Acoustic Excitation Digital Modulation Noise Reduction Method, System and Application for Laser Acoustic Vibration Detection

Abstract

This invention discloses a method, system, and application for acoustic excitation digital modulation noise reduction in laser vibration detection. It includes the following steps: Step 1, acquiring the single-point measurement timing parameters of the laser vibration measuring device; Step 2, measuring the response characteristics of the audio excitation device and determining the pre-stabilization time required for the audio excitation signal to reach a stable amplitude from activation; Step 3, constructing an on / off keying modulation function, generating a digital gating function based on the timing parameters and pre-stabilization time, and multiplying the audio excitation signal with the digital gating function in the time domain to generate a modulated intermittent audio excitation signal; Step 4, controlling the audio excitation device to play the intermittent audio excitation signal and simultaneously triggering the laser vibration measuring device to scan and acquire measurement points on the surface of the object under test according to the timing parameters. This invention, through on / off keying digital modulation technology, enables the audio excitation to be activated only during the effective detection period of the laser vibration measurement, thereby significantly reducing the average noise level and equipment power consumption.

Description

Technical Field

[0001] This invention relates to the field of laser vibration measurement and signal processing technology, and in particular to an acoustic excitation digital modulation noise reduction method, system and application for laser acoustic vibration detection. Background Technology

[0002] Laser Doppler vibration measurement technology has become an important means of detecting internal defects in building walls and composite materials due to its advantages such as long-distance measurement, wide bandwidth response and high sensitivity.

[0003] When detecting defects such as hollowness, external excitation is usually required to induce structural resonance in the test object. Traditional excitation methods often rely on mechanical vibrators or piezoelectric actuators, which not only introduce additional mass and alter the dynamic characteristics of the structure, but are also difficult to install on rough surfaces and may even pose a risk of damaging the test object. In contrast, acoustic excitation, as a completely non-contact excitation method, has the advantages of controllable frequency, no surface damage, and ease of implementation, making it the preferred solution in the field of non-destructive testing.

[0004] However, most existing laser acoustic vibration detection systems use continuous audio signals as the audio excitation device. Laser vibration measurement systems typically employ a point-by-point scanning mode, with each point acquisition cycle consisting of three phases: scanning, waiting, and detection. During the scanning and waiting phases, the laser vibrometer does not record effective data, but the acoustic excitation device remains in a high-power output state. This temporal asynchrony leads to serious problems: on the one hand, prolonged high-decibel noise causes significant noise pollution to operators and the surrounding environment; on the other hand, continuous excitation during ineffective periods significantly increases the device's power consumption and heat generation, limiting the application life of portable devices. Summary of the Invention

[0005] The purpose of this invention is to provide a method, system, and application for acoustic excitation digital modulation noise reduction in laser vibratory detection. This invention utilizes on / off keying digital modulation technology to ensure that the audio excitation is activated only during the effective detection period of the laser vibratory meter, thereby significantly reducing the average noise level and device power consumption.

[0006] The technical solution of this invention: A method for acoustic excitation digital modulation noise reduction of laser acoustic vibration detection, comprising the following steps:

[0007] Step 1: Obtain the single-point measurement timing parameters of the laser vibration measurement equipment, including laser scanning time, mechanical relaxation waiting time, and vibration signal detection time;

[0008] Step 2: Measure the response characteristics of the audio excitation device and determine the pre-stabilization time required for the audio excitation signal to reach a stable amplitude from the start.

[0009] Step 3: Construct an on / off keying modulation function, generate a digital gating function based on the timing parameters and the pre-stabilization time, multiply the audio excitation signal with the digital gating function in the time domain to generate a modulated intermittent audio excitation signal;

[0010] Step 4: Control the audio excitation device to play the intermittent audio excitation signal, and simultaneously trigger the laser vibration measurement device to scan and collect the measurement points on the surface of the object under test according to the timing parameters; wherein, the intermittent audio excitation signal is configured to: be turned on only at the pre-stabilization time before the start of the vibration signal detection time, and continue until the end of the vibration signal detection time, while remaining silent during the remaining period of the laser scanning time and the mechanical relaxation waiting time.

[0011] In the above-described acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection, the digital gate function is defined as follows in step 3:

[0012] ;

[0013] In the formula: Represents a digital gating function;

[0014] The modulated intermittent audio excitation signal is represented as:

[0015] ;

[0016] In the formula: It is an intermittent audio excitation signal. This is the audio excitation signal.

[0017] The aforementioned acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection, wherein the audio excitation signal It is a continuous wideband noise signal or a linear frequency modulated signal.

[0018] The aforementioned acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection further includes a synchronization compensation step based on sound wave propagation delay in step 4:

[0019] Obtain the distance between the audio excitation device and the surface of the object under test. Calculate the propagation time delay of the sound wave to the surface of the object under test:

[0020] ;

[0021] In the formula, The speed at which sound travels through the air;

[0022] The audio activation time is adjusted according to the propagation delay to ensure that the time when the sound wave reaches the surface of the object under test is synchronized with the time when the laser vibration measuring device starts collecting data.

[0023] The aforementioned acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection also includes a ranging step based on intermittent audio excitation signals:

[0024] Record the audio off moment after the audio excitation device stops playing. And monitor the vibration signal energy collected by the laser vibration measurement equipment in real time. ;

[0025] When the vibration signal energy Recording time at decay to background noise level Calculate the distance between the object to be measured and the laser vibration measuring device:

[0026] ;

[0027] in, This is the speed at which sound travels through the air.

[0028] In the aforementioned acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection, the timing parameters are determined experimentally or obtained from the specifications of the laser vibration measuring device.

[0029] A system for implementing the aforementioned acoustically excited digital modulation noise reduction method includes:

[0030] Laser vibration measurement equipment is used to collect vibration signals from the surface of an object under test;

[0031] An audio excitation device for outputting an intermittent audio excitation signal modulated by on / off keying;

[0032] The control unit is used to generate the intermittent audio excitation signal and control the operation of the laser vibration measuring device and the audio excitation device.

[0033] In the aforementioned system, the control unit is further configured to perform acoustic wave propagation delay compensation and / or calculate the distance between the object under test and the laser vibration measuring device based on the vibration signal decay time.

[0034] In the aforementioned system, the laser vibration measuring device is a vibration meter based on the laser Doppler frequency shift principle.

[0035] An application of an acoustically excited digital modulation noise reduction method as described above in the detection of hollow areas in building walls or internal defects in composite materials.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. This invention uses a digital gating function to strictly control the on / off timing of audio excitation. Audio is activated only during the pre-stabilization period before vibration signal detection and until the end of detection, remaining silent during other periods. This effectively eliminates noise radiation during ineffective periods, significantly improving the operator's working environment and reducing interference with the surrounding environment. Through intermittent excitation design, this invention ensures that audio excitation only operates during effective periods, significantly reducing the average power consumption of the excitation device, minimizing heat generation, extending the battery life of portable devices, and broadening their application scenarios.

[0038] 2. This invention ensures measurement accuracy through a dual-time synchronization design: First, the audio signal is activated in advance based on the pre-stabilization time of the audio excitation device, ensuring the stability of the audio signal during the detection period and avoiding insufficient vibration excitation due to signal instability. Second, through sound wave propagation delay compensation, the propagation delay is calculated based on the distance between the audio excitation device and the object under test, adjusting the audio activation time to achieve precise synchronization between the arrival of the sound wave on the object under test and the start of the laser vibration measurement equipment. Simultaneously, intermittent excitation avoids interference from the excitation signal during invalid periods on the laser vibration measurement equipment, reducing speckle noise and measurement errors, and improving the accuracy and reliability of the detection data.

[0039] 3. This invention utilizes the signal attenuation characteristics of intermittent audio excitation. After the audio is turned off, by monitoring the time difference between the vibration signal energy decaying to the background noise level and combining it with the speed of sound propagation in the air, the distance between the object under test and the laser vibration measuring device can be calculated. No additional ranging hardware is required, which simplifies the system structure, reduces equipment costs, and improves the integration and practicality of the detection system.

[0040] 4. In application, this invention does not require direct contact with the test object, and can be adapted to complex test objects such as rough surfaces and fragile structures. It does not introduce additional mass or change the dynamic characteristics of the test object, and is especially suitable for non-destructive testing scenarios such as detection of hollow areas in building walls and detection of internal defects in composite materials, with a wider range of applications. Attached Figure Description

[0041] Figure 1 This is a basic principle diagram for detecting the vibration of the object under test.

[0042] Figure 2 It is a diagram showing the placement relationship between the laser vibration measuring equipment, the audio equipment, and the object under test.

[0043] Figure 3 This is a diagram showing the selection of the test point by the laser vibration measurement equipment.

[0044] Figure 4 It is the time axis of the scanning laser vibration measurement device and the audio excitation signal.

[0045] Figure 5This is a schematic diagram of the original audio excitation signal and the intermittent audio excitation signal after OOK modulation.

[0046] Figure 6 is a schematic diagram of signal propagation and time sequence when the distance between the audio excitation device and the object under test is less than the distance between the audio excitation device and the laser vibration measuring device. (a) is a schematic diagram of signal propagation when the distance between the audio excitation device and the object under test is less than the distance between the audio excitation device and the laser vibration measuring device, and (b) is the time sequence of the audio signal arriving at the object under test and the laser vibration measuring device in this scenario.

[0047] Figure 7 is a schematic diagram of signal propagation and time sequence when the distance between the audio excitation device and the object under test is greater than the distance between the audio excitation device and the laser vibration measuring device. (a) is a schematic diagram of signal propagation when the distance between the audio excitation device and the object under test is greater than the distance between the audio excitation device and the laser vibration measuring device. (b) is the time sequence of the audio signal arriving at the object under test and the laser vibration measuring device in this scenario.

[0048] Figure 8 These are comparison diagrams of the intermittent audio excitation signal control timing under two distance scenarios. (a) is a schematic diagram of the intermittent audio excitation signal control timing when the distance between the audio excitation device and the object under test is less than the distance between the audio excitation device and the laser vibration measuring device, and (b) is a schematic diagram of the intermittent audio excitation signal control timing when the distance between the audio excitation device and the object under test is greater than the distance between the audio excitation device and the laser vibration measuring device.

[0049] Figure 9 It is a real-time vibration signal attenuation diagram collected by laser vibration measurement equipment. Detailed Implementation

[0050] The present invention will be further described below with reference to the embodiments and accompanying drawings, but this should not be construed as limiting the present invention.

[0051] Example 1: An acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection.

[0052] The method of this invention is a digital modulation method based on on-off keying (OOK). This method divides the measurement process into three stages: scanning, waiting, and probing, according to the single-point acquisition timing of laser vibratory measurement. By modulating the original audio excitation signal, the audio is controlled to be on only during the probing stage and its required pre-stabilization time, while remaining silent during the scanning and waiting stages. This invention effectively eliminates invalid noise during non-acquisition periods while ensuring the accuracy of the vibration signal acquisition data, significantly reducing the average noise level and equipment power consumption throughout the measurement process.

[0053] The following is a detailed explanation of laser vibrometrics: The Doppler frequency shift effect in laser vibrometrics is the core physical principle for measuring the vibration velocity of an object. The Doppler effect refers to the phenomenon that the observed wave frequency changes when there is relative motion between the wave source and the object being measured. Assuming the incident laser frequency is fixed... The surface velocity of the object to be measured is Due to the movement of the object, the frequency of the reflected light changes. Relative to incident light An offset occurs, and the frequency of the offset is... The motion of an object causes a change in the optical path, and the phase difference is:

[0054] ;

[0055] In the formula, the angle of incidence is The phase change rate, or frequency shift, is:

[0056] ;

[0057] When the laser shines perpendicularly onto the object being tested Then the phase change rate, i.e. the frequency shift, is:

[0058] ;

[0059] By combining the Doppler frequency shift effect with an interference optical path, the frequency of the measured light changes due to the Doppler frequency shift. The reference light maintains its original frequency. When two beams of light interfere, the intensity of the superimposed light is:

[0060] ;

[0061] In the formula, For light intensity, and These are the complex amplitudes of the two coherent optical fields, and It is the light intensity of a single light field. The frequency difference between the two light fields For time, , representing the initial phase difference between the two light fields. To determine the vibration displacement, the vibration signal is extracted by demodulating the interference signal. λ is the wavelength.

[0062] The equipment used in the method of this invention includes an audio excitation device and a laser vibration measuring device, the principle of which is as follows: Figure 1As shown, an audio device is used as the audio excitation device, and the emitted audio signal is used as the input signal to excite the object under test. The object under test is represented by a gray rectangle in the figure. Finally, a laser vibrometer is used to collect the output signal. The laser vibrometer utilizes interference and Doppler frequency shift effects to collect vibration signals. The specific detection process is as follows:

[0063] Place the audio excitation device and the laser vibrometer at a suitable distance from the object to be measured. It is necessary to ensure that the vibration characteristics of the object under test are excited, such as Figure 2 As shown. Using a vibration measuring device, select the area to be probed on the object under test. Then, select points at equal intervals within the selected area as the test points, such as... Figure 3 As shown, the rectangle represents the area selected on the surface of the object to be tested, and the dots represent the prediction points. The scanning process will scan line by line and point by point in the direction of the arrows. Different input signals are designed for different objects to be tested, and the designed input signals are played using an audio device to excite the object to be tested. The laser vibration measurement device is started, and the position of the laser irradiation is adjusted by the mechanical platform. Each prediction point goes through three processes: scanning, waiting, and detection. Then the output signal of each point is collected, the audio is turned off, and the acquisition is completed.

[0064] In the aforementioned detection and acquisition process, the designed audio excitation signal typically uses a continuous wideband audio signal as the output signal to losslessly excite the test object. However, continuous audio will continuously generate noise. Since only the detection process requires a stable excitation signal, the audio signal used as the input signal can be modulated so that the audio excitation signal is played stably only during detection. This can eliminate the noise generated by the audio during other processes. However, the audio takes a certain amount of time to stabilize from the start, so a period of time needs to be reserved before detection as the audio waiting time to stabilize. The on / off keying method can be used to modulate the audio so that the audio can be controlled to start playing at an appropriate position and stop immediately after the detection process ends.

[0065] Based on the above description of laser vibration measurement technology, the present invention specifically includes the following steps:

[0066] Step 1: Obtain the single-point measurement timing parameters of the laser vibration measurement equipment, including laser scanning time, mechanical relaxation waiting time, and vibration signal detection time;

[0067] In this step, the scanning process is as follows: Figure 4 As shown, the time taken for the laser vibration measuring device to transfer the laser to the next point after collecting vibration data from one point via a mechanical pan-tilt head or scanning galvanometer is the laser scanning time t. 扫描When the scanning galvanometer of a laser vibrometer rapidly moves from one measurement point to the next, it experiences brief relaxation oscillations due to mechanical inertia. During this period (typically on the order of milliseconds to tens of milliseconds) before the oscillations decay to a steady state, the laser spot position is unstable. Any data acquisition performed during this time will introduce significant speckle noise and measurement errors. Therefore, this period is called the mechanical relaxation waiting time t. 等待 This period is considered an invalid data acquisition time. Furthermore, the vibration signal detection time t is set according to the detection requirements. 探测 (Typically 10ms-100ms, needing to cover the resonant response period of the analyte). The laser scanning time t mentioned above... 扫描 Mechanical relaxation waiting time t 等待 and vibration signal detection time t 探测 The data can be retrieved directly from the specifications of the laser vibration measuring equipment, or it can be measured experimentally.

[0068] Step 2: Measure the response characteristics of the audio excitation device and determine the pre-stabilization time required for the audio excitation signal to reach a stable amplitude from the start.

[0069] In this step, such as Figure 4 As shown, after the audio excitation device (speaker) is turned on, the speaker needs a settling time, i.e., a pre-stabilization time, to play a stable audio excitation signal. Since the speed of laser light in air is much greater than that of sound, signal acquisition only needs to consider the time t required for sound wave propagation. 时延 .

[0070] Step 3: Construct an on / off keying modulation function, generate a digital gating function based on the timing parameters and the pre-stabilization time, multiply the audio excitation signal with the digital gating function in the time domain to generate a modulated intermittent audio excitation signal;

[0071] In this step, when the sound wave reaches the surface of the object to be measured, the laser vibration measurement device starts to collect the signal simultaneously, and the designed audio signal duration is controlled according to the required signal collection duration.

[0072] The On-Off Keying (OOK) modulation method used in this embodiment is a simple digital modulation method that represents binary data (1 and 0) by controlling the on / off state of the carrier wave. The gate function is:

[0073] ;

[0074] After the audio excitation signal is digitally modulated using on / off keying, the audio is usually converted into a binary data stream and then transmitted via a high-frequency carrier (such as radio frequency or optical signals). If the original audio excitation signal used is... The audio modulated signal obtained after on / off keying digital modulation is:

[0075] ;

[0076] The modulation process is as follows Figure 5 As shown. The time taken for each point to undergo the three processes of scanning, waiting, and probing is as follows: , , Audio is activated during the scanning and waiting periods. After the audio is activated, there is a period of audio stabilization before the detection time begins. At the start of the detection time, the audio reaches a stable state, and data acquisition begins. By using on / off keying digital modulation, the continuous audio excitation signal can be replaced with an intermittent audio excitation signal required for data acquisition in laser vibrometers, thus reducing noise.

[0077] Step 4: Control the audio excitation device to play the intermittent audio excitation signal, and simultaneously trigger the laser vibration measurement device to scan and collect the measurement points on the surface of the object under test according to the timing parameters; wherein, the intermittent audio excitation signal is configured to: be turned on only at the pre-stabilization time before the start of the vibration signal detection time, and continue until the end of the vibration signal detection time, while remaining silent during the remaining period of the laser scanning time and the mechanical relaxation waiting time.

[0078] In this step, within a complete measurement cycle... The value includes the laser scanning time t. 扫描 Mechanical relaxation waiting time t 等待 and vibration signal detection time t 探测 Intermittent audio excitation signals are used only during the pre-stabilization phase before detection. Enabled and continues until the detection ends. The actual effective duration of audio activation within one cycle. for:

[0079] ;

[0080] The total acoustic energy released in one cycle for:

[0081] ;

[0082] In the formula, The instantaneous sound power of the audio excitation device when it is turned on;

[0083] Average sound power for:

[0084] ;

[0085] According to acoustic principles, the change in sound power level Defined as the logarithm of the power ratio. Based on the continuous excitation mode, the average noise reduction brought by the OOK modulation mode is:

[0086] ;

[0087] Therefore, compared to continuously played audio excitation, the optimized intermittent audio excitation signal has a lower duty cycle, and its average noise reduction in decibels can be expressed as:

[0088] ;

[0089] This formula demonstrates that precise control of audio is achieved through on / off keying digital modulation. Always on, avoiding [the problem] at t 扫描 and t 等待 The noise generated during the initial invalid period significantly reduces the average noise level throughout the entire measurement process.

[0090] When using a combination of laser vibrometer and acoustic excitation to collect vibration signals from an object, the laser vibrometer is inevitably affected by the acoustic source due to the limited distance between it and the speaker. This results in the collected data including the vibration signal of the vibrometer itself. Therefore, depending on the distance between the speaker and the object under test and the laser vibrometer, two scenarios are considered: when the distance between the speaker and the object under test is greater than the distance between the speaker and the vibrometer, and when the distance between the speaker and the object under test is less than the distance between the speaker and the vibrometer. In both scenarios, on / off keying digital modulation technology can be used to adjust the audio signal, collecting only the signal generated by the vibration of the object under test due to audio excitation.

[0091] like Figure 7 As shown, when the distance between the speaker and the object under test is greater than the distance between the speaker and the vibration measuring device, the audio excitation signal output by the speaker will reach the surface of the object under test first. At this time, the laser vibration measuring device starts to collect the signal until the sound wave reaches the vibration measuring device. If the distance from the speaker to the object under test is... The distance from the speaker to the laser vibration measurement device is The time it takes for the audio excitation signal output by the speaker to reach the surface of the object under test is... The time it takes for the audio excitation signal output by the speaker to reach the surface of the object under test. , This is the speed at which sound travels through the air.

[0092] like Figure 6As shown, when the distance between the speaker and the object under test is less than the distance between the speaker and the vibration measuring device, the audio signal output by the speaker will reach the laser vibration measuring device first. At this time, it is necessary to set the audio signal according to the distance between the excitation device, the object under test, and the laser vibration measuring device to ensure that the laser vibration measuring device will not be interfered with by the audio signal when the audio signal reaches the surface of the object under test.

[0093] In the two cases, the audio signal is modulated using on / off keying, as follows: Figure 8 As shown.

[0094] Furthermore, when using laser vibration measurement equipment, it is necessary to focus the laser emitted by the equipment on the object to be measured. In order to focus the laser on the object to be measured, a rangefinder is usually used or a rangefinder module is connected to the laser vibration measurement equipment to measure the distance. This method of measuring distance is relatively complicated and increases costs. This embodiment provides a simpler method that does not require connecting other external equipment and can also measure the distance between the instrument and the object to be measured.

[0095] Because the laser vibrometer continuously receives vibration signals from the object under test, the laser continues to collect data even after the audio playback stops. When the audio is turned off, the audio playback immediately stops, and the time at this point is recorded. However, it takes some time for the played audio to reach the object under test. During this time, the signal collected by the laser vibration measuring device still exists until the signal completely disappears, at which point the time is recorded. Signals acquired using laser vibration measurement equipment, such as Figure 9 As shown, this gives us the time it takes for sound to travel. The distance between the object to be tested and the laser vibration measuring device is:

[0096] ;

[0097] In the formula, This is the speed at which sound travels through the air.

[0098] Based on the above method, when using audio excitation, the output power of the excitation device can be reduced, and noise generation can be decreased. Furthermore, without connecting other ranging modules, the distance between the object under test and the laser vibratory measuring device can be measured relatively accurately.

[0099] To verify the effectiveness of the method of the present invention, an experiment was designed to measure the vibration signal of a building wall. The experiment compared the original audio excitation signal and the intermittent audio excitation signal modulated by on-off keying. Taking the detection of hollow areas in a building wall as an example, the specific implementation parameters and results are as follows:

[0100] Parameter settings: Ambient temperature 25℃, sound propagation speed U=346m / s; Laser vibration measurement equipment timing parameters: t扫描 =30ms, t 等待 =20ms, t 探测 =50ms; Pre-stabilization time of audio excitation device =10ms; Distance between the audio excitation device and the wall =2m, propagation delay t 时延 =5.78ms; The corrected audio turn-on time is 30ms+20ms-10ms-5.78ms=34.22ms, and the turn-off time is 30ms+20ms+50ms=100ms; The original audio signal is a continuous wideband signal of 200-5000Hz.

[0101] Results: Using the method of this invention, the average noise level decreased by 15.2 dB compared to the continuous excitation mode, and the power consumption of the equipment decreased by 65%. The signal-to-noise ratio of the vibration signal collected by the laser vibration measuring device was improved by more than 20%, and the vibration characteristics of the hollow area and the normal area were significantly different, which can accurately locate the hollow area. The distance error between the wall surface and the vibration measuring device obtained by vibration signal attenuation distance measurement is less than 3%, which meets the detection accuracy requirements.

[0102] Example 2: This example provides a system for implementing the method described in Example 1, including a laser vibration measurement device, an audio excitation device, and a control unit. The specific configuration and workflow are as follows:

[0103] The laser vibration measurement equipment uses a two-dimensional scanning laser vibration meter, which is based on self-mixing interferometry technology and can be adapted to rough concrete surfaces without the need for reflective paper. The frequency response range is DC-100kHz, the working distance is 0.5-10m, and the output accuracy reaches 1%, meeting the high-precision vibration signal acquisition requirements for defect detection. It integrates a high-definition camera and 3D scanning distance measurement function, and supports automatic point-by-point acquisition by preset scanning paths through PC software.

[0104] The audio excitation device uses the HBK4250 broadband volume velocity audio system, featuring a dual-microphone design for real-time and accurate measurement of output volume velocity, effectively reducing near-field interference. Its frequency response covers the entire range commonly used in acoustic testing, with built-in overload protection and adjustable output power, making it suitable for high-transmission-loss building component testing scenarios. It connects to a dedicated acoustic probe via a quick connector, radiating digitally modulated audio signals in a directional manner to ensure the audio excitation signal is concentrated on the area under test.

[0105] Control Unit: The core controller is an FPGA, which has both PS-side interface configuration and PL-side parallel data processing capabilities, with a timing control error of ≤50ns. It connects to laser vibration measurement equipment via an EtherCAT interface and to an audio excitation device via an RS485 interface to achieve real-time transmission of commands and data. It has a built-in on / off keying (OOK) modulation module that can generate programmable gating functions to complete the intermittent modulation of the original audio signal, while reducing system power consumption through dynamic clock gating technology.

[0106] The system workflow is as follows:

[0107] Parameter configuration: Input detection parameters (measuring point spacing, detection time, pre-stabilization time, etc.) through the host computer software, and the control unit will automatically calculate the audio excitation on / off timing and generate the corresponding digital gating function.

[0108] Excitation signal modulation: The control unit generates a 100-5000Hz wideband original audio signal, which is multiplied by the time domain of the gate function to obtain an intermittent modulation signal, which is amplified by the audio excitation device and directionally radiated to the test object.

[0109] Synchronous acquisition: The control unit sends a synchronous trigger signal, and the laser vibration measurement device starts acquiring vibration signals after the audio excitation stabilizes. It converts the scattered light signal generated by the vibration of the object under test into an electrical signal and transmits it back to the control unit in real time.

[0110] Data processing: The control unit filters and reduces noise from the collected vibration signals, extracts characteristic parameters such as frequency and amplitude, and outputs the detection results through the host computer software to complete defect identification and location.

[0111] Through the above specific implementation methods, those skilled in the art can adjust the relevant parameters according to the actual detection scenario to achieve low-noise, low-power, and high-precision laser acoustic vibration detection, which can be effectively applied to fields such as building wall hollow detection and composite material internal defect detection.

Claims

1. A method for acoustic excitation digital modulation noise reduction in laser acoustic vibration detection, characterized in that, Includes the following steps: Step 1: Obtain the single-point measurement timing parameters of the laser vibration measuring device. The timing parameters include laser scanning time, mechanical relaxation waiting time, and vibration signal detection time. The mechanical relaxation waiting time refers to the time required for the oscillation of the scanning galvanometer of the laser vibration measuring device to decay to a stable state due to mechanical inertia after it has moved. Step 2: Measure the response characteristics of the audio excitation device and determine the pre-stabilization time required for the audio excitation signal to reach a stable amplitude from the start. Step 3: Construct an on / off keying modulation function, generate a digital gating function based on the timing parameters and the pre-stabilization time, multiply the audio excitation signal with the digital gating function in the time domain to generate a modulated intermittent audio excitation signal; Step 4: Control the audio excitation device to play the intermittent audio excitation signal, and simultaneously trigger the laser vibration measurement device to scan and collect the measurement points on the surface of the object under test according to the timing parameters; wherein, the intermittent audio excitation signal is configured to: be turned on only at the pre-stabilization time before the start of the vibration signal detection time, and continue until the end of the vibration signal detection time, while remaining silent during the remaining period of the laser scanning time and the mechanical relaxation waiting time.

2. The acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection according to claim 1, characterized in that, In step 3, the digital gating function is defined as follows: ； In the formula: Represents a digital gating function; The modulated intermittent audio excitation signal is represented as: ； In the formula: It is an intermittent audio excitation signal. This is the audio excitation signal.

3. The acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection according to claim 1, characterized in that, The audio excitation signal It can be a continuous wideband signal or a linear frequency modulated signal.

4. The acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection according to claim 1, characterized in that, Step 4 also includes a synchronization compensation step based on the sound wave propagation delay: Obtain the distance between the audio excitation device and the surface of the object under test. Calculate the propagation time delay of the sound wave to the surface of the object under test: ； In the formula, The speed at which sound travels through the air; The audio activation time is adjusted according to the propagation delay to ensure that the time when the sound wave reaches the surface of the object under test is synchronized with the time when the laser vibration measuring device starts collecting data.

5. The acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection according to claim 1, characterized in that, It also includes a ranging step based on intermittent audio excitation signals: Record the audio off moment after the audio excitation device stops playing. And monitor the vibration signal energy collected by the laser vibration measurement equipment in real time. ; When the vibration signal energy Recording time at decay to background noise level Calculate the distance between the object to be measured and the laser vibration measuring device: ； in, This is the speed at which sound travels through the air.

6. The acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection according to claim 1, characterized in that, The timing parameters are determined experimentally or obtained from the specifications of the laser vibration measuring device.

7. A system for implementing the acoustic excitation digital modulation noise reduction method for laser acoustic vibration detection as described in any one of claims 1 to 6, characterized in that, include: Laser vibration measurement equipment is used to collect vibration signals from the surface of an object under test; An audio excitation device for outputting an intermittent audio excitation signal modulated by on / off keying; The control unit is used to generate the intermittent audio excitation signal and control the operation of the laser vibration measuring device and the audio excitation device.

8. The system according to claim 7, characterized in that: The control unit is also configured to perform acoustic wave propagation delay compensation and / or calculate the distance between the object under test and the laser vibration measurement device based on the vibration signal decay time.

9. The system according to claim 7, characterized in that: The laser vibration measuring device is a vibration meter based on the laser Doppler frequency shift principle.

10. The application of an acoustically excited digital modulation noise reduction method as described in any one of claims 1 to 6 in the detection of hollow areas in building walls or internal defects in composite materials.

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