Sensor for measuring peak value of shock wave in water based on fiber bragg grating

Abstract

An underwater shock wave peak value measuring sensor based on a fiber bragg grating comprises a base, a hose, a capillary tube, an optical fiber and a hollow ball. The upper end of the sealed hose is fixed to the bottom of the base, the capillary tube is arranged in the center of the hose, the upper end of the capillary tube is fixed to the base, the optical fiber is arranged in the center of the capillary tube, the upper end of the capillary tube is fixed to the base, the grating is arranged in the middle of the optical fiber, the hollow ball is arranged at the bottom of the capillary tube and located in the hose, the hose is filled with filling liquid, and a balancing weight is arranged at the bottom of the hose. The sensor is simple in structure, convenient to operate, capable of sensing shock waves from any direction underwater, high in reliability and frequency response, flexible, practical, capable of rapidly measuring the peak value of the shock waves in water and low in cost.

Description

Technical Field

[0001] This utility model relates to the field of underwater static blasting technology, and in particular to a sensor for measuring the peak value of underwater shock waves based on fiber optic gratings. Background Technology

[0002] The underwater shock wave generated by underwater blasting has important characteristics such as a steep pressure front, extremely high peak pressure, and exponential pressure decay. The damage caused by underwater shock waves to underwater structures is mainly due to the peak pressure. Therefore, accurate measurement of the peak value of underwater shock waves has significant engineering practical value. Currently, in the field of shock wave measurement, piezoelectric ceramic-based sensor technology is commonly used both domestically and internationally. However, sensors based on this technology are mostly expensive. For the niche domestic shock wave detection market, there is still considerable room for improvement in shock wave sensor technology.

[0003] Therefore, it is necessary to design a fiber optic grating-based sensor for measuring the peak value of underwater shock waves to overcome the above problems. Utility Model Content

[0004] To avoid the above problems, a fiber optic grating-based underwater shock wave peak measurement sensor is provided. It has a simple structure, is easy to operate, can sense shock waves from any direction underwater, has high reliability and frequency response, is flexible and practical, can quickly measure the peak value of underwater shock waves, and is inexpensive.

[0005] This utility model provides a water shock wave peak measurement sensor based on fiber optic grating, comprising: a base, a flexible tube, a capillary tube, an optical fiber, and a hollow sphere; the upper end of the sealed flexible tube is fixed to the bottom of the base, the capillary tube is located in the center of the flexible tube and its upper end is fixed to the base, the optical fiber is located in the center of the capillary tube and its upper end is fixed to the base, a grating is provided in the middle of the optical fiber, the hollow sphere is located at the bottom of the capillary tube and inside the flexible tube, the flexible tube is filled with a filling liquid, and a counterweight is provided at the bottom of the flexible tube.

[0006] Preferably, the filling liquid is silicone oil.

[0007] Preferably, the bottom end of the optical fiber is connected to the hollow sphere without stress or deformation. The weight of the hollow sphere and its buoyancy in the filling liquid are equal. The hollow sphere is made of stainless steel and has a port at the top for connection with the optical fiber and capillary. The hollow sphere can be suspended in the silicone oil by its own weight and buoyancy, so that it will not exert additional force on the optical fiber and capillary, thereby improving measurement accuracy.

[0008] Preferably, the grating is disposed within the optical fiber.

[0009] Preferably, the bottom of the hose is sealed with a plug, which is made of stainless steel.

[0010] Preferably, the capillary is made of a high-toughness material. It can be a plastic product, with an inner diameter the same as the optical fiber diameter and an outer diameter slightly larger than the inner diameter.

[0011] Preferably, the base is provided with a signal line for connection to the optical fiber.

[0012] Preferably, the bottom of the hose is provided with a hook for connecting a counterweight. The hook is mainly used to attach a large mass counterweight to ensure that the device remains vertical in the water.

[0013] Fiber Bragg gratings (FGBs) are a type of wavelength-selective reflector. The wavelength of the signal reflected in an optical fiber is affected by temperature and strain, resulting in wavelength drift. This wavelength drift is highly sensitive to changes in strain and temperature, making fiber Bragg gratings suitable for use as high-precision detection instruments. Furthermore, sensors manufactured using FGB technology are also more cost-effective.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] 1. While ensuring the effectiveness of use, the equipment cost has been greatly reduced.

[0016] 2. It has a simple structure and mature related technologies, which can be quickly applied to practice.

[0017] 3. Protecting the optical fiber in a highly elastic capillary steel tube can ensure the safety of the optical fiber, improve the anti-interference capability of the fiber Bragg grating, avoid the influence of noise on the detection results, and the combination of capillary tube and optical fiber can improve the measurement range of the shock wave peak value of the device to a certain extent.

[0018] 4. Based on the wave measurement principle of the device constructed using Bragg grating (FGB) technology and structural design, the sensor can detect shock waves from any direction underwater, with high reliability and frequency response, making it suitable for underwater explosion testing.

[0019] 5. It is easy to operate, can be reused, and has low operating costs. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the structure of a fiber optic grating-based underwater shock wave peak measurement sensor according to a preferred embodiment of the present invention.

[0021] Figure 2 for Figure 1 Cross-sectional view at point A in the middle;

[0022] Figure 3 for Figure 1 Cross-sectional view at point B in the middle;

[0023] Explanation of icon numbers:

[0024] 1. Base; 2. Flexible tube; 3. Capillary tube; 4. Optical fiber; 5. Grating; 6. Hollow sphere; 7. Hook; 8. End cap. Detailed Implementation

[0025] The technical solution of this utility model will now be clearly and completely described in conjunction with the accompanying drawings. In the description of this utility model, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings and are only for the convenience of describing this utility model and simplifying the description. They do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this utility model. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0026] In the description of this utility model, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this utility model based on the specific circumstances.

[0027] like Figures 1 to 3 As shown, this embodiment provides an underwater shock wave sensor based on fiber optic grating 5, comprising: base 1, hose 2, capillary tube 3, optical fiber 4, and hollow sphere 6.

[0028] The upper end of the flexible tube 2 is fixed to the bottom of the base 1, and the capillary tube 3 is located in the center of the flexible tube 2 with its upper end fixed to the base 1. Meanwhile, the optical fiber 4 is located in the center of the capillary tube 3 with its upper end fixed to the base 1. The base 1 is provided with a signal line for connecting to the optical fiber 4. A grating 5 is located in the middle of the optical fiber 4. The grating 5 is the core of the entire device. It is used to measure the peak value of the shock wave by generating the wavelength drift during deformation. The hollow sphere 6 is located at the bottom of the capillary tube 3 and suspended inside the flexible tube 2.

[0029] The flexible tube 2 is sealed, with a stainless steel plug 8 at its bottom. A hook 7 is located at the bottom of the plug 8 for connecting a counterweight, allowing the sensor to hang naturally in the water while remaining vertical. The weight of the counterweight is sufficient to resist the buoyancy of the water. The flexible tube 2 is filled with a silicone oil filling fluid. The capillary tube 3 has a certain rigidity and elasticity and can be made of steel or plastic. In this embodiment, the capillary tube 3 is a high-toughness plastic product. Under normal conditions, it is straight. The middle of the capillary tube 3 is hollow, allowing the optical fiber 4 to pass through. Its outer diameter is slightly larger than its inner diameter, suspending a hollow sphere 6 below it. The sphere is suspended in the flexible tube 2 by the capillary tube 3. The bottom end of the optical fiber 4 is connected to the hollow sphere without stress or deformation. The weight of the hollow sphere 6 and its buoyancy in the filling fluid are equal. The sphere 6 is suspended in the silicone oil by its own weight and the buoyancy it experiences, ensuring that the effects on the hollow sphere 6 are the same in all directions, thus guaranteeing measurement accuracy.

[0030] Working principle: During testing, the sensor suspends a counterweight in the water via hook 7 according to the required depth. The shock wave generated by the explosion acts on the flexible hose 2 and propagates through the silicone oil. Due to the incompressibility of the silicone oil, the force of the shock wave in the water acts on the hollow sphere 6. The hollow sphere 6 is displaced by the shock wave, which in turn causes the capillary tube 3 to bend and deform. The optical fiber 4 inside the capillary tube 3 also bends and deforms. The grating 5 is affected by the deformation, and the wavelength of its reflected light shifts. The strain of the optical fiber 4 and the capillary tube 3 can be calculated based on the wavelength shift. Then, the amplitude and pressure of the shock wave can be calculated using the strain, the elastic modulus of the optical fiber 4 and the capillary tube 3, etc.

[0031] The key points supporting the feasibility of the working principle are as follows:

[0032] (1) The pressure transmission path of the shock wave in water is: shock wave pressure > hose deformation > silicone oil (incompressible fluid) transmission pressure > acting on the hollow sphere > hollow sphere displacement.

[0033] (2) Displacement-strain conversion mechanism: The displacement of the hollow sphere 6 will pull the capillary 3, causing it to bend and deform; the capillary 3 has a certain stiffness and elasticity, and its bending deformation is related to the applied force (caused by the shock wave pressure); this is the core mechanical conversion.

[0034] (3) Strain-wavelength conversion mechanism: The fiber optic grating (FBG) is firmly attached to or embedded inside a bent capillary. The bending deformation of the capillary 3 is directly transmitted to the internal optical fiber, causing axial strain in the FBG. According to the FBG principle, axial strain directly and linearly causes the reflection center wavelength λ to change. B drift (Δλ) B This is a mature and highly sensitive sensing technology.

[0035] (4) Pressure-strain relationship can be established: Theoretically, a model (P∝ε) relating the pressure P exerted by the shock wave on the hose 2 to the strain ε at the root of the capillary can be established through mechanical analysis (treating the capillary 3 as a cantilever beam or similar structure). This is combined with the strain measured by FBG (ε∝Δλ). B Ultimately, the pressure (P∝Δλ) can be derived. B ).

[0036] (5) Temperature compensation potential: Although FBG is sensitive to both temperature and strain, the silicone oil environment, metal parts and the sealing and encapsulation measures of the hose 2 can isolate the influence of the external environment on FBG in a short time, or compensate for the wavelength drift caused by temperature by using a reference grating (which is not subjected to strain but is in the same temperature environment). This is a common practice in FBG applications.

[0037] Technical Principle: The fiber grating used in this novel application employs Bragg grating (FGB) technology, which is the working principle of the FBG strain sensor. When a broadband beam of light is incident on the center of the fiber grating, narrowband light of a specific wavelength is reflected by the periodic refractive index structure. The center wavelength of the reflected light is λ. B = 2neff∧. Where neff is the effective refractive index of the fiber grating, and ∧ is the grating period. When the FBG is subjected to strain and temperature, ∧ and neff will change, leading to a change in the center wavelength of the FBG. Simultaneously, the photoelastic effect will also cause a change in the grating's refractive index, thus causing a change in the center wavelength of the FBG. The relationship between the wavelength shift of the fiber grating and strain, temperature, and the photoelastic effect is as follows:

[0038]

[0039] In the formula, λ B Δλ is the center wavelength of the FBG. B This represents the change in FBG wavelength; The coefficient of thermal expansion of the optical fiber; The thermo-optic coefficient of the optical fiber; ΔT is the photoelastic coefficient of the optical fiber material; n is the refractive index of the optical fiber; ΔT is the temperature change of the FBG; and Δε is the strain change of the FBG.

[0040] Because this invention employs a multi-layered protective measure consisting of an external flexible tube 2, silicone oil, and a capillary tube 3, and since the thermal conductivity of these three components is not particularly high, it can be assumed that the fiber optic grating will not be affected by temperature changes during underwater shock wave measurements. This avoids the need for multiple gratings to monitor temperature changes in traditional FBG strain sensors, thus saving instrument costs. Therefore, the FBG strain change Δε in this invention is:

[0041]

[0042] Meanwhile, the capillary tube 3, optical fiber 4, and hollow sphere 6 can be considered as a cantilever beam of length L fixed at one end. The outer diameter of the capillary tube 3 is D, the inner diameter is D1, and the elastic modulus is E1; the diameter of the optical fiber 4 is D1, and the elastic modulus is E2. If the hollow sphere 6 is subjected to a force F, the strain value ε of the optical fiber 4 is:

[0043] Moment of inertia of capillary tube 3:

[0044] Moment of inertia of fiber 4:

[0045] The bending stiffness of the integrated capillary tube 3 and optical fiber 4 is:

[0046]

[0047] Since the fiber grating is located in the middle, the bending moment at the middle of the beam is:

[0048] The bending strain formula is:

[0049] In the formula, Y = D1 / 2 is the distance from the outermost edge of the optical fiber to the neutral axis.

[0050] Thus, the strain value ε is:

[0051] Therefore, when the strain change of FBG is Δε, and Δλ is generated... B When the wavelength drifts. From the above equation (8), it can be seen that the force F acting on the hollow sphere at point 6 is:

[0052] Furthermore, the magnitude of the force exerted by the shock wave on the instrument surface in the water can be obtained from this formula, thus yielding the peak value of the shock wave at the sensor location. Simultaneously, as shown in formula (9), increasing the values ​​of E1 and E2 can improve the Δλ... B While maintaining a relatively stable performance, the measurement range of the device can be increased. However, this will also reduce the device's sensitivity, making it suitable only for measuring water shock waves in the high-peak range. In practical applications, the device can be flexibly adjusted and selected according to specific needs.

[0053] In practical work, Δλ B The measured values ​​are those that can be monitored by the terminal instrument; all other physical quantities are fixed values. Users can determine these values ​​based on Δλ. BThe change in value is used to determine the magnitude of the force on the hollow sphere 6 using the formula described above. The force on the hollow sphere 6 is caused by the deformation of the hose 2 wall after the water shock wave acts on it, squeezing the internal silicone oil. The silicone oil flows, transmitting the force to the hollow sphere 6. The hollow sphere 6 causes the capillary 3 to bend and deform, which in turn causes the fiber grating to deform, resulting in a wavelength shift in the optical fiber. The terminal instrument calculates the peak value of the water shock wave using the formula described above, based on the amount of the shift.

[0054] Of course, from (P∝Δλ) B During the process, the force F on the hollow sphere 6 is transmitted through multiple media, so there is no perfect correspondence between the force F and the shock wave pressure P in the water. At this time, the production unit needs to conduct multiple sets of controlled variable tests under different temperatures, different liquid environments and different shock wave pressures to obtain the corresponding coefficient of F∝P under different working conditions, correct the final measurement results, and improve the accuracy of the sensor in monitoring the peak value of the shock wave in the water.

[0055] Meanwhile, due to the influence of inertial and hydrodynamic effects, the sensor of this invention has good accuracy and effectiveness in measuring the peak value of the shock wave, but it cannot accurately measure the waveform changes of the subsequent negative pressure wave. The main reason for this is that both the hollow sphere 6 and the capillary tube 3 have mass. Shock waves are high-frequency (kHz or even MHz) and transient (microsecond-level) events. The inertia of mass may cause the system response speed to be unable to keep up with the rapid changes of the shock wave, resulting in signal lag, distortion, or amplitude attenuation. The stiffness of the capillary tube 3 and the mass of the hollow sphere 6 together determine the natural frequency of the system. If the natural frequency is lower than the main frequency component of the shock wave, the measurement results will be severely distorted.

[0056] This sensor uses the Bragg grating (FGB) technique to test underwater blast shock waves.

[0057] In addition to the above explanation of the principle, the relationship between the force F on the hollow sphere 6 and the strain of the fiber optic grating can be obtained through experiments. Then, multiple sets of experiments are conducted on the sensor to fit the relationship between the force F on the hollow sphere 6 and the shock wave pressure P in the water. In actual detection, the strain of the fiber optic grating can be used to obtain the shock wave pressure and its changes in the water. This method is simple, practical, efficient and convenient.

[0058] Whether from the working principle and calculation formula or from the actual experimental analysis, the correspondence and correlation between the strain of the fiber optic grating, the force on the hollow sphere 6 and the shock wave pressure P in the water can be obtained, so that the sensor can be used to quickly and efficiently measure the peak value of the shock wave in the water.

[0059] This sensor has a simple structure, is easy to operate, can detect shock waves from any direction underwater, has high reliability and frequency response, is flexible and practical, can quickly measure the peak value of shock waves in water, and is inexpensive.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this utility model, and are not intended to limit it. Although the utility model has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this utility model.

Claims

1. A fiber grating based sensor for measuring the peak pressure of a shock wave in water, characterized in that, It comprises a base, a hose, a capillary tube, an optical fiber and a hollow ball; the upper end of the hose is fixed on the bottom of the base, the capillary tube is arranged in the center of the hose and its upper end is fixed on the base, the optical fiber is arranged in the center of the capillary tube and its upper end is fixed on the base, the middle part of the optical fiber is provided with a grating, the hollow ball is arranged at the bottom of the capillary tube and located in the hose, the hose is filled with a filling liquid, and the bottom of the hose is provided with a counterweight. The filling liquid is silicon oil.

2. A fiber-optic grating based water shock wave peak measurement sensor as claimed in claim 1, characterized in that: The bottom end of the optical fiber is connected with the hollow ball without stress and deformation, and the gravity of the hollow ball and the buoyancy in the filling liquid are equal.

3. A fiber-optic grating based water shock wave peak measurement sensor as claimed in claim 1, characterized in that: The grating is arranged in the optical fiber.

4. The fiber-optic grating-based water-shock-wave peak-measurement sensor as claimed in claim 1, characterized by: The bottom of the hose is provided with a plug which is made of stainless steel.

5. The fiber-optic grating-based water-shock-wave peak-measurement sensor as claimed in claim 1, characterized by: The capillary tube is made of high toughness material.

6. The fiber-optic grating-based water-shock-wave peak-measurement sensor as claimed in claim 1, characterized by: The base is provided with a signal line for connecting with the optical fiber.

7. The fiber-optic grating-based water-shock-wave peak-measurement sensor as claimed in claim 1, characterized by: The bottom of the hose is provided with a hook for connecting with the counterweight.

8. The fiber-optic grating-based water-shock-wave peak-measurement sensor as claimed in claim 1, characterized by: ​

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