Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a strain sensor using an optical fiber grating (FBG), and more particularly to a cable-shaped strain sensor. 2. Description of the Related Art In recent years, an optical fiber grating (FBG: Fiber) having a Bragg diffraction grating formed on an optical fiber.
A strain sensor using Bragg Grating has attracted attention for measuring strain in tunnels, buildings, bridges, and the like.  In this strain sensor, when light is incident on an optical fiber having an optical fiber grating portion (FBG portion) in the longitudinal direction from one end and the wavelength of the reflected light from the FBG portion is observed, the strain of the FBG is measured. The distortion is detected by using the fact that the reflection wavelength changes according to the distortion. That is, since the wavelength of the reflected light of the FBG changes with the period of the grating, the reflection wavelength shifts when the optical fiber expands and contracts. Therefore, if the change in the reflection wavelength is measured, the strain applied to the optical fiber can be measured. Therefore, the FBG portion is fixed to a strain generating member (a metal wire to which tension is applied) and the FBG portion is fixed.
It measures the change in the reflected wavelength from the BG, and detects the strain (ie, stress) of the strain generating member. A conventional strain sensor using an FBG generally has a structure in which an FBG portion is directly bonded to an object to be measured for strain, that is, a strain generating member with an adhesive. On the other hand, as shown in FIGS. 3 (a) and 3 (b), the FBG portion 12 of the optical fiber 11 is housed in a package 13, and a mounting portion 14 made of an aluminum plate is passed through a mounting hole 15 to be measured. A strain sensor having a structure in which a screw is fixed to an object has also been developed.  However, the strain sensor shown in FIG. 3 as a conventional technique has the following problems. (1) Since the optical fiber 11 is routed and laid, it cannot be applied to outdoor use without an exterior. (2) When a protective tube is applied to the optical fiber 11, it is necessary to fix the protective tube at the end of the package 13, so that the package 13 becomes large and the structure becomes complicated. (3) Only the distortion at the point where the FBG section 12 of the optical fiber 11 is fixed can be measured.
In order to measure strain of a fixed span, it is necessary to lay a wire and fix a strain sensor to the wire. (4) Since the reflection wavelength changes due to a change in the refractive index of the optical fiber 11 due to a change in temperature and expansion and contraction (change in the period of the grating), a separate sensor for measuring the temperature for correction is required. Accordingly, an object of the present invention is to solve the above-mentioned problems, to provide a strain sensor which can be easily laid outdoors, is easy to manufacture, and can easily measure strain of a fixed span, and further facilitates temperature correction. It is another object of the present invention to provide a strain sensor that can be implemented in any of the above.  In order to achieve the above object, the present invention is configured as follows. (1) In a strain sensor using an optical fiber grating according to the first aspect of the present invention, an optical fiber having a plurality of gratings (Bragg diffraction gratings) formed in a longitudinal direction is provided around a spacer. Slot (slot)
And only the grating portion is fixed to the groove of the spacer, and a sheath is provided around the spacer. Since the strain sensor of the present invention has a cable structure, it can be easily installed outdoors without a special exterior. Further, since the optical fiber is accommodated in the spacer groove, there is no need to provide a special protective tube to the optical fiber, and the structure is simplified. Further, it is easy to provide a plurality of optical fiber grating portions on the optical fiber, which is suitable for measuring strain of a fixed span. (2) According to a second aspect of the present invention, in the strain sensor according to the first aspect, the optical fiber grating portion is directly fixed to the groove of the spacer with a material having good adhesiveness to the grating portion and the spacer. It is characterized by having. According to this feature, since the optical fiber grating portion is directly fixed to the groove of the spacer, the manufacture becomes very easy. (3) The invention of claim 3 is the invention according to claim 1 or 2
In the strain sensor described above, a plurality of grooves are formed in the spacer, and the optical fibers are respectively inserted into the plurality of grooves, and some of the optical fibers have grating portions in the grooves. It is characterized in that it is not fixed. According to this feature, since the optical fiber having the grating fixed in the spacer groove and the optical fiber not fixed coexist, both the refractive index change and expansion and contraction (period change of the grating) of the optical fiber due to the temperature change are caused. Even in an environment where the reflection wavelength changes, by using an optical fiber not fixed to the groove as a temperature sensor,
Temperature correction of the strain sensor can be easily performed. Therefore, it is not necessary to separately prepare a temperature measurement sensor for temperature correction. (4) The invention of claim 4 is the invention according to claim 1 or 2
In the strain sensor described above, at least two grating portions having different reflection wavelengths are formed in the optical fiber, one grating portion is directly fixed to the groove of the spacer, and the other grating portion is formed of the spacer. It is not fixed to the groove. According to this feature, when multiplexed light enters the optical fiber, the temperature can be measured by the reflected light from the grating portion not fixed to the groove of the spacer. That is, strain and temperature can be measured simultaneously with one optical fiber. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on the illustrated embodiment. FIG. 1 is an internal configuration diagram of a strain sensor using an optical fiber grating according to one embodiment of the present invention, and FIG. 2 is a sectional view of the strain sensor. This strain sensor is in the form of a cable 8 as a whole. This cable 8 has a spacer 2 inside.
And a pressing tape 6 and a sheath 5 are sequentially arranged therearound.
Is provided. A tension member 4 is provided at the center of the spacer 2, and a plurality of twist grooves (slots) 7 are formed around the spacer 2 by S twisting or Z twisting.
It is formed in the direction of twist. One of the plurality of twist grooves 7 is used as a groove for accommodating the optical fiber 1a serving as a strain sensor main body, and the other one is used as the temperature measuring optical fiber 1
Used as a groove for storing b. First, an optical fiber 1 having an FBG portion (optical fiber grating portion) 3 formed at an intermediate portion of a required strip length.
a and 1b are prepared. Here, the FBG unit 3 is provided at least at one position apart from the optical fibers 1a and 1b in the axial direction.
The optical fiber 1a side is referred to as a strain measurement FBG section 3a, and the optical fiber 1b side is referred to as a temperature measurement FBG section 3b. In order to form the FBG portion 3 on an optical fiber, the coating on the portion where the grating is to be written is once removed (usually about 10 mm). Then, the grating is written on the core portion of the optical fiber by a known method such as a two-beam interference method, a prism interference method, and a phase grating method. After the writing is completed, recoating is performed with the same material as the coating material of the optical fiber. At this time, slippage of the optical fiber occurs when strain is applied to the general coating material, so that a polyimide resin is used for the coating material as a measure against this. However, the strain measuring FBG portion 3a may be recoated, but in order to increase the sensitivity, a structure in which the glass of the optical fiber is directly adhered to the object without recoating is preferable. That is, it is preferable that the optical fiber grating is fixed directly to the spacer with a material having good adhesiveness to the bare grating and the spacer without recoating. The optical fibers 1a and 1b on which the strain measuring FBG section 3a and the temperature measuring FBG section 3b are formed are housed and twisted in the twist groove 7 of the spacer 2 as shown in FIG. Next, the strain measuring FBG portion 3a is bonded to the spacer 2. The temperature measurement FBG portion 3b is left as it is without bonding. Then, the holding tape 6 is wound around the sheath 5 to complete the strain sensor having the cable structure. When this strain sensor is used, the cable 8 manufactured as described above is fixed to two points on the object to be measured, or when the slip of the sheath 5 becomes a problem, the spacer 5 including the tension member 4 is used. The part is the 2
Fix to a point. Then, a light source (not shown) is connected to one end of each of the optical fibers 1a and 1b, and the other end is opened. Then, the light in the wavelength band including the set Bragg reflection wavelength is output from the light source to the optical fibers 1a and 1b, and the strain measurement F corresponding to the strain applied to the object to be measured is output.
The change in the Bragg reflection wavelength of the reflected light from the BG unit 3a is measured by a measuring device (not shown) such as an optical spectrum analyzer to measure the distortion. Since the grating (temperature measuring FBG portion 3b) inserted for temperature measurement is not fixed to the spacer 2, the strain measuring FBG portion 3a and the temperature measuring FBG portion 3b
The difference between the wavelength shifts of the two gratings is the distortion amount corrected for the temperature change. In order to correct the distortion due to the linear expansion of the spacer 2, a method of calculating the expansion and contraction of the spacer 2 from the temperature change and subtracting this is used. <Other Embodiments> The present invention is not limited to the above embodiments. For example, in order to measure the distortion at multiple points, the grating is written into a plurality of portions of one optical fiber and cabling is performed while changing the period of the grating. Also, insert a dummy wire to make a cable,
A manufacturing method in which an optical fiber grating is connected to a dummy wire and pulled in, a sheath at the grating portion is attached, an optical fiber is adhered to a spacer, and a sheath is molded is also conceivable. In the above embodiment, two optical fiber gratings for strain sensor and temperature measurement are used. However, at least two grating portions having different reflection wavelengths are formed on one optical fiber. By fixing one grating portion to the spacer groove, it can be used as a strain sensor, and the other grating portion can be used for temperature measurement without being fixed to the spacer groove. In this case, the two grating sections are preferably provided as close as possible because one of them is for temperature correction.
It goes without saying that the light incident on the optical fiber is a multiplex light. <How to Use, Applied System, etc.> The strain sensor of this embodiment can be applied to a wide range of fields. For example, it can be applied as a sensor capable of measuring strain and temperature, or as a strain monitor applied to a communication cable. Regarding the wavelength transmitted through the grating, an optical fiber on which the grating is formed can be used as a communication line. As described above, according to the present invention, the following excellent effects can be obtained. Since the strain sensor of the present invention has a cable structure, it can be easily installed outdoors without a special exterior. Since the optical fiber is accommodated in the spacer groove in the cable, there is no need to provide a special protective tube for the optical fiber, and the structure is simplified. Further, it is easy to provide a plurality of optical fiber grating portions on the optical fiber, which is suitable for measuring strain of a fixed span. Further, in a mode in which the optical fiber grating portion is directly fixed to the spacer, the manufacture becomes very easy. Further, an optical fiber grating housed in a spacer groove different from the strain sensor can be used as a temperature sensor by not fixing the grating portion to the spacer groove, and temperature correction of the strain sensor can be easily performed. can do. Further, at least two grating portions having different reflection wavelengths are formed in one optical fiber, and one grating portion is fixed to the spacer groove, and the other grating portion is not fixed to the spacer groove. Optical fiber can measure both strain and temperature.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an internal configuration diagram showing one embodiment of a strain sensor using an optical fiber grating of the present invention. FIG. 2 is a cross-sectional view of a strain sensor using the optical fiber grating of the present invention. FIG. 3 is an external view of a strain sensor showing an example of a conventional technique. [Description of Signs] 1 Optical fiber 2 Spacer 3 FBG section (optical fiber grating section) 3a FBG section for strain measurement 3b FBG section for temperature measurement 4 Tension member 5 Sheath 6 Pressing tape 7 Twisted groove 8 Cable
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