CN113607652A - Workpiece superficial layered imaging method based on photoacoustic spectrum - Google Patents
Workpiece superficial layered imaging method based on photoacoustic spectrum Download PDFInfo
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- CN113607652A CN113607652A CN202110917994.3A CN202110917994A CN113607652A CN 113607652 A CN113607652 A CN 113607652A CN 202110917994 A CN202110917994 A CN 202110917994A CN 113607652 A CN113607652 A CN 113607652A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/04—Analysing solids
- G01N29/06—Visualisation of the interior, e.g. acoustic microscopy
- G01N29/0654—Imaging
- G01N29/0672—Imaging by acoustic tomography
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/44—Processing the detected response signal, e.g. electronic circuits specially adapted therefor
- G01N29/46—Processing the detected response signal, e.g. electronic circuits specially adapted therefor by spectral analysis, e.g. Fourier analysis or wavelet analysis
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/1702—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids
- G01N2021/1706—Systems in which incident light is modified in accordance with the properties of the material investigated with opto-acoustic detection, e.g. for gases or analysing solids in solids
Abstract
The invention belongs to the technical field of photoacoustic spectrometry nondestructive testing, and particularly relates to a workpiece superficial layered imaging method based on photoacoustic spectrometry. The method comprises the steps of exciting a photoacoustic signal on the surface of a workpiece by using pulse laser and a scanning galvanometer, screening peak value spectrum information in the photoacoustic signal acquired by an acoustic sensor through fast Fourier transform according to the depth distribution characteristics of thermal waves represented by the photoacoustic signal with different frequencies, taking scanning position information of pulse laser beams controlled by the galvanometer controller as pixel position information of an image, obtaining accumulation layers with different depths, and obtaining a layered image of a shallow surface area of the workpiece through differential processing of adjacent accumulation layers, thereby providing a brand-new imaging solution for surface/subsurface defect detection of the workpiece. The method can be widely applied to the off-line surface subsurface defect detection of the workpiece, and can also be applied to the on-line monitoring of various laser processing processes including metal 3D printing, so that the method has wide application prospect.
Description
Technical Field
The invention belongs to the technical field of photoacoustic spectrometry nondestructive testing, and particularly relates to a workpiece superficial layered imaging method based on photoacoustic spectrometry.
Background
The laser excitation acoustic technology combines the characteristics of laser space lossless propagation and almost lossless acoustic propagation in a solid workpiece, so that not only is the acoustic excitation in the workpiece convenient to adjust, but also the physical and structural characteristics of various materials can be carried in the process of converting optical energy into acoustic energy, and therefore, the laser excitation acoustic technology is developed and applied more and more. However, the energy conversion efficiency of photoacoustic is not high, and the photoacoustic signal is difficult to detect, which restricts the industrial popularization of photoacoustic technology. With the development of electronic information technology, especially digital technology, small signal detection becomes more and more popular, and photoacoustic technology is being developed and applied in the golden period with the promotion of electronic technology.
Up to now, photoacoustic spectroscopy techniques have mainly focused on: 1 identification of biological tissue/features, comprising: skin elasticity, blood glucose, tissue imaging, etc.; 2, gas detection, mainly improving the sensitivity of gas and gas in transformer oil in a photoacoustic cell; 3 the cross application of the artificial intelligence method in the field of photoacoustic spectrometry. Related inventions in the field of workpiece surface sub-surface imaging are not developed at present.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a workpiece superficial layering imaging technology based on a photoacoustic spectrum, which utilizes the characteristic of abundant frequency components of the pulsed photoacoustic spectrum and combines differential image processing according to the correlation between the photoacoustic signal frequency and the imaging depth to realize the layering imaging of the surface/subsurface of a sample.
The technical scheme adopted by the invention is as follows:
a workpiece superficial layering imaging method based on a photoacoustic spectrum is disclosed, as shown in figure 1, and comprises a pulse photoacoustic excitation module 1 and a photoacoustic spectrum layering imaging module 2; the pulse photoacoustic excitation module 1 comprises a pulse laser 11, a scanning galvanometer 12, a galvanometer controller 13, an acoustic sensor 14 and an acquisition card 15, wherein the pulse laser 11 emits laser in a pulse emission mode(ii) a The scanning galvanometer 12 is used for scanning and reflecting the laser beam emitted by the pulse laser 11, realizing focusing on the surface of a workpiece and carrying out imaging scanning; the galvanometer controller 13 is used for driving the scanning galvanometer 12, controlling the scanning work of the scanning galvanometer 12 and sending scanning position information to the photoacoustic spectrum layered imaging module 2; the acoustic sensor 14 is arranged on the other surface of the workpiece opposite to the surface for receiving the laser beam, detects the signal of the photoacoustic signal after passing through the workpiece, and outputs the photoacoustic vibration signal as an analog photoacoustic signal in an analog signal mode; the acquisition card 15 is used for acquiring the analog photoacoustic signal, converting the analog photoacoustic signal into a digital signal and outputting the digital photoacoustic signal; the laser beam performs photoacoustic excitation on a superficial region of the workpiece, the depth h of the superficial region is the maximum heat wave input depth, and h is 2 pi (D/2 f)min)1/2Where D is the thermal diffusivity of the material, fminIs the lowest frequency peak of the excitation photoacoustic spectrum;
the photoacoustic spectrum layered imaging module 2 comprises a fast Fourier transformer and n image output channels, wherein the fast Fourier transformer receives digital photoacoustic signals output by an acquisition card 15, fast Fourier changes the digital photoacoustic signals, then sequentially divides the frequency output by the fast Fourier transformer into n frequency sections from high to low according to a set threshold value, and sequentially sends all the frequency sections into each image input channel from high to low, so that the 1 st image channel corresponds to the highest frequency section and the nth image channel corresponds to the lowest frequency section; in each image channel, the average intensity of the input signal is used as the gray scale, the scanning position information of the galvanometer controller 13 is used as the pixel coordinate of the image, and the depth h is obtainediThe gray scale image sigma MAP of the shallow surface of the workpiecei，hi＝2π(D/2fi)1/2，fiIs the input frequency of the ith channel, i is 0,1, …, n, and then the grayscale image of the (i-1) th channel is subtracted from the grayscale image of the ith channel to obtain the output picture MAP of each channeliMAP, pictureiReflects the depth range of the workpiece superficial region as hi-1,hi]The case of a region.
The invention has the beneficial effects that:
the invention can obtain different depth layers by decomposing frequency characteristics on the basis of pump laser scanning imaging, and obtain layered imaging images by layer difference, so the invention not only can be widely applied to off-line surface subsurface defect detection of workpieces, but also can be applied to on-line monitoring of various laser processing processes including metal 3D printing, and has wide application prospect.
Drawings
FIG. 1 is a schematic diagram of pulsed photoacoustic spectroscopy workpiece superficial layer imaging;
fig. 2 is a schematic diagram of pulsed photoacoustic spectroscopy stainless steel superficial 8-layer layered imaging implementation.
Detailed Description
The technical scheme of the invention is described in detail in the following with reference to the accompanying drawings.
As shown in fig. 2, an example of an embodiment of the present invention is to perform layered imaging on the sub-surface of a stainless steel sample in 8 layers, which is implemented as follows:
1. the pulse photoacoustic excitation part 1 is composed of a semiconductor pulse laser 11, a scanning galvanometer 12, a galvanometer controller 13, a piezoelectric buzzer 14 and an acquisition card 15.
The semiconductor pulse laser 11 is used as an energy source for pulse photoacoustic excitation, the emitted laser adopts a pulse emission mode, and the energy time characteristic of the emitted laser directly influences the general characteristics of photoacoustic signals, so that different pulse lasers can excite different photoacoustic spectrums.
The scanning galvanometer 12 is used as a beam space scanning reflection device of the pulse laser 11, has the characteristic of focusing on the upper surface of the workpiece, and can complete imaging scanning of the upper surface of the workpiece in a certain range, including line scanning and column scanning.
The galvanometer controller 13 is used as a driving device of the scanning galvanometer 12, provides a driving signal for the scanning galvanometer 12, controls the scanning working state of the scanning galvanometer 12, and simultaneously provides scanning position information of the scanning galvanometer 12 to the photoacoustic spectrum layered imaging part 2 as imaging pixel position information.
The pulse laser beam is emitted by a pulse laser 11 and is reflected and controlled by a scanning galvanometer 12, so that the photoacoustic excitation of the upper surface of the workpiece is realized.
The workpiece superficial region is positioned on the surface and the subsurface of the workpiece, and the depth h of the workpiece superficial region is the maximum heat wave input depth, and h is 2 pi (D/2 f)min)1/2(wherein D is the thermal diffusivity of the material, fminIs the lowest frequency peak of the excitation photoacoustic spectrum).
In the embodiment, stainless steel is used as a sample to be detected, and the material of the sample to be detected comprises various samples to be detected such as metal, nonmetal, semiconductor and the like.
The piezoelectric buzzer 14 is a detector for photoacoustic signals, is attached to the back of the workpiece, detects the photoacoustic signals after passing through the sample, and outputs the photoacoustic vibration signals as analog photoacoustic signals in an analog signal mode.
The analog photoacoustic signal is an analog quantity signal output after the photoacoustic signal is detected by the acoustic sensor.
The acquisition card 15 is matched with the acoustic sensor, and converts the analog photoacoustic signals into digital signals meeting the digital signal specification after acquiring the analog photoacoustic signals, namely, the digital photoacoustic signals are output.
The digital photoacoustic signal is an output signal of the acquisition card and meets the specification of the digital signal.
2. The photoacoustic spectrum layered imaging part 2 is composed 210 of a fast Fourier transformer 21, a photoacoustic spectrum highest frequency peak channel 22, a photoacoustic spectrum second high frequency peak channel 23, a photoacoustic spectrum lowest frequency peak channel 24, a shallowest accumulation layer 25, a shallowest accumulation layer 26, a deepest accumulation layer 27, a shallowest layer 28, a shallowest layer 29, a deepest layer and the like.
The fast fourier transformer 21 performs fast fourier transformation on the digital photoacoustic signal, and outputs signals corresponding to different frequencies in different channels.
The channel 22 of the peak of the highest frequency of the photoacoustic spectrum is the peak of the frequency spectrum output by the fast fourier transformer 21, and is divided into 8 frequency segments according to the frequency when the signal intensity is not lower than a certain threshold, wherein the channel corresponding to the segment of the highest frequency is the channel 22 of the highest frequency of the photoacoustic spectrum, which is substantially a band-pass filter that can only pass through the highest frequency segmentWave filter with average frequency f1。
The photoacoustic spectrum secondary high-frequency peak channel 23 is in the output spectrum peak value of the fast Fourier transformer 21, and is divided into 8 frequency sections according to the frequency height under the condition that the signal intensity is not lower than a certain threshold value, wherein the signal channel corresponding to the frequency section with the second highest frequency is the photoacoustic spectrum secondary high-frequency peak channel 23 which is a band-pass filter which can only pass through the second high-frequency section, and the average frequency of the signal channel is f2。
The photoacoustic spectrum lowest frequency peak channel 24 is in the output frequency spectrum peak of the fast Fourier transformer 21, and is divided into 8 frequency sections according to the frequency level under the condition that the signal intensity is not lower than a certain threshold value, wherein the signal channel corresponding to the section of the lowest frequency is the photoacoustic spectrum lowest frequency peak channel 24 which is a band-pass filter capable of passing through the lowest frequency section substantially, and the average frequency is f8。
The shallowest accumulation layer 25 forms a gray scale image sigma MAP by using the average intensity of the photoacoustic signal output from the highest frequency peak channel of the photoacoustic spectrum as the gray scale and the scanning position information of the galvanometer controller as the pixel coordinate of the image1Because the corresponding optical sound spectrum frequency band is the highest, the image of the workpiece superficial area with the shallowest depth is obtained, and the image depth is [ o, h1]，h1＝2π(D/2f1)1/2。
The sub-shallow accumulation layer 26 forms a gray scale image sigma MAP by using the average intensity of the photoacoustic signal output from the highest frequency peak channel of the photoacoustic spectrum as the gray scale and the scanning position information of the (1-3) galvanometer controller as the pixel coordinate of the image2Since the corresponding photoacoustic spectral frequency band is second-order high, the depth of the image is second-order shallow (a picture of a workpiece superficial region whose image depth is [ o, h ]2]With an image depth h2＝2π(D/2f2)1/2。
The deepest accumulation layer 27 forms a gray scale image sigma MAP by using the average intensity of the photoacoustic signal output by the highest frequency peak channel of the photoacoustic spectrum as the gray scale and the scanning position information of the galvanometer controller as the pixel coordinate of the image8FromThe corresponding optical sound spectrum frequency band is the lowest, so that the image of the workpiece superficial region with the deepest depth is obtained, and the image depth is [ o, h8]With an image depth h8＝2π(D/2f8)1/2。
The shallowest layer 28 and the shallowest accumulated layer 25 correspond to each other, and reflect the condition of the shallowest surface of the workpiece superficial region, and the image is MAP1The imaging depth range is [ o, h ]1]。
The sub-shallow layer 29 is a MAP obtained by subtracting the shallowest accumulation layer from the sub-shallow accumulation layer 262Reflects the depth range of the workpiece superficial region as h1,h2]The area of (2).
The deepest layer 210 is a picture MAP obtained by subtracting the second-deepest accumulation layer from the deepest accumulation layer 278Reflects the depth range of the workpiece superficial region as h7,h8]The area of (2).
Claims (1)
1. A workpiece superficial layering imaging method based on a photoacoustic spectrum is characterized by comprising a pulse photoacoustic excitation module (1) and a photoacoustic spectrum layering imaging module (2); the pulse photoacoustic excitation module (1) comprises a pulse laser (11), a scanning galvanometer (12), a galvanometer controller (13), an acoustic sensor (14) and an acquisition card (15), wherein the pulse laser (11) emits laser in a pulse emission mode; the scanning galvanometer (12) is used for scanning and reflecting laser beams emitted by the pulse laser (11), realizing focusing on the surface of a workpiece and carrying out imaging scanning; the galvanometer controller (13) is used for driving the scanning galvanometer (12), controlling the scanning work of the scanning galvanometer (12) and simultaneously sending scanning position information to the photoacoustic spectrum layered imaging module (2); the acoustic sensor (14) is arranged on the other surface of the workpiece opposite to the surface for receiving the laser beam, detects a signal of the photoacoustic signal after passing through the workpiece, and outputs the photoacoustic vibration signal into an analog photoacoustic signal in an analog signal mode; the acquisition card (15) is used for acquiring the analog photoacoustic signal, converting the analog photoacoustic signal into a digital signal and outputting the digital photoacoustic signal; the laser beam performs photoacoustic excitation on a superficial region of the workpiece, and the depth h of the superficial region is the maximum thermal wave input depth，h＝2π(D/2fmin)1/2Where D is the thermal diffusivity of the material, fminIs the lowest frequency peak of the excitation photoacoustic spectrum;
the photoacoustic spectrum layered imaging module (2) comprises a fast Fourier transformer and n image output channels, wherein the fast Fourier transformer receives digital photoacoustic signals output by an acquisition card (15), the digital photoacoustic signals are subjected to fast Fourier change, then the frequency output by the fast Fourier transformer is sequentially divided into n frequency sections from high to low according to a set threshold value, all the frequency sections are sequentially sent to each image input channel from high to low, so that the 1 st image channel corresponds to the highest frequency section, and the nth image channel corresponds to the lowest frequency section; in each image channel, the average intensity of the input signal is used as a gray scale, the scanning position information of the galvanometer controller (13) is used as the pixel coordinate of the image, and the depth h is obtainediThe gray scale image sigma MAP of the shallow surface of the workpiecei，hi＝2π(D/2fi)1/2，fiIs the input frequency of the ith channel, i is 1,2, …, n, and then the grayscale image of the ith-1 channel is subtracted from the grayscale image of the ith channel to obtain the output picture MAP of each channeliMAP, pictureiReflects the depth range of the workpiece superficial region as hi-1,hi]The case of a region.
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