Structural health monitoring (SHM) has become an important research problem which has the potential to monitor and ensure the performance and safety of civil structures. Traditional wire-based SHM systems require significant time and cost for cable installation. With the recent advances in wireless communication technology, wireless SHM systems have emerged as a promising alternative solution for rapid, accurate and low-cost structural monitoring. This paper presents a newly designed integrated wireless monitoring system that supports real-time data acquisition from multiple wireless sensing units. The selected wireless transceiver consumes relatively low power and supports long-distance peer-to-peer communication. In addition to hardware, embedded multithreaded software is also designed as an integral component of the proposed wireless monitoring system. A direct result of the multithreaded software paradigm is a wireless sensing unit capable of simultaneous data collection, data interrogation and wireless transmission. A reliable data communication protocol is designed and implemented, enabling robust real-time and near-synchronized data acquisition from multiple wireless sensing units. An integrated prototype system has been fabricated, assembled, and validated in both laboratory tests and in a large-scale field test conducted upon the Geumdang Bridge in Icheon, South Korea.

This paper presents the design, simulation, and validation experiments of a passive (battery-free) wireless frequency doubling antenna sensor for strain and crack sensing. Since the length of a patch antenna governs the antenna’s resonance frequency, a patch antenna bonded to a structural surface can be used to measure mechanical strain or crack propagation by interrogating resonance frequency shift due to antenna length change. In comparison with previous approaches such as radio frequency identification, the frequency doubling scheme is proposed as a new signal modulation approach for the antenna sensor. The proposed approach can easily distinguish backscattered passive sensor signal (at the doubled frequency $2{f}$ ) from environmental electromagnetic reflections (at original reader interrogation frequency $f$ ). To accurately estimate the performance of the frequency doubling antenna sensor, a multi-physics coupled simulation framework is proposed to aid the sensor design while considering both the mechanical and electromagnetic behaviors. Two commercial software packages, COMSOL and Advanced Design System (ADS), are combined to leverage the features from each other. The simulated performance of the frequency doubling antenna sensor is further validated by experiments. The results show that the sensor is capable of detecting small strain changes and the growth of a small crack.

Folded patch antennas were investigated for the development of low-cost and wireless smart-skin sensors that monitor the strain in metallic structures. When the patch antenna is under strain/deformation, its resonance frequency varies accordingly. The variation can be easily interrogated and recorded by a wireless reader. The patch antenna adopts a specially chosen substrate material with low dielectric attenuation, as well as an inexpensive off-the-shelf radiofrequency identification (RFID) chip for signal modulation. Since the RFID chip harvests electromagnetic power from the interrogation signal emitted by the reader, the patch antenna itself does not require other (internal) power sources and, thus, serves as a battery-less (passive) and wireless strain sensor. In this preliminary investigation, a prototype folded patch antenna has been designed and manufactured. Tensile testing results show strong linearity between the interrogated resonance frequency and the strain experienced by the antenna. Through experiments, the strain sensing resolution is demonstrated to be under 50 μϵ, and the wireless interrogation distance is shown to be over a few feet for this preliminary prototype.