Xiaohua Yi

Passive wireless smart-skin sensor using RFID-based folded patch antennas

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.

A mobile sensing system for structural health monitoring: design and validation

This paper describes a new approach using mobile sensor networks for structural health monitoring. Compared with static sensors, mobile sensor networks offer flexible system architectures with adaptive spatial resolutions. The paper first describes the design of a mobile sensing node that is capable of maneuvering on structures built with ferromagnetic materials. The mobile sensing node can also attach/detach an accelerometer onto/from the structural surface. The performance of the prototype mobile sensor network has been validated through laboratory experiments. Two mobile sensing nodes are adopted for navigating on a steel portal frame and providing dense acceleration measurements. Transmissibility function analysis is conducted to identify structural damage using data collected by the mobile sensing nodes. This preliminary work is expected to spawn transformative changes in the use of mobile sensors for future structural health monitoring.

Passive wireless antenna sensor for strain and crack sensing—electromagnetic modeling, simulation, and testing

This research investigates a passive wireless antenna sensor designed for strain and crack sensing. When the antenna experiences deformation, the antenna shape changes, causing a shift in the electromagnetic resonance frequency of the antenna. A radio frequency identification (RFID) chip is adopted for antenna signal modulation, so that a wireless reader can easily distinguish the backscattered sensor signal from unwanted environmental reflections. The RFID chip captures its operating power from an interrogation electromagnetic wave emitted by the reader, which allows the antenna sensor to be passive (battery-free). This paper first reports the latest simulation results on radiation patterns, surface current density, and electromagnetic field distribution. The simulation results are followed with experimental results on the strain and crack sensing performance of the antenna sensor. Tensile tests show that the wireless antenna sensor can detect small strain changes lower than 20???, and can perform well at large strains higher than 10?000???. With a high-gain reader antenna, the wireless interrogation distance can be increased up to 2.1?m. Furthermore, an array of antenna sensors is capable of measuring the strain distribution in close proximity. During emulated crack and fatigue crack tests, the antenna sensor is able to detect the growth of a small crack.

Large-Deformation Analysis and Experimental Validation of a Flexure-Based Mobile Sensor Node

This paper presents a new magnetic wall-climbing car as a mobile sensor node for health monitoring and dynamic testing of large civil (ferromagnetic) structures. Unlike traditional design, where the distance between the front and rear wheel pairs is fixed, the electromagnetically driven compliant beam connecting the axles not only offers an effective means to negotiate corners when maneuvering on ferromagnetic surfaces, but also serves as a sensor attachment device. Specifically, this paper presents the design concept of a novel magnetic flexonic mobile node incorporating a compliant beam and permanent magnets, and a 2-D model for simulating the deformed shape of the compliant beam. Simulation results show that there exist consistent relations between input/output displacements and rotation angle for control implementation in sensor attachment and corner negotiation regardless of gravity direction or the critical force for buckling. Experiment results are also provided to validate the theoretical model and compare with the analysis for sensor attachment and corner negotiation.

Wireless sensing with smart skins

The ever-increasing need for perpetual ubiquitous cognition of our environment prompts the integration of numerous unobtrusive, extremely low-cost and passively powered wireless sensors into our surroundings. Smart skins, i.e. thin layers of modified materials on top of surfaces that surround our every-day lives, constitute ideal such sensor candidates for the ubiquitous awareness vision. In this paper we are presenting three different types of highly sensitive smart skin sensors for identity and genuineness authentication and seal proof, large metallic structural strain and crack detection and chemical gas sensing of ammonia and nitrogen dioxide. These low-profile smart skin prototypes share not only all the aforementioned desired characteristics but also exhibit high levels of accuracy and reliability in a flexible and rugged design.

Sensitivity Modeling of an RFID-Based Strain-Sensing Antenna With Dielectric Constant Change

An radiofrequency identification (RFID)-based folded patch antenna has been developed as a novel passive wireless sensor to measure surface strain and crack, for the structural health monitoring of metallic structures. Up to 2.5 m of read range is achieved by a proof-of-concept prototype patch antenna sensor with a strain sensitivity around -760 Hz/με, which is equivalent to a normalized strain sensitivity of -0.74 ppm/με. In this paper, we propose to consider the change of the substrate dielectric constant due to strain when modeling the antenna sensor. An enhanced strain sensitivity model is introduced for more accurately estimating the strain sensing performance of the hereby introduced smart skin antenna sensor. Laboratory experiments are carried out to quantify the dielectric constant change under strain. The measurement results are incorporated into a mechanics-electromagnetics coupled simulation model. Accuracy of the multi-physics coupled simulation is improved by integrating dielectric constant change in the model.

Thickness variation study of RFID-based folded patch antennas for strain sensing

This paper explores folded patch antennas 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 that also provides power for the antenna operation. The patch antenna adopts a specially selected substrate material with low dielectric constant, as well as an inexpensive off-the-shelf radiofrequency identification (RFID) chip for signal modulation. A thicker substrate increases RFID signal-to-noise ratio, but reduces the strain transfer efficiency. To experimentally study the effect of substrate thickness, two prototype folded patch antennas with different substrate thicknesses have been designed and manufactured. For both prototypes, tensile testing results show strong linearity between the interrogated resonance frequency and the strain experienced by the antenna. Longer interrogation range is achieved with the larger substrate thickness.

Sensitivity analysis of transmissibility functions for structural damage detection

In order to assess structural safety conditions, many vibration-based damage detection methods have been developed in recent years. Among these methods, transmissibility function analysis can utilize output data only, and proves to be effective in damage detection. However, previous research mostly focused on experimental validation of using transmissibility function for damage detection. Very few studies are devoted to analytically investigating its performance for damage detection. In this paper, a spring-mass-damper model with multiple degrees-of-freedom is formulated for further analytical studies on the damage sensitivity of transmissibility functions. The sensitivity of transmissibility function against structural mass and stiffness change is analytically derived and validated by numerical examples.

Embedded transmissibility function analysis for damage detection in a mobile sensor network

Structural health monitoring (SHM) and damage detection have attracted great interest in recent decades, in meeting the challenges of assessing the safety condition of large-scale civil structures. By wiring remote sensors directly to a centralized data acquisition system, traditional structural health monitoring systems are usually costly and the installation is time-consuming. Recent advances in wireless sensing technology have made it feasible for structural health monitoring; furthermore, the computational core in a wireless sensing unit offers onboard data interrogation. In addition to wireless sensing, the authors have recently developed a mobile sensing system for providing high spatial resolution and flexible sensor deployment in structural health monitoring. In this study, transmissibility function analysis is embedded in the mobile sensing node to perform onboard and in-network structural damage detection. The system implementation is validated using a laboratory 2D steel portal frame. Simulated damage is applied to the frame structure, and the damage is successfully identified by two mobile sensing nodes that autonomously navigate through the structure.

Thermal effects on a passive wireless antenna sensor for strain and crack sensing

For application in structural health monitoring, a folded patch antenna has been previously designed as a wireless sensor that monitors strain and crack in metallic structures. Resonance frequency of the RFID patch antenna is closely related with its dimension. To measure stress concentration in a base structure, the sensor is bonded to the structure like a traditional strain gage. When the antenna sensor is under strain/deformation together with the base structure, the antenna resonance frequency varies accordingly. The strain-related resonance frequency variation is wirelessly interrogated and recorded by a reader, and can be used to derive strain/deformation. Material properties of the antenna components can have significant effects on sensor performance. This paper investigates thermal effects through both numerical simulation and temperature chamber testing. When temperature fluctuates, previous sensor design (with a glass microfiber-reinforced PTFE substrate) shows relatively large variation in resonance frequency. To improve sensor performance, a new ceramic-filled PTFE substrate material is chosen for re-designing the antenna sensor. Temperature chamber experiments are also conducted to the sensor with new substrate material, and compared with previous design.