CN117554320A - Nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging and application method thereof - Google Patents
Nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging and application method thereof Download PDFInfo
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Abstract
The invention discloses a nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging and a using method thereof, wherein the system comprises a terahertz wave transmitting end, a terahertz wave receiving end, a horn antenna, a data acquisition module, a two-dimensional scanning translation stage, a sample and a main control computer; the terahertz wave transmitting end and the terahertz wave receiving end are connected with the data acquisition module through a circuit, the data acquisition module is connected with the main control computer through a circuit, the main control computer is connected with the two-dimensional scanning translation table through a circuit, and a sample is placed on the two-dimensional scanning translation table. The invention utilizes a synthetic aperture imaging method and adopts a range offset migration algorithm to process the acquired data, so that digital focusing can be realized, imaging resolution is improved to a great extent, and detection resolution is improved.
Description
Technical Field
The invention belongs to the technical field of nondestructive testing, and particularly relates to a nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging and a use method thereof.
Background
The nondestructive test is a method for checking and testing the structure, state and type, quantity, shape, property, position, size and distribution of defects in the test piece on the premise of not damaging or affecting the service performance of a detected object by utilizing the characteristics of sound, light, magnetism, electricity and the like of substances. The nondestructive testing technology plays a key role in guaranteeing the product quality and early warning the operation faults of equipment, is an essential effective tool for industrial development, and is undoubtedly important. Related scientific researchers have studied various detection methods, classified according to working principles and detection modes, more than 70 nondestructive detection methods exist nowadays, but each method has the characteristics and advantages and cannot be fully applied to any occasion.
Terahertz (THz) waves refer to electromagnetic waves having a frequency in the range of 0.1-10THz, which are located between millimeter waves and infrared rays in terms of frequency and between electrons and photons in terms of energy. Terahertz imaging technology is a promising direction in terahertz research, and mainly benefits from the unique properties of the band: the transmission power is strong, terahertz waves are easy to transmit through nonmetallic and nonpolar materials, such as ceramics, plastics, foams and other common materials which cannot be transmitted by infrared light; the photon energy is low, and harmful ionization is not generated; many molecules show obvious absorption and dispersion characteristics in a terahertz frequency band, and molecular fingerprint characteristic spectrum can be established in a terahertz region to identify substance components. In the last twenty years, the development of new technologies such as electronics and photonics promotes the rapid development of terahertz imaging technology, the application field of the technology is also more and more wide, good results are obtained in the fields of security inspection, radar detection, biomedical imaging, nondestructive detection, artwork research and the like, and the technology has the advantages which are incomparable with the traditional imaging technology.
Compared with nondestructive testing technologies of other wave bands, the terahertz imaging technology has the unique advantages that: compared with light waves, the light wave-resistant composite material has strong penetrability to nonpolar and nonmetallic materials, and unlike X rays, the dielectric medium cannot be distinguished obviously due to excessive penetrability; compared with microwaves, the space resolution is high, the imaging resolution reaches millimeter or sub-millimeter magnitude, and the recognition accuracy of most occasions can be met; compared with X-rays, the terahertz wave has very low photon energy, does not generate harmful photoionization in biological tissues, and has higher safety; compared with ultrasound, terahertz imaging belongs to non-contact nondestructive testing, has wider frequency band and is more suitable for testing materials with serious attenuation to sound waves. In conclusion, the terahertz imaging technology has the unique advantages of safety, effectiveness, high permeability, high resolution and the like, and is being developed into a novel nondestructive testing means, so that the terahertz imaging technology has important significance for research of the technology.
The imaging resolution ratio of the conventional terahertz time-domain spectrum imaging system is higher, but the imaging resolution ratio is limited by the power of an emission source, the penetration depth is lower, and nondestructive detection can only be carried out on thin samples such as films, coatings and the like. The terahertz linear frequency modulation continuous wave has a certain bandwidth, so that the distance measurement function can be realized, the radiation power is generally higher, the penetrating capacity of a sample is stronger, the terahertz linear frequency modulation continuous wave is suitable for detecting thicker samples, and the terahertz linear frequency modulation continuous wave has the advantages of high imaging speed, simple system, low cost and the like. However, the traditional focusing real aperture imaging method can realize high-resolution imaging only in the depth of field range due to focal depth limitation, the imaging resolution is obviously reduced when the depth of field is deviated from the depth of field range, and only partial depth information can be obtained at a time for thicker samples.
Nondestructive testing technology based on terahertz imaging basically adopts optical devices such as lenses to realize physical focusing so as to improve imaging resolution. In the CN114002160a, the off-axis parabolic mirror is used to focus the beam, the terahertz gaussian beam radiated by the transmitting antenna is collimated by the first off-axis parabolic mirror to obtain a parallel beam, then the plane wave is converted into a converging spherical wave by the second off-axis parabolic mirror to focus the converging spherical wave on the sample position, and the sample is scanned to obtain the acquired data of the corresponding position, and then the acquired data is processed to obtain the imaging result. In order to improve the resolution of nondestructive testing, the patent CN111879722a adopts an optical element such as a conical lens or a diffraction mirror to replace a common lens, so as to convert a collimated gaussian beam into a quasi-zero-order bessel beam, and further extend the depth of field of the system. The emitted terahertz wave is firstly incident on a lens for collimation, the collimated Gaussian beam passes through an optical element to generate a terahertz wave zero-order Bessel beam, the terahertz wave zero-order Bessel beam irradiates on a measured object, the measured object reflects the beam, the reflected beam is received by a detector through the same light path, and finally the reflected signal is processed to obtain a sample imaging result.
Nondestructive detection based on terahertz time-domain spectrum imaging system is suitable for detection of thinner samples only due to low emission power and small penetration depth, and the scanning time is long.
The terahertz frequency modulation continuous wave imaging system basically adopts a focusing real aperture imaging method to carry out nondestructive detection, the method needs to utilize optical elements such as lenses or reflectors to build a quasi-optical system to improve imaging resolution, the complexity of the system can be increased, the depth of field range is small when a common lens focuses, and only partial depth information can be detected at one time for thick samples.
Disclosure of Invention
Aiming at the problems of terahertz three-dimensional imaging requirements of thick samples and small focal depth of a focusing real-aperture imaging method, the invention provides a nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging and a using method thereof. The frequency modulation continuous wave radiation power is generally higher, the sample permeability is better, the frequency modulation continuous wave radiation power has a certain bandwidth, the distance or depth information of a target can be obtained, and the frequency modulation continuous wave radiation power is suitable for imaging thicker targets. The synthetic aperture imaging technology is a digital focusing process, terahertz wave beams emitted by a transmitting antenna are directly incident on a sample, scattered wave fronts are focused into a three-dimensional target reflectivity image through a synthetic aperture imaging algorithm, the method eliminates the requirement of physical focusing by using a lens or a reflecting mirror, and focusing can be realized in a large range by using the imaging algorithm, so that high-resolution imaging is obtained.
In order to achieve the above object, the technical solution of the present invention is:
a nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging comprises a terahertz wave transmitting end, a terahertz wave receiving end, a horn antenna, a data acquisition module, a two-dimensional scanning translation stage, a sample and a main control computer; the terahertz wave transmitting end and the terahertz wave receiving end are connected with the data acquisition module through a circuit, the data acquisition module is connected with the main control computer through a circuit, the main control computer is connected with the two-dimensional scanning translation table through a circuit, and a sample is placed on the two-dimensional scanning translation table.
Further, the terahertz transmitting end and the receiving end are combined by using a directional coupler, so that the receiving and transmitting integration is realized.
Further, the terahertz wave transmitting end and the horn antenna are used for radiating terahertz linear frequency modulation continuous wave signals.
Further, the terahertz wave receiving end and the horn antenna are used for receiving echo signals reflected by the sample.
Further, the data acquisition module is used for acquiring intermediate frequency signals.
Further, the two-dimensional scanning translation stage is used for controlling the two-dimensional movement of the sample to be detected.
Further, the main control computer is used for controlling the movement of the scanning translation stage, the acquisition of data and the imaging display.
The invention also provides a using method of the nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging, which comprises the following steps:
(1) Setting parameters such as start-stop frequency, sampling rate, scanning point number and the like after the system is calibrated;
(2) The method comprises the steps of connecting a main control computer with a two-dimensional scanning translation stage and a data acquisition module, controlling the two-dimensional scanning translation stage to move to a starting point through the main control computer, setting scanning width and stepping in X and Y directions, and setting a storage path of test data;
(3) The main control computer starts clicking scanning, and the two-dimensional scanning translation stage starts scanning and obtains corresponding echo data until the whole target area is scanned;
(4) And carrying out corresponding imaging algorithm processing on the stored test data to obtain an imaging result.
Further, in the step (3), a scanning mode adopts plane scanning.
Further, the imaging algorithm in the step (4) comprises the following steps:
1) According to the distance of the system to the imaging resolution delta d, the target distance is toward the thickness L to be measured z Number of points n= (L) for determining compensation distance z /Δd)+1；
2) Performing two-dimensional FFT on echo signals in azimuth to realize plane wave decomposition of bending wave front, converting signals from space domain to wave number domain of plane where aperture (antenna) is located, and obtaining E (k) x ,k y ,k)；
3) Selecting different compensation distances z i (i=1, 2, …, N) and multiplying the data of the corresponding range plane by the corresponding range migration factor, i.e.Back-propagating the wavefront from the aperture to the target location, yielding E' (k) x ,k y ,k i )；
4) For data E' (k) x ,k y ,k i ) Performing three-dimensional IFFT, and storing imaging result matrix A of target on corresponding compensation distance plane i ；
5) Imaging result matrix A of all saved different distance planes i And (5) performing projection synthesis to obtain the three-dimensional imaging of the target.
The beneficial effects of the invention are as follows:
(1) The invention simplifies the system structure and can improve the detection resolution;
(2) According to the invention, by using a synthetic aperture imaging algorithm, clear imaging is realized in a thick sample under the condition that optical devices such as lenses are not used for focusing the sample, so that accurate reconstruction can be realized on target three-dimensional imaging;
(3) The terahertz frequency modulation continuous wave is adopted, so that the distance measurement function can be realized due to the broadband, the detection distance is long, in addition, the radiation power is higher, the penetrating power to a sample is stronger, and thicker samples can be detected;
(4) According to the invention, a quasi-optical system is not required to be designed by adopting optical elements such as lenses and the like to perform physical focusing on a sample, so that an experimental device is simplified, and the complexity of testing is reduced;
(5) The invention utilizes a synthetic aperture imaging method and adopts a range offset migration algorithm to process the acquired data, so that digital focusing can be realized, imaging resolution is improved to a great extent, and detection resolution is improved.
Drawings
FIG. 1 is a schematic diagram of a nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging.
Fig. 2 is a schematic diagram of a sample undergoing planar scanning imaging.
Fig. 3 is a flow chart of a range offset migration algorithm for synthetic aperture imaging.
The marks in the figure: 1. a terahertz transmitting end and a terahertz receiving end; 2. a horn antenna; 3. a sample; 4. a two-dimensional scanning translation stage; 5. a data acquisition module; 6. and a main control computer.
Detailed Description
As shown in fig. 1, the nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging comprises a terahertz wave transmitting end, a terahertz wave receiving end 1, a horn antenna 2, a data acquisition module 5, a two-dimensional scanning translation table 4, a sample 3 and a main control computer 6. The terahertz wave transmitting end and the terahertz wave receiving end 1 are connected with the data acquisition module 5 through a circuit, the data acquisition module 5 is connected with the main control computer 6 through a circuit, the main control computer 6 is connected with the two-dimensional scanning translation table 4 through a circuit, and the sample 3 is placed on the two-dimensional scanning translation table 4. The terahertz wave transmitting end and the horn antenna 2 are used for radiating terahertz linear frequency modulation continuous wave signals, the terahertz wave signals are irradiated to the sample 3, the terahertz wave receiving end and the horn antenna 2 are used for receiving echo signals reflected by the sample, the data acquisition module 5 is used for acquiring intermediate frequency signals, the two-dimensional scanning translation table 4 is used for controlling the two-dimensional movement of the sample 3 to be detected, and the main control computer 6 is used for controlling the movement of the scanning translation table, the data acquisition and the imaging display.
The application method of the nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging comprises the following steps: (1) Setting parameters such as start-stop frequency, sampling rate, scanning point number and the like after the system is calibrated; (2) The method comprises the steps of connecting a main control computer with a two-dimensional scanning translation stage and a data acquisition module, controlling the two-dimensional scanning translation stage to move to a starting point through the main control computer, setting scanning width and stepping in X and Y directions, and setting a storage path of test data; (3) The main control computer starts clicking scanning, and the two-dimensional scanning translation stage starts scanning and obtains corresponding echo data until the whole target area is scanned; (4) And carrying out corresponding imaging algorithm processing on the stored test data to obtain an imaging result.
The whole principle of the nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging is as follows: the terahertz wave transmitting end and the horn antenna 2 radiate terahertz Gaussian beams, the terahertz Gaussian beams are directly incident to a sample 3 placed on the two-dimensional scanning translation table 4, reflected beams carrying sample information are received by the horn antenna 2 and the terahertz wave receiving end, and then intermediate-frequency echo signals are obtained through the data acquisition module 5.
The terahertz transmitting end and the receiving end are combined by using a directional coupler, so that the receiving and transmitting integration is realized.
It should be noted that, in the imaging scanning mode in the present invention, a planar scanning is adopted. The sample to be measured is fixed on a two-dimensional scanning translation stage, a receiving and transmitting antenna is placed parallel to a scanning plane and is separated from the scanning plane by a distance z0, after one-dimensional scanning is completed along the X direction, the two-dimensional scanning translation stage moves to the next row along the Y direction until all points along the Y direction are scanned, all acquired data can form an echo data matrix at the moment, then an imaging algorithm is utilized for processing, and a two-dimensional imaging diagram and a three-dimensional imaging diagram can be obtained, wherein a specific scanning imaging schematic diagram is shown in fig. 2.
Because the terahertz wave beam is divergent, the direct imaging result of the acquired data is seriously defocused and has low resolution. The invention adopts a synthetic aperture imaging algorithm to improve the imaging resolution, and the synthetic aperture imaging algorithm is a range offset migration algorithm. The algorithm flow chart is shown in fig. 3, and the process can be summarized as the following steps:
(1) According to the distance of the system to the imaging resolution delta d, the target distance is toward the thickness L to be measured z Number of points n= (L) for determining compensation distance z /Δd)+1；
(2) Performing two-dimensional FFT on echo signals in azimuth to realize plane wave decomposition of bending wave front, converting signals from space domain to wave number domain of plane where aperture (antenna) is located, and obtaining E (k) x ,k y ,k)；
(3) Selecting different compensation distances z i (i=1, 2, …, N) and multiplying the data of the corresponding range plane by the corresponding range migration factor, i.e.Back-propagating the wavefront from the aperture to the target location, yielding E' (k) x ,k y ,k i )；
(4) Then for data E' (k) x ,k y ,k i ) Performing three-dimensional IFFT, and storing imaging result matrix A of target on corresponding compensation distance plane i ；
(5) Imaging result matrix A of all saved different distance planes i And (5) performing projection synthesis to obtain the three-dimensional imaging of the target.
For the compensation distance z as part of the correction factor i The function of (2) is to compensate the bending effect of spherical wave on the corresponding distance plane, thereby realizing complete focusing on the distance plane, while corresponding to other distance planesAfter compensation, there may be different degrees of phase error in the target echo. Therefore, on the premise of measuring the target distance, the distance migration correction factor is dynamically adjusted by using a distance migration algorithm, and the compensation distance is accurately positioned, so that the accurate reconstruction of the scattering intensity information of the target can be realized theoretically.
Claims (10)
1. The nondestructive testing system based on terahertz frequency modulation continuous wave synthetic aperture imaging is characterized by comprising a terahertz wave transmitting end, a terahertz wave receiving end, a horn antenna, a data acquisition module, a two-dimensional scanning translation stage, a sample and a main control computer; the terahertz wave transmitting end and the terahertz wave receiving end are connected with the data acquisition module through a circuit, the data acquisition module is connected with the main control computer through a circuit, the main control computer is connected with the two-dimensional scanning translation table through a circuit, and a sample is placed on the two-dimensional scanning translation table.
2. The nondestructive testing system based on terahertz frequency-modulated continuous wave synthetic aperture imaging according to claim 1, wherein the terahertz transmitting end and the receiving end are combined by a directional coupler to realize the transceiving integration.
3. The non-destructive inspection system based on terahertz frequency-modulated continuous wave synthetic aperture imaging of claim 1, wherein the terahertz wave transmitting end and the horn antenna are used for radiating terahertz chirped continuous wave signals.
4. The nondestructive testing system based on terahertz frequency-modulated continuous wave synthetic aperture imaging according to claim 1, wherein the terahertz wave receiving end and the horn antenna are used for receiving echo signals reflected by a sample.
5. The nondestructive testing system based on terahertz frequency-modulated continuous wave synthetic aperture imaging according to claim 1, wherein the data acquisition module is used for acquiring intermediate frequency signals.
6. The nondestructive testing system based on terahertz frequency-modulated continuous wave synthetic aperture imaging according to claim 1, wherein the two-dimensional scanning translation stage is used for controlling the two-dimensional movement of the sample to be tested.
7. The non-destructive testing system based on terahertz frequency-modulated continuous wave synthetic aperture imaging of claim 1, wherein the master control computer is used for controlling the movement of the scanning translation stage, the acquisition of data and the imaging display.
8. A method of using a terahertz frequency-modulated continuous wave synthetic aperture imaging-based nondestructive testing system according to any one of claims 1-7, comprising the steps of:
(1) Setting parameters such as start-stop frequency, sampling rate, scanning point number and the like after the system is calibrated;
(2) The method comprises the steps of connecting a main control computer with a two-dimensional scanning translation stage and a data acquisition module, controlling the two-dimensional scanning translation stage to move to a starting point through the main control computer, setting scanning width and stepping in X and Y directions, and setting a storage path of test data;
(3) The main control computer starts clicking scanning, and the two-dimensional scanning translation stage starts scanning and obtains corresponding echo data until the whole target area is scanned;
(4) And carrying out corresponding imaging algorithm processing on the stored test data to obtain an imaging result.
9. The method for using a terahertz frequency modulated continuous wave synthetic aperture imaging-based nondestructive testing system according to claim 8, wherein the scanning mode in the step (3) adopts plane scanning.
10. The method of using a terahertz frequency modulated continuous wave synthetic aperture imaging-based nondestructive testing system according to claim 8, wherein the imaging algorithm in step (4) comprises the following steps:
1) According to the distance of the system to the imaging resolution delta d, the target distance is toward the thickness L to be measured z Number of points n= (L) for determining compensation distance z /Δd)+1；
2) Performing two-dimensional FFT on echo signals in azimuth to realize plane wave decomposition of bending wave front, converting signals from space domain to wave number domain of plane where aperture (antenna) is located, and obtaining E (k) x ,k y ,k)；
3) Selecting different compensation distances z i (i=1, 2, …, N) and multiplying the data of the corresponding range plane by the corresponding range migration factor, i.e.Back-propagating the wavefront from the aperture to the target location, yielding E' (k) x ,k y ,k i )；
4) For data E' (k) x ,k y ,k i ) Performing three-dimensional IFFT, and storing imaging result matrix A of target on corresponding compensation distance plane i ；
5) Imaging result matrix A of all saved different distance planes i And (5) performing projection synthesis to obtain the three-dimensional imaging of the target.
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