CN112484656A - Optical fiber type convergence meter and using method thereof - Google Patents
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
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Abstract
The invention discloses an optical fiber type convergence meter and a using method, wherein the optical fiber type convergence meter comprises a banded flexible substrate, a protective layer, a transmission optical cable, an optical fiber grating array and an optical fiber grating demodulator, wherein the transmission optical cable is adhered to the surface of the banded flexible substrate or embedded in the protective layer, and one end of the transmission optical cable is connected with the optical fiber grating demodulator; the fiber grating array is composed of a plurality of fiber gratings etched on the optical fibers in the transmission optical cable and is arranged on the transmission optical cable along the length direction; the optical fiber type convergence meter is characterized in that the change value of the central wavelength of each optical fiber grating is monitored in real time, the wavelength change is converted into the curvature value of the convergence meter, and the two-dimensional shape of the optical fiber convergence meter is reconstructed through the two-dimensional curve, namely the deformation curve of the inner wall of the cross section of the roadway.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to an optical fiber type convergence meter for monitoring the deformation of the cross section of an underground tunnel and a using method thereof.
Background
With the continuous development of national economic construction, various engineering facilities such as dams, bridges, tunnels, high-rise buildings and the like are also under rapid construction, and after the facilities are constructed, in order to ensure the normal operation of the facilities, the spatial position and the deformation of a key part need to be regularly detected for a long time to obtain the position variation, and long-term accumulated data are analyzed to determine the bearing capacity of the existing structure, use durability and the like, ensure that the engineering structure is in a good working state and predict the safety degree of the structure, so that the deformation monitoring of displacement, settlement and the like of various types of large engineering facilities is necessary from the aspects of safety and economy.
Underground tunnels such as tunnels, mine tunnels and galleries can deform during construction and operation, and monitoring of the convergence deformation of the sections of the underground tunnels is particularly important for ensuring the safety of the underground tunnels. The existing common tunnel section deformation monitoring methods mainly comprise two categories, one category is non-contact, and the other category mainly comprises methods such as a total station, a laser convergence meter, a digital close-range photogrammetry and the like, which have higher requirements on the visibility conditions of a test site and can not realize long-term, real-time and on-line measurement; the other type is contact type, and mainly comprises a convergence meter, a Basette convergence measuring system and an optical fiber type convergence system, wherein the convergence meter realizes automatic measurement at present, the precision can reach 0.01mm, but the interference on engineering construction and operation is large, and the device is not suitable for long-term monitoring; the Basette convergence measuring system adopts paired electrolyte sensors to measure the displacement change of a known length (or an arm lever) so as to obtain the relative displacement between measuring points, has the characteristics of high precision, automation and the like, is installed in a roadway and tightly attached to the wall of the tunnel, has no sight requirement, can almost generate the profile of the section of the roadway in real time, but is difficult to install, long in debugging time consumption and high in cost, cannot stably work for a long time due to the influence of high temperature and high humidity of underground engineering, is difficult to maintain in the operation period, and basically cannot be repaired; the existing optical fiber type convergence system mainly adopts a distributed optical fiber strain and temperature sensing mechanism (CN 102384725B) based on Brillouin scattering, an optical fiber is taken as a sensor, the optical fiber is installed on a tunnel section by gluing or fixed-point fixing, and the optical fiber is converted into convergence state change information of the tunnel section by calibrating the correlation between a sensor gauge length convergence value and a sensing optical fiber strain change amount to realize tunnel deformation monitoring.
In recent years, with the rapid development of the fiber grating sensing technology, some researchers try to apply the fiber grating to convergent displacement contact measurement, but only realize local measurement of tunnel deformation, and a long-term monitoring system which can be used for measuring the whole deformation of the cross section of an underground tunnel is not formed.
Disclosure of Invention
The invention aims to design an optical fiber type convergence meter and a using method thereof, which are used for monitoring the deformation of the cross section of an underground tunnel.
The purpose of the invention can be realized by adopting the following technical scheme: an optical fiber type convergence meter and a using method thereof, the optical fiber type convergence meter comprises a banded flexible substrate, a protective layer, a transmission optical cable, an optical fiber grating array and an optical fiber grating demodulator, wherein the banded flexible substrate and the protective layer are fixed together; the transmission optical cable is arranged on the surface of the banded flexible substrate or embedded in the protective layer, and one end of the transmission optical cable extends out of the protective layer to be connected with the fiber bragg grating demodulator; the fiber grating array is composed of a plurality of fiber gratings etched on the optical fibers in the transmission optical cable; the fiber grating demodulator demodulates the central wavelength variation value of each fiber grating in the fiber grating array in real time.
The fiber gratings on the fiber grating array are distributed at equal intervals, or different intervals are customized according to actual test requirements.
The fiber grating array is composed of high-reflectivity fiber gratings with different central wavelengths or composed of low-reflectivity weak gratings with the same central wavelength.
More than one fiber grating is arranged in the fiber grating array for temperature sensing and compensating the temperature influence when the other fiber gratings are used for strain measurement.
The strip-shaped flexible substrate is made of steel, PVC or shape memory alloy.
The protective layer is made of epoxy resin glue, silica gel or vulcanized rubber.
The use method of the optical fiber type convergence meter comprises the following steps:
s1: installing an optical fiber type convergence meter close to the inner wall of the underground tunnel, and winding the optical fiber type convergence meter around the inner wall of the cross section of the underground tunnel by a circle;
s2: one end of the transmission optical cable is connected to a fiber grating demodulator, and the central wavelength change value of the fiber grating is monitored in real time through the fiber grating demodulator;
s3: converting the wavelength variation of the grating into a convergence curvature value of the position of the fiber grating according to a proportional relation between the wavelength variation and the curvature of the fiber grating calibrated in advance;
s4: and constructing a two-dimensional shape of the optical fiber type convergence meter, namely a deformation curve of the inner wall of the cross section of the underground tunnel by using the position information and the curvature value of each optical fiber grating collected on the optical fiber type convergence meter through a two-dimensional curve reconstruction algorithm to obtain the convergence deformation information of vault settlement and side wall displacement of the cross section of the underground tunnel.
Compared with the prior art, the invention has the beneficial effects that:
(1) the invention combines the advantages of strong anti-interference capability of the fiber bragg grating, simple structure, small volume and easy implantation into various materials without affecting the material characteristics, adopts a flexible belt-shaped structure, can be arranged by being clung to the surface of the inner wall of the underground tunnel, has no requirement on the visual field condition, and can realize long-term, real-time and on-line monitoring and early warning of the cross section deformation of the underground tunnel such as a tunnel, a mine tunnel, a gallery and the like from the construction to the operation period;
(2) the fiber bragg grating excellent strain and temperature sensor is integrated, the temperature compensation function is realized, and the high-precision monitoring of the deformation of the underground roadway can be realized;
(3) the multichannel fiber bragg grating demodulator is adopted to demodulate sensing signals, a plurality of fiber type convergence meters can be simultaneously arranged at different axial positions of the underground tunnel, a large-range and distributed underground tunnel deformation monitoring network is constructed, and compared with a distributed fiber strain sensing system, the multichannel fiber bragg grating demodulator can greatly reduce the cost and improve the testing precision.
Drawings
FIG. 1 is a schematic structural diagram of a fiber-optic convergence meter of the present invention;
FIG. 2 is a schematic view of a structure in which a transmission cable is attached to the surface of a ribbon-shaped flexible substrate;
FIG. 3 is a schematic view of a structure of a transmission cable embedded in a protective layer;
FIG. 4 is a schematic view of the installation and layout of the optical fiber type convergence gauge in the underground tunnel;
FIG. 5 is a schematic diagram of a fiber grating;
FIG. 6 is a graph of an input spectrum of a fiber grating;
FIG. 7 is a graph of the transmission spectrum of a fiber grating;
FIG. 8 is a graph of the excitation wavelength variation to be measured of the reflection spectrum of a fiber grating;
FIG. 9 is a schematic diagram of a bending mechanism microstructure unit of a curvature sensor based on FBG;
FIG. 10 is a schematic diagram of bending deformation of a micro-structural unit of a bending mechanism of a FBG-based curvature sensor;
FIG. 11 is a schematic diagram of a two-dimensional planar curve fit;
in the figure: 1. the device comprises a banded flexible substrate, 2 parts of a protective layer, 3 parts of a transmission optical cable, 31-3N parts of a fiber bragg grating array, 4 parts of a fiber bragg grating demodulator, 5 parts of an underground roadway, 6 parts of an optical fiber type convergence meter.
Detailed Description
The following detailed description of the embodiments of the present invention will be described in conjunction with the accompanying drawings, which are included for the purpose of illustration only and are not to be construed as limiting the invention, and in order to better illustrate the following embodiments, some components of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
As shown in fig. 1, 2, 3, and 4, an optical fiber type convergence meter includes a ribbon-shaped flexible substrate 1, a protective layer 3, a transmission optical cable 3, an optical fiber grating array 31-3N, and an optical fiber grating demodulator 4, where the ribbon-shaped flexible substrate 1 is fixed with the protective layer 2, the ribbon-shaped flexible substrate 1 is made of steel, PVC, or a shape memory alloy, and the protective layer 2 is made of epoxy resin adhesive, silica gel, or vulcanized rubber; the transmission optical cable 3 is arranged on the surface of the banded flexible substrate 1 or embedded in the protective layer 2, and one end of the transmission optical cable 3 extends out of the protective layer 2 to be connected with the fiber bragg grating demodulator 4; the fiber grating array 31-3N is composed of a plurality of fiber gratings etched on the optical fibers in the transmission optical cable 3, the fiber gratings can be arranged at equal intervals, different intervals can be customized according to actual test requirements, the fiber grating array 31-3N is composed of high-reflectivity fiber gratings with different central wavelengths or composed of low-reflectivity weak gratings with the same central wavelength, and at least one fiber grating in the fiber grating array is used for temperature sensing and compensating temperature influence when the other fiber gratings are used for strain measurement; the fiber grating demodulator 4 demodulates the central wavelength variation value of each fiber grating in the fiber grating array 31-3N in real time.
The use method of the optical fiber type convergence meter comprises the following steps:
s1: installing an optical fiber type convergence meter 6 close to the inner wall of the underground tunnel 5, and winding the inner wall of the cross section of the underground tunnel 5 for a circle;
s2: one end of a transmission optical cable 3 is connected to a fiber grating demodulator 4, and the central wavelength change value of the fiber grating is monitored in real time through the fiber grating demodulator 4;
s3: converting the wavelength variation of the grating into a convergence curvature value of the position of the fiber grating according to a proportional relation between the wavelength variation and the curvature of the fiber grating calibrated in advance;
s4: the two-dimensional shape of the optical fiber type convergence meter 6, namely the deformation curve of the inner wall of the cross section of the underground tunnel 5 is constructed by the position information and the curvature value of each optical fiber grating collected on the optical fiber type convergence meter 6 through a two-dimensional curve reconstruction algorithm, so that the convergence deformation condition of the cross section of the underground tunnel 5, such as vault settlement, side wall displacement and other information, is obtained.
The measurement principle of the optical fiber type convergence meter 6 for monitoring the cross section deformation of the underground tunnel 5 is as follows:
the Fiber Bragg Grating (FBG) is a passive device of optical Fiber, which utilizes the photosensitivity of doped (such as germanium, phosphorus, etc.) optical Fiber, and makes the interaction between the external incident photons and the doped ions in the Fiber core through a certain process method to cause the periodic permanent change of the refractive index of the Fiber core along the axial direction of the optical Fiber, so as to form a spatial phase Grating in the Fiber core, which essentially forms a narrow-band reflective filter or mirror in the Fiber core, and only the light wave satisfying the Grating Bragg condition, i.e. formula (1), can be reflected, as shown in fig. 5, 6, 7, 8:
λB＝2neffΛ (1)
wherein λBIs the central wavelength of the grating, i.e. the Bragg wavelength, neffIs the effective index of the fiber core and Λ is the grating period.
As shown in the formula (1), the Bragg wavelength of the fiber grating depends on the grating period Λ and the effective refractive index neffAny physical process that changes these two parameters will cause a shift in the Bragg wavelength of the grating, which when measured causes a change in the stress state of the sensing grating, will result in neffOr Λ, to produce a corresponding shift in the center reflection wavelength of the sensing grating, the shift being determined by equation (2), i.e.:
ΔλB＝2Δneff·Λ+2neff·ΔΛ (2)
under the condition of not considering the temperature influence, under the action of uniform axial strain of the fiber grating, by analyzing the Bragg central wavelength shift caused by the elasto-optical effect and the strain effect, the longitudinal strain sensitivity expression of the fiber grating can be obtained as follows:
in the formula, p11、p12The elasto-optic constant, i.e. the longitudinal and transverse refractive index changes caused by longitudinal strain.
(3) The formula can be further simplified as follows:
wherein p iseIs an effective elasto-optic coefficient, about 0.216. And (4) monitoring the shift condition of the central reflection wavelength through the real-time fiber grating according to the formula (4), so that the change condition of the axial strain borne by the grating can be obtained.
The optical fiber type convergence meter 6 adopts a ribbon structure, only material deformation caused by bending is caused in actual use, and the conditions such as distortion and the like are not considered, so that the deformation measurement of the optical fiber type convergence meter 6 is simplified into the problem of two-dimensional plane curve shape sensing; selecting a section of microstructure unit with length L and thickness 2h on a convergence meter, as shown in FIGS. 9 and 10, setting the microstructure unit to meet an ideal deformation condition, assuming that the curvature radius of the bending deformation of the microstructure unit is R and the corresponding central angle is theta; in the flexible bending deformation zone, the bending inner side of the structural infinitesimal is shortened under the action of compression, the length is changed into L-delta L, the stress state is one-way compression, the outer side is stretched under the action of tension, and the length is changed into L + delta L; the length of a layer between two deformation zones of shortening and extending is constant all the time, namely the strain quantity is zero, namely a strain neutral layer, the length of the strain neutral layer is kept constant, and according to a material mechanics formula, under a pure bending condition, the length of the strain neutral layer is as follows:
L＝R·θ (5)
L+ΔL＝(R+h)·θ (6)
L-ΔL＝(R-h)·θ (7)
the equations (5) to (8) can be solved
Wherein C is the bending curvature of the microstructure unit, the FBG is adhered to the upper surface (FBG1) or the lower surface (FBG2) of the microstructure unit, and the following formulas (4) and (9) are combined to obtain:
for any determined FBG, λB、peSince h is constant, the curvature C is linear with the FBG center wavelength variation offset, and equation (10) can be further rewritten as follows:
C＝K·ΔλB (11)
in the formula, K is the sensitivity of curvature change to FBG wavelength variation and can be measured by experimental calibration; the bending curvature sensing of each FBG on the fiber-optic type convergence gauge 6 can be realized according to the equation (11).
The optical fiber type convergence meter 6 adopts a belt-shaped structure, is laid for a circle along the inner wall of the cross section of the underground tunnel 5, can be considered that the optical fiber type convergence meter 6 is only bent and deformed in the cross section of the tunnel, and has no distortion, so that the research of a two-dimensional plane curve reconstruction algorithm is only considered according to discrete curvature information, any section of curve on the convergence meter is selected as shown in figure 11,
in the figure Qn、Qn+1Are respectively two points on the curve, SnIs the arc length between two points, thetan、θn+1The included angles between the tangent lines of the two points and the positive direction of the X axis are included angles, namely tangent direction angles. When Q is knownnCoordinate value and arc length S of pointnThen Q can be pushed outn+1Then the whole curve is reconstructed by the method. The fitting recursion equation of the curve is established as follows:
according to known conditions, since Qn、Qn+1Between them is a circular arc, so that QnAnd Qn+1The intersection angle between two tangent lines and two connecting lines is equal, so there is QnTo Qn+1Vector of (2)
Comprises the following steps:
in the formula
Is a curve at QnThe vector of the tangent line of (a),
is a curve at Qn+1The tangent vector of (c).
QnAnd Qn+1Chord length L therebetweennComprises the following steps:
Ln＝2×Sn×sin(γ/2)/γ (13)
wherein gamma is
And
the calculation formula of the included angle is as follows:
γ＝|θn+1-θn| (14)
as can be seen from the geometric relationship calculation, γ is equal to the arc angle corresponding to the arc segment, and therefore:
then there are:
accordingly, the coordinate position of the (n + 1) th point relative to the nth point can be obtained, that is, the coordinate of the position of the (n + 1) th point is recurred according to the coordinate position of the nth point, and the recurred formula is as follows:
knowing the position information of each point FBG on the optical fiber type convergence meter 6, combining the formula (12) to obtain the arc angle theta of the corresponding segment, and then setting initial conditions: first point coordinate Q0(x0,y0) And corresponding tangential direction angle theta0Then, a two-dimensional shape curve of the whole optical fiber type convergence meter 6, namely a deformation curve of the inner wall of the cross section of the underground tunnel 5 can be reconstructed according to the formula (17) by combining spline interpolation and other methods, so that the convergence deformation condition of the cross section of the underground tunnel 5, such as vault settlement, side wall displacement and other information, can be obtained; when the structure is deformed, the corresponding FBG wavelength is changed, and based on the curve shape reconstruction method, the continuous, real-time and on-line monitoring of the section deformation of the underground tunnel 5 can be realized.
The invention is not limited to the details of the prior art, and it should be understood that the above-described embodiments of the invention are only examples for clearly illustrating the technical solutions of the invention, and are not intended to limit the specific embodiments of the invention. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention claims should be included in the protection scope of the present invention claims.
Claims (7)
1. An optical fiber type convergence meter and a using method thereof are characterized in that: the optical fiber type convergence meter comprises a banded flexible substrate (1), a protective layer (2), a transmission optical cable (3), an optical fiber grating array (31-3N) and an optical fiber grating demodulator (4); the belt-shaped flexible substrate (1) and the protective layer (2) are fixed together; the transmission optical cable (3) is arranged on the surface of the banded flexible substrate (1) or embedded in the protective layer (2), and one end of the transmission optical cable (3) extends out of the protective layer (2) to be connected with the fiber bragg grating demodulator (4); the fiber bragg grating array (31-3N) is formed by a plurality of fiber bragg gratings etched on optical fibers in the transmission optical cable (3); the fiber grating demodulator (4) demodulates the central wavelength variation value of each fiber grating in the fiber grating array (31-3N) in real time.
2. The fiber optic convergence gauge of claim 1, wherein: the fiber gratings on the fiber grating array (31-3N) are distributed at equal intervals.
3. The fiber optic convergence gauge of claim 1, wherein: the fiber grating array (31-3N) is composed of high-reflectivity fiber gratings with different central wavelengths or low-reflectivity weak fiber gratings with the same central wavelength.
4. The fiber optic convergence gauge of claim 1, wherein: more than one fiber grating in the fiber grating array (31-3N) is used for temperature sensing and compensating the temperature influence when the other fiber gratings are used for strain measurement.
5. An optical fiber type convergence gauge according to claim 1, wherein the flexible ribbon substrate (1) is made of steel, PVC or shape memory alloy.
6. The fiber optic convergence gauge of claim 1, wherein: the protective layer (2) is made of epoxy resin glue, silica gel or vulcanized rubber.
7. A method of using the fiber optic convergence meter of any one of claims 1-6, comprising: the method comprises the following steps:
s1: the optical fiber type convergence meter (6) is tightly attached to the inner wall of the underground tunnel (5) and winds the inner wall of the cross section of the underground tunnel (5) for a circle;
s2: one end of the transmission optical cable (3) is connected to a fiber grating demodulator (4), and the central wavelength change value of the fiber grating is monitored in real time through the fiber grating demodulator (4);
s3: converting the wavelength variation of the grating into a convergence curvature value of the position of the fiber grating according to a proportional relation between the wavelength variation and the curvature of the fiber grating calibrated in advance;
s4: and (3) constructing a two-dimensional shape of the optical fiber type convergence meter (6), namely a deformation curve of the inner wall of the cross section of the underground tunnel (5), by using the position information and the curvature value of each optical fiber grating collected on the optical fiber type convergence meter (6) through a two-dimensional curve reconstruction algorithm to obtain the convergence deformation information of vault settlement and side wall displacement of the cross section of the underground tunnel (5).
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113776450A (en) * | 2021-10-09 | 2021-12-10 | 武汉市勘察设计有限公司 | A ground deformation monitoring system and monitoring method based on optical fiber technology |
| CN114353686A (en) * | 2021-09-10 | 2022-04-15 | 重庆交通大学 | Intelligent acquisition method and related device of curvature distribution of tunnel lining |
| CN115014223A (en) * | 2022-05-25 | 2022-09-06 | 汕头大学 | Submarine cable deformation monitoring system based on sensing grating array |
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