This paper presents a passive wireless long-range displacement sensor that is based on the circular patch antenna, and the detecting range of the sensor can be customized. The sensor consists of a chipped circular antenna with two opened rectangular windows, a substrate, and a ground plate with a sloping channel. No bonding between the antenna and the ground plate allows for the chipped antenna to slide along the sloping channel. The channel will drive the current flow on the plate once the chip is activated, increasing the effective electrical length and, consequently, decreasing the resonant frequency of the circular antenna. The sensing mechanism equates the measuring displacement to the relative movement of the antenna with respect to the ground that achieves the measurement of long-range displacement and, thus, the proposed sensor can avoid stress damage to the antenna due to excessive deformation. Three different range sensors were simulated in the the Ansoft high frequency structure simulator (HFSS). The results show that the resonance frequency of the antenna has a linear relationship with the varying chute depth beneath the chip. Three sensors were fabricated, and the experimental results also validated that the sensitivity of the sensor can be adjusted.

Currently, the reader based on the frequency modulated continuous wave (FMCW) radar used to dynamically interrogate the chipless radio frequency identification (RFID) sensor needs to collect the envelope of the echo signal through the oscilloscope, which entails expensive acquisition equipment and occupies a considerable amount of space. This article introduces a low-cost FMCW reader for the chipless RFID sensor’s dynamic interrogation. The reader mixes the received signal with the local oscillator signal and down-converts it into a low-frequency signal, which can be directly sampled by a low-cost analog to digital converter (ADC). Then, the sensor’s resonant frequency can be extracted from the amplitude and phase variation of the reader’s sampled signal, respectively within a single interrogation signal period. The working mechanism of the reader is analysed theoretically, followed by verification through the simulations performed in the advanced design system (ADS). Hilbert-based algorithms are presented for restoring the amplitude and phase variations of the sampled signal in the time domain. Then, the high-speed reader is employed to wirelessly query a crack sensor, and the measurement results acquired from the reader exhibit excellent concordance with those obtained from the vector network analyzer (VNA), demonstrating the accuracy and reliability of the reader. Combined with the other types of sensors, the reader can easily read information, such as acceleration, dynamic temperature, and humidity, providing massive dynamic data for structural health monitoring (SHM) and showing great potential in Internet of Things (IoT).

Conventional deformation sensors are inconvenient to deploy in large areas due to the employment of cables and power supplies. This paper 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.

A waterproofing layer is necessary to maintain the performance of a patch antenna sensor, especially when embedded in a wet environment. The influence of varying levels of damaged waterproofing layers on the accuracy and sensitivity of sensors has been analyzed in this paper. The behavior of a patch antenna sensor with a possible damaged waterproofing layer is described by an equivalent model with a covered dielectric load. Theoretical calculation and simulation in HFSS ver.15 have been utilized to confirm the influence of the leakage. The antenna sensor with a damaged waterproofing layer tends to have a smaller initial resonant frequency and poor accuracy. Considering three antenna sensors with undamaged, half-damaged, and fully damaged waterproofing layers, an experiment is conducted to verify the influence of damage on sensor performance. Compared with the undamaged waterproofing layer case, the case with a fully damaged waterproofing layer has an initial resonant frequency that is 50% smaller, and the error increases from 10% to 33%. The identification method for the damaged condition of the waterproofing layer is summarized based on the irregularity of the return loss (S11) and resonant frequency.