Monolithic patch antennas for deformation measurements are designed to be stressed. To avoid the issues of incomplete strain transfer ratio and insufficient bonding strength of stressed antennas, this paper presents a passive wireless crack sensor based on an unstressed patch antenna. The rectangular radiation patch of the proposed sensor is partially covered by a radiation sub-patch, and the overlapped length between them will induce the resonate frequency shift representing the crack width. First, the cavity model theory is adopted to show how the resonant frequencies of the crack sensor are related to the overlapped length between the patch antenna and the sub-patch. This phenomenon is further verified by numerical simulation using the Ansoft high-frequency structure simulator (HFSS), and results show a sensitivity of 120.24 MHz/mm on average within an effective measuring range of 1.5 mm. One prototype of proposed sensor was fabricated. The experiments validated that the resonant frequency shifts are linearly proportional to the applied crack width, and the resolution is suitable for crack width measuring.

This paper presents a passive displacement sensor based on a normal mode helical antenna. The sensor consists of an external helical antenna and an inserting dielectric rod. First, the perturbation theory is adopted to demonstrate that both the electric intensity and magnetic intensity have a noticeable gradient change within the in-and-out entrance of the helical antenna, which will cause the sensor to experience a resonant frequency shift. This phenomenon was further verified by numerical simulation using the Ansoft high frequency structure simulator (HFSS), and results show the linear correlation between the retrieved resonant frequency and the displacement. Two sets of proposed sensors were fabricated. The experiments validated that the resonant frequency shifts are linearly proportional to the applied displacement, and the sensing range can be adjusted to accommodate the user’s needs.

Bolts are widely used in civil engineering, and the detection of bolt loosening is of great significance to ensure the safety of a structure. This paper introduces a new method for detecting bolt loosening using a customized detachable strain sensor based on a patch antenna. A patch antenna with overlapping sub-patch is proposed to measure the longitudinal elongation of the entire bolt shaft, indicating the loosening state of the bolt. When the bolt is fastened, the elongation of the bolt under tension will change the combined length of the underlying patch and the radiation sub-patch, consequently increasing or decreasing the resonant frequency of the antenna. The resonant frequency of the antenna can be measured by the vector network analyzer. Furthermore, with wireless interrogation of the strain sensor based on the patch antenna, the proposed method can also be used in the wireless detection of bolt loosening. The authors conducted a finite element analysis of the bolt and the electromagnetic simulations of the antenna. They designed the detection sensor and conducted a series of experimental tests to demonstrate how a bolt under different applied preloads can be effective and feasible under the proposed method.

Traditional acceleration sensors use cables to transport data and batteries to supply power, which may increase failure risk, deployment inconvenience, and maintenance difficulty. This article proposes a passive and wireless acceleration sensing system, namely, a newly designed patch-antenna-based sensing node interrogated by frequency-modulated continuous-wave (FMCW) radar. The acceleration is correlated with the location of a cantilever beam, whose oscillation alters the additional capacitance of the patch antenna and consequently the resonant frequencies. The testing range can be adjusted by changing the material and dimensions of the cantilever beam, which is designed up to ±110 m/s 2 with a sampling rate of 500 Hz to meet the requirements of different conditions. The working mechanism of the sensing system is analyzed theoretically and then verified by the simulation in COMSOL Multiphysics. Then, an experiment is carried out to evaluate the performance of the sensing system. Both the resonant frequency and remaining power can be utilized to extract the acceleration response, and results show a good fit with the actual response (i.e., an average error of 4.5%). Empowered by FMCW Radar, the passive accelerometer may have great potential in identifying structure strain mode, impedance-based detection of bolt loosening, automation in construction, etc.

In order to ensure the safety and preserve the value of historical buildings, inclination is an essential parameter during the continuous structural health monitoring process. However, the wire and price of a traditional sensor limit application. This paper proposes a low-cost inclination sensor based on a patch antenna with a reconfigurable water load. Only the water directly on the antenna is considered effective. The different volume of the effective water load, which is determined by the inclination of the attached surface, will affect the effective permittivity of the dielectric plate of the patch antenna, further causing a variation in the resonant frequency. Therefore, the proposed antenna sensor can monitor the inclination of the attached surface by interrogating the resonant frequency. The working mechanism is first clarified by theoretically investigating the relationship between the dielectric properties and the inclination of the covering medium. The antenna sensor is then simulated using High-Frequency Structure Simulator ver.15 (HFSS 15), which helps to determine geometric parameters and confirm accuracy and sensitivity. An experiment has been conducted based on the design verified in the simulation. The inclination detection shows a correlation coefficient of 0.9771 with a sensitivity of 7.92 MHz/°, indicating a potential for real application.