Microwave sensing for non-destructive evaluation of anisotropic materials with application in wood industry
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Microwave non-destructive testing of wood is an active research field, but, despite remarkable advances reported in the literature to date, the wood testing devices are not widely implemented in industry. This thesis aims to progress the knowledge on wood testing by investigating two of the key issues: microwave propagation through dried wood and sensor design. Two microwave antennas with focused beam are designed and implemented. First antenna is a commonly used horn with a dielectric lens, offering a broadband solution, operating over the 8 to 12.4 GHz frequency band. The second solution is a novel metal plate lens antenna with beam forming in the near field zone. A successful beam forming and focusing is achieved, but a narrowband characteristic prevented application of this sensor for microwave wood testing considered in this thesis. A microwave system for a free-space measurement of wood properties is, in its various forms, applied to measurement of wood properties, considering wood as an anisotropic, heterogeneous and multiphase dielectric. Microwave free-space transmission measurement methods are considered, analysing error sources and available mitigation techniques. A focused-beam transmission measurement setup with free-space calibration has been identified as an optimum solution for microwave wood testing. The properties of this measurement system are analysed, having in mind its application for wood measurement in industrial environment. The samples for the study are carefully chosen to cover a range of features frequently met in practice. The ‘actual’ sample properties, against which the performance microwave measurements are judged, are determined using visual inspection and CT scan. The theoretical background on electromagnetic wave propagation through anisotropic media is considered. Of particular interest is depolarisation of a linear plane wave in anisotropic media, which is also demonstrated experimentally. A simple case of grain inclination in a plane is considered first, demonstrating experimentally that grain inclination directly relates to the level of depolarisation. This is then applied to a general case, in which the grain is inclined in three-dimensional space. It is shown that the technique has a good correlation with visually inspected grain angle values, but additional sensor calibration is recommended. Heterogeneity of the sample is analysed using the same set of sensors, but in different arrangement. The aim was to detect variations in wood structure and investigate a method for automated categorisation of wood samples, based on the type of defect. The categorisation of samples is considered as a way to combat a great variability in sample properties and allow easier and more accurate empirical modelling. The microwave transmission measurement data are compared with CT scans and visual inspection of samples. Good results are achieved, not only for samples with distinctive defects such as knots, but for samples with needle flecks, resin pockets and change in annual ring arrangement along the axial direction. Heterogeneity study is then extended to include an analysis of effects which gradual variations in wood structure have on the measured microwave signal. The obtained results show that phase of the microwave transmission coefficient can be used as a good indicator of slow variation in sample density. The study also includes an analysis of free space calibration and broadband transmission measurement, investigating its positive sides such as improved accuracy, as well as its negative sides such as complexity which these procedures introduce in an industrial process. Techniques for combating residual error are investigated, offering the frequency averaging as an easily implemented option. The importance of working over a frequency bandwidth is demonstrated, for dealing with phase periodicity as well as combating measurement uncertainty. Response calibration is considered as an affordable option which can remove some of the systematic errors, yet is less disruptive for the industrial process. Furthermore, both moisture content and density distribution are considered, as well as bulk properties, averaged over the whole sample volume. It has been demonstrated that both moisture and density of wood contribute to the changes in microwave transmission coefficient. Measured data reveal a polarisation dependence of the moisture related transmission magnitude, which may be used as additional information in attempt to distinguish between the contributors. This was further investigated on the set of samples observed at several moisture content values. The correlation between bulk density and microwave measured density improves when samples with knots are omitted, demonstrating advantage of sample categorisation. In the final section of the thesis, the scattering experiment is performed, measuring the transmission through the wood when transmitting and the receiving antenna axes are at the right angle. This experiment shows that maximum transmission in this direction correlates best with the arrangement of annual rings in the sample, indicating possible existence of guided modes in the layered media. This finding is significant as it demonstrates the complexity of microwave propagation model for the sample with such complex structure.