Browsing by Author "Eskelinen, Pekka, Prof., Aalto University, Department of Electrical Engineering and Automation, Finland"

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Studies on characterization of dielectric composite materials using radar and other microwave sensors
(2016) Olkkonen, Martta-Kaisa
School of Electrical Engineering | Doctoral dissertation (article-based)
This thesis describes several new advances in measurement of the properties of materials using microwave techniques. The developments were made through an interdisciplinary approach to explore the properties of composite materials such as biofuel, concrete, asphalt and the rock inclusions of asphalt. In this thesis, a composite material has been defined as inhomogeneous material with inclusions that have dimensions comparable to the measurement wavelength. Resonators were chosen to characterize materials in which the properties, such as moisture, are linked directly to the real or imaginary part of the measured signal, using either a linear or parabolic statistical fitting. The concepts were tested using online moisture measurement of biofuel employing a stripline cavity resonator at approximately 360 MHz, attached above and below the conveyor belt. In a similar manner, a resonant coaxial surface sensor operating at approximately 1.7 GHz was used to test the same concepts by determining the moisture content of concrete. Investigations into the use of radar techniques for material characterization employed a radar device (13–17 GHz), which was developed by colleagues to achieve a resolution (2–3 cm) sufficient for gaining an image of the structure of only the surface layer (< 5 cm). It is suggested that the use of polyoxymethylene (POM) or similar dielectric is preferable for radar calibration, as the reflectivity is of the same order as the material under test unlike in traditionally used metal sheets, which can cause saturation of the receiver. The permittivity of the asphalt aggregates was measured using both the resonator and waveguide methods as a precursor to measuring composites. Then, a scanner (7–17 GHz), which measured the transmission scattering parameters of a sample, was used as a laboratory reference method. This produced a two-dimensional color image of the permittivity and hence structure of the composite material with a resolution of 5 mm x 5 mm. Phase error due to multiple reflections was compensated by fitting a line to the fluctuating phase through the minima and maxima of the amplitude. Another signal processing routine was developed to filter out the multiple reflections so that the frequency-dependent permittivity was derived based on only the single-pass propagation through a concrete slab over 0.8–12 GHz. Further, a new algorithm was developed to perform direct interpretation of the GPR data. A color map was superimposed on top of a conventional grey-scale GPR image to distinguish materials based on their reflection-related phase.