All-fiber laser ultrasonic monitoring system

Abstract

The invention discloses an all-fiber laser ultrasonic monitoring system. The system comprises a laser excitation unit, an excitation end optical fiber, an optical fiber grating sensor, a demodulation unit, a data acquisition unit and a control unit. The excitation function and the receiving function are realized through the optical fiber and are embedded into a to-be-monitored material. Due to the fact that the cylindrical structure of the optical fiber, the excitation end and the receiving end have direction sensitivity at the same time, under the cooperative influence of the two ends, the all-optical-fiber laser ultrasonic monitoring system is high in sensitivity to a linear transmission path between excitation and receiving two points and within a small deflection angle range, and interference of ultrasonic signals in other directions is small; therefore, the easy-to-damage position of the complex geometric component can be distributed and controlled in a targeted manner, and the monitoring algorithm is simplified.

Description

Technical Field

[0001] The present invention relates to the field of structural health monitoring, and in particular to an all-fiber laser ultrasonic monitoring system. Background Art

[0002] Ultrasonic signals are widely used in the field of structural health monitoring due to their large propagation energy and high frequency. Traditional ultrasonic sensors are mainly based on piezoelectric ceramics (lead zirconate titanate, PZT). However, they are susceptible to electromagnetic interference, have a large additional weight, are difficult to embed inside materials, and are difficult to reuse on a large scale. In addition, guided wave monitoring is omnidirectionally sensitive and can receive scattered, reflected and other signals from multiple directions, making data processing more complicated. Fiber optic sensors are resistant to electromagnetic interference, have a small size and light weight, are easy to embed into materials, and can be multiplexed. In addition, optical fibers have directional correlation, high sensitivity along the axial direction, and are insensitive to signals in other directions. They can be used to monitor certain vulnerable parts in a targeted manner to simplify the damage analysis algorithm.

[0003] At present, the research on using optical fiber as a receiver to collect ultrasonic signals is very extensive. Among them, fiber Bragg grating sensors (FBG) are commonly used. Early fiber optic ultrasonic sensing technology was mainly used to monitor passive ultrasonic signals, such as acoustic emission and impact signals. The long drift of ultrasonic waves can be converted into high-frequency energy changes through broadband light source demodulation, tunable laser demodulation and other methods to achieve low-energy, high-frequency ultrasonic signal monitoring. However, the excitation signal of active ultrasound still relies on piezoelectric materials. There are few studies on using optical fiber to excite ultrasonic signals, and there is still a gap in embedded all-fiber ultrasonic monitoring. If the limitations of optical fiber excitation signals can be solved, 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, and efficient health monitoring in complex service environments can be achieved. Summary of the invention

[0004] In view of the above-mentioned deficiencies in the prior art, the present invention provides an all-fiber laser ultrasonic monitoring system.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution adopted by the present invention is:

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

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

[0008] The excitation end optical fiber is embedded in the material to be monitored; the excitation end optical fiber is used to transmit the laser into the monitoring material to perform light-heat-acoustic energy conversion and generate an ultrasonic signal;

[0009] The fiber grating sensor is embedded in the material to be monitored; the fiber grating sensor is used to collect ultrasonic signals;

[0010] The demodulation unit is used to demodulate the collected ultrasonic signal and convert it into a voltage signal 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 a control signal to realize the synchronous operation of the laser excitation unit and the data acquisition unit.

[0013] Furthermore, multiple excitation end optical fibers are embedded in the material to be monitored, the laser excitation unit and the multiple excitation end optical fibers are connected through a spectrometer, and multi-point fiber grating sensors with the same monitoring points as the excitation end optical fibers are embedded in the material to be monitored to form a multi-channel optical fiber monitoring network, and the multi-channel optical fiber monitoring network is used to locate damage in the material to be monitored.

[0014] Furthermore, a multi-channel optical fiber monitoring network is used to locate damage to the material to be monitored. The specific process is: obtaining the signal attenuation percentage of each channel in the multi-channel optical fiber monitoring network before and after damage, and determining the damage position of the material to be monitored as between the channel with the largest signal attenuation percentage and the channel with the second largest signal attenuation percentage.

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

[0016] Furthermore, the excitation end optical fiber and the fiber grating sensor are arranged 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 the excitation and reception functions through optical fiber, and has multiple inherent advantages of optical fiber: anti-electromagnetic interference characteristics, small size, light weight, and easy to embed into the material;

[0019] (2) The present invention proposes for the first time to couple both the excitation end optical fiber and the receiving end optical fiber, i.e., the fiber grating sensor, into the interior of the material. Compared with the existing air coupling or surface mounting methods, the fiber grating sensor can be protected by the material, has higher stability, is less susceptible to external interference, reduces energy dissipation, and helps maintain the integrity of the optical signal. In addition, the flexible nature of the optical fiber makes the present invention applicable to geometric components with complex shapes 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 by optical fiber and embedded in the material to be monitored. Since the excitation and reception of the optical fiber 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, and is less disturbed by the ultrasonic signals scattered, reflected and transmitted in other directions. Therefore, it can carry out targeted control of the vulnerable positions of complex geometric components, thereby improving the efficiency of structural monitoring of complex geometric components;

[0021] (4) The present invention uses multiple excitation end optical fibers embedded in the material to be monitored, uses a splitter to connect the laser excitation unit and the multiple excitation end optical fibers, and embeds the same number of multi-point fiber grating sensors as the excitation end optical fibers into the material to be monitored, giving full play to the multiplexing of optical fibers and building a multi-channel optical fiber monitoring network. The changes in the characteristic parameters of the received ultrasonic signals of different channels can be compared to achieve damage assessment. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a structural schematic diagram of an all-fiber laser ultrasonic monitoring system;

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

[0024] Figure 3 This is a diagram of ultrasonic detection results of the directional correlation test under the condition that the angle between the excitation and receiving points is 0° with the end point of the excitation end optical fiber as the center;

[0025] Figure 4 This is a diagram of ultrasonic detection results of the directional correlation test under the condition that the angle between the excitation and receiving points is 30° with the end point of the excitation end optical fiber as the center;

[0026] Figure 5 This is a diagram of ultrasonic detection results of the directional correlation test under the condition that the angle between the excitation and receiving points is 60° with the end point of the excitation end optical fiber as the center;

[0027] Figure 6 This is a diagram of ultrasonic detection results of the directional correlation test under the condition that the angle between the excitation and receiving points is 90° with the end point of the excitation end optical fiber as the center;

[0028] Figure 7 This is a multi-channel directivity test result diagram;

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

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

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

[0032] Fig.11 This is a damage monitoring result diagram of the fourth channel all-fiber laser ultrasonic monitoring system of the present invention;

[0033] Among them: 1. Spectral splitter; 2. Multi-point fiber grating sensor. DETAILED DESCRIPTION

[0034] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations 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 optical fiber, a fiber 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 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 repeated triggering. The laser excitation unit in the present invention adopts a master oscillator power amplifier laser to meet the above characteristics. The master oscillator power amplifier laser can adjust the three optical parameters of the laser output power, pulse width and repetition frequency. The output end optical fiber of the master oscillator power amplifier laser is a bare optical fiber output.

[0038] In an optional embodiment of the present invention, the excitation end optical fiber is embedded in the material to be monitored; the excitation end optical fiber is used to transmit the laser to the inside of the monitoring material for light-heat-acoustic energy conversion and generate an ultrasonic signal. The excitation end optical fiber is connected to the output end optical fiber of the laser excitation unit to transmit the laser emitted by the laser excitation unit. The excitation end optical fiber and the output end optical fiber of the laser excitation unit both meet the requirements of low loss and large energy output, and there is no large energy leakage at the local fusion point of the two optical fibers. The ultrasonic signal is generated by local and rapid thermal expansion in the excitation end optical fiber and propagates inside the material to be monitored. The specific principle is: high-energy laser is coupled to the optical fiber, the laser is bound in the optical fiber, and is immune to interference from external signals, and the waveguide structure of the optical fiber makes the output beam have excellent spatial distribution quality; when the laser energy is absorbed to a certain depth of the material, the material is instantly heated, resulting in local thermal expansion, causing slight deformation and displacement; then the local stress is released in an uneven form to form an ultrasonic signal. By continuing to adjust 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 evaporate the material or generate plasma without going through the liquid stage. This process is called ablation. The ablation process may produce local instantaneous high temperature and high pressure, leading to the formation of ultrasonic waves.

[0039] The excitation end optical fiber is a bare optical fiber or an optical fiber coated with a material having high light absorption and high thermal expansion coefficient. The specific implementation example of the present invention uses a bare optical fiber.

[0040] like Figure 2 As shown, CFRP is a carbon fiber reinforced composite material plate. The present invention uses multiple excitation end optical fibers to be embedded into the material to be monitored, uses a splitter 1 to connect the laser excitation unit and the multiple excitation end optical fibers, and embeds the same number of serial fiber grating sensors 2 as the excitation end optical fibers at the monitoring points to the material to be monitored to form a multi-channel optical fiber monitoring network, and uses the multi-channel optical fiber monitoring network to locate the damage of the material to be monitored.

[0041] The present invention utilizes a multi-channel optical fiber monitoring network to locate damage to the material to be monitored. The specific process is: obtaining the signal attenuation percentage of each channel before and after damage in the multi-channel optical fiber monitoring network, and determining the damage position of the material to be monitored as between the channel with the largest signal attenuation percentage and the channel with the second largest signal attenuation percentage.

[0042] Specifically, the present invention uses a hot pressing method to embed the excitation end optical fiber between the 4th and 5th layers of the 8-layer carbon fiber laminate, with unidirectional plying, and the excitation end optical fiber is arranged along the direction of the carbon fiber. The excitation end optical fiber is a double-clad optical fiber, four of which are embedded in the 8-layer carbon fiber laminate, and the multi-channel excitation end optical fibers are recorded as CH1-CH4.

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

[0044] The ultrasonic signal propagates inside the material to be monitored. When it encounters the damaged position, it is scattered, reflected and transmitted. The waveform changes and is finally received by the Bragg fiber grating sensor. Therefore, the waveform itself contains relevant damage information. The present invention can evaluate the damage in the material by analyzing the changes in the characteristic parameters of the ultrasonic signal, such as amplitude and arrival time.

[0045] The excitation end optical fiber and the fiber grating sensor are arranged on the same straight line or within a small deflection angle range. The present invention can give full play to the strong directional correlation of the present invention by arranging the excitation end optical fiber and the fiber grating sensor on the same straight line or within a small deflection angle range. Specifically, when the excitation end optical fiber adopts a bare optical fiber, the small deflection angle is 30°.

[0046] Specifically, during operation, the present invention arranges the grating area of ​​the fiber Bragg grating at the receiving end on the optical path of the excitation end, and the multi-point fiber Bragg grating sensors are recorded as CH1'-CH4'. Based on this, the present 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 collected ultrasonic 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 the present invention include a wavelength tunable laser, a circulator, a photodetector, a preamplifier, and an oscilloscope. The demodulation principle is to lock the laser wavelength at 3dB to obtain a change in the grating spectrum reflectivity, which is then input into the photodetector to be converted into an electrical signal, and finally recorded by an oscilloscope. In an optional embodiment of the present invention, the data acquisition unit converts the voltage signal into a digital signal through a data acquisition card and stores it.

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

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

[0051] The experimental steps of directional ultrasonic monitoring of the optical fiber ultrasonic excitation end of the present invention are as follows:

[0052] S1, embedding: strip the outer coating layer of the excitation end optical fiber and embed it into a pure epoxy resin board;

[0053] S2, excitation end system connection: the pigtail of the excitation end optical fiber is connected to the output end optical fiber of the main oscillator power amplifier laser;

[0054] S3, receiving end system connection: use PZT to collect ultrasonic signals and connect to the oscilloscope to output voltage signals;

[0055] S4, trigger setting: the main oscillator power amplifier laser realizes signal synchronization control through the control unit;

[0056] S5, excitation end parameter setting: the main oscillator power amplifier laser input 100% output power, 200ns pulse width; use the signal generator to synchronize the control pulse output frequency to 500Hz;

[0057] S6. Signal acquisition: With the end point of the excitation end optical fiber as the center, record the test results at the angles between the excitation and reception points of 0°, 30°, 60°, and 90°. The monitoring results are as follows: Figure 3 , Figure 4 , Figure 5 and Figure 6 As shown, it can be seen that the amplitude of the ultrasonic signal decreases as the angle increases.

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

[0059] S1. Connection of the excitation end system: embed a multi-channel optical fiber monitoring network into the CFRP, connect the input end of the optical splitter to the output end optical fiber of the main oscillator power amplifier laser, and connect the output end of the optical splitter to the four optical fibers of the excitation end to control the optical transmission path;

[0060] S2, receiving end system connection: the receiving end uses a multi-point Bragg fiber grating sensor and is connected to the demodulation unit;

[0061] S3, trigger setting: the main oscillator power amplifier laser and the data acquisition unit are connected to the control unit to realize signal synchronization control;

[0062] S4, excitation end parameter setting: the main oscillator power amplifier laser input 100% output power, 200ns pulse width; use the signal generator to synchronously control the pulse output frequency to 500Hz;

[0063] S5. Parameter setting at the receiving end: The demodulation unit sequentially locks the 3dB positions of the spectra of the four FBGs with different central wavelengths to collect 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 collect the results of ultrasonic signals in turn, such as Figure 7 The test 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 proves that the all-fiber laser ultrasonic monitoring system of the present invention has strong directional correlation.

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

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

[0068] S2, receiving end system connection: the receiving end uses a multi-point Bragg fiber grating sensor and is connected to the demodulation unit;

[0069] S3, prefabricated damage: a hole is opened in the middle of the prepared carbon fiber laminate embedded with the multi-channel optical fiber monitoring network;

[0070] S4, trigger setting: the main oscillator power amplifier laser and the data acquisition unit are connected to the control unit to realize signal synchronization control;

[0071] S5, setting of excitation end parameters: using a spectrometer to control the optical path, respectively exciting the ultrasonic signal in the CH1-CH4 channels; inputting 100% output power and 200ns pulse width to the main oscillator power amplifier laser; using a signal generator to synchronously control the output frequency of the pulse to be 500Hz;

[0072] S6, receiving end parameter setting: the demodulation unit sequentially locks the 3dB position of the spectrum of the four FBGs with different central wavelengths to collect 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 grating sensor on the same path collects the ultrasonic signal; the same is true for channels 2, 3, and 4, which only collect the ultrasonic signal of the corresponding channel. Figure 8-Figure 11 The ultrasonic signal results collected by the four channels before and after the injury respectively show that the amplitude attenuation of the CH2' and CH3' channels is greater than that of the CH1' and CH4' channels. Therefore, the present invention determines that the injury position is between channels 2 and 3, thereby achieving a preliminary judgment of the injury.

[0075] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. An all-fiber laser ultrasonic monitoring system, characterized in that: It includes a laser excitation unit, an excitation end optical fiber, a fiber grating sensor, a demodulation unit, a data acquisition unit and a control unit; The laser excitation unit is used to emit laser to the excitation end optical fiber; The excitation end optical fiber is embedded in the material to be monitored; the excitation end optical fiber is used to transmit the laser into the monitoring material to perform light-heat-acoustic energy conversion and generate an ultrasonic signal; The fiber Bragg grating sensor is embedded in the material to be monitored; the fiber Bragg grating sensor is used to collect ultrasonic signals; The demodulation unit is used to demodulate the collected ultrasonic signal and convert it into a voltage signal for output; The data acquisition unit converts the voltage signal into a digital signal and stores it; The control unit is used to output a control signal to realize the synchronous operation of the laser excitation unit and the data acquisition unit.

2. The all-fiber laser ultrasonic monitoring system according to claim 1, characterized in that: A plurality of excitation end optical fibers are embedded in the material to be monitored, and the laser excitation unit and the plurality of excitation end optical fibers are connected through a spectrometer. A multi-point fiber grating sensor with the same monitoring point as the excitation end optical fiber is embedded in the material to be monitored to form a multi-channel optical fiber monitoring network, and the multi-channel optical fiber monitoring network is used to locate the damage of the material to be monitored.

3. The all-fiber laser ultrasonic monitoring system according to claim 1, characterized in that: A multi-channel optical fiber monitoring network is used to locate the damage of the material to be monitored. The specific process is: obtain the signal attenuation percentage of each channel before and after damage in the multi-channel optical fiber monitoring network, and determine the damage position of the material to be monitored as between the channel with the largest signal attenuation percentage and the channel with the second largest signal attenuation percentage.

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

5. The all-fiber laser ultrasonic monitoring system according to claim 1, characterized in that: The excitation end optical fiber and the fiber grating sensor are arranged on the same straight line or within a set small deflection angle range.

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