2 research outputs found
Three-Dimensional Microwave Imaging for Concealed Weapon Detection Using Range Stacking Technique
Three-dimensional (3D) microwave imaging has been proven to be well suited for concealed weapon detection application. For the 3D image reconstruction under two-dimensional (2D) planar aperture condition, most of current imaging algorithms focus on decomposing the 3D free space Green function by exploiting the stationary phase and, consequently, the accuracy of the final imagery is obtained at a sacrifice of computational complexity due to the need of interpolation. In this paper, from an alternative viewpoint, we propose a novel interpolation-free imaging algorithm based on wavefront reconstruction theory. The algorithm is an extension of the 2D range stacking algorithm (RSA) with the advantages of low computational cost and high precision. The algorithm uses different reference signal spectrums at different range bins and then forms the target functions at desired range bin by a concise coherent summation. Several practical issues such as the propagation loss compensation, wavefront reconstruction, and aliasing mitigating are also considered. The sampling criterion and the achievable resolutions for the proposed algorithm are also derived. Finally, the proposed method is validated through extensive computer simulations and real-field experiments. The results show that accurate 3D image can be generated at a very high speed by utilizing the proposed algorithm
Microwave NDT&E using open-ended waveguide probe for multilayered structures
Ph. D. Thesis.Microwave NDT&E has been proved to be suitable for inspecting of dielectric structures due
to low attenuation in dielectric materials and free-space. However, the microwave responses
from multilayered structures are complex as an interrogation of scattering electromagnetic
waves among the layers and defects. In many practical applications, electromagnetic analysis
based on analytic- and forward structural models cannot be generalised since the defect shape
and properties are usually unknown and hidden beneath the surface layer.
This research proposes the design and implementation of microwave NDT&E system for
inspection of multilayered structures. Standard microwave open-ended rectangular waveguides
in X, Ku and K bands (frequency range between 8-26.5 GHz) and vector network analyser
(VNA) generating sweep frequency of wideband monochromatic waves have been used to
obtain reflection coefficient responses over three types of challenging multilayered samples: (1)
corrosion progression under coating, (2) woven carbon fibre reinforced polymer (CFRP) with
impact damages, and (3) thermal coated glass fibre reinforced polymer (GFRP) pipe with inner
flat-bottom holes. The obtained data are analysed by the selected feature extraction method
extracting informative features and verify with the sample parameters (defect parameters). In
addition, visualisation methods are utilised to improve the presentation of the defects and
material structures resulting in a better interpretation for quantitative evaluation.
The contributions of this project are summarised as follows: (1) implementation of microwave
NDT&E scanning system using open-ended waveguide with the highest resolution of 0.1mm x
0.1 mm, based on the NDT applications for the three aforementioned samples; (2) corrosion
stages of steel corrosion under coating have been successfully characterised by the principal
component analysis (PCA) method; (3) A frequency selective based PCA feature has been used
to visualise the impact damage at different impact energies with elimination of woven texture
influences; (4) PCA and SAR (synthetic aperture radar) tomography together with time-offlight extraction, have been used for detection and quantitative evaluation of flat-bottom hole
defects (i.e., location, size and depth).
The results conclude that the proposed microwave NDT&E system can be used for detection
and evaluation of multilayered structures, which its major contributions are follows.
(1) The early stages (0-12month) of steel corrosion undercoating has been successfully
characterised by mean of spectral responses from microwave opened rectangular
waveguide probe and PCA.
(2) The detection of low energy impact damages on CFRP as low as 4 Joules has been
archived with microwave opened rectangular waveguide probe raster scan together with
SAR imaging and PCA for feature extraction methods.
(3) The inner flat-bottom holes beneath the thermal coated GFRP up to 11.5 mm depth has
been successfully quantitative evaluated by open-ended waveguide raster scan using
PCA and 3-D reconstruction based on SAR tomography techniques. The evaluation
includes location, sizing and depth.
Nevertheless, the major downside of feature quantities extracted from statistically based
methods such as PCA, is it intensely relies on the correlation of the input dataset, and thus
hardly link them with the physical parameters of the test sample, in particular, the complex
composite architectures. Therefore, there are still challenges of feature extraction and
quantitative evaluation to accurately determine the essential parameters from the samples. This
can be achieved by a future investigation of multiple features fusion and complementary
features.Ministry of Science and Technology of Royal Thai Government
and Office of Educational Affairs, the Royal Thai Embass