JP2001201411A - Optical fiber sensor and optical fiber sensor multiple point measuring system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber sensor, using a small and cheap bending type optical fiber which is hardly affected by temperature and is capable of measuring physical quantities such as strain, force, stress, displacement, etc., and an optical fiber sensor multiple point measuring system, capable of multiple point measurement of strain and the like by using the optical fiber sensors and use the system for detecting danger such as caving-in and collapse in a tunnel. SOLUTION: An optical fiber type strain gauge 1 has a gauge base 10, having on the whole a square shape formed by an H-shape space 13, a first groove 12 and a second groove 14 formed almost linearly without eccentricity, by providing a specific interval G in the H-shape space 13 along the longitudinal direction of the gauge base 10 and an optical fiber 16 arranged in the first and the second grooves 12 and 14 and having a bend part 16A in the interval G. The width dimensions of the first and the second grooves 12 and 14 are formed 3.2 times as large as the outer diameter dimension(d) of the optical fiber.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber type strain gauge utilizing bending loss of an optical fiber.
Force, stress, strain, an optical fiber sensor that performs measurement of physical quantities such as displacement, etc., and performing tunnel multi-point measurement by using a plurality of the optical fiber sensors,
The present invention relates to an optical fiber sensor multi-point measurement system for detecting a danger such as collapse.

[0002]

2. Description of the Related Art Conventionally, an electric resistance type strain gauge has been generally used for measuring a strain or the like of a machine or a structure. In recent years, an optical fiber type strain sensor using an optical fiber has also been proposed and put to practical use.

That is, a sensor function, which is one of the functions provided in intelligent materials / structures, is required to be capable of online measurement for a long period of time. Optical fibers with features such as non-induction, explosion-proof, and corrosion resistance are promising components, and have been put into practical use as optical fiber strain sensors that measure strain using various characteristics of optical fibers. ing.

As a strain sensor using such an optical fiber, for example, a so-called Fabry-Perot type optical fiber type strain gauge (first conventional example), a collection of lectures at the 37th Annual Meeting of the Japan Society of Mechanical Engineers (・) No. 96-1
Published paper "Bending type optical fiber strain gauge" described in Koichi Egawa et al. (Second conventional example), Japanese Patent Laid-Open No. 9-149.
No. 27 gazette (third conventional example), Processe
ding of 19th Meeting on L
lightwave Sensing Technology
gy, May 1997, various strain sensors have been proposed, such as the one described in Toshio Kurashima et al. (fourth conventional example), "Measurement of strain distribution in concrete structures using optical fiber sensors".

Among them, the strain sensor using a so-called bent optical fiber, such as the second conventional example, applies a small bend to the optical fiber and applies a force to the bend to attenuate the transmitted light to reduce the strain. Is detected. That is, the loss of the amount of transmitted light differs depending on the radius of curvature of the optical fiber, and no loss occurs when the radius of curvature is large. For a radius of curvature at which a certain loss occurs, the loss increases as the radius of curvature decreases, and the loss decreases as the radius of curvature increases. Using such a principle, a strain gauge using a bent optical fiber changes the radius of curvature by a force applied to the gauge, and detects strain from a change in the amount of transmitted light.

[0006]

The conventional electric resistance type strain gauge has a problem that it is not suitable for use under the influence of an electromagnetic field. Also, in the case of multi-point measurement, the cable must inevitably become thick, which is inconvenient to handle, and furthermore, problems such as a decrease in insulation are inevitable.
On the other hand, in the strain sensor using the optical fiber described above,
For example, in the case of the first conventional example, high precision is required for machining and assembling the gauge components, but the production of such high precision increases the cost. In addition, a so-called OTDR (Optical Tim)
It cannot be used for multipoint measurement using e-Domain Reflectometry (optical time domain reflection measurement method).

Further, the bending type optical fiber type strain gauge according to the second conventional example uses an optical fiber having a different refractive index difference in an elliptical or quarter-arc bent shape in combination. It is very difficult to accurately form the shape on the gauge base, and no consideration is given to the temperature compensation.

Furthermore, the optical fiber type strain gauge according to the third conventional example is difficult to reduce the size of the gauge, is not suitable for measuring local stress or strain, and cannot compensate for temperature of the gauge alone. There are also problems.

Furthermore, in the fourth prior art,
An average strain of about 1 m in distance can be measured along the optical fiber, but local stress or strain cannot be measured with high sensitivity. In addition, the influence of temperature is about ten times that of an electric resistance strain gauge, so that the influence of temperature cannot be ignored.

An object of the present invention is to provide a compact and inexpensive manufacturing,
An optical fiber sensor that can measure a physical quantity such as strain, force, stress, displacement, etc., which is hardly affected by temperature, using a bent optical fiber, and is capable of measuring strain using a plurality of the optical fiber sensors. The present invention is to provide a multi-point optical fiber sensor measurement system capable of performing multi-point measurement, and the like, and to use the optical fiber sensor multi-point measurement system to detect dangers such as falling or falling down of a tunnel. It is in.

[0011]

In order to achieve the above object, the present invention provides a novel and useful optical fiber sensor and a plurality of optical fiber sensors as a strain gauge or a force sensor using a so-called bent optical fiber. We have invented an optical fiber sensor multi-point measurement system that can perform multi-point measurement using a single device.

That is, according to the first aspect of the present invention, an optical fiber sensor according to the first aspect of the present invention has a substantially straight line without eccentricity at a predetermined interval along one direction of the structure. And first and second grooves formed in the first and second grooves, and at least one of the first and second grooves having a bent portion within the space between the first and second grooves.
And a width dimension of the first and second grooves is formed to be at least twice as large as an outer diameter dimension of the optical fiber.

Since the optical fiber may be disposed so as to have a bent portion in the first and second grooves formed substantially linearly, the bent portion of the optical fiber can be easily formed.
In the case where the same bent shape is provided on the gauge base of the strain gauge, or in the case where it is provided directly on the strain-generating portion of the elastic body of the force sensor, molding can be performed at low cost.

Further, since the width of the first and second grooves is at least twice as large as the outer diameter of the optical fiber, the optical fiber is arranged in such a wide groove. The difference in local properties near the edges of the layer and the groove shape hardly appears in the characteristics.

Further, the structure has a step which is orthogonal to the first and second grooves and forms the interval. In the step, the optical fiber is not fixed but freely movable. May be.

In the optical fiber sensor, the structure is constituted by an optical fiber type strain gauge comprising a gauge base, or directly to a strain-generating portion of an elastic body constituting the structure without attaching a strain gauge. The optical fiber may be constituted by a force sensor or the like formed with a bent portion.

On the other hand, in the optical fiber sensor multipoint measuring system according to the second aspect of the present invention, a plurality of the above optical fiber sensors are arranged at predetermined intervals. Using at least one optical fiber inserted into the plurality of optical fiber sensors to form one optical transmission path as a whole, and performing multipoint measurement of strain by the plurality of optical fiber sensors. I do.

Further, in the optical fiber sensor multipoint measuring system according to the sixth aspect, a plurality of the optical fibers are further connected by using an optical switch.

Thus, multi-point measurement using OTDR is possible. In addition, although a physical quantity to be focused on is measured by a sensor, abnormalities such as large deformation and cutting can be detected in transmission optical fibers arranged before and after the sensor.
Further, for example, it is possible to measure an object such as a tunnel whose measurement range by a sensor extends over several tens of kilometers.

[0020]

Embodiments of the present invention will be described below in detail with reference to the drawings. The optical fiber sensor according to the first embodiment of the present invention includes an optical fiber type strain gauge. That is, as shown in FIG. 1, an optical fiber type strain gauge 1 of the present embodiment includes a gauge base 10 having an H-shaped void 13 formed therein and having a rectangular shape as a whole, and a longitudinal direction of the gauge base 10. A first groove 12 and a second groove 14 which are formed substantially linearly without eccentricity while leaving a predetermined interval G in the H-shaped gap portion 13.
And a gap G disposed in the first and second grooves 12 and 14.
And an optical fiber 16 having a bent portion 16A therein. The width dimension of the first and second grooves 12 and 14 is 3.2 times (3.2d) the outer diameter (diameter) dimension d of the optical fiber.
It is formed in the size of. The first and second grooves 12
And 14 have a width d of the outer diameter (diameter) of the optical fiber.
It is desirable that the size is at least twice (2.0d), preferably 2.5 times (2.5d) or more.

FIG. 2 shows an optical fiber type strain gauge 1 shown in FIG.
FIG. 3 is a side view showing the measurement object together with the measurement object. The gap base 13 extends in the direction perpendicular to the axial direction of the optical fiber 16 in the first and second grooves 12 and 14.
A thin portion 17 located around the gap portion 13 and a thick portion 18 thicker than the thin portion 17 are defined by the step portions 15 formed on both sides of the thin portion 17. On the thin portion 17 side, a welded portion 20 for fixing the gauge base 10 and the measuring object 19 such as a steel material by, for example, spot welding is formed along the step portion 15.

Further, a narrow portion 21 is formed in the thin portion 17 of the gauge base 10. Thereby, the rigidity of the thin portion 17 is reduced, and the gauge base 10 is easily deformed according to the strain of the measurement object 19.

In the present embodiment, the gauge base 10 has a longitudinal dimension of about 20 mm to 30 mm, the thick portion 18 has a thickness of 0.6 mm, and the thin portion 17 has a thickness of 0.2 mm.
mm. In addition, the first groove 12 and the second groove 14
Is 3 mm.

Next, a method of using the optical fiber type strain gauge 1 of the present embodiment will be described.

Optical fiber type strain gauge 1 of the present embodiment
Transmits the strain of the measurement object 19 to the bent portion 16A of the optical fiber 16 provided as a detection unit and detects the distortion. That is,
In order to use the optical fiber type strain gauge 1 of the present embodiment, as shown in FIGS. 1 and 2, the gauge base 10 is fixed to an object 19 to be measured, and one end of the optical fiber 16 is connected to an LED.
The other end is connected to a light source (not shown), and the other end is connected to a light detection side (not shown) such as a PD.

When a strain is generated by applying a stress to the measuring object 19, the gauge base 10 is similarly pulled or compressed. As a result, the gap G between the gaps 13 increases in the case of tension and decreases in the case of compression. The gap G is increased or decreased by increasing or decreasing the gap G between the gaps 13.
3, the amount of bending of the optical fiber 16 changes, causing a change in the amount of transmitted light. In other words, in the case of tension, the radius of curvature of the bent portion 16A increases, and the loss decreases. As a result, the amount of transmitted light increases, whereas in the case of compression, the radius of curvature of the bent portion 16A decreases, so that the loss increases. As a result, the amount of transmitted light decreases. Therefore, this light amount change is
The amount of strain can be measured by detecting with a detecting means including the above.

As described above, according to the optical fiber type strain gauge 1 of the present embodiment, not only the tensile strain but also the compressive strain can be measured, and the displacement of the crack can also be measured. In addition, the radius of curvature of the optical fiber 16 within the gap G of the gap 13 can be changed to change the detection sensitivity according to the type of the measurement object 19. For example, if the measurement object 19 is concrete with small strain, the radius of curvature may be small, and if the measurement object 19 is iron with large strain, the radius of curvature may be large.

Here, in the optical fiber type strain gauge 1 of the present embodiment, as shown in FIGS. 1 and 2, a direction perpendicular to the axial direction of the optical fiber 16 in the first and second grooves 12, 14. The gauge base 10 and the measuring object 19 are fixed by performing spot welding on the welding portion 20 formed on the thin portion 17 side along the step portion 15 formed so as to extend. When the strain gauge 1 and the measuring object 19 are fixed by the fixing force acting along the direction orthogonal to the axial direction of the optical fiber 16 in this manner, the gauge length is reduced by the distance between the fixed portions of the optical fiber 16 in the axial direction. Become. As a result, the tensile stiffness is reduced and the internal force is reduced, so that the shear stress at the portion where the measurement object 19 is fixed is suppressed, and the variation in gauge characteristics is reduced.

Further, since the gauge length is the distance between the fixing portions in the axial direction of the optical fiber 16, the uncertainty of the gauge length in temperature compensation using linear expansion is eliminated.

Further, the first and second grooves 12 and 12 are simply moved along the step portion 15 by, for example, moving a spot welder.
Since the welded portion 20 along the direction orthogonal to the axial direction of the optical fiber 16 in the portion 14 can be welded to the measurement object 19, the work of attaching the strain gauge can be facilitated.

Next, an optical fiber sensor according to a modification of the above embodiment will be described with reference to FIG. The optical fiber sensor 3 of the present modified example attaches the optical fiber type strain gauge 1 of the above-described first embodiment to the strain generating portion 32 of the elastic body 30 by spot welding as shown in FIG.
The elastic body 30 functions as a sensor by being fixed to a measurement object (not shown) using the fixing holes 30A and 30B. As shown in FIG. 3, incident light that has entered the optical fiber 16 from the left side in FIG. 3 is detected as transmitted light in the right direction in FIG. Now, arrows 32L and 32R are added to the strain generating portion 32.
When a tensile load is applied or a displacement occurs in the tensile direction, as shown in FIG.
As the radius of curvature of A (see FIG. 1) increases, the loss decreases, and as a result, the amount of transmitted light increases. Therefore, the amount of distortion can be measured by detecting this change in the amount of light with a detecting means including a PD or the like.

Next, an optical fiber sensor according to a second embodiment of the present invention will be described with reference to FIG. The optical fiber sensor according to the second embodiment is constituted by a force sensor in which a bent portion of an optical fiber is formed directly on a strain-generating portion of an elastic body without attaching a strain gauge. That is, as shown in FIG. 4, the force sensor according to the present embodiment includes a beam-type load cell 40 made of an elastic body and an optical fiber 41 attached to the beam-type load cell 40. Although not shown, the beam-type load cell 40 is fixed to an arbitrary fixing point by a fixing bolt to form a cantilever, and has a load application point on its free end side to which a load to be measured is applied. I have. Therefore, when a load is applied to the load application point from above, the beam-type load cell 40
The upper side is pulled and the lower side is compressed. As shown in FIG. 4, the beam-type load cell 40 has a stepped portion 40A along the width direction from both upper and lower surfaces of the beam-type load cell 40.
40B are formed. Also, the step portions 40A and 40B
Between them, a sensing unit 42 is provided. As shown in FIG. 4, a concave portion 44 is provided on the upper surface side of the sensing section 42 along the width direction of the beam type load cell 40. On the other hand, a recess 46 is also provided on the lower surface side of the sensing unit 42 along the width direction of the beam-type load cell 40. Also, inside the sensing unit 42,
As shown in FIG. 4, a hollow portion 43 is provided, and the hollow portion 43 is formed such that the concave portions 44 and 46 are configured to be thin. That is, here, the concave portions 44 and 46
Are formed by thin portions defined by hollows 43 and respective recesses along the width direction of the upper and lower surfaces of the beam type load cell 40. The recesses 44 and 46
The upper and lower surfaces of the beam-type load cell 40 are formed symmetrically. Therefore, when a load is applied to the above-mentioned load application point from above, the tensile stress concentrates in the concave portion 44, whereas the concave portion 46 , Compressive stress concentrates.

On the other hand, on the upper surface side of the beam type load cell 40, first and second fiber insertion grooves 48A and 48B are formed along the length direction, and the first fiber insertion groove 48A From an input / output terminal (not shown) provided on the fixed end side end surface of the mold load cell 40 to the step portion 40A on the upper surface side of the beam type load cell 40, the second fiber insertion groove 48B is not shown from the step portion 40A. The output end is provided on the upper surface of the beam type load cell 40. Conversely, first and second fiber insertion grooves (not shown) are formed on the lower surface side of the beam-type load cell 40 along the length direction thereof.
The second fiber insertion groove extends from an input / output terminal (not shown) provided on the fixed end side end surface of the beam type load cell 40 to a step portion 41A on the lower surface side of the beam type load cell 40.
It is provided on the lower surface of the beam type load cell 40 from the step portion 41B to the output end (not shown). An optical fiber 41 is inserted into the first and second fiber insertion grooves 48A and 48B on the upper surface side of the beam type load cell 40, and the optical fiber 41 has a bent portion 41a in a step portion 40A. Another optical fiber having the same outer diameter (diameter) as the optical fiber 41 is inserted into the first and second fiber insertion grooves on the lower surface side of the beam type load cell 40. Has a bent portion (not shown) in the step portion 40B.

Here, in the second embodiment of the present invention, the width dimensions of the first and second fiber insertion grooves 48A and 48B on the upper surface side and the first and second fiber insertion grooves on the lower surface side. Is 3.2 times (3.2) the outer diameter (diameter) dimension d of the optical fiber 41 and one other optical fiber (not shown).
d). It is desirable that the width dimension is at least twice (2.0 d), preferably 2.5 times (2.5 d) or more the outer diameter (diameter) dimension d of the optical fiber 50 or the like.

FIGS. 5 and 6 are views for explaining a method of forming the bent portion of the optical fiber in the optical fiber force sensor according to the second embodiment of the present invention, and FIG. 5 is shown in FIG. It is the conceptual diagram which looked at the optical fiber sensor from the upper part.

In order to form the bent portion of the optical fiber in the optical fiber force sensor according to the second embodiment, first, as shown in FIG.
Set at the bottom of Next, a round bar (a round bar fixing jig (not shown) is placed on the strain generating portion) for imparting bending is set. Subsequently, the transmitted light of the optical fiber is detected by the PD (in a state where the optical fiber is on a straight line). Further, according to the setting sensitivity, the round bar is moved along the slide groove of the fixing jig to form a bend (the above operation is performed while monitoring with the PD. The setting sensitivity is determined after the round bar is removed. The change in the set sensitivity due to the springback of the optical fiber is considered in advance). Next, fix the position of the round bar and set the groove (W)
Is filled with an adhesive to fix the optical fiber. After the adhesive is cured, the round bar and the fixing jig are removed (the amount of transmitted light is checked by PD). By this procedure, an elastic curve of an optical fiber having a predetermined curvature is obtained in the sensor using the set groove width of the straight groove, and a bent portion of the bent optical fiber sensor can be realized. In this way, both grooves can be machined without replacement of the workpiece at the time of machining the grooves, and good results have been obtained in the optical fiber type strain cage and the optical fiber force sensor prototyped in terms of characteristics. By using the bent portion of the present invention, an inexpensive optical fiber strain cage and an optical fiber force sensor can be realized.

In the first and second embodiments described above, the width dimension of the (fiber insertion) groove is 3.2 times (3.2 d) the outer diameter (diameter) dimension d of the optical fiber. Is formed. The width dimension of the groove is preferably at least twice (2.0 d), preferably 2.5 times (2.5 d) or more the outer diameter (diameter) dimension d of the optical fiber. In order to clarify the significance of adopting such a groove width, a description will be given in comparison with an example in which the first and second grooves are inclined so that a highly sensitive bent portion can be manufactured at low cost.

FIG. 7 shows an example in which the above-mentioned inclined groove is provided (for example, see Japanese Patent Application Laid-Open No. 11-287626). By having such an inclined groove, bends 16B and 16C are formed in addition to the bend 16A, which has the effect of increasing the sensitivity. On the other hand, since the edge portion of the gap G (step portion) contributes to the sensitivity, a change in output due to instability such as stress relaxation of the adhesive layer near the edge portion is concerned. Therefore, especially in the field of civil engineering measurement where long-term stability is required, it is necessary to keep the aging of the adhesive low, and the selection of the adhesive is an important issue. Further, it is difficult to uniformly control the edge shape after the adhesive is cured.

On the other hand, in the first and second embodiments of the present invention, a bent portion where the influence of the instability of the adhesive layer near the edge portion is relatively small is formed. That is, the width dimension of the groove is at least 2.0 times (2.0d), preferably 2.5 times (2.0.times.) The outer diameter (diameter) dimension d of the optical fiber.
5d) By making the size not less than 5d), the bent portions 16B, 16B
The radius of curvature of C was made large, and a shape having almost no sensitivity was obtained. Note that a similar effect can be obtained even when the above-described inclined groove is provided. However, in the present invention,
A straight groove was adopted because of the workability of processing (both grooves can be machined without changing the work). In addition, by adopting these shapes, it is possible to reduce variations in characteristics between sensors due to non-uniformity of the bonding state near the edge portion.

In order to prove the superiority of the present invention described above, the following analysis method was adopted.

That is, the following is described in Tomo et al., Such as Tomo, Vol. 63, No. 615 (1997-11), "Measurement of Strain by Bending Optical Fiber System", Transactions of the Japan Society of Mechanical Engineers. The light propagation loss α ⁺ (dB / m) per unit length of an optical fiber having a uniform bending curvature is equal to Marcuse.
Is analyzed as follows.

[0042]

(Equation 1)

Here,

[0044]

(Equation 2)

R: radius of curvature a: core radius Δ = ((n1−n2) / n1) n1: core refractive index n2: cladding refractive index λ: wavelength of light u: transverse standardization propagation constant of core w: cladding The following characteristic equation of the optical fiber is given between u and w.

[0046]

(Equation 3)

[0047]

(Equation 4)

J and k are the Bessel function and the modified Bessel function of the second kind, respectively. The total light propagation loss α (dB) when the curvature varies depending on the location of the optical fiber is expressed by the equation α ( ^* ) in the path of the optical fiber. Given by integrating along

[0049]

(Equation 5)

On the other hand, the radius of curvature (R) of the optical fiber can be obtained from the displacement (y) of the fiber according to equation (6).

[0051]

(Equation 6)

Here, the light propagation loss when a wavelength of 1310 nm is used in a single mode fiber is described in the above (1).
It calculated | required using Formula (5).

The radius of curvature of the fiber is G = 3 m.
m, the bent portion shape was analytically determined using the finite element method, and the groove width as a parameter.

FIG. 8 is a graph showing the effect of the bent portion on the transmission loss in the optical fiber sensor according to the first and second embodiments of the present invention. How the ratio of the light propagation loss changes according to the distance to the optical fiber diameter is shown for each groove width ratio to the optical fiber diameter. In the figure, the abscissa axis shows from the edge to the center of the bent portion.

Here, a displacement equivalent to a strain of 1000 × 10 −6 is applied to the gap G, and the ratio of the light propagation loss according to the distance from the groove end to the center with respect to the loss of the entire bend is represented by the light. It was determined for each groove width ratio to the fiber diameter.

As is apparent from FIG.
Are 1.17 (d), 1.33 (d),
In the case of 1.67 (d), the influence of the groove end (edge) portion is large, while 2 (d), 2.33 (d), and 2.67.
In the case of (d) and 3.2 (d), the influence of the groove end (edge) is kept small. Therefore, the optical fiber diameter d
It has been found that it is desirable for the groove width ratio to be at least twice.

FIG. 9 is a graph showing the effect of the bent portion on the transmission loss in the optical fiber sensor according to the first and second embodiments of the present invention. It shows how the ratio of light propagation loss at the groove end changes according to the width ratio. Here, the horizontal axis is plotted by taking the groove width ratio with respect to the optical fiber diameter in order to see the effect of the edge portion.

As is apparent from FIG.
In the case where the groove width ratio to 1.67 (d) is about 2%, the effect of the groove end (edge) is almost 2%.
In the case of 2.33 (d), 2.67 (d), and 3.2 (d), the influence of the groove end (edge) is suppressed to about 1% or less. Therefore, it is further clear from FIG. 9 that it is desirable that the groove width ratio to the optical fiber diameter d be at least twice.

FIG. 10 is a graph in which the sensor sensitivity was examined and plotted on a graph for the case where the groove width ratio to the optical fiber diameter d was 3.2 (d). When the radius of curvature at the center increased, the amount of transmitted light also increased. As the radius of curvature increases and the radius of curvature decreases, the amount of transmitted light also decreases. However, it has been found that good characteristics can be obtained within a practically used range. For the optical fiber having a groove width ratio to the optical fiber diameter d of 2 (d), 2.33 (d), and 2.67 (d), substantially the same characteristics as those of 3.2 (d) are obtained. It is inferred.

Further, the optical fiber sensor according to the first embodiment of the present invention was examined for its sensitivity characteristics as a sensor. FIG. 11 is a graph showing load characteristics when an optical fiber type strain gauge constituting the optical fiber sensor according to the first embodiment of the present invention is attached to a test piece (cantilever). Here, when an optical fiber type strain gauge having a groove width ratio of 3.2 (d) to the optical fiber diameter d shown in FIG. 1 is attached to a test piece (cantilever), the output with respect to a change in load is output. I examined how it changes.

As shown in FIG. 11, in both the tension and the compression, the output monotonously changed with the change in the load, and it was found that sufficient sensitivity was obtained as a sensor.

Next, an optical fiber sensor multipoint measuring system according to a third embodiment of the present invention will be described.

The optical fiber sensor multipoint measuring system of the present embodiment comprises a plurality of optical fiber strain gauges 1 (of which the groove width ratio is 3.2 times the optical fiber diameter) of the first embodiment. A single optical fiber is inserted through the plurality of optical fiber strain gauges 1 at predetermined intervals, and this optical fiber is connected to an OTDR measuring device to connect the plurality of optical fiber strain gauges 1. Is used to perform multi-point measurement of strain.

Now, the multipoint measuring system of the present embodiment
Optical fiber type strain gauges 1a, 1b, 1c,.
1n are arranged at predetermined intervals, and the optical fiber 60 is inserted through the plurality of optical fiber type strain gauges. As shown in FIG.
It is connected to a DR measuring instrument and performs multipoint measurement of strain with a plurality of optical fiber strain gauges.

That is, in the multipoint measuring system of the present embodiment, as shown in FIG. 12, a laser diode (LD) 62 driven by a pulse oscillator 61 outputs an optical pulse, and the optical pulse is a directional coupler. Optical fiber 6 through 63
Incident at 0. The rear Rayleigh scattered light or Fresnel reflected light generated in each of the optical fiber strain gauges 1a, 1b, 1c,..., 1n in the optical fiber 60 returns to the incident end. The light returning to the incident end enters the light receiving element (PD) 64 through the directional coupler 63 and is converted into an electric signal. After the converted electric signal is amplified to a required level by the amplifier 65, it is analyzed in the time domain by the analysis processing unit / display unit 66, and the analysis result is displayed. For example, an optical fiber type strain gauge 1a, 1b, 1c,..., 1n is fixed to a buried pipe or the like as a detecting unit every 1 m, and connected to such an OTDR measuring device to constitute a strain multipoint measuring system. For example, it is possible to simultaneously measure the position where the damage is present and the strain amount.

Further, an extended example of the optical fiber sensor multipoint measuring system according to the third embodiment will be described.

As shown in FIG. 13, the multipoint measuring system according to this extended example has optical fiber strain gauges 1a, 1a.
.., 1n (each having a groove width ratio of 3.2 times the optical fiber diameter) are connected in series to one optical fiber 60 in the form of 1, 2, 3,. , And a plurality of optical fibers 1, 2, 3,..., M are connected using the optical switch 130. The optical switch 130 is an OT
The OTDR measurement device 13 is connected to the DR measurement device 132.
2 is connected to a controller and display 134. When the distance between the sensors is smaller than the resolution of the OTDR measuring device 132, the loop 1 of the optical fiber between the sensors is used.
35 may be provided.

Next, an optical fiber sensor multipoint measuring system according to a fourth embodiment of the present invention will be described.

The optical fiber sensor multi-point measuring system of this embodiment uses the multi-point measuring system shown in FIG. 13 for prior detection of danger such as falling or falling of a tunnel.

As shown in FIG. 14, the optical fiber sensor multipoint measuring system of this embodiment has a single optical fiber 142 (FIBER: 1, FIBER: 2,...).
FIBER: 16), the bending type optical fiber sensor 1 is provided at a maximum of 5 places, and the optical switch module (optical channel selector) 143A is used to switch up to 16 optical fibers. Monitoring is performed using a dedicated line, public line, LAN, etc., and abnormality detection, alarm, and location of the abnormality are automatically performed.

In the detection, the optical loss due to the strain (stress) applied to the bending type optical fiber sensor is determined by the OTDR method (Optic method).
al Time Domain Reflectome
(try: optical time domain reflection measurement method), and the distortion is determined from the optical loss. The measurement and analysis processing time is set to a minimum of about 4 seconds per one place [measurement / analysis time per one optical fiber is set to 15 seconds and channel switching is set to about 1 second].

Here, the minimum installation interval of the sensor on one optical fiber was set to be at least about 12 m. The measurement is performed at predetermined time intervals, and the amount of strain at each sensor position is displayed as a time-series signal (the horizontal axis is date and time). When the strain exceeds a preset level (threshold), an alarm is displayed and the location of the abnormality is notified. Measured data is stored sequentially. OTDR before conversion to strain, if necessary
It also monitors and saves measurement data. In addition, when the optical fiber cable is greatly deformed or broken at a place other than the optical sensor installation location, the location can be specified from the OTDR measurement data before conversion into strain by the present system. When optical connectors are attached to both ends of an optical fiber and switching is performed by an optical switch to perform OTDR measurement from both ends, even if the optical fiber cable is broken, it is possible to measure up to the break point by monitoring from both ends.

For the sensor section 141, the optical fiber type strain gauge 1 of the first embodiment shown in FIG. 1 is used. When bending (bending) is applied to the optical fiber, the light propagation mode of the light wave in the optical fiber changes, the waveguide mode becomes partially a radiation mode, and the light wave is radiated from the inside of the optical fiber to the outside. Loss occurs. The sensor section 141 measures a strain using a change in light loss due to the degree of bending of the optical fiber.

As the optical fiber cable 142, a single mode optical fiber is used.

The optical fiber sensor multipoint measuring device 143 is
It comprises an optical switch module 143A and an OTDR module 143B. The optical switch module 143A receives the optical pulse from the OTDR module 143B and enters the optical fiber 142, and the OTDR module 14 receives the reflected return light from the optical fiber sensor 141.
An optical switch module for switching the connection to 3B, which has a maximum of 16 switching channels. That is, the optical fiber connected to the common channel (one channel) is connected to one of 16 channels (optical fiber to which 16 sensors are attached). The switching time is about 1 second. The OTDR module 143B is an OTDR module.
This is a part for measuring light loss in the sensor unit 141 by the DR method. The OTDR module 143B repeatedly emits an optical pulse to the connected optical fiber, and when the optical pulse propagates through the optical fiber, it is scattered or reflected at each point in the longitudinal direction of the optical fiber, and the OTDR module 1B.
The light intensity and the position of the backscattered light returning to 43B are measured. If the optical fiber is bent, light radiation loss occurs, and the light intensity of the propagating light sharply decreases, and the backscattered light also sharply decreases in proportion thereto. The present bent optical fiber sensor pays attention to an optical loss effect caused by bending (bending) of the optical fiber, and applies it to multipoint measurement.

A maximum of five sensors can be installed with one optical fiber. The minimum optical fiber length between the sensors is about 12 m, and the measurement length is several tens km. The OTDR measurement time for one optical fiber is a minimum of 10 seconds, and considering the switching time of an optical switch and the analysis processing by a computer, the minimum measurement interval per sensor is about 4 seconds.

The measurement system control computer 144 uses an FA computer and includes a main body, a keyboard, a display, and an uninterruptible power supply.

The optical fiber sensor multi-point measuring device 143
Setting of measurement conditions, conversion of OTDR measurement data to strain, storage of measurement data (measurement condition data, OTDR measurement data for measurement date and time, strain conversion data), and the like are realized as functions of measurement software. As a function of the measurement software, OT is displayed on the computer display.
DR measurement data, distortion of each sensor with respect to the measurement date and time, display of an alarm when the distortion exceeds a preset level (threshold), and display of measurement conditions are performed. The enlargement and reduction of the measurement data graph can be performed during measurement.

[0079]

As described above, when measuring physical quantities such as force, stress, strain, displacement, etc., when using for disaster prevention of public works such as tunnel fall and collapse, and landslide, bridge, plant equipment, lifeline (buried) It is possible to provide an optical fiber sensor and an optical fiber sensor multi-point measuring system suitable for use in predicting the life of a large structure such as a gas conduit or the like, and diagnosing equipment, and when used in a long-distance measurement system.

In particular, according to the optical fiber sensor of the first to fourth aspects, the same bent shape can be inexpensively directly provided on the gauge base of the optical fiber type strain gauge or on the strain-generating portion such as an elastic body. Can be molded. In addition, the difference in the local properties near the edges of the adhesive layer and the groove shape hardly appears in the characteristics.

On the other hand, according to the optical fiber sensor multi-point measurement system according to the fifth and sixth aspects, multi-point measurement using OTDR is possible. In addition, although a physical quantity to be focused on is measured by a sensor, abnormalities such as large deformation and cutting can be detected in transmission optical fibers arranged before and after the sensor. Further, for example, it is possible to measure an object such as a tunnel whose measurement range by a sensor extends over several tens of kilometers.

[Brief description of the drawings]

FIG. 1 is a plan view showing an optical fiber type strain gauge constituting an optical fiber sensor according to a first embodiment of the present invention.

FIG. 2 is a sectional view illustrating an optical fiber type strain gauge constituting the optical fiber sensor according to the first embodiment of the present invention.

FIG. 3 is a plan view showing an optical fiber sensor according to a modification of the first embodiment of the present invention, in which the optical fiber type strain gauge shown in FIG. 1 is attached to a strain generating portion.

FIG. 4 is a diagram illustrating an optical fiber force sensor according to a second embodiment of the present invention in which an optical fiber is directly adhered to a strain generating portion of an elastic body.

FIG. 5 is a perspective view for explaining a method of forming a bent portion of an optical fiber in an optical fiber force sensor according to a second embodiment of the present invention.

FIG. 6 is a view for explaining a method of forming a bent portion of an optical fiber in an optical fiber force sensor according to a second embodiment of the present invention.

FIG. 7 shows, as a comparative example, a state of formation of a bent portion in the case where an inclined groove is formed, in order to explain the effect of the bent portion on transmission loss in the optical fiber sensors according to the first and second embodiments of the present invention. FIG.

FIG. 8 is a graph showing an influence of a bent portion on a transmission loss in the optical fiber sensor according to the first and second embodiments of the present invention. How the ratio of the light propagation loss changes in accordance with the relationship is shown for each groove width ratio to the optical fiber diameter.

FIG. 9 is a graph showing an effect of a bent portion on transmission loss in the optical fiber sensor according to the first and second embodiments of the present invention. It shows how the ratio of the light propagation loss at the groove end changes accordingly.

FIG. 10 is a graph showing the relationship between the amount of transmitted light and the radius of curvature at the center of the bent portion in the optical fiber sensors according to the first and second embodiments of the present invention.

FIG. 11 is a graph showing load characteristics when an optical fiber strain gauge constituting the optical fiber sensor according to the first embodiment of the present invention is attached to a test piece (cantilever).

FIG. 12 is a diagram showing an optical fiber sensor multipoint measurement system according to a third embodiment of the present invention.

FIG. 13 is a diagram illustrating an optical fiber sensor multipoint measurement system according to a modification of the third embodiment of the present invention.

FIG. 14 shows a system configuration in which the optical fiber sensor multipoint measurement system according to a modification of the third embodiment of the present invention shown in FIG. FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical fiber type strain gauge 10 Gauge base 13 H type gap 12 First groove 14 Second groove G interval 16A Bend 16 Optical fiber d Outside diameter (diameter) of optical fiber

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Taro Uesugi 3-5-1, Chofugaoka, Chofu-shi, Tokyo Inside Kyowa Dengyo Co., Ltd. (72) Inventor Eriko Fujishima 3-5-1, Chofugaoka, Chofu-shi, Tokyo Shares F-term in the company Kyowa Dengyo (reference) 2F065 AA65 CC23 FF41 GG07 JJ18 LL02 2F076 BA01 BB09 BD06 2H038 AA03 AA05

Claims (6)

[Claims] 1. A first and a second groove formed substantially linearly without being eccentric at predetermined intervals along one direction of a structure, and are disposed in the first and the second grooves. And at least one optical fiber having a bent portion within the interval, wherein the width dimension of the first and second grooves is formed to be at least twice as large as the outer diameter dimension of the optical fiber. An optical fiber sensor, comprising: 2. The optical fiber sensor according to claim 1, wherein said structure further includes a step which is orthogonal to said first and second grooves and forms said space, wherein said optical fiber is within said step. An optical fiber sensor characterized in that it is not fixed and is configured to be freely movable. 3. An optical fiber sensor according to claim 1, wherein said optical fiber sensor is constituted by an optical fiber type strain gauge comprising a gauge base. 4. The optical fiber sensor according to claim 1, wherein the optical fiber sensor has a bent portion of the optical fiber directly on a strain generating portion of an elastic body constituting the structure without attaching a strain gauge. An optical fiber sensor formed by molding a fiber. 5. An optical fiber sensor according to claim 1, wherein a plurality of optical fiber sensors are arranged at predetermined intervals, and each of said plurality of optical fiber sensors is inserted into said plurality of optical fiber sensors to form one optical transmission path as a whole. An optical fiber sensor multi-point measurement system, wherein one optical fiber is used, and multi-point measurement of strain is performed by the plurality of optical fiber sensors. 6. The optical fiber sensor multipoint measuring system according to claim 5, wherein a plurality of said optical fibers are connected using an optical switch.