A fiber grating displacement sensor with large range and high sensitivity and fixed at both ends

Abstract

The invention relates to the field of fiber grating sensors, and discloses a large-range and high-sensitivity two-end fixed fiber grating displacement sensor, comprising a shell, a movable probe is arranged at one end of the shell, a displacement transmission rod is arranged horizontally in the center of the shell, one end of the displacement transmission rod is fixed to the back of the probe, and the other end is fixed to the inner wall of the shell through a transmission rod reset device; the upper and lower surfaces of the displacement transmission rod are both provided with slopes with a longitudinal section of a right triangle, and two adjacent slopes are symmetrically arranged; two groups of sensing units are arranged inside the shell, the sensing units include a probe, an optical fiber and a probe reset device, the probe is fixed to the inner wall of the shell through the probe reset device, the needle of the probe contacts the slope, one end of the optical fiber is fixed to the probe, and the other end is tightened and pasted to the inner surface of the shell and then led out, and a grating is engraved on the optical fiber; the sensor disclosed by the invention takes into account the requirements of large range and high sensitivity in displacement measurement, and has the advantages of simple structure, flexible design and small size.

Description

Technical Field

[0001] The invention relates to the field of fiber grating sensors, and in particular to a large-range and high-sensitivity two-end fixed fiber grating displacement sensor. Background Art

[0002] There are many geomechanical challenges in the process of marine natural gas hydrate exploitation. Such as large amounts of sand production, wellbore shrinkage and instability, reservoir collapse after sediment particle deficiency, and large-scale submarine landslides, all of which have an important impact on the safe and efficient development of hydrates. Some scholars believe that the decomposition of hydrates caused by hydrate exploitation has little effect on submarine landslides in terms of space and time scales. Other scholars believe that hydrate decomposition is an important factor in triggering submarine landslides based on the analysis of three-dimensional seismic data. Therefore, it is of great significance to study whether the creep generated by hydrate decomposition will cause submarine landslides for the safe and efficient development of hydrates.

[0003] Displacement is often used to describe the amount of creep. At present, displacement sensors based on electrical and mechanical principles occupy most of the displacement detection market. These technologies have a long history of development and are highly mature. However, for submarine hydrate mining projects, electrical displacement sensors are active devices and have certain safety hazards. They also share the same shortcomings as mechanical displacement sensors, such as being easily disturbed by the external environment, having a short lifespan, and being unable to detect at long distances. Therefore, they are not suitable for displacement detection of submarine geological creep.

[0004] Fiber Bragg grating sensor technology uses the central wavelength of light reflection as the detection object and optical fiber as the propagation medium. It has the advantages of being passive, high precision, long life, anti-electromagnetic interference, and high resolution, and is suitable for harsh environments and long-term detection requirements. Common fiber Bragg grating displacement sensor structures include cantilever beam structure, spring structure, fixed-end structure or their combination structures. Through comparison, it is found that the fiber Bragg grating displacement sensor with cantilever beam and spring structure has the disadvantages of large volume and low sensitivity; the fixed-end structure displacement sensor has the problems of small range and low precision due to the small linear area; and other combination structures or combined structure displacement sensors have the disadvantages of larger volume, more complex structure, and higher cost of the sensor due to redundant accessories. Therefore, there is an urgent need for a fiber Bragg grating displacement sensor with a large range, high sensitivity, high precision, small volume, low cost and simple structure to meet the detection needs of submarine geological creep. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a large-range and high-sensitivity two-end fixed fiber Bragg grating displacement sensor, which has the advantages of small size, flexible design and simple structure.

[0006] To achieve the above object, the technical solution of the present invention is as follows:

[0007] A large-range and high-sensitivity, two-end fixed fiber Bragg grating displacement sensor comprises a shell, a movable probe is arranged at one end of the shell, a displacement transfer rod is arranged horizontally in the center of the shell, one end of the displacement transfer rod is fixed to the back of the probe, and the other end is fixed to the inner wall of the shell through a transfer rod reset device; the upper and lower surfaces of the displacement transfer rod are both provided with slopes with a longitudinal section of a right triangle, and two adjacent slopes are arranged symmetrically, and the slopes located on the upper and lower surfaces of the displacement transfer rod are arranged in parallel; two groups of sensing units are arranged inside the shell, and the two groups of sensing units are symmetrically arranged about the displacement transfer rod, and the sensing units include a probe, an optical fiber and a probe reset device, the probe is fixed to the inner wall of the shell through the probe reset device, the needle of the probe contacts the slope, one end of the optical fiber is fixed to the probe, and the other end is tightened and pasted to the inner surface of the shell and then led out, and a grating is engraved on the optical fiber.

[0008] In the above scheme, the transmission rod resetting device includes a support rod and a spring 1 sleeved on the support rod, the displacement transmission rod is sleeved on the support rod, the end of the support rod away from the probe is fixed on the inner wall of the outer shell, one end of the spring 1 is fixed on the inner wall of the outer shell, and the other end is connected to the displacement transmission rod.

[0009] In the above solution, the probe resetting device is a cantilever beam, one end of which is fixed to the inner wall of the housing away from the probe, and the other end is connected to the probe.

[0010] In the above scheme, the probe resetting device includes a shell fixed to the inner wall of the shell, and a spring 2 arranged in the shell, one end of the spring 2 is fixed to the shell, and the other end is connected to the probe, four strip grooves are longitudinally opened on the inner wall of the shell, and four slides are arranged on the side of the probe, and the slides are located in the strip grooves.

[0011] In the above solution, a groove is longitudinally provided on the slope surface, and the needle head of the probe is located in the groove.

[0012] In the above solution, the probe is disc-shaped, the back side of which is fixedly connected to the displacement transmission rod, and the edge of which is in close contact with the inner wall of the shell and can slide on the inner wall of the shell.

[0013] Through the above technical solution, the present invention provides a large-range and high-sensitivity two-end fixed fiber Bragg grating displacement sensor with the following beneficial effects:

[0014] 1. The probe of the present invention adopts a large-area disc shape, which increases the contact area with the object to be measured, avoids the situation where a needle-shaped probe may damage the surface of the object to be measured, and the disc-shaped probe will be suitable for more application environments.

[0015] 2. The present invention utilizes a displacement transmission rod with a periodically arranged slope surface to not only change the direction of displacement transmission, but also change the sensitivity by adjusting the ratio of the right-angle side lengths of the slope surface. Increasing the number of periods of the slope surface can expand the measuring range and realize large-range and high-sensitivity displacement detection.

[0016] 3. The fiber Bragg grating displacement sensor with a fixed structure at both ends of the present invention has high sensitivity, and the symmetrical placement of two sensing units will reduce the volume of the sensor. The two grating reflection center wavelengths have the same drift to temperature, and the influence on temperature is eliminated by subtracting the two reflection center wavelengths.

[0017] 4. The present invention adopts two signal processing methods. The first signal processing method has greater sensitivity in a single cycle, and the second signal processing method has a larger range and higher accuracy in a single cycle because it converts the nonlinear region into a linear region.

[0018] 5. The sensor of the present invention has the advantages of flexible design, high sensitivity, simple structure and small size. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below.

[0020] Figure 1 This is a schematic diagram of a large-range and high-sensitivity two-end fixed fiber Bragg grating displacement sensor disclosed in Example 1 of the present invention.

[0021] Figure 2 This is a schematic diagram of a large-range and high-sensitivity two-end fixed fiber Bragg grating displacement sensor disclosed in Example 2 of the present invention.

[0022] Figure 3 Schematic diagram of the cross-sectional area of ​​the probe resetting device housing.

[0023] Figure 4 Schematic diagram of the probe structure.

[0024] Figure 5 It is a stereoscopic diagram of the displacement transfer rod.

[0025] Figure 6 The first signal processing method is used to calculate the strain-displacement curve of the optical fiber. (A) represents the original reflection center wavelength drift-displacement function curve, (B) represents the function curve after filtering, and (C) represents the function curve after linear fitting. In the figure, M and N represent the reflection center wavelength drift-displacement function curves of optical fiber 1 and optical fiber 2 after strain, respectively.

[0026] Figure 7The optical fiber strain-displacement curve of the second signal processing method, (A) represents the original reflection center wavelength drift-displacement function curve, (B) represents the linear curve obtained after the square term in the difference elimination function. In the figure, M and N represent the reflection center wavelength drift-displacement function curves of optical fiber 1 and optical fiber 2 after strain, respectively.

[0027] In the figure, 1, shell; 2, probe; 3, displacement transmission rod; 4, support rod; 5, spring 1; 6, slope; 7, probe 1; 8, probe 2; 9, optical fiber 1; 10, optical fiber 2; 11, grating 1; 12, grating 2; 13, cantilever beam; 14, shell; 15, spring 2; 16, strip groove; 17, slide; 18, groove. DETAILED DESCRIPTION

[0028] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0029] The present invention provides a large-range and high-sensitivity two-end fixed fiber Bragg grating displacement sensor, such as Figure 1 and Figure 2 As shown, it includes a shell 1, and a movable probe 2 is set at one end of the shell 1. In this embodiment, the probe 2 is disc-shaped, and its edge is in close contact with the inner wall of the shell 1 and can slide on the inner wall of the shell 14.

[0030] A displacement transmission rod 3 is horizontally arranged at the center of the housing 1. One end of the displacement transmission rod 3 is fixed to the back of the probe 2, and the other end is fixed to the inner wall of the housing 1 through a transmission rod reset device. The transmission rod reset device includes a support rod 4 and a spring 5 sleeved on the support rod 4. The displacement transmission rod 3 is sleeved on the support rod 4. The end of the support rod 4 away from the probe 2 is fixed to the inner wall of the housing 1. One end of the spring 5 is fixed to the inner wall of the housing 1, and the other end is connected to the displacement transmission rod 3. When the probe 2 drives the displacement transmission rod 3 to move to the right, the displacement transmission rod 3 can slide on the support rod 4 and compress the spring 5. When the external force is removed, the probe 2 will be reset under the action of the spring 5.

[0031] The upper and lower surfaces of the displacement transmission rod 3 are both provided with a slope 6 with a right triangle longitudinal section, and the two adjacent slopes 6 are symmetrically arranged, and the slopes 6 located on the upper and lower surfaces of the displacement transmission rod 3 are arranged in parallel. The two adjacent slopes 6 can form a wave crest, and a complete wave crest is a cycle. Overall, the upper and lower surfaces of the displacement transmission rod 3 are tooth-shaped structures, and the tooth-shaped structures of the upper and lower surfaces differ by half a cycle.

[0032] Two groups of sensing units are arranged inside the housing 1. The two groups of sensing units are symmetrically arranged about the displacement transmission rod 3. The sensing units include probes, optical fibers and probe reset devices. The probes are fixed to the inner wall of the housing 1 through the probe reset device. The needle of the probe contacts the slope 6. One end of the optical fiber 9 is fixed to the probe. The other end is tightened and pasted to the inner surface of the housing 14 and then led out. A grating is engraved on the optical fiber. For ease of description, the two groups of sensing units are respectively sensing unit 1 and sensing unit 2, the corresponding two probes are respectively probe 1 7 and probe 2 8, the two optical fibers are respectively optical fiber 1 9 and optical fiber 2 10, the two probe reset devices are respectively probe reset device 1 and probe reset device 2, and the grating is divided into grating 1 11 and grating 2 12.

[0033] like Figure 1 As shown, in the embodiment 1 of the present invention, the probe resetting device is a cantilever beam 13, one end of the cantilever beam 13 is fixed to the inner wall of the housing 1 away from the probe 2, and the other end is connected to the probe. When the probe moves up and down along the slope 6, the cantilever beam 13 can vibrate up and down, and the maximum deflection of the end of the cantilever beam 13 should be greater than the height of the slope 6 to ensure that the probe can safely and repeatedly make vertical jumps.

[0034] like Figure 2 As shown, in embodiment 2 of the present invention, the probe resetting device includes a housing 14 fixed to the inner wall of the housing 1, and a spring 15 disposed in the housing 14, one end of the spring 15 is fixed to the housing 1, and the other end is connected to the probe, as shown in FIG. Figure 3 As shown, four strip grooves 16 are longitudinally formed on the inner wall of the housing 14. Figure 4 As shown, four slides 17 are arranged on the side of the probe, and the slides 17 are located in the strip groove 16 and can slide up and down in the strip groove 16. During the up and down movement of the probe, the spring 2 15 can provide a reset effect.

[0035] Specifically, Figure 5 As shown, a groove 18 is longitudinally provided on the slope surface 6, and the needle head of the probe is located in the groove 18, which can prevent the probe from tilting and deviating on the slope surface 6 when moving.

[0036] The working principle of the displacement sensor of the present invention is as follows:

[0037] When probe 2 senses external displacement, probe 2, displacement transmission rod 3 and slope 6 slide to the right, probe 1 7 moves downward, optical fiber 1 9 contracts, and the reflection center wavelength generated by the grating is blue-shifted. Probe 2 8 also moves downward, but optical fiber 2 10 is stretched, and the reflection center wavelength generated by grating 2 12 is red-shifted; when the displacement is continuously applied, probe 1 7 and probe 2 8 move upward, but optical fiber 1 9 is stretched and optical fiber 2 10 contracts until they return to their initial state, which is one cycle. Then, the relationship between the displacement and the drift of the reflection center wavelength within one cycle is calibrated, and the drift and displacement curves of the reflection center wavelengths of the two gratings are plotted in the same coordinate system, with the drift of the reflection center wavelength as the vertical coordinate and the displacement as the horizontal coordinate, as shown in the figure. Figure 6 (A) and Figure 7 As shown in (A), the displacement can be measured by subsequent signal processing methods, as shown in Figure 6 (B)(C) and Figure 7 (B) When the external displacement is cancelled, the transmission rod reset device performs reset.

[0038] In embodiments 1 and 2, only one cycle of displacement measurement is shown. The present invention can also achieve a larger range of measurement by increasing the number of cycles of the slope surface 6. At the same time, in order to improve the sensitivity of the sensor, within the safety range of the optical fiber, the height of the slope surface 6 is determined by combining the Pythagorean theorem and the maximum strain of the optical fiber, and the sensitivity of the sensor to detect displacement is adjusted by adjusting the ratio of the right angle sides of the slope surface 6.

[0039] In the present embodiments 1 and 2, the thickness of the disc-shaped probe 2 and the cross-sectional area of ​​the displacement transmission rod 3 cannot be too small, and they need to be free from deformation when subjected to force.

[0040] In the present embodiments 1 and 2, the width of the slope 6 should be equal to the width of the displacement transmission rod 3. The slope 6 has three functions: first, converting the lateral displacement Δx into the longitudinal displacement Δy; second, setting the sensitivity magnification according to the ratio of the right angle sides; and third, increasing the number of slope cycles to expand the displacement range of the sensor.

[0041] In the present embodiments 1 and 2, increasing the number of slope surfaces 6 can increase the measuring range of the sensor without reducing the sensitivity.

[0042] In the present embodiments 1 and 2, lubricating oil is applied to reduce the wear of the probe and the groove 18 of the slope 6 and increase the service life.

[0043] In the present embodiments 1 and 2, the strain range of the optical fiber should be controlled within 0-0.9%.

[0044] In the present embodiments 1 and 2, the two signal processing methods used both use the difference in the drift of the reflection center generated by the upper and lower optical fiber strains, thereby eliminating the influence of temperature on displacement detection.

[0045] The present invention involves two signal processing methods, and the following two signal processing methods are both applicable to embodiments 1 and 2.

[0046] (1) The first signal processing method: When the probe is subjected to external displacement and generates a lateral displacement Δx to the right, the periodically arranged slopes on the upper and lower surfaces of the displacement transmission rod convert the lateral displacement Δx into a longitudinal displacement Δy. The ratio of the right-angle sides of the slope is also the sensitivity magnification K of the sensor. In a single cycle, the larger the K value, the larger the range, and the smaller the sensitivity. The smaller the K value, the smaller the range, and the larger the sensitivity. The relationship between the K value and the range is positively correlated, and the relationship with the sensitivity is negatively correlated. The expression is:

[0047] Δx=K×Δy

[0048]

[0049] Where S is the inherent sensitivity of the fixed sensor at both ends, and S' is the sensitivity of the sensor after slope conversion.

[0050] When an external lateral displacement Δx is applied, the relationship between the tensile strain Δε of the optical fiber and the external displacement Δx is:

[0051]

[0052] In the formula, K, s, and x0 represent the magnification, the length of the optical fiber between the probe and the fixed point P, and the applied prestress displacement, respectively. These numbers are constants after the sensor structure is determined, so the above formula is a linear function. Substituting the relationship between strain and the grating reflection center wavelength drift Δλ = N × Δε, we get:

[0053] Where N represents the strain sensitivity coefficient, which becomes a constant as the FBG fiber parameters are determined.

[0054] It is worth noting that the above equation is valid only when the measured displacement Δx is small, so that the small segment of nonlinear curve can be approximated.

[0055]

[0056] It is regarded as a linear curve and is used for displacement measurement.

[0057] Experimental verification shows that the sensor has a large linear displacement area and sensitivity in the part far from the y-axis. Therefore, it is necessary to apply a pre-displacement x0 or add a filter to skip the nonlinear area of ​​the sensor reflection center wavelength drift-displacement curve. These shortcomings make the displacement range that the fiber Bragg grating displacement sensor can detect in a single cycle very small, only within 2mm, due to the existence of the nonlinear area when the sensor uses the above formula for signal processing.

[0058] In order to overcome the disadvantage of limited range, the present invention places multiple slope periods on the upper and lower surfaces of the displacement transfer rod. When the displacement transfer rod moves to the right, the probes above and below the center line will move up and down periodically, and the optical fiber will also be stretched or contracted periodically, so that one grating center wavelength is always red-shifted and the other grating center wavelength is blue-shifted. Subsequently, the grating strain and displacement curves are plotted separately in the same coordinate system. Finally, the displacements measured in multiple periods are superimposed through signal processing to obtain a large-range fiber Bragg grating displacement sensor, which solves the problem of small sensor range. Figure 6 The relationship between the grating reflection center wavelength drift and displacement after multiple cycles of signal processing is shown. The original reflection center wavelength drift-displacement function curves of the two optical fibers after strain are obtained, such as Figure 6 As shown in (A), the curve contains nonlinear regions and approximate linear regions. Then a filter is used to eliminate the nonlinear region in the curve, leaving only the approximate linear region, as shown in Figure 6 Then, the obtained approximate linear region is linearly fitted, and finally the linear curve of the reflection center wavelength drift-displacement is obtained, as shown in Figure 6 As shown in (C), M and N represent the reflection center wavelength drift-displacement function curves of optical fiber 1 and optical fiber 2 after being strained.

[0059] (2) The second signal processing method: Since the reflection center wavelength drift-displacement curve is in a square relationship, the square term is the main factor causing nonlinearity, so eliminating the square term in the function can get a linear function. The second signal processing method is based on the above idea, making the initial phases of the two reflection center wavelength drift-displacement functions differ by half a period. After the difference, the square term in the curve can be subtracted, and after sorting, the original nonlinear function is converted into a linear function. The specific implementation method in this design is as follows: let the period of the upper surface slope of the displacement transfer rod lag behind the lower surface slope by half a period, and get the original reflection center wavelength drift-displacement expression and curve. The curve also contains nonlinear regions and approximate linear regions, such as Figure 7 Then the obtained reflection center wavelength drift-displacement relationship is subtracted, and the reflection center wavelength drift-displacement function curve is obtained after sorting, showing a good linear relationship, as shown in Figure 7 As shown in (B). The specific mathematical analysis is as follows:

[0060] The strain expressions of fiber 1 and fiber 2 are as follows:

[0061]

[0062]

[0063] In the formula, Δε1 and Δε2 represent the strains of fiber 1 and fiber 2, respectively, and s, y, and l represent the length of fiber suspension, longitudinal displacement, and phase length of the slope leading or lagging, respectively. Square the above formula and subtract it to get the following formula:

[0064]

[0065] Since the optical fiber strain is extremely small, the squared value is much smaller than the original value, so it can be ignored. After sorting, the result is shown as follows:

[0066]

[0067] In the formula, Δε 1-2 It represents the strain of fiber 1 minus the strain of fiber 2, s represents the length of the fiber in the housing, and l represents the length of the slope of the upper surface of the displacement transmission rod leading or lagging behind the slope of the lower surface by half a cycle. When the sensor parameters are determined, s and l are both constants. Therefore, the relationship between the strain and displacement of the optical fiber of the displacement sensor is proportional, eliminating the nonlinear region in the original curve, but the maximum sensitivity is slightly reduced. Substituting the relationship between strain and reflection center wavelength drift, we get:

[0068]

[0069] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A high-sensitivity, large-range, two-end fixed fiber Bragg grating displacement sensor, characterized in that: It comprises a shell, a movable probe is arranged at one end of the shell, a displacement transfer rod is arranged horizontally in the center of the shell, one end of the displacement transfer rod is fixed to the back of the probe, and the other end is fixed to the inner wall of the shell through a transfer rod reset device; the upper and lower surfaces of the displacement transfer rod are periodically provided with slopes with a longitudinal section of a right triangle, and the two adjacent slopes are symmetrically arranged, the slopes located on the upper and lower surfaces of the displacement transfer rod are arranged in parallel, the two adjacent slopes can form a wave crest, and a complete wave crest is a cycle; two groups of sensing units are arranged inside the shell, the two groups of sensing units are symmetrically arranged about the displacement transfer rod, the sensing units comprise a probe, an optical fiber and a probe reset device, the probe is fixed to the inner wall of the shell through the probe reset device, the needle of the probe contacts the slope, one end of the optical fiber is fixed to the probe, and the other end is tightened and pasted to the inner surface of the shell and then led out, and a grating is engraved on the optical fiber.

2. The large-range and high-sensitivity two-end fixed fiber Bragg grating displacement sensor according to claim 1, characterized in that: The transmission rod resetting device includes a support rod and a spring 1 sleeved on the support rod, the displacement transmission rod is sleeved on the support rod, one end of the support rod away from the probe is fixed on the inner wall of the shell, one end of the spring 1 is fixed on the inner wall of the shell, and the other end is connected to the displacement transmission rod.

3. The large-range and high-sensitivity two-end fixed fiber Bragg grating displacement sensor according to claim 1, characterized in that: The probe resetting device is a cantilever beam, one end of which is fixed on the inner wall of the shell away from the probe, and the other end is connected to the probe.

4. The large-range and high-sensitivity two-end fixed fiber Bragg grating displacement sensor according to claim 1, characterized in that: The probe resetting device includes a shell fixed to the inner wall of the shell, and a second spring arranged in the shell, one end of the second spring is fixed to the shell, and the other end is connected to the probe, four strip grooves are longitudinally opened on the inner wall of the shell, four slides are arranged on the side of the probe, and the slides are located in the strip grooves.

5. The large-range and high-sensitivity two-end fixed fiber Bragg grating displacement sensor according to claim 1, characterized in that: A groove is longitudinally formed on the slope surface, and the needle head of the probe is located in the groove.

6. The large-range and high-sensitivity two-end fixed fiber Bragg grating displacement sensor according to claim 1, characterized in that: The probe is disc-shaped, with its back fixedly connected to the displacement transmission rod, and its edge in close contact with the inner wall of the shell and capable of sliding on the inner wall of the shell.

Images