ABSTRACT
The application of different types of microwave resonators for sensing cracks in metallic structures has been subject of many studies. While most studies have been focused on improving the sensitivity of planar crack sensors, the theoretical foundation of the topic has not been treated in much detail. The major objective of this study is to perform an exhaustive study of the principles and theoretical foundations for crack sensing based on planar microwave resonators, especially defective ground structures (DGS) including complementary split ring resonators (CSRRs). The analysis is carried out from the equivalent circuit model as well as the electromagnetic (EM) field perspectives, and guidelines for the design of crack sensors with high sensitivity are developed. Numerical and experimental validation of the provided theoretical analysis is another aim of this article. With this aim, the developed guidelines are used to design a crack sensor based on a single-ring CSRR. It is shown that the sensitivity of the proposed sensor is almost three times higher than the sensitivity of a conventional double-ring CSRR. Moreover, it is demonstrated that folded dumbbell-shape DGS resonators can be used to achieve even higher sensitivities. The CSRR-based crack sensors presented in this study and other studies available in the literature are only sensitive to cracks with a specific orientation. To address this limitation, a modified version of the DGS is proposed to sense cracks with arbitrary orientations at the cost of lower sensitivity. The performance of all the presented sensors is validated through EM simulation, equivalent circuit model extraction, and measurement of the fabricated prototypes.
ABSTRACT
Background: This research is focused on the design of highly sensitive microfluidic sensors for the applications in liquid dielectric characterizations including biomedical samples. Methods: Considering the narrow-band operation of microfluidic sensors based on microwave resonators, in this study, microfluidic sensors based on the variation of transmission phase in microwave transmission lines (TLs) are proposed. It is shown that among different microwave TLs, slot-lines are an appropriate type of TL for sensing applications because a major portion of the electromagnetic (EM) field passes above the line, where a microfluidic channel can be easily devised. Results: The proposed concept is presented and the functionality of the proposed sensor is validated through full-wave EM simulations. Moreover, the effects of the dimensions of the microfluidic channel and the thickness of the substrate on the sensitivity of the sensor are studied. Furthermore, taking the advantages of differential circuits and systems into account, a differential version of the microfluidic sensor is also presented. It is shown that the sensitivity of the sensor can be adjusted according to the application. Specifically speaking, the sensitivity of the proposed microfluidic sensor is almost linearly proportional to the length of the channel, i.e., the sensitivity can be doubled by doubling the channel length. Conclusions: In this research, it is shown that using slot-line TLs highly sensitive microfluidic sensors can be designed for the applications in liquid dielectric characterizations, especially for biomedical samples where small variations of permittivity have to be detected.
