JP2001194109A - Displacement measuring apparatus using rayleigh scattering - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus for continuously and safely measuring the displacement of a civil engineering structure by use of Rayleigh scattering occurring in an optical fiber, while amplifying microscopic displacements. SOLUTION: The apparatus detects the displacement of the civil engineering structure 12 from changes in the backscattered light of a light pulse input to the optical fiber 11. The optical fiber 11 is fixedly connected to the displacement amplifying rod 23 of a displacement amplifying means 21 provided in a displacement detecting part. When it is assumed that a crack 29 occurs in the civil engineering structure 12 and the structure 12 is displaced by x, the distance over which the fiber supporting point 27 of the displacement amplifying means 21 is displaced is amplified for detection. When the optical fiber 11 is pulled by the displacement amplifying rod 23, it is curved at an acute angle and the amount of backscattered light decreases sharply. This is measured by an optical time domain reflection measuring instrument 13. The distance to the place of occurrence of the crack 29 is determined from light reception time and the velocity of light within the optical fiber 11.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a displacement measuring apparatus utilizing Rayleigh scattering for measuring the displacement of civil engineering structures or the like using backscattered light from an optical fiber (hereinafter referred to as Rayleigh scattering).

[0002]

2. Description of the Related Art Conventionally, to inspect deformation (strain) of tunnels, bridges, and other civil engineering structures, a person walks on a dark tunnel on foot and looks for displacement such as cracks by visual inspection. A test mortar was applied to the generated location, and displacement was confirmed by the state of the test mortar.

[0003]

In the conventional method of visually inspecting a person on a patrol by walking, there is a problem that a long-distance inspection requires a lot of time and continuous observation is not always possible. . Further, in a tunnel or the like where cracks or the like have already occurred, there is a problem that it is too dangerous to observe the progress of the tunnel because it is difficult to know when a fall or a landslide will occur. Also, in civil engineering structures and bedrock, when the progress of cracks is mm or less, it is extremely difficult not only to visually judge the progress but also to grasp changes over time. There was a problem.

[0004] The present invention provides an apparatus for continuously and safely measuring the displacement of a long civil engineering structure such as a tunnel by utilizing the Rayleigh scattering of an optical fiber, and amplifying the small displacement to reliably measure the displacement. The purpose is to do so.

[0005]

SUMMARY OF THE INVENTION The present invention relates to an optical fiber 1;
1 is stretched around a civil engineering structure 12, and this optical fiber 11
In a device for detecting the displacement of the civil engineering structure 12 by a change in the backscattered light of the light pulse input to one end of the civil engineering structure, a displacement amplifying means 21 is provided at a position where the displacement of the civil engineering structure 12 is detected. The optical fiber 11 is connected and fixed to a displacement amplifying rod 23 of the means 21. The displacement amplifying means 21 comprises a displacement transmitting member 22 and a displacement amplifying rod 23.
Are connected rotatably at a connection point 24, and the fixed point 25 of the displacement transmitting member 22 is connected to the civil structure 12 with a crack 29 interposed therebetween.
The fixed point 26 of the displacement amplifying rod 23 is fixedly attached to one of the wall surfaces.
5 is fixedly attached to the wall surface of the civil engineering structure 12 on the opposite side to the fixed point 26, the fixed point 26 is set as close as possible to the connection point 24, and the tip of the displacement amplification rod 23 is Fiber support point 27 for supporting
Is a displacement measuring device using Rayleigh scattering.

In the above construction, the civil engineering structure 1
Assuming that a crack 29 has occurred in 2 and has been displaced and moved by x, the displacement movement distance at the fiber support point 27 is amplified and detected.

When the optical fiber 11 is pulled by the displacement amplification rod 23, it is bent at a sharp end 46 at an acute angle. Due to the sharp bending of the optical fiber 11, the amount of scattered light backward decreases drastically. This is measured by the optical time domain reflectometer 13. At this time, the distance to the point where the crack 29 occurs is determined from the light receiving time and the speed of light in the optical fiber 11.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Scattering phenomena in the optical fiber 11 include Rayleigh scattering, Brillouin scattering, and Raman scattering, and the spectrum of these scattered lights is shown in FIG. Rayleigh scattering is caused by density fluctuation in a medium of a component having the same frequency as incident light. Brillouin scattering is
It is caused by the interaction of a component slightly different in frequency from the incident light with the sound wave in the medium. Raman scattering is caused by an interaction between a component having a frequency slightly different from that of incident light and molecular vibration in a medium.

The present invention relates to an apparatus for measuring the displacement of civil engineering structures by utilizing Rayleigh scattering among the scattering phenomena in the optical fiber 11 as described above. The principle of the present invention will be described with reference to FIG. Will be described. Reference numeral 11 denotes an optical fiber, and the optical fiber 11 is attached to a position to be measured on a long civil engineering structure 12 such as a tunnel. The proximal end portion 14 of the optical fiber 11 is provided with an optical time domain reflectometer 13
Is combined with

In such a configuration, when an optical pulse 17 is sent from the optical time domain reflectometer 13 to the optical fiber 11, the optical pulse 17 propagates inside the optical fiber 11. Here, it is assumed that a displacement occurs in a measurement target portion such as a wall surface of the civil engineering structure 12, and a bent portion 16 is generated in the optical fiber 11 with the displacement. Then, a part of the light pulse 17 becomes the Rayleigh scattered light 18 at the bent portion 16, a part of the light pulse 17 is returned to the base end 14 side, and the rest becomes a traveling light pulse 19 and the end Propagation to the part 15 side.

At the bent portion 16, a larger light loss occurs than at the straight portion, so that the slope of the light loss curve 20 becomes larger at this portion. Also, the end 15 of the optical fiber 11
Significant light loss also occurs at the site. This light loss curve 20
Is a display unit of the optical time domain reflectometer 13, in which the vertical axis represents light loss and the horizontal axis represents the installation distance of the optical fiber 11.

The laying distance of the optical fiber 11 is determined by the length of the civil engineering structure 12, the distance from the civil engineering structure 12 to the installation place of the optical time domain reflectometer 13, and the like.
It is possible up to about 10 km.

Rayleigh scattered light 1 in optical fiber 11
In order to catch 8, it is necessary to bend the optical fiber 11 to a diameter of about 7 mm by displacement. However, the displacement of the civil engineering structure 12 is very small, and it is difficult to directly capture the Rayleigh scattered light 18 from the bending of the optical fiber 11.

Therefore, in the present invention, a displacement of 1 mm or less can be detected by amplifying and detecting the displacement of the civil engineering structure 12. The displacement amplifying means 21 that makes this possible will be described with reference to FIGS. 2 to 5 as an example in the case where the civil engineering structure 12 is a tunnel. Optical fiber 11
As shown in FIG. 2, several tens of cm
The displacement amplifying means 21 is attached to a location where displacement is to be measured, such as a location where a crack 29 occurs in the civil engineering structure 12. The displacement amplifying means 21 includes a portion for amplifying and detecting the displacement and a portion for bending the optical fiber 11 more reliably by the displacement.

As shown in FIG. 3, the displacement amplifying means 21 has a displacement transmitting member 22 and a displacement amplifying rod 23 rotatably connected at a connecting point 24, and a fixed point 25 of the displacement transmitting member 22 is The fixed point 26 of the displacement amplifying rod 23 is fixedly attached to one wall surface of the civil structure 12 sandwiching the crack 29, and the civil structure opposite to the fixed point 25 sandwiching the crack 29. It is fixedly attached to the wall surface of the object 12. The fixed point 26 is set as close to the connection point 24 as possible. Further, a fiber support point 27 for fixedly supporting the optical fiber 11 is provided at the tip of the displacement amplification rod 23. The fiber support point 27
The bending generators 30 are provided on both sides of the optical fiber 11, and the optical fibers 11 are fixed to the civil engineering structure 12 by the fiber fixtures 28 at the respective outer positions.

The bending generator 30 will be described with reference to FIGS. The bending generator 30 includes a corrosion-resistant body 35 having a side length of about 5 to 10 cm and a thickness of about 1 cm, and a cover plate 36 to cover the body 35.
A first concave portion 39 and a second concave portion 40 are formed in the vessel 35 at a depth several times the diameter of the optical fiber 11 and communicate with each other. A guide projection 42 is provided. A gap between the outer circumference of the guide projection 42 and the inner circumference of the first recess 39 forms a guide passage 43 having substantially the same diameter as the optical fiber 11, and both end portions of the guide projection 42 have a sharp end. It is a part 46.

Insertion holes 37 are formed to communicate from the left and right sides of the first recess 39 to the outside.
Is a constricted portion 3 whose middle is approximately the same as the diameter of the optical fiber 11.
8 and the opening portions at both ends of the constricted portion 38
Spreads like a trumpet and communicates with the first recess 39,
It communicates with the outside. The holding body 44 is attached by a washer 49 and a screw 45 so as to face the outer periphery of the guide projection 42 and at substantially the same interval as the diameter of the optical fiber 11.

At the approximate center of the second concave portion 40, the winding portion 4 is provided with an interval at which the optical fiber 11 can be wound several times.
1 is provided, and a mounting hole 48 penetrating the winding portion 41 and the cover plate 36 is formed. Reference numeral 47 denotes an assembly hole for mounting the container 35 and the cover plate 36.

In this state, the optical fiber 11 is connected to the container 35.
The constricted portion 38, the first concave portion 3 from one of the insertion holes 37
9 and further through the guide passage 43 to the second recess 4.
The wire is wound once or twice around the winding portion 41 inside the inner portion 0, is again fitted into the first concave portion 39 through the other guide passage 43, and is taken out from the other insertion hole 37 through the narrow portion 38.

In order to set the optical fiber 11 in the bending generator 30, the optical fiber 11 has a sufficiently large curvature so that the bending loss is as small as possible inside the first concave portion 39 and the second concave portion 40. Fitted with a radius. Next, the holding body 44 is applied to a portion where the optical fiber 11 intersects the guide passages 43, 43, and the optical fiber 11 is fixed by tightening with the washer 49 and the screw 45.

Similarly, the optical fibers 11 are fitted and fixed in the bodies 35 of all the bending generators 30. At this time,
The optical fiber 11 is fixed by a fiber fixing device 28 without applying a pulling force.

Next, the operation when the crack 29 occurs in the civil engineering structure 12 will be described. In FIG. 3, the optical fiber 1
It is assumed that a crack 29 has occurred in No. 1. Displacement transmitting member 22
Is fixed at the fixed point 25, and the displacement amplification rod 23 is fixed at the fixed point 26. If the crack 29 is displaced and moved by x in the direction of the arrow shown in the figure, the fixed point 26 is also displaced and moved by x. . However, the connecting point 24 is not displaced because it is connected to the displacement transmitting member 22.

Here, assuming that the distance from the connection point 24 to the fixed point 26 is a and the distance between the connection point 24 and the fiber support point 27 is b, the displacement movement distance y at the fiber support point 27 is y = x · b / a. If b> a, the amplification is b / a times (at least about 10 times).

When the optical fiber 11 is pulled by the displacement amplification rod 23, the optical fiber 11
As shown by the dashed line in FIG.
The optical fiber 11 is bent at an acute angle at the position, or may be cut at the position of the sharp end 46 in some cases.

The bending or cutting of the optical fiber 11 sharply increases the bending loss of the optical fiber 11, and the amount of scattered light backward decreases sharply. This is measured by an OTDR (optical fiber reflectometer) in the optical time domain reflectometer 13. Since the amount of light detected by the optical time domain reflectometer 13 is weak, the averaging process is performed many times to obtain a waveform as shown in FIG. The distance to the disaster occurrence point is determined from the light receiving time at this time and the speed of light in the optical fiber 11.

When the optical time domain reflectometer 13 detects a displacement such as a crack 29 in the civil engineering structure 12 in this way, it notifies the manager or the like via an internal transmission circuit or the like.

In the embodiment shown in FIG. 3, the position of the fixed point 26 of the displacement amplification rod 23 is provided between the connection point 24 and the fiber support point 27, so that the amplified displacement y is equal to the displacement x. In this case, the bending generator 30 on the left side in the figure becomes the bending action point. However, the position of the fixed point 26 may be the opposite direction across the connection point 24. In this case, the displacement after amplification is increased. The y is in the opposite direction to the displacement x, and the bending generator 30 on the right side in the figure becomes the bending action point.

The bending generator 30 is shown in FIGS.
The structure is not limited to the structure shown in FIG. 6, but may be a simple structure as shown in FIG.
Is pulled from the large diameter L1 to the small diameter L2 so that the bending loss of the optical fiber 11 sharply increases and the amount of scattered light backward decreases drastically.

Further, the displacement amplifying means 21 is not limited to one mainly composed of a displacement transmitting member 22 and a displacement amplifying rod 23 as shown in FIG.
A mechanism that can amplify and detect the displacement by another mechanism may be used.

[0030]

As compared with the conventional method in which a person walks on foot and performs a visual inspection, long-distance inspection work can be continuously and constantly observed in a short time. Also, except when the optical fiber 11 and the displacement amplifying means 21 are installed, it is not necessary for a human to directly enter the tunnel 12 where the crack 29 is generated, so that it is safe even if a falling board or a landslide occurs.

Cracks 2 in civil engineering structure 12 and bedrock
Even if the progress of 9 is a minute unit of mm or less, the displacement amplifying means 21 can determine the progress state extremely accurately,
It is easy to grasp changes over time. The bend generator 30 accommodates and fixes the middle of the optical fiber 11 with a curvature such that the bending loss is as small as possible, and a guide projection 42 having a sharp end 46 is provided at this fixed position. Since it has a configuration, there is no mechanically movable part, and it operates reliably even if it is installed for a long time.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a principle operation of a displacement measuring device using Rayleigh scattering according to the present invention.

FIG. 2 is a partially cutaway explanatory view showing a state in which a displacement measuring device using Rayleigh scattering according to the present invention is installed in a civil engineering structure 12.

FIG. 3 is an enlarged explanatory view of the displacement amplification means 21 according to the present invention.

4 is a cross-sectional view of the bending generator 30. FIG.

FIG. 5 is a sectional view of the bending generator 30 taken along line AA in FIG. 4;

FIG. 6 is an explanatory view of another embodiment of the bending generator 30.

FIG. 7 is a scattered light spectrum for explaining a scattering phenomenon in the optical fiber 11;

[Explanation of symbols]

11: Optical fiber, 12: Civil structure, 13: Optical time domain reflectometer, 14: Base end, 15: End, 16 ...
Bent part, 17: light pulse, 18: Rayleigh scattered light, 19
... traveling light pulse, 20 ... light loss curve, 21 ... displacement amplification means, 22 ... displacement transmission member, 23 ... displacement amplification rod, 24
... Connection point, 25 ... Fixed point, 26 ... Fixed point, 27 ... Fiber support point, 28 ... Fiber fixing tool, 29 ... Crack, 3
0: bending generator, 35: body, 36: cover plate, 37: insertion hole, 38: constricted portion, 39: first concave portion, 40: second concave portion, 41: winding portion, 42: guide projection, 43 ... Guide passage, 44 ... Holder, 45 ... Screw, 46 ... Sharp end
47 ... assembly holes, 48 ... mounting holes, 49 ... washers.

Claims (5)

[Claims] 1. An apparatus in which an optical fiber 11 is stretched around a civil engineering structure 12, and a displacement of the civil engineering structure 12 is detected by a change in backscattered light of a light pulse input to one end of the optical fiber 11. , The civil engineering structure 12
A displacement amplifying means 21 is provided at a displacement detecting position of the optical fiber 11 and the optical fiber 11
A displacement measuring device using Rayleigh scattering, characterized by being connected and fixed. 2. The displacement amplifying means 21 includes a displacement transmitting member 22.
And a displacement amplification rod 23 are rotatably connected at a connection point 24, and a fixed point 25 of the displacement transmission member 22 is fixedly attached to one wall surface of the civil engineering structure 12 sandwiching the crack 29, Further, the fixed point 26 of the displacement amplification rod 23 is fixedly attached to the wall surface of the civil engineering structure 12 opposite to the fixed point 25 sandwiching the crack 29. 2. The displacement measuring apparatus using Rayleigh scattering according to claim 1, wherein a fiber support point 27 for supporting the optical fiber 11 is provided at a position close to the distal end of the displacement amplification rod 23. . 3. The displacement amplifying means 21 includes a displacement transmitting member 22.
And a displacement amplification rod 23 are rotatably connected at a connection point 24, and a fixed point 25 of the displacement transmission member 22 is fixedly attached to one wall surface of the civil engineering structure 12 sandwiching the crack 29, Further, the fixed point 26 of the displacement amplification rod 23 is fixedly attached to the wall surface of the civil engineering structure 12 opposite to the fixed point 25 sandwiching the crack 29. A fiber support point 27 that supports the optical fiber 11 is provided at the distal end of the displacement amplification rod 23, and a bending generator 30 is provided on each side of the fiber support point 27. 2. The displacement measuring apparatus using Rayleigh scattering according to claim 1, wherein the optical fibers 11 are fixed to the civil engineering structure 12 at respective outer positions. 4. The displacement amplifying means 21 includes a displacement transmitting member 22.
And a displacement amplification rod 23 are rotatably connected at a connection point 24, and a fixed point 25 of the displacement transmission member 22 is fixedly attached to one wall surface of the civil engineering structure 12 sandwiching the crack 29, Further, the fixed point 26 of the displacement amplification rod 23 is fixedly attached to the wall surface of the civil engineering structure 12 opposite to the fixed point 25 sandwiching the crack 29. A fiber support point 27 that supports the optical fiber 11 is provided at the distal end of the displacement amplification rod 23, and a bending generator 30 is provided on each side of the fiber support point 27. Further, the optical fiber 11 is fixed to the civil engineering structure 12 at each of the outer positions, and the bending generator 30 accommodates the optical fiber 11 with a curvature such that bending loss is as small as possible in the middle of the optical fiber 11. Is fixed, the displacement measuring apparatus utilizing a Rayleigh scattering according to claim 1, characterized by being provided with a guide projection 42 having a sharp tip portion 46 to the fixed position. 5. The bending generator 30 includes a corrosion-resistant container 3.
5 and a cover plate 36, and the first recess 3
9 and the second concave portion 40 are formed so as to communicate with each other.
6 is provided, and an insertion hole 37 having a constricted portion 38 is formed in the middle from the left and right sides of the first concave portion 39 to the outside, and the guide convex portion 42 is formed in the second concave portion 40. 5. The displacement measuring apparatus using Rayleigh scattering according to claim 4, wherein the holding members 44 are provided at substantially the same interval as the diameter of the optical fiber sensor 32 therebetween.