CN112858182A - Laser ultrasonic signal defect echo signal extraction method based on waveform separation - Google Patents
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Abstract
The invention discloses a method for extracting a laser ultrasonic signal defect echo signal based on waveform separation, which mainly comprises the following steps: 1. preprocessing the laser ultrasonic signals of a plurality of scanning points to eliminate the interference of abnormal signals; 2. wavelet packet decomposition, 3, extraction and reconstruction of surface wave signals: 4. extracting and reconstructing longitudinal wave signals; 5. obtaining a defect echo signal in the ultrasonic signal mainly containing the surface wave signal by neighborhood subtraction; 6. and obtaining a defect echo signal in the longitudinal wave signal by neighborhood subtraction. The method realizes that the defect echo signal with specific components is extracted and reconstructed from the laser ultrasonic signal with the complex mode.
Description
Technical Field
The invention belongs to the field of laser ultrasonic detection, and particularly relates to a laser ultrasonic signal defect echo signal extraction method based on waveform separation.
Background
Laser ultrasonic detection technology is used as a non-contact nondestructive detection means, and is increasingly applied to online detection in the metal manufacturing process. The detection omission and error detection events of the tiny defects and the defects with large embedding depth in the laser ultrasonic detection process are caused by severe temperature field change, arc interference, overlarge detection surface fluctuation, large defect embedding depth and the like in the metal additive manufacturing process. It is needed to find a feature extraction method capable of realizing defect echo signals in a laser ultrasonic detection process.
Chinese patent, publication No. CN111795931A, discloses a reconstruction extraction method for laser ultrasonic defect detection diffraction echo signals, which effectively avoids the influence of high-amplitude incident signals on Empirical Mode (EMD) decomposition and ensures that useful signals are not lost; the end effect existing when EMD decomposes the noise-added signal is overcome, and the decomposition effect of EMD is improved; the incident signals with low amplitude and low frequency are removed to the maximum extent, and the useful signals are prevented from being submerged in the incident signals; the useful signals containing defect information are limited and retained, and high-frequency noise signals from the environment are discarded, so that the purposes of accurately outputting the useful signals and accurately judging the defects are achieved.
Although the method can accurately extract and reconstruct the defect echo signal of the surface wave component in the laser ultrasonic signal and abandon the noise signal from the environment, the method only extracts and reconstructs the defect diffraction wave signal of the surface wave component in the laser ultrasonic signal, and cannot extract and reconstruct the defect echo signal of other specific components in the laser ultrasonic signal with a complex mode.
Disclosure of Invention
The invention provides a laser ultrasonic signal defect echo signal extraction method based on waveform separation, and aims to solve the problem that the defect echo signal with specific components cannot be extracted and reconstructed from a laser ultrasonic signal with a complex mode in the existing laser ultrasonic detection process.
The specific technical scheme of the invention is as follows:
the method for extracting the defect echo signal of the laser ultrasonic signal based on waveform separation comprises the following implementation steps:
step 1: preprocessing the laser ultrasonic signals of a plurality of scanning points to eliminate the interference of abnormal signals;
step 2: wavelet packet decomposition
A: processing the laser ultrasonic signal of any scanning point after pretreatment by adopting a wavelet packet decomposition algorithm, selecting a wavelet basis function as sym8 in the processing process, selecting the number of decomposition layers as 5 according to the frequency band distribution characteristics of the surface wave signal so as to obtain the wavelet packet coefficients of different nodes in the fifth layer, and sequentially arranging the wavelet packet coefficients of different nodes in the fifth layer from small to large according to the frequency;
b: processing the laser ultrasonic signal of any scanning point after pretreatment by adopting a wavelet packet decomposition algorithm, selecting a wavelet basis function as sym8 in the processing process, selecting the number of decomposition layers as 4 according to the frequency band distribution characteristic of a longitudinal wave signal so as to obtain the wavelet packet coefficients of different nodes in the fourth layer, and sequentially arranging the wavelet packet coefficients of different nodes in the fourth layer from small to large according to the frequency;
step 3, extracting and reconstructing surface wave signals:
performing zero setting processing on all the wavelet packet coefficients of the No. 1 node on the 5 th layer and other node wavelet packet coefficients except the wavelet packet coefficient of the No. 2 node after the wavelet packet decomposition of the part A in the step 2, and extracting and reconstructing all the wavelet packet coefficients of the node on the layer to obtain a reconstructed ultrasonic signal mainly containing a surface wave signal;
step 4, extraction and reconstruction of longitudinal wave signals:
performing zero setting processing on all the wavelet packet coefficients of other nodes except the wavelet packet coefficient of the No. 4 node on the basis of the wavelet packet coefficient of the No. 2 node and the wavelet packet coefficient of the No. 3 node on the No. 4 layer after the wavelet packet of the part B is decomposed in the step 2, and extracting and reconstructing all the wavelet packet coefficients of the node on the layer to obtain an ultrasonic signal which is reconstructed and mainly comprises a longitudinal wave signal;
and 5: obtaining a defect echo signal in the surface wave signal by neighborhood subtraction;
repeating the steps 2 and 3 to obtain laser ultrasonic signals mainly containing surface wave signals of all scanning points, and subtracting the laser ultrasonic signal mainly containing the surface wave signal of the last scanning point from the laser ultrasonic signal mainly containing the surface wave signal of the current scanning point to obtain a defect echo signal;
step 6: obtaining a defect echo signal in the longitudinal wave signal by neighborhood subtraction;
and (4) repeatedly executing the steps 2 and 4 to obtain the laser ultrasonic signals of all the scanning points, which mainly contain the longitudinal wave signals, and subtracting the laser ultrasonic signal of the previous scanning point, which mainly contains the longitudinal wave signals, from the laser ultrasonic signal of the current scanning point, so as to obtain the defect echo signal.
Further, in the step 2: the frequency band distribution characteristic of the surface wave signal is the frequency band distribution range of the surface wave signal, and the value range of the frequency band distribution characteristic is 0-3.9 MHz; and the step 3 matched with the method comprises the following steps: the wavelet packet coefficient frequency of the No. 1 node is 0-1.95MHz, and the wavelet packet coefficient frequency of the No. 2 node is 1.95-3.9 MHz.
Further, in the step 2: the frequency band distribution characteristic of the longitudinal wave signal is the frequency band distribution range of the longitudinal wave signal, and the value range of the frequency band distribution characteristic is 3.9MHz-15.6 MHz; and step 4, matching with the step: the wavelet packet coefficient frequency of the No. 2 node is 3.9MHz-7.8MHz, the wavelet packet coefficient frequency of the No. 3 node is 7.8MHz-11.7MHz, and the wavelet packet coefficient frequency of the No. 4 node is 11.7MHz-15.6 MHz.
Further, the specific way of eliminating the abnormal signal in step 1 is as follows: the amplitude of the probe optical signal at each scanning point is divided by the reference optical signal, and the expression is as follows:
wherein S isiDetecting the amplitude of the optical signal; diFor the reference optical signal amplitude, the probe optical signal and the reference optical signal are both generated by the ultrasonic signal of the scanning point.
The invention has the beneficial effects that:
1. the method is based on the distribution characteristics of the frequency band of the surface wave signal and the frequency band of the longitudinal wave signal, simultaneously utilizes a wavelet packet decomposition mode to reconstruct, reconstructs wavelet packet coefficients of different nodes by selecting different decomposition layers to realize the characteristic extraction and reconstruction of the surface wave signal and the longitudinal wave signal, further realizes the extraction and reconstruction of the defect echo signals of the two waveforms, and is simple and small in calculated amount.
2. By utilizing the characteristic that the appearance time of non-defect waveforms in ultrasonic signals of adjacent scanning points is basically consistent, data preprocessing is carried out by adopting a field subtraction mode, the artifacts formed by the non-defect waveforms in the A scanning signals in focusing imaging are effectively inhibited, wherein the artifacts include but are not limited to swept plane longitudinal waves, surface direct waves, multiple bottom waves, mode converted waves and boundary echoes, the defect imaging characteristics are prevented from being submerged by the artifacts, and therefore the effective identification of the defect characteristics is realized.
Drawings
FIG. 1 is a flow chart of an embodiment;
fig. 2 is a diagram of a raw laser ultrasonic signal.
Fig. 3 is a diagram of a laser ultrasonic signal mainly containing a surface wave component after surface wave reconstruction.
Fig. 4 is a diagram showing a laser ultrasonic signal mainly containing a longitudinal wave component after reconstruction of a longitudinal wave.
Fig. 5 is a diagram showing a defect echo signal mainly containing a surface wave component.
Fig. 6 is a diagram showing a defect echo signal mainly containing a longitudinal wave component.
Detailed Description
The method of the present invention is further described below with reference to the accompanying drawings and examples.
The embodiment provides a method for extracting a laser ultrasonic signal defect echo signal based on waveform separation, which is specifically implemented as shown in fig. 1:
1. two-dimensional array for acquiring laser ultrasonic signals
The laser ultrasonic detection system is adopted to carry out the detection experiment of the test block manufactured by the arc welding additive material, the size of the test block is 90 multiplied by 80 multiplied by 6, the defect size phi is 0.5mm, the scanning step length is 0.2mm, the scanning distance is 32mm, and the number of scanning points is160, the sampling frequency is 125MHz, the amplitude signal wave of each scanning point comprises 2500 characteristic points (namely the number of the sampling points is 2500), the excitation energy is 42.3mJ, the distance between an excitation probe and a receiving probe is 8mm, and the acquired original laser ultrasonic signals are stored into a two-dimensional array B according to the acquisition sequenceij(ii) a Wherein i is 160; j is 2500; as shown in fig. 2, waveforms containing various components in the original laser ultrasonic signal include surface waves, longitudinal waves, mode-converted waves, boundary echoes, bottom waves, defect echoes of surface wave components, defect echoes of longitudinal wave components, and the like;
2. laser ultrasonic signal pretreatment:
in the detection process, each scanning point of the laser ultrasonic system can simultaneously generate two signals, namely an AC signal (namely a detection light signal) and a DC signal (namely a reference light signal), and the amplitude of the detection light signal is divided by the amplitude of the reference light signal, so that abnormal signals caused by overlarge surface fluctuation of a detection test block and poor centering effect of a receiving probe in the detection process are eliminated;
wherein the amplitude of the AC signal is SiIndicating that the DC signal takes DiRepresents;
3. wavelet packet decomposition
Adopting wavelet packet decomposition algorithm to carry out pretreatment on laser ultrasonic signal X of any scanning pointkProcessing, wherein k belongs to i, in the processing process, a wavelet basis function is preferably selected as sym8, through multiple pairs of analysis, the frequency band of the surface wave signal is mainly concentrated at 0-3.9MHz, so that the number of decomposition layers is selected to be 5, thereby obtaining wavelet packet coefficients of different nodes in the fifth layer, and the wavelet packet coefficients of different nodes in the fifth layer are sequentially arranged from small to large according to the frequency;
adopting wavelet packet decomposition algorithm to carry out pretreatment on laser ultrasonic signal X of any scanning pointkThe wavelet basis function is preferably sym8, and the frequency band of longitudinal wave signal is mainly concentrated on 3.9-15.6 MHz by multiple analysis, so that it is selectedThe number of the decomposition layers is 4, so that wavelet packet coefficients of different nodes in the fourth layer are obtained, and the wavelet packet coefficients of different nodes in the fourth layer are sequentially arranged from small to large according to frequency;
it should be noted that: when wavelet packet decomposition is performed, several layers of decomposition are performed, and the number of nodes in the layer is 2nA plurality of; each node has a corresponding wavelet packet coefficient that determines the magnitude of the frequency.
4. Extraction and reconstruction of surface wave signals
As the wavelet packet coefficient frequency of the No. 1 node of the fifth layer wavelet packet decomposition is 0-1.95MHz, and the wavelet packet coefficient frequency of the No. 2 node is 1.95-3.9MHz, the other node wavelet packet coefficients except the wavelet packet coefficient of the No. 1 node and the wavelet packet coefficient of the No. 2 node in the layer are subjected to zero setting treatment, then all the node wavelet packet coefficients in the layer are extracted and reconstructed, and an ultrasonic signal Q1 mainly containing a surface wave signal is reconstructedk；
5. Extraction and reconstruction of longitudinal wave signals
Because the wavelet packet coefficient frequency of the No. 2 node decomposed by the wavelet packet of the 4 th layer is 3.9MHz-7.8MHz, the wavelet packet coefficient frequency of the No. 3 node is 7.8MHz-11.7MHz, and the wavelet packet coefficient frequency of the No. 4 node is 11.7MHz-15.6MHz, all the other node wavelet packet coefficients except the wavelet packet coefficients of the above three nodes in the 4 th layer are set to zero, all the node wavelet packet coefficients in the 4 th layer are extracted and reconstructed to reconstruct an ultrasonic signal Q2 mainly comprising a longitudinal wave signalk；
6. Repeating the steps 3 and 4 to finish the laser ultrasonic signal Q1 mainly containing the surface wave signal of all the scanning pointsiAs shown in fig. 3; repeating the steps 3 and 5 to finish the laser ultrasonic signal Q2 mainly containing longitudinal wave signals of all scanning pointsiAs shown in fig. 4;
7. neighborhood subtraction
Direct waves and bottom waves in the A-scan signals can generate large interference and artifacts in a focusing imaging process, particularly for laser ultrasonic signals, the signals have various waveforms including but not limited to swept longitudinal waves, surface direct waves, multiple bottom waves, mode converted waves, boundary echoes and other wave patterns, and the defect features can be submerged by direct imaging.
Considering that the positions of the waveforms such as direct waves, bottom waves and the like in the adjacent A-scan signals are basically fixed in the scanning detection process, the laser ultrasonic signal Q1 mainly containing surface wave signals at the current scanning point is usediLaser ultrasonic signal Q1 mainly containing surface wave signal and obtained by subtracting last scanning pointi-1Suppressing the interference of direct waves and bottom waves to the focusing imaging, highlighting the defect echo signals in the laser ultrasonic signals mainly containing surface wave signals, as shown in fig. 5, improving the focusing imaging effect, and similarly, obtaining the laser ultrasonic signal Q2 mainly containing longitudinal wave signals at the current scanning pointiLaser ultrasonic signal Q2 mainly containing longitudinal wave signal and subtracted from last scanning pointi-1As shown in fig. 6, interference of direct waves and bottom waves on focusing imaging is suppressed, defect echo signals in laser ultrasonic signals mainly containing longitudinal wave signals are highlighted, and the effect of focusing imaging is improved.
Claims (4)
1. A laser ultrasonic signal defect echo signal extraction method based on waveform separation is characterized in that:
step 1: preprocessing the laser ultrasonic signals of a plurality of scanning points to eliminate the interference of abnormal signals;
step 2: wavelet packet decomposition
A: processing the laser ultrasonic signal of any scanning point after pretreatment by adopting a wavelet packet decomposition algorithm, selecting a wavelet basis function as sym8 in the processing process, selecting the number of decomposition layers as 5 according to the frequency band distribution characteristics of the surface wave signal so as to obtain the wavelet packet coefficients of different nodes in the fifth layer, and sequentially arranging the wavelet packet coefficients of different nodes in the fifth layer from small to large according to the frequency;
b: processing the laser ultrasonic signal of any scanning point after pretreatment by adopting a wavelet packet decomposition algorithm, selecting a wavelet basis function as sym8 in the processing process, selecting the number of decomposition layers as 4 according to the frequency band distribution characteristic of a longitudinal wave signal so as to obtain the wavelet packet coefficients of different nodes in the fourth layer, and sequentially arranging the wavelet packet coefficients of different nodes in the fourth layer from small to large according to the frequency;
step 3, extracting and reconstructing surface wave signals:
performing zero setting processing on all the wavelet packet coefficients of the No. 1 node on the 5 th layer and other node wavelet packet coefficients except the wavelet packet coefficient of the No. 2 node after the wavelet packet decomposition of the part A in the step 2, and extracting and reconstructing all the wavelet packet coefficients of the node on the layer to obtain a reconstructed ultrasonic signal mainly containing a surface wave signal;
step 4, extraction and reconstruction of longitudinal wave signals:
performing zero setting processing on all the wavelet packet coefficients of other nodes except the wavelet packet coefficient of the No. 4 node on the basis of the wavelet packet coefficient of the No. 2 node and the wavelet packet coefficient of the No. 3 node on the No. 4 layer after the wavelet packet of the part B is decomposed in the step 2, and extracting and reconstructing all the wavelet packet coefficients of the node on the layer to obtain an ultrasonic signal which is reconstructed and mainly comprises a longitudinal wave signal;
and 5: obtaining a defect echo signal in the ultrasonic signal mainly containing the surface wave signal by neighborhood subtraction;
repeating the steps 2 and 3 to obtain laser ultrasonic signals mainly containing surface wave signals of all scanning points, and subtracting the laser ultrasonic signal mainly containing the surface wave signal of the last scanning point from the laser ultrasonic signal mainly containing the surface wave signal of the current scanning point to obtain a defect echo signal;
step 6: obtaining a defect echo signal in the longitudinal wave signal by neighborhood subtraction;
and (4) repeatedly executing the steps 2 and 4 to obtain the laser ultrasonic signals of all the scanning points, which mainly contain the longitudinal wave signals, and subtracting the laser ultrasonic signal of the previous scanning point, which mainly contains the longitudinal wave signals, from the laser ultrasonic signal of the current scanning point, so as to obtain the defect echo signal.
2. The method for extracting the defect echo signal of the laser ultrasonic signal based on the waveform separation as claimed in claim 1, wherein: in the step 2: the frequency band distribution characteristic of the surface wave signal is the frequency band distribution range of the surface wave signal, and the value range of the frequency band distribution characteristic is 0-3.9 MHz; and the step 3 matched with the method comprises the following steps: the wavelet packet coefficient frequency of the No. 1 node is 0-1.95MHz, and the wavelet packet coefficient frequency of the No. 2 node is 1.95-3.9 MHz.
3. The method for extracting the defect echo signal of the laser ultrasonic signal based on the waveform separation as claimed in claim 1, wherein: in the step 2: the frequency band distribution characteristic of the longitudinal wave signal is the frequency band distribution range of the longitudinal wave signal, and the value range of the frequency band distribution characteristic is 3.9MHz-15.6 MHz; and step 4, matching with the step: the wavelet packet coefficient frequency of the No. 2 node is 3.9MHz-7.8MHz, the wavelet packet coefficient frequency of the No. 3 node is 7.8MHz-11.7MHz, and the wavelet packet coefficient frequency of the No. 4 node is 11.7MHz-15.6 MHz.
4. The method for extracting the defect echo signal of the laser ultrasonic signal based on the waveform separation as claimed in claim 1, wherein: the specific way of eliminating the abnormal signal in the step 1 is as follows: the amplitude of the probe optical signal at each scanning point is divided by the reference optical signal, and the expression is as follows:
wherein S isiDetecting the amplitude of the optical signal; diFor the reference optical signal amplitude, the probe optical signal and the reference optical signal are both generated by the ultrasonic signal of the scanning point.
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