JP2001281077A - Optical fiber strain sensor - Google Patents

Abstract

(57) [Problem] To provide an optical fiber strain sensor which overcomes the above-mentioned drawbacks of a strain sensor using a communication optical fiber. SOLUTION: This optical fiber strain sensor detects a strain generated in an optical fiber due to Brillouin scattering of incident light, and is fixed to an outer cover 14 which covers the optical fiber 2 in a state where the optical fiber 2 is stretched and strained. Have been.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a strain sensor using an optical fiber.

[0002]

2. Description of the Related Art Structural distortion is measured in various industrial fields. Its purpose is mechanical testing of structural materials, structural maintenance and disaster prevention. Measurement of stress-strain curves,
It is used for monitoring deformation and detecting abnormal points. As a strain detecting means, there is an electrical measurement represented by a resistance wire strain meter. However, because of the point measurement and the susceptibility to electromagnetic noise, the number of measurement and the number of locations are limited in actual applications in terms of cost. Recently, in order to overcome these drawbacks, methods of measuring strain as a distribution using an optical fiber have been proposed (Japanese Patent Application Laid-Open Nos. Hei 6-273270, Hei 10-90018, Hei 10-90120). Such). This measurement system measures in a time-division manner a frequency shift amount having a distortion dependency of backscattered light (Brillouin scattering) of light incident on an optical fiber, and knows a magnitude of the distortion and a place where the distortion occurs as a distribution. be able to. By applying this measurement system, it has become possible to measure strain as a distribution relatively inexpensively. Its measurement capability is a distance resolution of 1 m and a distortion resolution of 0.1 mm / 1.
m, the measurable distance is 50 km, and the allowable dynamic range (total transmission loss level by the optical fiber) is about 2 dB at the lowest value depending on the measurement conditions.

[0003]

The optical fiber (optical fiber strain sensor) used for strain detection in the strain measuring system is a single mode (SM) optical fiber for optical fiber communication (Japanese Patent Laid-Open No. 10-197298). Gazette). The optical fiber for communication has a positive margin of about 0.2% with respect to the outer cover in consideration of the installation environment. Sunahachi,
The optical fiber is designed so that the optical performance of the optical fiber does not decrease with 0.2% elongation deformation.

[0004] A method of giving a positive excess length to an optical fiber will be described. To give a positive excess length to an optical fiber of the optical fiber 5 shown in FIG. 2, a bare optical fiber 3 (as shown in FIG. And a core 2), and is covered with a resin 4. In the case of the optical fiber core 7 shown in FIG. 3, the optical fiber 5 is undulated and covered with a resin 6. In the case of the optical fiber cord 10 shown in FIG.
The optical fiber core 7 is covered with an outer sheath in a state where the length is changed. In the case of the optical cable 13 shown in FIG. 5, an optical cord 10 (or an optical fiber) is twisted around the tension member 11 as an axis and covered with an outer sheath 12.

On the other hand, as shown in FIGS. 10 and 11, an optical fiber containing a metal tube is used as an optical fiber strain sensor in consideration of the use environment (Japanese Patent Laid-Open No. Hei 10-90018). The strain detection optical fiber cable 21 shown in FIG. 10 is obtained by inserting a bare optical fiber 3 covered with a resin 14 into a metal tube 19. The strain detecting optical fiber cable 21 shown in FIG.
Inserts a bare optical fiber 3 coated with a resin 14 into a metal tube 19 and inserts the bare optical fiber into the metal tube 1.
9 is fixed with resin.

In any case, there is no technical consideration regarding the extra length, and if these optical fibers are used for strain detection, the initial sensitivity becomes dull by the positive extra length, and it becomes difficult to identify the place where the elongation has occurred. . This means that the capability inherent in the measuring device itself is not exhibited.

FIG. 12 shows an outline of a strain test apparatus. In FIG. 12, a strain measuring device 24 is connected to one end of a strain detecting optical fiber 25. Strain detection optical fiber 25
The load applying machine 26 for applying a load to the vehicle includes a fixed portion 27 and a movable portion 28. The measurement portion of the strain detection optical fiber 25 is gripped by the fixed portion 27, and the movable portion 28 is moved to apply a predetermined tensile load to the strain detection optical fiber 28.

FIG. 13 shows the result of a 0.3% strain test (extending 6 m to 6.018 mm) of the conventional strain detecting optical fiber cable shown in FIGS. 10 and 11 using the strain test apparatus shown in FIG. . In the strain detecting optical fiber cable of FIG. 10, the position at which the strain occurs is shifted and the value lacks accuracy. The strain detection optical fiber cable of FIG.
The value decreases as the distortion detection position spreads. FIG.
In the strain detection optical fiber cable of No. 0, since the contained optical fiber is not fixed, it is gradually drawn into the load range, and as a result, strain occurs near the R portion where the movement of the optical fiber is poor (see FIG. 12). It seems to be. FIG.
It is considered that the strain detecting optical fiber cable of No. 1 originally has the sensitivity reduced by the extra length inherent in the optical fiber and also draws the extra length of the unloaded portion near both ends of the load range.

The present invention provides an optical fiber strain sensor which overcomes the above-mentioned drawbacks of a strain sensor using a communication optical fiber.

[0010]

SUMMARY OF THE INVENTION An optical fiber strain sensor according to the present invention is an optical fiber strain sensor for detecting a strain generated in an optical fiber due to Brillouin scattering of incident light. The optical fiber is fixed to the outer sheath covering the optical fiber in a state where the optical fiber is covered.

In the optical fiber strain sensor of the present invention, since the optical fiber is fixed to the outer cover in a state where the optical fiber is stretched and strained, the optical fiber expands and contracts integrally with the outer cover. Therefore, there is no dead zone for distortion detection,
Also, the accuracy is improved.

In the above optical fiber strain sensor, the environment resistance is improved by fixing the optical fiber over the entire line with a metal tube as the outer cover. In addition, since the outer sheath is metal and the optical fiber is intermittently fixed, the increase in transmission loss due to side pressure on the optical fiber due to the shrinkage of the outer sheath when distortion occurs reduces accurate long-range distortion measurement. enable. Furthermore, the intermittent fixing by simple caulking is performed, and the manufacturing cost of the optical fiber strain sensor is reduced.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION An optical fiber strain sensor according to the present invention is fixed to an outer sheath covering the optical fiber in a state where the optical fiber is stretched and strained. The magnitude of the elongation strain is
It is 0.01 to 2%, preferably 0.1 to 1%. The lower limit of 0.01% is the detection limit of the strain meter, and the strain is 2%.
If it exceeds, the optical fiber may be broken. Also,
When the content is 0.1% or more in the preferable range, the extra length can be given uniformly, and 1% is the upper limit of the practical range of strain measurement.

A resin or metal is used as a material for the outer skin. As the resin, vinyl chloride resin, polyethylene,
Nylon and others are used. As the metal, stainless steel, Inconel (trade name of Inco Corporation), copper, titanium, aluminum or the like is used.

FIG. 6 shows an optical fiber strain sensor 1 according to the present invention.
5 is shown. A core 2 is covered with a clad 1 and a bare optical fiber is covered with a resin 14. FIG. 14 shows a method for manufacturing the optical fiber strain sensor 15. The bare optical fiber is sent out from the bare optical fiber supply reel 30 while tension is applied to the bare optical fiber by the dancer roll 33. The sent bare optical fiber is covered with a resin coating device 31, cooled by a cooling device 32, and then wound by an optical fiber strain sensor winding device 34.

FIG. 7 shows an optical fiber strain sensor 2 according to the present invention.
1 is shown. A bare optical fiber comprising a core 2 and a clad 1 is covered with a resin 14. The bare optical fiber covered with the resin 14 is inserted into a metal tube 19.
The bare optical fiber covered with the resin 14 is fixed to the metal tube 19 over the entire length of the metal tube 19 with a fixing resin 20 (for example, a thermosetting epoxy resin). FIG. 15 shows a method of manufacturing the optical fiber strain sensor 21. A coated optical fiber supply reel 30 is applied while tension is applied to a resin-coated bare optical fiber (a quartz-based single mode optical fiber having a normal resin coating) by a dancer roll 33.
And feeds the coated optical fiber to the resin coating device 31 via the guide roll 41. On the other hand, a plate-shaped metal hoop is sent out from the metal hoop supply device 43 and is formed by a forming roll 44 into a C-shaped tubular body that opens upward. The resin is applied by the resin application device 31 while inserting the coated optical fiber into the tubular body from the opening. Then, it is formed into a tube by the final forming roll 45, heated by the heating device 42, cooled by the cooling device 32, and then cooled by the optical fiber distortion sensor winding device 4.
Wind up with 6.

FIG. 8 shows an optical fiber strain sensor 2 according to the present invention.
3 is shown. The optical fiber strain sensor 23 covers the bare optical fiber 3 with the resin 14 and is inserted into the metal tube 19. The metal tube 19 is compressed from the outside in the inner diameter direction, and caulked to fix the resin-coated optical fiber to the metal tube 19. The interval between the caulking 22 in the longitudinal direction of the optical fiber is, for example, 1 m or more, depending on the strain measurement conditions. FIG. 16 shows a method of manufacturing the optical fiber strain sensor 23. The resin-coated optical fiber inserted into the metal tube is sent out from the metal tube-containing optical fiber supply device 50, the optical fiber with the metal tube is caulked by the caulking device 52, and then wound up by the optical fiber strain sensor winding device 53. The caulking device 52 guides and holds the optical fiber containing the metal tube with the guide roll 51, and caulks with the caulking jig 54 driven by the air cylinder 55. The caulked position of the caulked optical fiber with metal tube is detected by the caulking position detector 56, and based on the caulked position, the optical fiber with metal tube is fed out from the optical fiber supply device with metal tube 50 by the caulking interval. The wound optical fiber with a metal tube is subjected to elongation strain at each caulking interval in a separate process.

[0018]

[Embodiment 1] FIG. 6 shows an embodiment of the present invention.
FIG. 14 shows a manufacturing method. When coating the UV curable resin on the quartz single-mode bare fiber comprising the core and the clad, the bare fiber was wound while applying a predetermined tension (indicated by an arrow). Outer diameter of bare optical fiber: 0.125 mm Outer diameter of coating: 0.25 mm Tension: 2.0 N Initial strain: 0.2%

[Embodiment 2] FIG. 7 shows an embodiment of the present invention. FIG. 15 shows a manufacturing method. When inserting a silica-based single-mode optical fiber with ordinary resin coating into a metal tube, apply epoxy resin (heat-curing type) to the optical fiber until the resin is cured and the metal tube and optical fiber are fixed. A predetermined tension was applied. Bare optical fiber outer diameter: 0.125 mm Resin coating outer diameter: 0.9 mm Resin material: Nylon Metal tube outer diameter: 1.8 mm Metal tube inner diameter: 1.4 mm Metal tube material: Stainless steel Tension: 9.0 N Initial strain: 0 0.3%

[Embodiment 3] FIG. 8 shows an embodiment of the present invention. FIG. 16 shows a manufacturing method. A quartz single-mode optical fiber having a usual resin coating is inserted into a metal tube (using the method disclosed in Japanese Patent Application Laid-Open No. 62-44010), and the extra length is adjusted to minus 0.1% (Japanese Patent Application Laid-Open (JP-A) no. 63-1
After that, the metal tube and the optical fiber were caulked at a predetermined interval, and tension was applied to the metal tube together with a predetermined plastic strain. Thereafter, the tension was released. A plastic strain of 0.2% was applied to 4m-9m of the total length of 22m. Thereafter, a 0.2% elongation strain test was performed on 8 m to 14 m as shown in FIG. The result shows a normal value as shown in FIG. Outer diameter of bare optical fiber: 0.125 mm Outer diameter of resin coating: 0.9 mm Resin material: Nylon Outer diameter of metal tube: 2.0 mm Inner diameter of metal tube: 1.6 mm Metal tube material: Stainless steel Staking distance: 1 m Inner diameter of swaging part: 0 0.7mm Tension: 210N Initial strain: 0.1%

The first embodiment is applied at a relatively low cost when no environmental resistance or mechanical strength is required. In the second embodiment, when mechanical strength and environmental resistance are required, a metal tube is used as a protective tube. The third embodiment is similar to the first and third embodiments.
When the outer sheath and the optical fiber are fixed, the present invention is applied to a case where the transmission loss of the optical fiber increases due to the side pressure on the optical fiber due to the contraction of the outer sheath when the distortion occurs, and a long distance cannot be measured. As shown in FIG. 17, the load range (400
m) transmission loss of about 2 dB, Example 3 is 0.25 dB
It is. Assuming that the dynamic range is 2 dB, the measurable distance is 400 m in the second embodiment and 3200 m in the third embodiment. It has been greatly improved. FIG. 13 shows the results of a test performed according to the test procedure shown in FIG. It can be seen that the position and accuracy of the distortion are accurately detected. (Examples 1 and 3 are omitted because the graphs almost overlap with Example 2.)

[0022]

According to the present invention, it is possible to obtain accurate distortion positions and values. In addition, the measuring distance can be extended, and the capability of the strain measuring instrument can be fully exhibited. Even in actual industrial application, it is possible to provide an inexpensive sensor having excellent environmental resistance. The type of optical fiber and resin used, the material of the metal tube,
It goes without saying that the outer diameter, wall thickness, caulking distance, and the like can be appropriately selected according to the application environment. Further, the metal tube may be coated with a resin to obtain chemical resistance and insulation.

[Brief description of the drawings]

FIG. 1 is a sectional view of a bare optical fiber.

FIG. 2 is a sectional view of an optical fiber.

FIG. 3 is a sectional view of an optical fiber.

FIG. 4 is a sectional view of an optical fiber code.

FIG. 5 is a sectional view of an optical fiber cable.

FIG. 6 is a sectional view of the optical fiber strain sensor of the present invention.

FIG. 7 is a cross-sectional view of the optical fiber strain sensor of the present invention.

FIG. 8 is a sectional view of the optical fiber strain sensor of the present invention.

FIG. 9 is a diagram showing an example of distortion at the time of manufacture and at the time of measurement according to the present invention.

FIG. 10 is a sectional view showing an example of a conventional optical fiber strain sensor.

FIG. 11 is a sectional view showing another example of the conventional optical fiber strain sensor.

FIG. 12 is a diagram illustrating a strain test method for an optical fiber strain sensor.

FIG. 13 is a diagram showing the results of a load test by comparing the present invention example and the conventional example.

14 is a view illustrating a method for manufacturing the optical fiber strain sensor of the present invention shown in FIG.

FIG. 15 is a view illustrating a method of manufacturing the optical fiber strain sensor of the present invention shown in FIG. 7;

FIG. 16 is a view illustrating a method of manufacturing the optical fiber strain sensor of the present invention shown in FIG.

FIG. 17 is a diagram showing the effect of intermittent fixing on transmission loss.

[Explanation of symbols]

 1: Cladding part 2: Core part 3: Bare optical fiber 4: Resin coat 5: Optical fiber 6: Resin coat 7: Optical fiber core 8: Outer sheath 9: Kevlar fiber 10: Optical fiber cord 11: Tension member 12: Outer sheath 13: Optical fiber cable 14: Resin 15: Resin sheath type sensor 19: Metal tube 20: Fixing resin 21: Metal tube sheath type sensor 22: Caulking part 23 : Metal tube sheath caulking type sensor 24: Strain measuring device 25: Test strain sensor 26: Load loader 27: Fixed end 28: Movable end 30: Bare fiber supply spool 31: Resin coating device 32: Cooling device 33: Dancer roll 34: Optical fiber distortion sensor winding device 40: Resin-coated optical fiber supply device 41: Guide roll 42: Heating device 43: Metal hoop supply device 44: Composition Forming roll 45: Final forming roll 46: Optical fiber strain sensor winding device 50: Metal tube-containing optical fiber supply device 51: Guide roll 52: Caulking device 53: Optical fiber strain sensor winding device 54: Caulking Jig 55: Air cylinder 56: Caulking position detector

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification FI FI Theme Court ゛ (Reference) G02B 6/44 341 G02B 6/44 341 (72) Inventor Noriyasu Funayama 3-4 Tsukiji 3-chome, Chuo-ku, Tokyo No. Nippon Steel Welding Industry Co., Ltd. (72) Inventor Shuichi Ueno 3-4, Tsukiji, Chuo-ku, Tokyo Nippon Steel Welding Industry Co., Ltd. F-term (reference) 2F065 AA01 AA65 DD03 DD04 FF33 FF41 LL02 PP01 2G086 DD05 2H050 AC09 AD06 BB03Q BB09Q BB10Q BB26Q

Claims (4)

[Claims] 1. An optical fiber strain sensor for detecting a strain generated in an optical fiber due to Brillouin scattering of incident light, wherein the optical fiber is fixed to an outer sheath covering the optical fiber in a state where the optical fiber is stretched and strained. An optical fiber strain sensor. 2. The optical fiber according to claim 1, wherein the optical fiber is fixed to a metal sheath over the entire length.
An optical fiber strain sensor according to claim 1. 3. The optical fiber strain sensor according to claim 1, wherein said optical fiber is intermittently fixed to a metal sheath. 4. The optical fiber strain sensor according to claim 3, wherein the optical fiber is fixed to the outer cover by caulking.