A fiber optic two-dimensional strain sensor

Abstract

The present invention discloses a two-dimensional optical fiber strain sensor, belonging to the field of optical fiber sensing technology, comprising an FBG strain sensor. The FBG strain sensor has an optical fiber groove along its length, an optical fiber is disposed within the groove, and a plurality of optical fiber bonding points are disposed on the optical fiber. A diamond structure is disposed at each end of the FBG strain sensor, wherein a first grating and a second grating are disposed within the two diamond structures, respectively. A circular structure is symmetrically disposed about the center of the FBG strain sensor, a third grating is disposed between the two circular structures, and a sensing arm is disposed between the diamond structure and the circular structure. The present invention can measure strain in two directions, and has improved sensitivity in both directions. The FBG strain sensor of the present invention can measure strain in both the axial and radial directions of the optical fiber within a two-dimensional plane, while determining the direction of the strain applied by the first and second gratings within the diamond structure.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber sensing, and in particular relates to an optical fiber two-dimensional strain sensor. Background Art

[0002] By monitoring the temperature, strain, force, displacement or vibration of key mechanical components, the health status of the monitored mechanical equipment can be estimated through specific algorithms. One of the most important physical parameters to be monitored in the field. ) sensor has the advantages of small size, corrosion resistance, and anti-electromagnetic interference ( ) and multiplexing capabilities. With appropriate packaging, the effect on The above strain is converted into The change of the reflected central wavelength is demodulated by a certain spectrum demodulation method. Multi-directional measurement of strain sensors requires the deployment of fiber Bragg gratings in each strain direction to be measured, and the strain sensitivity is generally low. For example, the FBG six-dimensional strain sensor for monitoring frozen soil can obtain the frozen soil change process by monitoring the strain in six directions; through a circular substrate, the six different directions are packaged , which can realize 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 they are packaged and assembled through fiber-reinforced polymer to realize three-dimensional measurement of the road surface; this is a two-dimensional strain sensor, which is assembled through the sensor group in After installation and measurement, the strain data is consistent with the resistance strain gauge.

[0003] A single one-dimensional strain sensor is only sensitive to strain in one direction, a characteristic that limits its use. When multidimensional strain measurement is required in practical applications, the FBG multidimensional strain sensor requires deploying FBG strain sensors in each measurement direction. However, the sensor string layout is relatively complex, and the strain sensitivity is low, typically lower than that of bare fiber Bragg gratings. Summary of the Invention

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

[0005] To achieve the above-mentioned object, the present invention provides the following solution: The present invention provides a two-dimensional optical fiber strain sensor, comprising an FBG strain sensor, wherein an optical fiber groove is provided along the length direction of the FBG strain sensor, an optical fiber is provided in the optical fiber groove, and a plurality of optical fiber bonding points are provided on the optical fiber, and a diamond structure is provided at each end of the FBG strain sensor, and a first grating ( ) and the second grating ( ), the center of the FBG strain sensor is symmetrically provided with a round-shaped structure, and a third grating is provided between the two round-shaped structures ( ), a sensing arm is provided between the diamond-shaped structure and the circular structure. A certain wavelength spacing is required.

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

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

[0008] Preferably, The direction is enhanced by the sensor arm, the principle is the right triangle amplification principle: under the premise of the hypotenuse remaining unchanged, the two right angles of the right triangle are changed, and the amount of change is different. Longer than the right angle side Big changes; The theoretical magnification of the sensor arm can be obtained Related to the sensor arm angle:

[0009] .

[0010] Preferably, the strain of the FBG sensor and the strain at the measured point The relationship is:

[0011]

[0012] For optical fiber Length between bonding points, For optical fiber The length change between bonding points, is the length between the points to be measured, is the length change between the points to be measured;

[0013] The sensor sensitivity is thus defined as:

[0014]

[0015] in represent The center wavelength of the reflected light, is the central wavelength drift of the sensing grating, is the effective elastic-optical coefficient.

[0016] The present invention discloses the following technical effects: the present invention can realize the measurement of strain in two directions, and the sensitivity in both directions is improved. The FBG strain sensor of the present invention can measure the strain in both the axial and radial directions of the optical fiber in a two-dimensional plane, and at the same time determine the direction of the strain through the first and second gratings in the diamond structure. When the FBG strain sensor is subjected to the axial strain of the optical fiber, the first grating in the diamond structure ( ) and the second grating ( ) is subjected to tensile strain, the wavelength ( ) drifts to the right; when the strain is in the radial direction of the fiber, the fiber Bragg grating in the diamond region exhibits compressive strain, and the wavelength ( ) is shifted to the left. The direction of strain on the FBG sensor can be distinguished by observing the wavelength drift direction of the first and second gratings in the diamond structure. And the third grating ( ) sensitivity enhancement, 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 accompanying drawings, which constitute part of this application, are intended to provide a further understanding of this application. The exemplary embodiments and descriptions of this application are intended to explain this application and do not constitute an improper limitation on this application. In the accompanying drawings:

[0018] Figure 1 Schematic diagram of the structure of the optical fiber two-dimensional strain sensor of the present invention;

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

[0020] Figure 3 Schematic diagram of the relationship between the strain amplification factor and the angle of the sensing arm of the present invention;

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

[0022] Figure 5 Schematic diagram of strain under different boundary load conditions in Example 1;

[0023] Figure 6 Schematic diagram of the strain amplification factor under different boundary load conditions in Example 1.

[0024] In the figure: 1. Diamond structure; 2. Sensing arm; 3. Fiber optic groove; 4. Ring structure; 5. Welding point; 6. Fiber optic bonding point; 7. First grating; 8. Second grating; 9. Third grating. DETAILED DESCRIPTION

[0025] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0026] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0027] Reference Figure 1-Figure 2 As shown, this embodiment provides a fiber optic two-dimensional strain sensor, including an FBG strain sensor. An optical fiber groove 3 is provided along the length direction of the FBG strain sensor, an optical fiber is provided in the optical fiber groove 3, and a plurality of optical fiber bonding points 6 are provided on the optical fiber. Diamond structures 1 are provided at both ends of the FBG strain sensor, a first grating 7 and a second grating 8 are provided in the two diamond structures 1, respectively. A meandering structure 4 is symmetrically provided at the center of the FBG strain sensor, a third grating 9 is provided between the two meandering structures 4, and a sensing arm 2 is provided between the diamond structure 1 and the meandering structure 4.

[0028] The present invention can measure strain in two directions, and the sensitivity in both directions is improved. The FBG strain sensor of the present invention can measure strain in both the axial and radial directions of the optical fiber within a two-dimensional plane, while simultaneously determining the direction of the strain applied via the first grating 7 and the second grating 8 within the diamond structure 1. When the FBG strain sensor is subjected to axial strain of the optical fiber, the first grating 7 and the second grating 8 within the diamond structure 1 are subjected to tensile strain, and the wavelength shifts to the right. When the strain is radial, the fiber Bragg grating in the diamond region exhibits compressive strain, and the wavelength shifts to the left. The direction of the strain applied to the FBG sensor can be distinguished by observing the direction of wavelength shift of the first grating 7 and the second grating 8 in the diamond structure 1. Furthermore, by enhancing sensitivity through the ring structure 4 and the sensing arm 2, the sensitivity of the optical fiber to axial and radial strain is optimized according to the angle of the sensing arm 2, achieving an optimal sensitivity design.

[0029] Under the action of directional tensile stress, 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 tensile stress, and the wavelength shifts to the right side of the spectrum. The strain size is read by wavelength or spectrum demodulation; Under the action of directional compressive stress, 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 strain size is read by wavelength or spectrum demodulation. Under the action of directional tensile stress, 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 strain size is read by wavelength or spectrum demodulation. Under directional compressive stress, the first and second gratings 7 and 8 experience tensile stress, causing their wavelengths to shift to the right of the spectrum. The third grating 9 experiences compressive stress, causing its wavelengths to shift to the left of the spectrum. Spectral demodulation is used to read the strain magnitude. The direction of the strain can be determined by observing the wavelength shifts of the first, second, and third gratings 7, 8, and 9. Strain data is obtained by demodulating the third grating 9.

[0030] Under the action of directional stress, the sensitivity of the sensor becomes more sensitive as the angle of the sensing arm 2 increases, and the extreme value appears at around 17 degrees. After that, the strain sensitivity decreases as the angle of the sensing arm 2 increases. Under the action of directional stress, the sensitivity of the sensor decreases with the increase of the angle of the sensor arm 2, and an extreme point appears at about 19 degrees. The influence of the angle design of the sensor arm 2 on the sensitivity is verified, and the angle of the sensor arm 2 is arrive When the strain magnification factor is between 0 and 1, the sensitivity is increased. The relationship between the strain magnification factor and the angle of the sensing arm 2 is as follows: Figure 3 shown.

[0031] In a further optimized solution, 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 the multiple welding points 5.

[0032] Further optimizing the scheme, the sensitivity enhancement formula of the circular structure 4 is:

[0033]

[0034] in is the strain at the measured point, For external force, is Young's modulus, is the cross-sectional area.

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

[0036] Further optimize the plan, The direction is enhanced by the sensing arm 2, the principle is the right triangle amplification principle: under the premise that the hypotenuse of a right triangle remains unchanged, the two right angles are changed, and the amount of change is different. Longer than the right angle side Big changes; The theoretical magnification of sensor arm 2 can be obtained Related to the angle of sensor arm 2:

[0037] .

[0038] Further optimization scheme, the strain of FBG sensor and the strain at the measured point The relationship is:

[0039]

[0040] For optical fiber Length between bonding points, For optical fiber The length change between bonding points, The length between the points to be measured is the length change between the points to be measured;

[0041] The sensor sensitivity is thus defined as:

[0042]

[0043] in represent The center wavelength of the reflected light, is the central wavelength drift of the third grating, is the effective elastic-optical coefficient.

[0044] Diamond structure 1: When the FBG sensor is subjected to axial ( ) strain, the rhombus grating area will be subjected to tensile strain, and the wavelength will shift to the right; when Affected by the fiber radial direction ( When strain occurs, the grating area in the diamond-shaped region experiences compressive strain, causing the wavelength to shift to the left. The grating in the diamond-shaped region can be used to distinguish the direction of strain. Sensing arm 2 primarily transmits radial stress, while fiber groove 3 protects the optical fiber. The meandering structure 4 concentrates strain in areas with a smaller cross-sectional area, enhancing sensitivity. The combination of sensing arm 2, diamond structure 1, and meandering structure 4 enables two-dimensional, omnidirectional strain measurement.

[0045] ①, ②, ③, ④, ⑤, and ⑥ are optical fiber bonding points 6. Between ① and ⑤, and between ④ and ⑥, are diamond-shaped gratings responsible for stress direction determination, namely the first grating 7 and the second grating 8, respectively. Between ② and ③, is a circular grating responsible for strain measurement, namely the third grating 9.

[0046] Example 1

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

[0048] Welding point 5 ( ) between the strain to be measured workpiece Directional actual strain ;

[0049] Welding point 5 ( ) between the strain to be measured workpiece Directional actual strain ;

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

[0051] , is the strain of the optical fiber during axial tension;

[0052] , is the strain of the optical fiber during radial stretching;

[0053] The strain magnification factor is defined as:

[0054] , , is the strain magnification factor during axial tension;

[0055] , , is the strain magnification factor during radial stretching;

[0056] Numerical simulation:

[0057] Directional stretching simulation: The left end of the object to be tested is fixed and constrained, and a boundary load is applied to the right end.

[0058] Directional stretching simulation: The lower end of the object to be tested is fixedly constrained, and a boundary load is applied to the upper end.

[0059] The strain and strain magnification factor under different boundary load conditions can be calculated as follows: Figure 5-Figure 6 shown.

[0060] According to the sensitivity definition of grating strain sensor, the sensitivity of bare fiber is generally 1.2 , which can be calculated 、 The directional strain sensitivity is 1.77 and 1.764 , the strain magnification is , , .

[0061] In the description of the present invention, it should be understood that the terms "longitudinal", "transverse", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are 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 cannot be understood as a limitation on the present invention.

[0062] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined 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), and a plurality of optical fiber bonding points (6) are provided on the optical fiber. A rhombus structure (1) is provided at each end of the FBG strain sensor, a first grating (7) and a second grating (8) are provided in each of the two rhombus structures (1), a circular structure (4) is provided symmetrically at the center of the FBG strain sensor, a third grating (9) is provided between the two circular structures (4), and a sensing arm (2) is provided between the rhombus structure (1) and the circular structure (4). Welding points (5) are respectively provided at both ends of the sensing arm (2) and the outer side of the diamond structure (1); The sensitivity enhancement principle of the loop structure (4) is the strain concentration caused by the reduction of cross-sectional area; Under the action of directional stress, the sensitivity of the sensor becomes more sensitive as the angle of the sensing arm (2) increases.

2. The optical fiber two-dimensional strain sensor according to claim 1, characterized in that: The direction is enhanced by the sensing arm (2), the principle of which is the right triangle amplification principle: under the premise that the hypotenuse of a right triangle remains unchanged, the amount of change of the two right angles is different, and the shorter right angle is Longer than the right angle side Big changes; The theoretical magnification of the sensor arm (2) can be obtained Related to the angle of the sensing arm (2): 。 3. The optical fiber two-dimensional strain sensor according to claim 1, characterized in that: Strain of the sensor and the strain at the measured point The relationship is: is the length between the bonding points of the third grating (9), is the length variation between the bonding points of the third grating (9), is the length between the points to be measured, is the length change between the points to be measured; The sensor sensitivity is thus defined as: in represent The center wavelength of the reflected light, is the central wavelength drift of the sensing grating, is the effective elastic-optical coefficient.

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