An all-fiber laser-ultrasound monitoring system

Abstract

The application discloses a kind of all-fiber laser ultrasonic monitoring systems.The system includes laser excitation unit, excitation end optical fiber, fiber grating sensor, demodulation unit, data acquisition unit and control unit.Two kinds of functions of excitation and reception in the application are realized by optical fiber, and embedded into the material to be monitored.Due to the cylindrical structure of the optical fiber, the excitation end and the receiving end have direction sensitivity at the same time.Under the synergistic effect of the two, the all-fiber laser ultrasonic monitoring system of the application is sensitive to the linear transmission path between the excitation and reception points and the small deflection angle range, and is less affected by the ultrasonic signals in other directions, so it can control the vulnerable position of complex geometric components, and simplify the monitoring algorithm.

Description

Technical Field

[0001] This invention relates to the field of structural health monitoring, and more specifically to an all-fiber laser ultrasonic monitoring system. Background Technology

[0002] Ultrasonic signals, characterized by high propagation energy and high frequency, are widely used in structural health monitoring. Traditional ultrasonic sensors are mainly based on piezoelectric ceramics (lead zirconate titanate, PZT). However, they are susceptible to electromagnetic interference, have significant added weight, are difficult to embed within materials, and are challenging to reuse on a large scale. Furthermore, guided wave monitoring is sensitive in all directions, receiving scattered and reflected signals from multiple directions, making data processing complex. Fiber optic sensors, on the other hand, are resistant to electromagnetic interference, small in size and light in weight, easily embedded within materials, and can be multiplexed. Moreover, optical fibers exhibit directional dependence, high sensitivity along the axial direction, and insensitivity to signals in other directions, allowing for targeted monitoring of specific vulnerable areas and simplifying damage analysis algorithms.

[0003] Currently, research on using optical fibers as receivers to acquire ultrasonic signals is widespread. Among these, fiber optic grating (FBG) sensors are commonly used. Early fiber optic ultrasonic sensing technology was mainly used to monitor passive ultrasonic signals, such as acoustic emission and impact signals. Methods such as broadband light source demodulation and tunable laser demodulation can convert long-drift ultrasonic waves into high-frequency energy changes, enabling the monitoring of low-energy, high-frequency ultrasonic signals. However, the excitation signal for active ultrasound still relies on piezoelectric materials. Research on using optical fibers to excite ultrasonic signals is limited, and embedded all-fiber ultrasonic monitoring remains a gap. If the limitations of optical fiber excitation signals can be overcome, an all-fiber laser ultrasonic monitoring system can be established, and the mapping relationship between ultrasonic signal characteristic parameters and damage can be obtained, the intrinsic problems of traditional ultrasonic piezoelectric materials can be fundamentally avoided, enabling efficient health monitoring in complex service environments. Summary of the Invention

[0004] To address the aforementioned shortcomings in the prior art, this invention provides an all-fiber laser ultrasonic monitoring system.

[0005] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows:

[0006] A full-fiber laser ultrasonic monitoring system includes a laser excitation unit, an excitation end optical fiber, a fiber optic grating sensor, a demodulation unit, a data acquisition unit, and a control unit;

[0007] The laser excitation unit is used to emit laser light into the optical fiber at the excitation end;

[0008] An excitation fiber is embedded inside the material to be monitored; the excitation fiber is used to transmit laser light into the monitored material to perform light-to-thermal-to-acoustic energy conversion and generate ultrasonic signals.

[0009] A fiber Bragg grating sensor is embedded inside the material to be monitored; the fiber Bragg grating sensor is used to acquire ultrasonic signals.

[0010] The demodulation unit is used to demodulate the acquired ultrasonic signals and convert them into voltage signals for output.

[0011] The data acquisition unit converts the voltage signal into a digital signal and stores it.

[0012] The control unit is used to output control signals to achieve synchronous operation of the laser excitation unit and the data acquisition unit.

[0013] Furthermore, multiple excitation-end optical fibers are embedded inside the material to be monitored. The laser excitation unit and the multiple excitation-end optical fibers are connected through a beam splitter. A multi-point fiber optic grating sensor with the same monitoring point as the excitation-end optical fiber is embedded inside the material to be monitored, forming a multi-channel optical fiber monitoring network. The multi-channel optical fiber monitoring network is then used to locate the damage to the material to be monitored.

[0014] Furthermore, a multi-channel fiber optic monitoring network is used to locate the damage to the material under test. The specific process is as follows: obtain the signal attenuation percentage of each channel in the multi-channel fiber optic monitoring network before and after damage, and determine the damage location of the material under test between the channel with the largest signal attenuation percentage and the channel with the second largest signal attenuation percentage.

[0015] Furthermore, the excitation end fiber is made of bare fiber or fiber coated with a material with high light absorption and high thermal expansion coefficient.

[0016] Furthermore, the excitation fiber and the fiber optic grating sensor are positioned on the same straight line or within a set small deflection angle range.

[0017] The present invention has the following beneficial effects:

[0018] (1) The present invention realizes both excitation and reception functions through optical fiber, which has many intrinsic advantages of optical fiber: anti-electromagnetic interference characteristics, small size, light weight, and easy embedding into the material.

[0019] (2) This invention proposes for the first time to couple both the excitation fiber and the receiving fiber, i.e., the fiber optic grating sensor, into the material. Compared with existing air coupling or surface mounting methods, it can be protected by the material, has higher stability, is not easily affected by external forces, reduces energy dissipation, helps maintain the integrity of the optical signal, and the flexible nature of the optical fiber makes this invention applicable to complex geometric components such as curved structures, thus improving the application scenarios of monitoring.

[0020] (3) Both the excitation and reception functions of the present invention are realized through optical fibers and embedded in the material to be monitored. Since the excitation and reception of optical fibers are both directionally sensitive, under the synergistic influence of the two, the all-fiber laser ultrasonic monitoring system of the present invention is highly sensitive to the linear transmission path between the excitation and reception points and within a small deflection angle range. It is less affected by the ultrasonic signal interference from scattering, reflection and transmission in other directions. Therefore, it can target the easily damaged locations of complex geometric components, thereby improving the efficiency of structural monitoring of complex geometric components.

[0021] (4) This invention employs multiple excitation-end optical fibers embedded within the material to be monitored, uses a beam splitter to connect the laser excitation unit and the multiple excitation-end optical fibers, and embeds a multi-point fiber optic grating sensor with the same number of monitoring points as the excitation-end optical fibers within the material to be monitored. This fully utilizes the reusability of optical fibers to build a multi-channel optical fiber monitoring network. Furthermore, it allows for comparison of changes in the characteristic parameters of the received ultrasonic signals from different channels, thereby achieving damage assessment. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of an all-fiber laser ultrasonic monitoring system.

[0023] Figure 2 This is a schematic diagram of the structure of a multi-channel all-fiber laser ultrasonic monitoring system;

[0024] Figure 3 This is a diagram showing the ultrasonic detection results of directional correlation testing with the excitation and receiving points centered at the end of the excitation fiber and the angle between the two points being 0°.

[0025] Figure 4 This is a diagram showing the ultrasonic detection results of directional correlation testing with the excitation and receiving points centered at the end of the excitation fiber and an angle of 30° between them.

[0026] Figure 5 This is a diagram showing the ultrasonic detection results of directional correlation testing with the excitation and receiving points centered at the end of the excitation fiber and an angle of 60° between them.

[0027] Figure 6 This is a diagram showing the ultrasonic detection results of directional correlation testing with the excitation and receiving points centered at the end of the excitation fiber and an angle of 90° between them.

[0028] Figure 7 This is a graph showing the results of multi-channel directional testing;

[0029] Figure 8 This is a diagram showing the damage monitoring results of the first-channel all-fiber laser ultrasonic monitoring system of the present invention.

[0030] Figure 9 This is a diagram showing the damage monitoring results of the second-channel all-fiber laser ultrasonic monitoring system of the present invention.

[0031] Figure 10 This is a diagram showing the damage monitoring results of the third-channel all-fiber laser ultrasonic monitoring system of the present invention.

[0032] Figure 11 This is a diagram showing the damage monitoring results of the fourth-channel all-fiber laser ultrasonic monitoring system of the present invention.

[0033] Among them: 1. Optical splitting device; 2. Multi-point fiber optic grating sensor. Detailed Implementation

[0034] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0035] like Figure 1 As shown, an all-fiber laser ultrasonic monitoring system includes a laser excitation unit, an excitation end fiber, a fiber optic grating sensor, a demodulation unit, a data acquisition unit, and a control unit.

[0036] In an optional embodiment of the present invention, the laser excitation unit is used to emit a laser to the excitation end optical fiber.

[0037] Specifically, the laser at the excitation end needs to meet the requirements of high energy, short pulse, and repetitive triggering. In this invention, the laser excitation unit uses a master oscillating power amplifier laser, which meets the above characteristics. The master oscillating power amplifier laser can adjust three optical parameters: laser output power, pulse width, and repetition frequency. The output fiber of the master oscillating power amplifier laser is a bare fiber output.

[0038] In an optional embodiment of the present invention, the excitation-end optical fiber is embedded inside the material to be monitored. The excitation-end optical fiber is used to transmit laser light into the monitored material for optical-thermal-acoustic energy conversion and to generate an ultrasonic signal. The excitation-end optical fiber is connected to the output optical fiber of the laser excitation unit to transmit the laser light emitted by the laser excitation unit. Both the excitation-end optical fiber and the output optical fiber of the laser excitation unit meet the requirements of low loss and high energy output, and there is no significant energy leakage at the splice point of the two optical fibers. The ultrasonic signal is generated in the excitation-end optical fiber by local and rapid thermal expansion and propagates inside the monitored material. The specific principle is as follows: a high-energy laser is coupled into the optical fiber, and the laser is confined within the optical fiber, making it immune to interference from external signals. The waveguide structure of the optical fiber gives the output beam excellent spatial distribution quality. When the laser energy is absorbed to a certain depth in the material, it instantly heats the material, causing local thermal expansion, resulting in minute deformation and displacement. Then, the local stress is released in a non-uniform form, forming an ultrasonic signal. By continuously adjusting the optical parameters of the laser and increasing the laser energy density to reach the ablation threshold of the material, the high-energy pulse can directly cause the material to evaporate or generate plasma without going through a liquid stage. This process is called ablation. During ablation, localized instantaneous high temperatures and pressures may be generated, leading to the formation of ultrasonic waves.

[0039] The excitation end fiber can be bare fiber or fiber coated with a material with high light absorption and high thermal expansion coefficient. In the specific embodiment of this invention, bare fiber is used.

[0040] like Figure 2 As shown, CFRP is a carbon fiber reinforced composite material plate. In this invention, multiple excitation-end optical fibers are embedded inside the material to be monitored. A beam splitter 1 is used to connect the laser excitation unit and the multiple excitation-end optical fibers. The same number of cascaded fiber grating sensors 2 with the same number of monitoring points as the excitation-end optical fibers are embedded inside the material to be monitored to form a multi-channel optical fiber monitoring network. The multi-channel optical fiber monitoring network is used to locate the damage in the material to be monitored.

[0041] This invention utilizes a multi-channel optical fiber monitoring network to locate damage in materials under test. The specific process is as follows: obtain the signal attenuation percentage of each channel in the multi-channel optical fiber monitoring network before and after damage, and determine the damage location of the material under test between the channel with the largest signal attenuation percentage and the channel with the second largest signal attenuation percentage.

[0042] Specifically, this invention employs a hot-pressing method to embed the excitation fiber between the 4th and 5th layers of an 8-layer carbon fiber laminate, using unidirectional layup, with the excitation fiber arranged along the carbon fiber direction. The excitation fiber is a double-clad fiber, with four fibers embedded within the 8-layer carbon fiber laminate; these multi-channel excitation fibers are designated CH1-CH4.

[0043] In an optional embodiment of the present invention, a fiber optic grating sensor is embedded inside the material to be monitored; the fiber optic grating sensor is used to receive ultrasonic signals, and due to geometric and photoelastic effects, the ultrasonic waves modulate the grating period and refractive index, thereby affecting the grating spectral drift.

[0044] Ultrasonic signals propagate inside the material being monitored. When the ultrasonic signal encounters a damage location, it is scattered, reflected, and transmitted, causing changes in waveform. Finally, the waveform is received by a Bragg fiber grating sensor. Therefore, the waveform itself contains relevant damage information. This invention can assess the damage in the material by analyzing changes in characteristic parameters of the ultrasonic signal, such as amplitude and arrival time.

[0045] The excitation fiber and the fiber Bragg grating sensor are positioned on the same straight line or within a set small deflection angle range. This invention fully utilizes the strong directional correlation of the invention by positioning the excitation fiber and the fiber Bragg grating sensor on the same straight line or within a set small deflection angle range. Specifically, when bare fiber is used at the excitation end, the small deflection angle is 30°.

[0046] Specifically, in the operation of this invention, the grating area of ​​the fiber optic grating at the receiving end is arranged on the optical path at the excitation end. The multi-point fiber optic grating sensor is denoted as CH1'-CH4'. Based on this, this invention can obtain a 4×4 multi-channel fiber optic monitoring network.

[0047] In an optional embodiment of the present invention, the demodulation unit is used to demodulate the acquired ultrasound signal.

[0048] Specifically, the demodulation system of the demodulation unit requires a high signal-to-noise ratio, resistance to environmental interference, and high stability of the locked wavelength within the laser ultrasonic monitoring time. The main instruments of the demodulation unit in this invention include a wavelength-tunable laser, a circulator, a photodetector, a preamplifier, and an oscilloscope. The demodulation principle involves locking the laser wavelength at 3dB to obtain the change in the reflectivity of the grating spectrum, then inputting it into the photodetector to convert it into an electrical signal, which is finally recorded by the oscilloscope. In an optional embodiment of this invention, the data acquisition unit converts the voltage signal into a digital signal and stores it via a data acquisition card.

[0049] In an optional embodiment of the present invention, the control unit is used to output control signals to realize synchronous control of the laser excitation unit and the data acquisition unit.

[0050] The control unit in this invention is a computer terminal.

[0051] The experimental steps for directional ultrasonic monitoring at the fiber optic ultrasonic excitation end of this invention are as follows:

[0052] S1. Embedding: The outer coating layer of the excitation end optical fiber is stripped off and embedded in a pure epoxy resin board;

[0053] S2. Excitation end system connection: The pigtail of the excitation end fiber is connected to the output fiber of the master oscillating power amplifier laser;

[0054] S3. Receiver system connection: Use PZT to acquire ultrasonic signals and connect it to an oscilloscope to output voltage signals;

[0055] S4. Trigger setting: The main oscillation power amplifier laser is controlled synchronously via the control unit;

[0056] S5. Excitation end parameter settings: input 100% output power of the main oscillation power amplifier laser, pulse width of 200ns; use a signal generator to synchronously control the output frequency of the pulse to 500Hz;

[0057] S6. Signal Acquisition: Centered on the end of the excitation fiber, record the test results at angles of 0°, 30°, 60°, and 90° between the excitation and reception points. The monitoring results are as follows: Figure 3 , Figure 4 , Figure 5 and Figure 6 As shown, the amplitude of the ultrasonic signal decreases with increasing angle.

[0058] The directionality testing steps of the all-fiber laser ultrasonic monitoring system of the present invention are as follows:

[0059] S1. Excitation end system connection: Embed a multi-channel fiber optic monitoring network into the CFRP, connect the input end of the beam splitter to the output end of the master oscillating power amplifier laser, and connect the output end of the beam splitter to the four excitation end fibers to control the optical transmission path.

[0060] S2. Receiver system connection: The receiver uses a multi-point Bragg fiber grating sensor, which is connected to the demodulation unit;

[0061] S3. Trigger setting: The main oscillation power amplifier laser and the data acquisition unit are connected to the control unit to achieve signal synchronization control;

[0062] S4. Excitation end parameter settings: input 100% output power of the main oscillation power amplifier laser, pulse width of 200ns; use a signal generator to synchronously control the output frequency of the pulse to 500Hz;

[0063] S5. Receiver parameter setting: The demodulation unit sequentially locks the 3dB position of the spectrum of different center wavelengths of the four FBGs to acquire the ultrasonic signal.

[0064] S6. Signal Acquisition: Connect the ultrasonic signal to the preamplifier and then record it with an oscilloscope.

[0065] When CH1 is used as the excitation end, the four gratings sequentially acquire the ultrasonic signal results, such as... Figure 7 As shown. Experimental results show that when the excitation and reception are on the same path, the amplitude of the CH1' ultrasonic signal is more than 20 times that of the CH2'-CH4' ultrasonic signal. This demonstrates that the all-fiber laser ultrasonic monitoring system of this invention has strong directional correlation.

[0066] The perforation damage monitoring and testing steps of the all-fiber laser ultrasonic monitoring system of the present invention are as follows:

[0067] S1. Excitation end system connection: Connect the input end of the beam splitter to the output end of the main oscillating power amplifier laser via optical fiber, and connect the output end of the beam splitter to the four optical fibers of the excitation end to control the optical transmission path;

[0068] S2. Receiver system connection: The receiver uses a multi-point Bragg fiber grating sensor, which is connected to the demodulation unit;

[0069] S3, Pre-damage: Make a hole in the center of the carbon fiber laminate that has been prepared and embedded with a multi-channel optical fiber monitoring network;

[0070] S4. Trigger setting: The main oscillation power amplifier laser and the data acquisition unit are connected to the control unit to achieve signal synchronization control;

[0071] S5. Excitation end parameter settings: Use a beam splitter to control the optical path and excite ultrasonic signals in channels CH1-CH4 respectively; input 100% output power and 200ns pulse width to the main oscillation power amplifier laser; use a signal generator to synchronously control the output frequency of the pulse to 500Hz;

[0072] S6. Receiver parameter setting: The demodulation unit sequentially locks the 3dB position of the spectrum of different center wavelengths of the four FBGs to acquire the ultrasonic signal.

[0073] S7. Signal Acquisition: Connect the ultrasonic signal to the preamplifier and then record it with an oscilloscope.

[0074] When one channel is excited, only the fiber Bragg grating sensor on the same path acquires the ultrasonic signal; the same applies to channels 2, 3, and 4, where only the ultrasonic signal of the corresponding channel is acquired. Figures 8-11 The results show the ultrasound signals acquired by the four channels before and after the injury. The results indicate that the signal amplitude attenuation in channels CH2' and CH3' is greater than that in channels CH1' and CH4'. Therefore, this invention determines that the injury location is between channels 2 and 3, thus achieving a preliminary assessment of the injury.

[0075] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. A full-fiber laser ultrasonic monitoring system, characterized in that, It includes a laser excitation unit, a beam splitter, multiple excitation end optical fibers, a multi-point fiber optic grating sensor, a demodulation unit, a data acquisition unit, and a control unit; The laser excitation unit is used to emit laser light to the beam splitter. The input end of the beam splitter is connected to the laser excitation unit, and the multiple output ends of the beam splitter are respectively connected to the multiple excitation optical fibers to form multiple excitation optical paths. The plurality of excitation-end optical fibers are at least partially embedded inside the material to be monitored. Each of the excitation-end optical fibers has an excitation endpoint located inside the material to be monitored. The excitation-end optical fibers are used to transmit laser light to the corresponding excitation endpoint and to convert the laser light into light-heat-sound energy at the excitation endpoint to generate an ultrasonic signal. The multi-point fiber optic sensor is at least partially embedded inside the material to be monitored. The multi-point fiber optic sensor includes an optical fiber and multiple grating regions disposed on the optical fiber. The multiple grating regions are all located inside the material to be monitored and are used to acquire ultrasonic signals. The number of the multiple excitation end optical fibers is the same as the number of multiple grating regions. When one channel is excited, only the fiber Bragg grating sensor on the same path collects the ultrasonic signal. Similarly, for the other channels, only the ultrasonic signal of the corresponding channel is collected. Each of the grating regions is positioned on the excitation optical path of the excitation endpoint of its corresponding excitation optical fiber, and is used to collect the ultrasonic signal generated by the corresponding excitation endpoint and propagated to the grating region through the material to be monitored; The excitation endpoint of one of the excitation optical fibers and a corresponding grating area form an independent same-path monitoring channel, and in each independent same-path monitoring channel, the excitation endpoint and the corresponding grating area are positioned on the same straight line or within a set small deflection angle range. The demodulation unit is used to demodulate the ultrasonic signal acquired by the multi-point fiber optic grating sensor and convert it into a voltage signal output. The data acquisition unit is used to convert the voltage signal into a digital signal and store it; The control unit is used to output control signals to achieve synchronous operation of the laser excitation unit and the data acquisition unit; The multiple independent co-path monitoring channels are used to locate the damage to the material under test. The damage location includes: obtaining the signal attenuation percentage of each independent co-path monitoring channel before and after damage, and determining the damage location of the material under test between the independent co-path monitoring channel with the largest signal attenuation percentage and the independent co-path monitoring channel with the second largest signal attenuation percentage.

2. The all-fiber laser ultrasonic monitoring system according to claim 1, characterized in that, The excitation end fiber is a bare fiber or a fiber coated with a material with high light absorption and high thermal expansion coefficient.

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