Ultrasonic guided wave phased array imaging method based on waveform covariance

Abstract

Disclosed is an ultrasonic guided wave phased array imaging method based on waveform covariance, which belongs to the field of ultrasonic nondestructive testing technology. The method includes performing dispersion and amplitude compensation on phased array acquisition signals; extracting signal wave packets belonging to the current imaging point from all signals; calculating the waveform covariance coefficients between all wave packets; and calculating imaging indicators based on the waveform covariance coefficients. The method can realize damage imaging of ultrasonic guided wave waveguide structures, and has the advantages of low background noise and pseudo-point interference in imaging results.

Description

Technical field:

[0001] The invention belongs to the technical field of ultrasonic nondestructive testing, and in particular relates to an ultrasonic guided wave phased array imaging method. Background technology:

[0002] Compared with ultrasonic body waves, ultrasonic guided waves can propagate over longer distances and can perform nondestructive testing on the waveguide structure along the entire propagation path under a single excitation. They are sensitive to both surface and internal defects of the waveguide structure. The nondestructive testing and structural health monitoring method based on ultrasonic guided waves has the advantages of being pollution-free and having high detection efficiency. It is a nondestructive testing and structural health monitoring method with good application prospects. In addition, for some structures or areas that cannot be reached by traditional ultrasonic body wave detection methods, the detection method based on ultrasonic guided waves can be used as an effective alternative.

[0003] Phased array is a method that uses multiple array elements to excite and receive signals, which can collect more information about the structure being tested. Compared with single array element detection, it has higher detection accuracy and signal-to-noise ratio. Among the existing ultrasonic guided wave phased array imaging methods, there are mainly two categories: imaging methods based on signal amplitude and imaging methods based on signal phase. These methods do not fully utilize all the information in the signal, resulting in high background noise and pseudo-point interference in their imaging results, and the imaging performance still needs to be improved. Summary of the invention:

[0004] In order to further improve the imaging performance of the existing ultrasonic guided wave phased array imaging method, the object of the present invention is to provide a new ultrasonic guided wave phased array imaging method to achieve high-quality imaging of defects in waveguide structures.

[0005] The objective of the present invention is achieved through the following technical solutions. A waveform covariance-based ultrasonic guided wave phased array imaging method comprises the following steps:

[0006] Step S1: performing frequency dispersion and amplitude compensation on the signal collected by the phased array according to the imaging point position;

[0007] Step S2: extracting the signal wave packet belonging to the current imaging point from the dispersion and amplitude compensated signal;

[0008] Step S3: Calculate the waveform covariance coefficients between all signal wave packets;

[0009] Step S4: Calculate imaging indicators based on the waveform covariance coefficient, and use the imaging indicators as pixel values ​​to obtain imaging results.

[0010] In the waveform covariance-based ultrasonic guided wave phased array imaging method, in step S1, the dispersion and amplitude compensation method includes:

[0011]

[0012] in, represents the frequency domain signal obtained after dispersion and amplitude compensation, S n (ω) represents the frequency domain form of the nth signal collected by the ultrasonic guided wave phased array, (x, y) represents the coordinates of the imaging point, Indicates the location of the excitation array element corresponding to the nth signal The distance to the imaging point (x, y) is the distance from the imaging point (x, y) to the receiving element. The product of the distances, e is a scientific constant, j is an imaginary unit, is the location of the excitation element corresponding to the nth signal The distance to the imaging point (x, y) is the distance from the imaging point (x, y) to the receiving element. The sum of the distances, k(ω) represents the wave number of the ultrasonic guided wave at the angular frequency ω.

[0013] In the waveform covariance-based ultrasonic guided wave phased array imaging method, in step S2, the extraction of the signal wave packet is performed in the following manner:

[0014]

[0015] in is the frequency domain signal obtained after dispersion and amplitude compensation The time domain form of is the extracted signal wave packet, and T represents the effective length of the signal wave packet in the time domain.

[0016] In the ultrasonic guided wave phased array imaging method based on waveform covariance, the frequency domain signal obtained after dispersion and amplitude compensation is The time domain form Obtained by inverse Fourier transform.

[0017] In the waveform covariance-based ultrasonic guided wave phased array imaging method, the time domain effective length T of the signal wave packet is: T=1 / f, where f is the carrier frequency of the excitation signal in the ultrasonic guided wave phased array.

[0018] In the waveform covariance-based ultrasonic guided wave phased array imaging method, in step S4, the imaging index is calculated by:

[0019]

[0020] Where I(x, y) represents the imaging pixel value at the image point (x, y). represents the waveform covariance coefficient between the pth signal wave packet and the qth signal wave packet extracted at the imaging point (x, y), where p and q are both positive integers, and w p，q is the weight factor, N is the number of signals, and Respectively and The mean value at time t, L is the number of points in the signal within time 0≤t≤T, t i represents the i-th discrete time point.

[0021] In the ultrasonic guided wave phased array imaging method based on waveform covariance, the weight factor w p，q The value of is:

[0022] The present invention has the following beneficial effects:

[0023] The invention discloses an ultrasonic guided wave phased array imaging method based on waveform covariance, which simultaneously utilizes the amplitude and phase information of the signal within the effective time T, has the advantages of small background noise and pseudo-point interference in the imaging result, and can achieve high-quality imaging. Description of the drawings:

[0024] In order to more clearly illustrate the embodiments of the present invention, the drawings required to be used in the embodiments are briefly introduced below.

[0025] Figure 1 is a time domain waveform diagram of all 56 signals acquired by the ultrasonic guided wave phased array according to an embodiment of the present invention;

[0026] Figure 2 It is a flow chart of an ultrasonic guided wave phased array imaging method based on waveform covariance provided by an embodiment of the present invention;

[0027] Figure 3 is a typical imaging result diagram obtained by the method of the present invention provided in an embodiment of the present invention;

[0028] Figure 4 This is an imaging result diagram obtained by the existing full-focus imaging method provided by an embodiment of the present invention;

[0029] Figure 5 This is an imaging result diagram obtained by the existing imaging method based on symbol coherence factor provided by an embodiment of the present invention. Specific implementation method:

[0030] The following is combined with Figures 1 to 5 The specific embodiments of the present invention are further described in detail. The described embodiments are only used to explain the present invention but not to limit the scope of the present invention.

[0031] Reference Figure 1 , which is a time domain waveform diagram of all 56 signals obtained by phased array acquisition for implementing an ultrasonic guided wave phased array imaging method based on waveform covariance in this embodiment. The structure under test is an aluminum alloy plate with a geometric size of 610mm×1215mm×2mm. The ultrasonic guided wave phase control adopted is a ring array containing 8 array elements, the diameter of the ring array is 15mm, and the 8 array elements are distributed at equal angles. During signal acquisition, the ring array is placed at the center of the aluminum alloy plate and used as the coordinate origin, so that the specific coordinates of each array element can be determined. The excitation signal is a 5-cycle sinusoidal signal modulated by a Hanning window and with a carrier frequency of f=80kHz. During signal acquisition, the time length of each signal is 0.8ms, and the sampling frequency of the signal is 10MHz. Each array element takes turns as an exciter to excite the target excitation signal, and the remaining 7 array elements are used as signal receivers for signal acquisition, so that N=8×7=56 signals can be acquired. In this way, the coordinates of the excitation array elements corresponding to all 56 signals can be determined And collect array element coordinates Where i=1, 2,…, 56.

[0032] Reference Figure 2 , a flowchart of an ultrasonic guided wave phased array imaging method based on waveform covariance is provided for implementing the present invention, and an ultrasonic guided wave phased array imaging method based on waveform covariance comprises the following steps:

[0033] Step S1: performing frequency dispersion and amplitude compensation on the signal collected by the phased array according to the imaging point position;

[0034] Step S2: extracting the signal wave packet belonging to the current imaging point from the dispersion and amplitude compensated signal;

[0035] Step S3: Calculate the waveform covariance coefficients between all wave packets;

[0036] Step S4: Based on the waveform covariance coefficient, the imaging index is calculated, and the imaging result is obtained by using the index as the pixel value. Imaging is actually to assign a value to each imaging point, and use the value as the pixel value of the image at that point. When the pixel values ​​of all points are determined, an image can be obtained.

[0037] To put it bluntly, imaging is to determine the imaging index of each imaging point, and this index has the ability to indicate damage;

[0038] Preferably, in an embodiment of the method, the dispersion and amplitude compensation method in step S1 includes:

[0039]

[0040] in, represents the frequency domain signal obtained after dispersion and amplitude compensation; S n (ω) represents the frequency domain form of the nth signal collected by the ultrasonic guided wave phased array, which can be Figure 1 The time domain signal in is obtained by Fourier transform; (x, y) represents the coordinates of the imaging point, Indicates the location of the excitation array element corresponding to the nth signal The distance to the imaging point (x, y) is the distance from the imaging point (x, y) to the receiving element. The product of the distances; e is a scientific constant; j is an imaginary unit; is the location of the excitation element corresponding to the nth signal The distance to the imaging point (x, y) is the distance from the imaging point (x, y) to the receiving element. k(ω) represents the wave number of the ultrasonic guided wave at the angular frequency ω. In this embodiment, k(ω) is the wave number of the A0 mode in the 2 mm thick aluminum plate, which can be obtained by solving the Lamb wave dispersion equation. In this embodiment, n = 1, 2, ..., 56, the excitation array element position and receiving element position Determined by the coordinates of the circular array. For example, for the nth signal, its corresponding excitation array element and receiving array element can be found, and the coordinates of these two array elements are and The coordinates (x, y) of the imaging point are divided into a series of square grids by discretizing the imaging area, and the center coordinates of the grids are the coordinates of the imaging. In this embodiment, the imaging area selects a square area of ​​500mm×500mm around the center of the board, and the size of the imaging grid is 2mm×2mm.

[0041] In one embodiment of the method, the extraction of the signal wave packet in step S2 is performed in the following manner:

[0042]

[0043] in is a frequency domain signal The time domain form of can be obtained by inverse Fourier transform, is the extracted signal wave packet, and T represents the effective length of the wave packet in the time domain.

[0044] In one embodiment of the method, the imaging index in step S4 is calculated as follows:

[0045]

[0046] Where I(x, y) represents the imaging pixel value at the image point (x, y). represents the waveform covariance coefficient between the pth signal wave packet and the qth signal wave packet extracted at the imaging point (x, y), where p and q are both positive integers, and w p，q is the weight factor, N is the number of signals, and Respectively and The mean value at time t, L is the number of points in the signal within time 0≤t≤T, t i Indicates the ith discrete time point. In this embodiment, the number of signals N=56.

[0047] Preferably, in one embodiment of the method, the effective time domain length of the wave packet includes T=1 / f, where f is the carrier frequency of the excitation signal in the phased array. In this embodiment, T=1 / f=0.0125ms.

[0048] Preferably, in one embodiment of the method, the weight factor w p，q The values ​​include

[0049] Reference Figure 3 , which is a typical imaging result diagram obtained by the method provided by an embodiment of the present invention. In the figure, the pixel value is normalized to the maximum value, the logarithm with the base of 10 is taken and then multiplied by 20 to obtain the pixel value in decibels, and displayed in the range of -20 to 0dB. The darker the color bar on the right, the higher the pixel value, and the greater the probability of a defect.

[0050] Reference Figure 4 , is an imaging result diagram obtained by the existing all-focus imaging method provided by an embodiment of the present invention. In this embodiment, the calculation formula of the all-focus imaging method is: Among them, I TFM (x, y) represents the imaging pixel value at position (x, y) of the all-focus imaging method, N is the number of signals, for exist The value of is the effective time domain width of the excitation signal in this embodiment, It is the nth signal wave packet intercepted after frequency dispersion and amplitude compensation according to the method. Figure 4 and Figure 3 The processing method is the same as that of , which is to normalize the pixel value to the maximum value, take the logarithm with base 10 and multiply it by 20 to get the pixel value in decibels, and display it in the range of -20 to 0dB. Figure 4 As Figure 3 A comparison result of Figure 4 Compared with Figure 3Therefore, from the comparison results of this embodiment, it can be seen that the method of the present invention has a better ability to suppress pseudo points and background noise than the existing full focusing method.

[0051] Reference Figure 5 , is an imaging result diagram obtained by the existing imaging method based on symbol coherence factor provided by an embodiment of the present invention. In this embodiment, the calculation formula of the imaging method based on symbol coherence factor is: and

[0052]

[0053] Where N is the number of signals, for exist The value of is the effective time domain width of the excitation signal in this embodiment, The nth signal wave packet intercepted after frequency dispersion and amplitude compensation according to the method described is shown. Figure 5 and Figure 3 The processing method is the same as that of , which is to normalize the pixel value to the maximum value, take the logarithm with base 10 and multiply it by 20 to get the pixel value in decibels, and display it in the range of -20 to 0dB. Figure 5 As Figure 3 Another comparison result, Figure 5 Compared with Figure 3 Therefore, from the comparison results of this embodiment, it can be seen that this method has a better ability to suppress pseudo points and background noise than the existing imaging method based on symbol coherence factor.

[0054] Although the embodiments of the present invention are described above in conjunction with the accompanying drawings, the present invention is not limited to the above specific embodiments and application fields, and the above specific embodiments are only illustrative and instructive, rather than restrictive. A person of ordinary skill in the art can also make many forms under the guidance of this specification and without departing from the scope of protection of the claims of the present invention, all of which belong to the protection of the present invention.

Claims

1. An ultrasonic guided wave phased array imaging method based on waveform covariance, characterized in that: The method comprises the following steps: Step S1: performing frequency dispersion and amplitude compensation on the signal collected by the phased array according to the imaging point position, wherein the frequency dispersion and amplitude compensation method includes: ， in, represents the frequency domain signal obtained after dispersion and amplitude compensation, represents the frequency domain form of the nth signal collected by the ultrasonic guided wave phased array, represents the coordinates of the imaging point, Indicates the location of the excitation array element corresponding to the nth signal To imaging point The distance from the imaging point To the receiving array location The product of the distances, e is a scientific constant, j is an imaginary unit, is the location of the excitation element corresponding to the nth signal To imaging point The distance from the imaging point To the receiving array location The sum of the distances, Indicates the angular frequency of ultrasonic guided waves The wave number at Step S2: extracting the signal wave packet belonging to the current imaging point from the dispersion and amplitude compensated signal. The extraction of the signal wave packet is performed in the following manner: ， in is the frequency domain signal obtained after dispersion and amplitude compensation The time domain form of is the extracted signal wave packet, Represents the effective length of the signal wave packet in the time domain; Step S3: Calculate the waveform covariance coefficients between all signal wave packets; Step S4: Calculate imaging index based on waveform covariance coefficient, and use the imaging index as pixel value to obtain imaging result. The imaging index is calculated by: ， in, Represents image point The imaging pixel value at position, Indicates imaging point The waveform covariance coefficient between the p-th signal wave packet and the q-th signal wave packet extracted at the position, p and q are both positive integers, is the weight factor, N is the number of signals, and Respectively and About Time The mean of For the signal at time The points within represents the i-th discrete time point.

2. The waveform covariance-based ultrasonic guided wave phased array imaging method according to claim 1, characterized in that: Frequency domain signal after dispersion and amplitude compensation The time domain form Obtained by inverse Fourier transform.

3. The waveform covariance-based ultrasonic guided wave phased array imaging method according to claim 1, characterized in that: The effective length of the signal wave packet in the time domain for: ,in is the carrier frequency of the excitation signal in the ultrasonic guided wave phased array.

4. The ultrasonic guided wave phased array imaging method based on waveform covariance according to claim 1, characterized in that: The weight factor The value of is: .

Images