Depolarizing Chipless RFID Sensor Tag for Characterization of Metal Cracks Based on Dual Resonance Features

In structural health monitoring (SHM) applications, chipless RFID sensor systems are potential for acquiring multiple information of metal defects, such as defect orientation and size, due to its broadband operation. However, the issue of limited readability of chipless RFID tags in the real environment and particularly on a large metallic platform raises the need for applying the cross-polarization reading, and thus depolarizing sensor tags are required. This paper introduces a depolarizing chipless RFID sensor tag design for characterization of metal cracks based on dual resonance features. The proposed chipless RFID sensor tag integrates a modified circular patch antenna with notched edges as the crack sensor and diagonal bent dipoles as the tag ID. The notches on the diagonal edges make the circular patch able to depolarize the incident waves and generates two resonances for sensing purpose. To obtain the frequency signature of the sensor tag, a chipless RFID sensor reader is developed from an ultra-wideband (UWB) radar module applying the short-time Fourier transform (STFT). The simulation and experimental studies show that the dual resonance features generated by the depolarizing circular patch can be used to indicate the crack orientation as well as to characterize the crack width in millimeter. With the sensor's readability over a large metallic platform and the usability of the dual resonance features in crack characterization, the developed chipless RFID system is potential for real implementations of the internet of things (IoT)-based SHM.