In recent few years, the antenna and sensor communities have witnessed a considerable integration of radio frequency identification (RFID) tag antennas and sensors because of the impetus provided by internet of things (IoT) and cyber-physical systems (CPS). Such types of sensor can find potential applications in structural health monitoring (SHM) because of their passive, wireless, simple, compact size, and multimodal nature, particular in large scale infrastructures during their lifecycle. The big data from these ubiquitous sensors are expected to generate a big impact for intelligent monitoring. A remarkable number of scientific papers demonstrate the possibility that objects can be remotely tracked and intelligently monitored for their physical/chemical/mechanical properties and environment conditions. Most of the work focuses on antenna design, and significant information has been generated to demonstrate feasibilities. Further information is needed to gain deep understanding of the passive RFID antenna sensor systems in order to make them reliable and practical. Nevertheless, this information is scattered over much literature. This paper is to comprehensively summarize and clearly highlight the challenges and state-of-the-art methods of passive RFID antenna sensors and systems in terms of sensing and communication from system point of view. Future trends are also discussed. The future research and development in UK are suggested as well.

Chipped radio-frequency identification (RFID) sensor systems have been studied for structural health monitoring (SHM) applications. However, the use of chip in sensor tags and its standardized narrowband operation contribute shortcomings in cost, durability, and detection capability. This paper presents a novel use of the frequency signature-based chipless RFID for metal crack detection and characterization operating in ultra-wideband frequency. The vision is to implement a lowcost and high-temperature-resistant passive wireless sensor able to monitor the crack on a metallic structure with multiparameter detection. We propose a chipless RFID sensor tag integrating four tip-loaded dipole resonators as a 4-bit ID encoder and a circular microstrip patch antenna (CMPA) resonator as a crack sensor. The radar cross section spectrum of the chipless RFID sensor tag generates four resonant frequencies from the dipole resonators and a resonant frequency from the CMPA resonator. Simulation and experimental results show that the resonant frequency shift of the CMPA is a useful feature to indicate the crack orientation and the crack width on a metallic structure. The direction of the resonant frequency shift represents the orientation of the crack, while the magnitude of the resonant frequency shift is proportional to the width of the crack. Furthermore, the experimentation with a natural fatigue crack sample proves that the proposed sensor tag is capable of detecting submillimeter cracks.

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Chipless RFID sensors attract attention to structural health monitoring (SHM) because of its advantages of being low-cost, wireless, passive, and having multiple resonances for sensing. Its application for corrosion sensing, however, receives little attention and faces challenges in terms of sensitivity and reliability. This paper proposes a chipless RFID sensor for corrosion characterization based on frequency selective surface (FSS) and feature fusion. An FSS pattern on a substrate is designed to generate three resonances within 2-6 GHz. The ability of the FSS to characterize corrosion thickness was simulated and validated in the experiments. The experimental results using dedicated corrosion undercoating samples show that the FSS based chipless RFID sensor can be used to characterize corrosion, where the three resonance frequency features provide sensitivity and consistent monotonic relations to the corrosion progression. Furthermore, feature fusion using simple sum and confidence weighted averaging (CWA) can enhance the sensitivity and reliability of the sensor. With the lowprofile and printability of the sensor, this work paves the way for smart coatings for corrosion sensing and monitoring on metallic structures.