A Wireless Passive Sensor Based on U-Shaped Resonators for Bidirectional Deformation Sensing

Conventional deformation sensors are inconvenient to deploy in large areas due to the employment of cables and power supplies. This article proposes a wireless passive sensor based on U-shaped resonators for bidirectional deformation sensing. The proposed sensor consists of two ultra-wideband (UWB) antennas for receiving the inquiry signal and retransmitting the coded signal, and a sensing element for encoding bidirectional deformation information in the signal. The two UWB antennas are set to cross-polarization to avoid interference from ambient reflected signals. The sensing element consists of a microstrip line for signal transmission, a U-shaped resonator for horizontal deformation monitoring at low frequency band, and a U-shaped resonator for vertical deformation monitoring at high frequency band. The deformation alters the overlapped length between the U-shaped resonator and its corresponding movable sub-patch, thereby shifting its resonant frequency. The absence of force between the U-shaped resonator and its corresponding movable sub-patch serves to prevent sensor damage from excessive force and to ensure a complete strain transfer ratio. The equivalent circuit model of the sensor is established to reveal its sensing mechanism, and the relationships between horizontal deformation and resonance frequency of low frequency band, and vertical deformation and resonance frequency of high frequency band, are studied by simulation in CST Microwave Studio. To verify the performance, the sensor is fabricated and installed on the concrete surface, and a series of wireless experiments are conducted. The experimental results demonstrate the feasibility of the sensor in simultaneously monitoring both horizontal and vertical deformation.