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Chip-based radio frequency identification (RFID) sensor systems have been investigated for structural health monitoring applications. However, the use of chips in sensor tags has drawbacks in terms of cost, durability, and detection capability. In this paper, we propose a reconfigurable RFID-based chipless tag sensor for strain detection, which detects structural strain by observing the offset of the resonant frequency of the antenna and incorporates an encoding unit for tag identification. To realize the reconfigurability of the antenna, a PIN diode is added to control the antenna, and the frequency reconfigurability of the antenna is realized by controlling the turn on/off of the PIN diodes. The Kalman filter algorithm is used to realize the information fusion of multiple sensors to improve the sensor detection accuracy, and the background noise data enhancement is used to expand the original data samples to fuse the information of the three sensors’ data, and the relative error after fusion is reduced by about 36% compared with the relative error before fusion compared with separate measurements. The antenna was fabricated by an etching process and measured using vector network analyzer to verify the accuracy of the antenna simulation results.

This paper presents a capacitive displacement sensor based on a capacitively fed inverted-F antenna (CFIFA) for displacement detection. The sensor is composed of a grounded L-shape patch and a rectangular upper patch, forming a capacitor between them. The asymmetric dipole model is adopted to explain the frequency shift and current distribution of the proposed antenna sensor at its first-order resonance. The numerical simulation of the CFIFA using the Ansoft high-frequency structure simulator (HFSS) software is carried out to optimize the dimensional parameters, allowing the antenna to perform better. Two sets of CFIFAs are fabricated and tested for verification. Results show that the CFIFA has a good linear relationship between its first resonant frequency and the relative displacement, and is capable of a long range of displacement measuring.

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.

This paper presents an angle sensor based on a sector ring patch antenna (SRPA) for loose bolt detection. The sensor is comprised of two parts, a ring substrate sandwiched between a ground plane and a sector ring radiation patch, and a sector ring substrate covered by a radiation patch. The two radiation patches are partially overlapped and the overlapped areas are in contact allowing the electric current to flow within the combined radiation patch. The resonant cavity theory is adopted to explain the relationship between the frequency shift and the overlapped angle. The Ansoft high frequency structure simulator software is used to simulate SRPA, and the dimensional parameters of the antenna are optimized to make the antenna perform better. For verification, two sets of SRPAs are fabricated and tested.

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&rsquo, s needs.

In the field of structural health detection, since the sensor needs to be directly bonded to the measured object, it is difficult to stably transmit the RFID information and realize the passive wireless structure strain detection. To circumvent these difficulties, this paper innovatively proposes a dual-resonant circular microstrip patch antenna (CMPA) sensor with chipless RFID tag. The information of the RFID tag can be changed that allows the sensor to be deployed on a large scale. The separating sensor includes chipless RFID tag layer and dual-resonant CMPA layer, which can simultaneously reflect RFID tag information and strain information.The design of separated layers can minimize the resonant frequency shift of ID tags when stress is implemented on CMPA strain part. Current distribution analysis and equivalent circuit are used to study the strain size and direction. Experiments show that the offset of the resonant frequency of the sensor antenna has linear relationship with the lateral and longitudinal strains. The correlation coefficients are 0.9284 and 0.9235 respectively, and the sensitivity coefficient of the lateral and longitudinal strain is 528 MHz ϵ −1 and 384 MHz ϵ −1. At the same time, the readability of the RFID tag information is maintained.

An efficient methodology using static test data and changes in natural frequencies is proposed to identify the damages in structural systems. The methodology consists of two main stages. In the first stage, the Damage Signal Match (DSM) technique is employed to quickly identify the most potentially damaged elements so as to reduce the number of the solution space (solution parameters). In the second stage, a particle swarm optimization (PSO) approach is presented to accurately determine the actual damage extents using the first stage results. One numerical case study by using a planar truss and one experimental case study by using a full-scale steel truss structure are used to verify the proposed hybrid method. The identification results show that the proposed methodology can identify the location and severity of damage with a reasonable level of accuracy, even when practical considerations limit the number of measurements to only a few for a complex structure.

Development of a health monitoring system for construction has become an important task for structural damage identification. Since the building structure has many uncertain factors, the method of using modal analysis to identify the structural damage is of low accuracy. A method applying a grey system to the structural damage identification is presented. The grey system for the frequency variance rate and the stiffness change is established with the first-order single-argument grey system (GM(1, 1)). GM(1, 1) prediction is used to reflect the global function of the structural dynamic fingerprints and find the relation of the frequency variance rate and the stiffness change. Vibration tests for frame structures were carried out, with many cases considered, including single-damage and multi-damage ones with different degrees and locations. The results show that for shear buildings, the damage degree and location can be determined by using the grey system and measuring the frequency change.