Optical fiber two-dimensional strain sensor

Abstract

The invention discloses an optical fiber two-dimensional strain sensor, and belongs to the technical field of optical fiber sensing, the optical fiber two-dimensional strain sensor comprises an FBG strain sensor, an optical fiber groove is arranged in the FBG strain sensor along the length direction, an optical fiber is arranged in the optical fiber groove, a plurality of optical fiber bonding points are arranged on the optical fiber, and rhombic structures are respectively arranged at two ends of the FBG strain sensor. A first grating and a second grating are respectively arranged in the two rhombic structures, concentric-square-shaped structures are symmetrically arranged in the center of the FBG strain sensor, a third grating is arranged between the two concentric-square-shaped structures, and sensing arms are arranged between the rhombic structures and the concentric-square-shaped structures. According to the invention, the strain measurement in two directions can be realized, and the sensitivity in the two directions is improved. The FBG strain sensor can measure the strain in the axial direction and the radial direction of the optical fiber in a two-dimensional plane, and meanwhile, the strain direction is judged through the first grating and the second grating in the rhombic structure.

Description

Technical Field

[0001] The present invention belongs to the technical field of fiber optic sensing, and particularly relates to a fiber optic two-dimensional strain sensor. Background Art

[0002] By monitoring the temperature, strain, force, displacement or vibration of key mechanical components, the health state of the monitored mechanical equipment can be estimated through specific algorithms. Strain is one of the most important monitored physical parameters in modern mechanical equipment SHM. Sensors based on fiber Bragg gratings (FBGs) have the advantages of small size, corrosion resistance, electromagnetic interference (EMI) resistance and multiplexing ability. By properly packaging the FBG, the strain acting on the FBG can be converted into a change in the reflection center wavelength of the FBG and demodulated through a certain spectral demodulation method. Currently, the common FBG strain sensors on the market require fiber Bragg gratings to be arranged in each strain direction to be measured for multi-directional measurement, and the strain sensitivity is generally low. For example, the FBG six-dimensional strain sensor for monitoring frozen soil. By monitoring the strain in six directions, the change process of frozen soil can be obtained; through a circular substrate, FBGs are packaged in six different directions to achieve strain monitoring in six directions within a plane; through the analysis of the highway pavement layer, vertical strain sensors and horizontal strain sensors are designed, and are packaged and assembled through fiber-reinforced polymers to achieve three-dimensional measurement of the pavement; this is a two-dimensional strain sensor, which is installed and measured by a sensor group at 0°, 45°, and 90°, and the strain data is consistent with that of the resistance strain gauge.

[0003] A single one-dimensional strain sensor is only sensitive to the strain in one direction, and this characteristic limits the use of FBG strain sensors. When multi-dimensional strain measurement is required in practical applications, the FBG multi-dimensional strain sensors adopted need to arrange FBG strain sensors in each measurement direction, but the arrangement of the sensor string is relatively complex, and the strain sensitivity is low, usually lower than that of the bare fiber Bragg grating. Summary of the Invention

[0004] The purpose of the present invention is to provide a fiber optic two-dimensional strain sensor to solve the problems existing in the above-mentioned prior art.

[0005] To achieve the above purpose, the present invention provides the following solution: The present invention provides a fiber optic two-dimensional strain sensor, including an FBG strain sensor. A fiber groove is provided in the FBG strain sensor along the length direction. An optical fiber is provided in the fiber groove. A plurality of optical fiber bonding points are provided on the optical fiber. Diamond structures are respectively provided at both ends of the FBG strain sensor. A first grating (λ 1 ) and a second grating (λ 2 ) are respectively provided in the two diamond structures. A loop structure is symmetrically provided at the center of the FBG strain sensor. A third grating (λ3 ) There is a sensing arm between the rhombic structure and the loop structure. Generally, λ 1 , λ 2 , λ 3 requires a certain wavelength interval.

[0006] Preferably, welding points are respectively provided at both ends of the sensing arm and on the outer side of the rhombic structure.

[0007] Preferably, the sensitivity enhancement principle of the loop structure is the strain concentration caused by the reduction of the cross-sectional area.

[0008] Preferably, the sensitivity is enhanced in the Y direction through the sensing arm (2), and the principle is the right triangle amplification principle: in a right triangle, with the hypotenuse unchanged, when changing the two right sides, the change amounts are different, and the short right side a changes more than the long right side b; a 2 +b 2 =(a - Δa) 2 +(b + Δb) 2 It can be obtained that the theoretical amplification factor β of the sensing arm (2) is related to the angle of the sensing arm (2):

[0009]

[0010] Preferably, the strain ε of the FBG sensor FBG and the relationship with the strain ε to be measured is:

[0011]

[0012] L FBG is the length between the bonding points of optical fiber III, ΔL FBG is the change amount of the length between the bonding points of optical fiber III, L is the length between the points to be measured, and ΔL is the change amount of the length between the points to be measured;

[0013] Therefore, the definition of the sensor sensitivity:

[0014]

[0015] where λ B represents the central wavelength of the FBG reflected light, Δλ B is the drift amount of the central wavelength of the sensing grating, and p e is the effective elasto-optic coefficient.

[0016] The present invention discloses the following technical effects: The present invention can achieve the measurement of strains in two directions, and the sensitivities in both directions are improved. The FBG strain sensor in the present invention can measure the strains in two directions of the optical fiber axis and the optical fiber radial direction in a two-dimensional plane, and at the same time, the first grating and the second grating in the rhombic structure are used to judge the direction of the applied strain. When the FBG strain sensor is subjected to the strain in the optical fiber axis direction, the first grating (λ1 ) and the second grating (λ 2 ) are subjected to tensile strain, and the wavelength (λ 1， λ 2 ) drifts to the right; when the strain is in the radial direction of the optical fiber, the fiber grating in the diamond region exhibits compressive strain, and the wavelength (λ 1， λ 2 ) shifts to the left. The strain direction experienced by the FBG sensor is distinguished by observing the wavelength drift directions of the first and second gratings in the diamond structure. And the third grating (λ 3 ) is sensitized through the loop structure and the sensing arm. The axial and radial strain sensitivities of the optical fiber are optimized according to the angle of the sensing arm, and the optimal sensitivity design can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings forming a part of this application are used to provide a further understanding of this application. The schematic embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation of this application. In the drawings:

[0018] Figure 1 is a schematic structural diagram of the fiber two-dimensional strain sensor of the present invention;

[0019] Figure 2 is a schematic structural diagram of the welding point distribution of the present invention;

[0020] Figure 3 is a schematic diagram showing the relationship between the strain amplification coefficient and the angle of the sensing arm of the present invention;

[0021] Figure 4 is a schematic structural diagram of Embodiment 1 of the present invention;

[0022] Figure 5 is a schematic diagram of the strain under different boundary load conditions in Embodiment 1;

[0023] Figure 6 is a schematic diagram of the strain amplification coefficient under different boundary load conditions in Embodiment 1.

[0024] In the figure: 1. Diamond structure; 2. Sensing arm; 3. Optical fiber groove; 4. Loop structure; 5. Welding point; 6. Optical fiber bonding point; 7. First grating; 8. Second grating; 9. Third grating. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0026] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] Referring to Figure 1 - Figure 2 As shown, this embodiment provides a fiber optic two-dimensional strain sensor, including an FBG strain sensor. Along the length direction inside the FBG strain sensor, there is an optical fiber groove 3. Inside the optical fiber groove 3, there is an optical fiber. There are multiple optical fiber bonding points 6 on the optical fiber. At both ends of the FBG strain sensor, there are respectively diamond structures 1. Inside the two diamond structures 1, there are respectively a first grating 7 and a second grating 8. A loop structure 4 is symmetrically arranged at the center of the FBG strain sensor. Between the two loop structures 4, there is a third grating 9. Between the diamond structure 1 and the loop structure 4, there is a sensing arm 2.

[0028] The present invention can achieve the measurement of strain in two directions, and the sensitivity in both directions is improved. The FBG strain sensor in the present invention can measure the strain in two directions, namely the axial direction and the radial direction of the optical fiber, in a two-dimensional plane. At the same time, the first grating 7 and the second grating 8 inside the diamond structure 1 are used to judge the direction of the applied strain. When the FBG strain sensor is subjected to axial strain of the optical fiber, the first grating 7 and the second grating 8 inside the diamond structure 1 are subjected to tensile strain, and the wavelength drifts to the right; when the applied strain is the radial direction of the optical fiber, the fiber grating in the diamond region shows compressive strain, and the wavelength shifts to the left. By observing the wavelength drift direction of the first grating 7 and the second grating 8 in the diamond structure 1, the direction of the strain received by the FBG sensor can be distinguished. And through the loop structure 4 and the sensing arm 2 for sensitivity enhancement, the axial and radial strain sensitivities of the optical fiber can be optimized according to the angle of the sensing arm 2, and the best sensitivity design can be achieved.

[0029] Under the action of tensile stress in the X direction, the first grating 7 and the second grating are subjected to tensile stress, and the wavelength shifts to the right side of the spectrum. The third grating 9 is subjected to tensile stress, and the wavelength shifts to the right side of the spectrum. The magnitude of the strain is read through wavelength or spectrum demodulation; under the action of compressive stress in the X direction, the first grating 7 and the second grating 8 are subjected to compressive stress, and the wavelength shifts to the left side of the spectrum. The third grating 9 is subjected to compressive stress, and the wavelength shifts to the left side of the spectrum. The magnitude of the strain is read through wavelength or spectrum demodulation. Under the action of tensile stress in the Y direction, the first grating 7 and the second grating 8 are subjected to compressive stress, and the wavelength shifts to the left side of the spectrum. The third grating 9 is subjected to tensile stress, and the wavelength shifts to the right side of the spectrum. The magnitude of the strain is read through wavelength or spectrum demodulation; under the action of compressive stress in the Y direction, the first grating 7 and the second grating 8 are subjected to tensile stress, and the wavelength shifts to the right side of the spectrum. The third grating 9 is subjected to compressive stress, and the wavelength shifts to the left side of the spectrum. The magnitude of the strain is read through spectrum demodulation. By observing the wavelength shift direction of the first grating 7, the second grating 8, and the third grating 9, the strain direction is identified. The strain data is obtained by demodulating the third grating 9.

[0030] Under the action of the Y-direction stress, the sensitivity of the sensor becomes more sensitive as the angle of the sensing arm 2 increases. The extreme value appears at around 17 degrees, and then the strain sensitivity decreases as the angle of the sensing arm 2 increases. Under the action of the X-direction stress, the sensitivity of the sensor decreases as the angle of the sensing arm 2 increases, and an extreme point appears at around 19 degrees. It is verified that the angle design of the sensing arm 2 affects the sensitivity, and when the angle of the sensing arm 2 is between 10° and 40°, it has the effect of increasing the sensitivity. The relationship between the strain amplification coefficient and the angle of the sensing arm 2 is as Figure 3 shown.

[0031] For the further optimization scheme, welding points 5 are respectively provided at both ends of the sensing arm 2 and on the outer side of the diamond structure 1. The FBG strain sensor is welded to the object to be measured through multiple welding points 5.

[0032] For the further optimization scheme, the sensitivity enhancement formula of the loop structure 4 is:

[0033]

[0034] where ε is the strain at the point to be measured, F is the applied force, E is the Young's modulus, and A is the cross-sectional area.

[0035] Sensitivity enhancement principle of the loop structure 4: The strain is concentrated in the area with a smaller cross-sectional area, that is, concentrated in the loop structure 4, and the grating is arranged in this area to play a sensitivity enhancement role.

[0036] For the further optimization scheme, the sensitivity enhancement in the Y direction is achieved through the sensing arm 2. The principle is the right triangle amplification principle: on the premise that the hypotenuse of a right triangle remains unchanged, when changing the two right sides, their change amounts are different, and the short right side a changes more than the long right side b; a 2 +b 2 =(a - Δa) 2 +(b + Δb) 2 It can be obtained that the theoretical amplification factor β of the sensing arm 2 is related to the angle of the sensing arm 2:

[0037]

[0038] For the further optimization scheme, the relationship between the strain εFBG of the FBG sensor and the strain ε at the point to be measured is:

[0039]

[0040] L FBG is the length between the bonding points of optical fiber Ⅲ, ΔL FBG is the change amount of the length between the bonding points of optical fiber Ⅲ, L is the length between the points to be measured, and ΔL is the change amount of the length between the points to be measured;

[0041] Therefore, the definition of the sensor sensitivity:

[0042]

[0043] where λ B represents the central wavelength of the FBG reflected light, and Δλ B is the central wavelength drift of the third grating, and p e is the effective elasto-optic coefficient.

[0044] Rhombus structure 1: When the FBG sensor is subjected to axial (X-) strain, the grating area passing through the rhombus will be subjected to tensile strain, and the wavelength will shift to the right; when the FBG is subjected to radial (Y-) strain of the optical fiber, the grating area in the rhombus area will be subjected to compressive strain, and the wavelength will drift to the left. The strain direction can be distinguished by the grating in the rhombus area. The sensing arm 2 mainly plays a role in transmitting the radial stress, the optical fiber groove 3 mainly plays a role in protecting the optical fiber, and the loop structure 4 mainly means that the strain is concentrated in the area with a smaller cross-sectional area. The strain is concentrated in the loop structure 4, playing a role in sensitivity enhancement. Two-dimensional omnidirectional strain measurement can be achieved through the sensing arm 2, the rhombus structure 1, and the loop structure 4.

[0045] ①, ②, ③, ④, ⑤, ⑥ are the optical fiber bonding points 6. Between ① and ⑤, and between ④ and ⑥ are the grating areas of the rhombus region, responsible for judging the stress direction, which are the first grating 7 and the second grating 8 respectively; between ② and ③ is the grating area of the loop region, responsible for measuring the strain magnitude, which is the third grating 9.

[0046] Example 1

[0047] Referring to Figure 4 , the angle of the sensing arm 2 is selected to be 26.8 degrees. At this time, the strain sensitivities of the sensor for the axial and radial directions are the same. The encapsulation structure material is set as structural steel. Through the Comsol simulation software, it can be calculated that the strain sensitivities in the two directions of the optical fiber are the same, both about 1.8 pm / με. And within a certain strain range, the linearity is good.

[0048] The strain between the X welding points 5(A, B) is the actual strain ε x ;

[0049] The strain between the Y welding points 5(C, D) is the actual strain ε y ;

[0050] The strain measured by the fiber optic sensor is expressed as:

[0051] ε FBG , where X is the optical fiber strain during axial tension;

[0052] ε FBG , where Y is the optical fiber strain during radial tension;

[0053] Define the strain amplification factor as:

[0054] γX = ε FBG , x / ε x , γ X is the strain amplification coefficient during axial tension;

[0055] γ y = ε FBG , y / ε y , γ y is the strain amplification coefficient during radial tension;

[0056] Numerical simulation:

[0057] Tensile simulation in the X direction: The left end of the object to be measured is fixed and constrained, and a boundary load is applied to the right end.

[0058] Tensile simulation in the y direction: The lower end of the object to be measured is fixed and constrained, and a boundary load is applied to the upper end.

[0059] The strain and strain amplification coefficient under different boundary load conditions can be calculated as shown in Figure 5 - Figure 6 the following. According to the definition of the sensitivity of the grating strain sensor, the sensitivity of the bare optical fiber is generally 1.2 pm / με. The strain sensitivities in the X and Y directions can be calculated as 1.77 pm / με and 1.764 pm / με respectively. At this time, the strain amplification factor is γ X = 1.475 ， γ y = 1.47, γ X ≈ γ y .

[0060] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention.

[0061] The above-described embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. An optical fiber two-dimensional strain sensor, characterized in that: The invention comprises an FBG strain sensor, wherein an optical fiber groove (3) is provided in the FBG strain sensor along the length direction, an optical fiber is provided in the optical fiber groove (3), a plurality of optical fiber bonding points (6) are provided on the optical fiber, a rhombus structure (1) is provided at both ends of the FBG strain sensor, a first grating (7) and a second grating (8) are provided in the two rhombus structures (1), a return structure (4) is symmetrically provided at the center of the FBG strain sensor, a third grating (9) is provided between the two return structures (4), and a sensing arm (2) is provided between the rhombus structure (1) and the return structure (4).

2. The optical fiber two-dimensional strain sensor according to claim 1, characterized in that: Welding points (5) are respectively provided at both ends of the sensor arm (2) and the outer side of the diamond structure (1).

3. The optical fiber two-dimensional strain sensor according to claim 1, characterized in that: The sensitivity enhancement principle of the circular structure (4) is the strain concentration caused by the reduction of the cross-sectional area.

4. The optical fiber two-dimensional strain sensor according to claim 1, characterized in that: The Y direction is enhanced by the sensor arm (2), based on the principle of right triangle amplification: under the premise that the hypotenuse of a right triangle remains unchanged, if the two right angles are changed, the change amount is different, and the short right angle side a changes more than the long right angle side b; 2 +b 2 =(a-Δa) 2 +(b+Δb) 2 It can be obtained that the theoretical magnification β of the sensor arm (2) is related to the angle of the sensor arm (2):

5. The optical fiber two-dimensional strain sensor according to claim 1, characterized in that: The strain ε of the FBG sensor FBG The relationship between the strain ε of the measured point is: L FBG ΔL is the length between the bonding points of optical fiber III. FBG is the length variation between the bonding points of optical fiber III, L is the length between the points to be measured, and ΔL is the length variation between the points to be measured; The sensor sensitivity is thus defined as: where λ B represents the central wavelength of the reflected light from the FBG, Δλ B is the drift of the central wavelength of the sensing grating, p e is the effective elastic-optical coefficient.

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