the outer periphery thereof has a bridging member coated so as to bridge the outer coating layers on both sides which are not removed or the coating layers. Characteristic strain sensor. 3. The strain sensor according to claim 2, wherein the detection unit includes the bridging member and an assisting member fixed to a substantially central portion of the bridging member. 4. The strain sensor according to claim 3, wherein the bridging member is made of an elastic soft member, and the assisting member is made of an elastic ring member. 5. The strain sensor according to claim 1, wherein the grating section and the detecting section of the optical fiber core wire are provided at a plurality of positions along a length direction of the optical fiber core wire. The strain sensor is characterized in that the fiber gratings of the grating portions provided at the plurality of locations have different reflection peak wavelengths. 6. The strain sensor according to claim 5, and an OTDR having a broadband light source including reflection peak wavelengths of a plurality of grating portions of the strain sensor, wherein one end of an optical fiber core wire of the strain sensor is connected to the strain sensor. OTDR
The strain sensing device is characterized by detecting the strain due to an external force applied to the grating unit via the sensing unit by measuring the distance-reflected light amount characteristic of the strain sensor.