Development of a resonant repeater tag for the enhancement of sensitivity and specificity in a wireless eddy current sensing scheme
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Eddy current sensing has been successfully used in various applications from testing heat exchange tubes for nuclear power plants to assessing dielectric thickness on printed circuit boards. However, in civil infrastructures cosmetic or cementitious surface material often keeps the probe or reader coil from accessing conductive medium inside the structure, resulting in reduced coupling as the distance increases between the DUT (device under test) and probe. Thus, the direct application of existing eddy current sensing technique is not very useful to detect flaws in civil infrastructures. To address this weak coupling problem, a simple scheme is proposed in which a resonant passive repeater tag is placed between the reader coil and the conducting test target. The feasibility of detecting defects like cracks or fractures in conductive medium using a passive resonant tag and measuring the impedance as a method of interrogation is shown. The electromagnetic waves are transmitted into and detected from a resonant tag and the conductive medium underneath the tag without direct physical contact using a reader coil above the resonant repeater tag. Experimental data taken from simple setups to demonstrate the advantage of the proposed scheme are presented. In addition, the theoretical background, such as the self and mutual inductance, and image theory, are discussed extensively. It is also shown that the theories can be applicable to build the equivalent circuit with the proper calibration process. The analyses have been carried out to characterize the responses resulting from the various experiments. Furthermore, new measurands, the effective normalized inductance and resistance, were devised and employed to reanalyze the same experimental data. An effort to construct equivalent circuit model of the system has been made to correctly predict the response without the actual experiment. To improve the process of building the equivalent circuit, the total 4 types of tags and 6 types of DUT are built and tested. The analysis is also given for the constructed equivalent circuit model.