A review of passive wireless sensors for structural health monitoring

Wireless sensors for Structural Health Monitoring (SHM) is an emerging new technology that promises to overcome many disadvantages pertinent to conventional, wired sensors. The broad field of SHM has experienced significant growth over the past two decades, with several notable developments in the area of sensors such as piezoelectric sensors and optical fibre sensors. Although significant improvements have been made on damage monitoring techniques using these smart sensors, wiring remains a significant challenge to the practical implementation of these technologies. Wireless SHM has recently attracted the attention of researchers towards un-powered and more effective passive wireless sensors. This article presents a review of some of the underlying technologies in the field of wireless sensors for SHM - with a focus on the research progress towards the development of simple, powerless, yet effective and robust wireless damage detection sensors. This review examines the development of passive wireless sensors in two different categories: (1) use of oscillating circuits with the help of inductors, capacitors and resistors for damage detection; and (2) use of antennas, Radio Frequency Identification (RFID) tags and metamaterial resonators as strain sensors for wireless damage monitoring. An assessment of these electromagnetic techniques is presented and the key issues involved in their respective design configurations are discussed

Antenna integration for wireless and sensing applications

As integrated circuits become smaller in size, antenna design has become the size limiting factor for RF front ends. The size reduction of an antenna is limited due to tradeoffs between its size and its performance. Thus, combining antenna designs with other system components can reutilize parts of the system and significantly reduce its overall size. The biggest challenge is in minimizing the interference between the antenna and other components so that the radiation performance is not compromised. This is especially true for antenna arrays where the radiation pattern is important. Antenna size reduction is also desired for wireless sensors where the devices need to be unnoticeable to the subjects being monitored. In addition to reducing the interference between components, the environmental effect on the antenna needs to be considered based on sensors' deployment. This dissertation focuses on solving the two challenges: 1) designing compact multi-frequency arrays that maintain directive radiation across their operating bands and 2) developing integrated antennas for sensors that are protected against hazardous environmental conditions. The first part of the dissertation addresses various multi-frequency directive antennas arrays that can be used for base stations, aerospace/satellite applications. A cognitive radio base station antenna that maintains a consistent radiation pattern across the operating frequencies is introduced. This is followed by multi-frequency phased array designs that emphasize light-weight and compactness for aerospace applications. The size and weight of the antenna element is reduced by using paper-based electronics and internal cavity structures. The second part of the dissertation addresses antenna designs for sensor systems such as wireless sensor networks and RFID-based sensors. Solar cell integrated antennas for wireless sensor nodes are introduced to overcome the mechanical weakness posed by conventional monopole designs. This can significantly improve the sturdiness of the sensor from environmental hazards. The dissertation also introduces RFID-based strain sensors as a low-cost solution to massive sensor deployments. With an antenna acting as both the sensing device as well as the communication medium, the cost of an RFID sensor is dramatically reduced. Sensors' strain sensitivities are measured and theoretically derived. Their environmental sensitivities are also investigated to calibrate them for real world applications.Ph.D.Committee Chair: Tentzeris, Emmanouil; Committee Member: Akyildiz, Ian; Committee Member: Allen, Mark; Committee Member: Naishadham, Krishna; Committee Member: Peterson, Andrew; Committee Member: Wang, Yan

Antenna sensing for wearable applications

As wearable technologies are growing fast, there is emerging trend to increase functionality of the devices. Antennas which are primarily component in communication systems can offer attractive route forward to minimize the number of components functioning as a sensing element for wearable and flexible electronics. Toward development of flexible antenna as sensing element, this thesis investigates the development of the flexible and printed sensing NFC RFID tag. In this approach, the sensor measurement is supported by the internal sensor and analog-to-digital convertor (ADC) of the NFC transponder. Design optimisation, fabrication and characterization of the printed antenna are described. Besides, the printed antenna, NFC transponder and two simple resistive sensors are integrated to form a fully flexible sensing RFID tag demonstrating applicability in food and health monitoring. This thesis also presents development of two antenna sensors by using functional materials: (i) An inductor-capacitor (LC) resonant tank based wireless pressure sensor on electrospun Poly-L-lactide (PLLA) nanofibers-based substrate. The screen-printed resonant tank (resonant frequency of ~13.56 MHz) consists of a planar inductor connected in parallel with an interdigitated capacitor. Since the substrates is piezoelectric, the capacitance of the interdigitated capacitor varies in response to the applied pressure. To demonstrate a potential application of developed pressure sensor, it was integrated on a compression bandage to monitor sub-bandage pressure. (ii) To investigate the realization of sensing antenna as temperature sensor simple loop antenna is designed and in this study unlike the first study that the sensing element was the substrate, the conductive body of the antenna itself is considered as a functional material. In this case, a small part of a loop antenna which originally was printed using silver paste is replaced by Poly(3,4-ethylenedioxythiophene): polystyrene (PEDOT: PSS). The sensing mechanism is based on the resonant frequency shift by varying temperature. While using functional materials is useful for realization of antenna sensor, another approach also is presented by developing stretchable textile-based microstrip antennas on deformable substrate which can measure joint angles of a human limb. The EM characteristics of the meshed patch antenna were compared with its metallic counterpart fabricated with lithography technique. Moreover, the concept of stretchable UHF RFID-based strain sensor is touched in the final part of this thesis

Development of microstrip patch antenna strain sensors for wireless structural health monitoring

Current developments in the design and manufacturing of composite materials along with their superior mechanical characteristics have resulted in the extensive use of these materials in advanced structures for aerospace industry. In recent years, several different attempts have been made to develop Structural Health Monitoring (SHM) systems and, as a result, various SHM techniques have been introduced. Currently, however, none of these techniques are capable of monitoring the condition of complex operational aerospace structures. The number of sensors and its wiring pose significant problems because of the increased signal processing demand and heightened system unreliability, respectively. Current available wireless sensors are not efficient enough to be used in SHM for aerospace structures primarily because of cost and battery power limitations. The aim in this research was to investigate the feasibility of using microstrip patch antennas as a new type of strain sensor and develop the required techniques for wireless strain measurement without these aforementioned problems. Analytical, computational (finite element analysis) and experimental tests conducted in this research demonstrated that microstrip patch antennas can be used to reliably measure strain. As a result of investigating different microstrip patch antenna configurations in this study, novel antenna sensors were designed, simulated and tested. These antenna sensors (circular, slotted circular and meandered circular microstrip patch antennas) showed good sensitivity with strain with acceptable linearity between strain and the shift in its resonant frequency. In particular, the meandered circular microstrip patch antenna was shown to have localised and omni-directional strain measurement capabilities. In this research, it was shown that by using carefully selected techniques, the resonant frequency of circular microstrip patch antennas can be detected and measured wirelessly. As a result, the feasibility of wireless strain measurement for different aerospace materials (aluminium, carbon fibre reinforced polymer and glass fibre reinforced polymer) was demonstrated. Several diagnostic-related parameters here were investigated and a pivotal parameter discovered was the distance between the sensor and the reader. This technology can be further developed to be used in SHM techniques for wireless measurement of strain and possibly wireless detection of damage in aerospace structures

Transparent and Flexible Radio Frequency (RF) Structures

With increasing demand for a wearable devices, medical devices, RFID, and small devices, there is a growing interest in the field of transparent and flexible electronics. In order to realize optically transparent and flexible microwave components, novel materials can be used. The combination of new materials and radio frequency (RF) structures can open interesting perspectives for the implementation of cost effective wireless communication system and wearable device design. The transparent and flexible RF structures can facilitate its application in the transparent and curved surfaces. In this dissertation, we present several demonstrations, all based on optically transparent and flexible materials and structures. We firstly demonstrate an optically transparent, flexible, polarization-independent, and broadband microwave absorber. The bow-tie shaped array which possesses double resonances is designed and measured. The combined resonances lead to more than 90% total absorption covering a wide frequency range from 5.8 to 12.2 GHz. Due to the use of thin metal and PDMS, the whole structure is optically transparent and flexible. Secondly, we demonstrate a new method for fabricating transparent and stretchable radiofrequency small antennas by using stretchable micromesh structures. Size reduction is achieved by using the zeroth-order resonant (ZOR) property. The antennas consist of a series of tortuous micromesh structures, which provides a high degree of freedom for stretching when encapsulated in elastomeric polymers and is optically transparent. Accordingly, these antennas can be stretched up to 40% in size without breaking. The resonant frequency of the antennas is linearly reconfigurable from 2.94 GHz to 2.46 GHz upon stretching. Next, we describe an ultra-low profile and flexible triple-polarization antenna. It is realized by using ZOR array antenna with high port-to-port isolation. This flexible antenna is fabricated with a flexible substrate and silver nanowire vias to be used in various wearable applications. Lastly, we demonstrate a dual-band tri-polarized antenna based on half-mode hexagonal (HMH) SIW structure. CRLH HMHSIW antenna and ZOR HMHSIW antenna are designed to have dual-band operating frequencies. This novel antenna can provide much improved wireless communication efficiency for the WBAN system under various incident field angles and polarizations.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147562/1/tjang_1.pd

Design And Practical Implementation Of Harmonic-Transponder Sensors

Harmonic radar is a nonlinear detection technology that transmits and receives radio-frequency (RF) signals at orthogonal frequencies, so as to suppress the undesired clutters, echoes and electromagnetic interreferences due to multipath scattering. Its implementation generally comprises a nonlinear tag (i.e, a harmonic transponder), which picks the interrogation signal at specific fundamental frequency (f0) and converts it into a high/sub-harmonic signal (nf0). Such a technology has been successfully applied to tracking small insects and detection of electrically-small objects in the rich-scattering environment. Similarly, a harmonic sensor is used to interrogate electrically-small and passive sensors, of which the magnitude and peak frequency of output harmonics (e.g., second harmonic) are functions of the parameter to be sensed. A harmonic tag or sensor comprises one or multiple antennas, a frequency modulator, a sensor, a microchip and matching networks. Here, we propose and experimentally validate compact, low-cost, low-profile, and conformal hybrid-fed microstrip antennas for the harmonics-based radar and sensor systems. The proposed 98 microstrip antennas are based on a simple single-layered and hybrid-feed structure. By optimizing the feed position and the geometry of microstrip patch, the fundamental mode and particular higher-order modes can be excited at the fundamental frequency and the second harmonic. We have derived the analytical expressions for calculating the antennas’ resonant frequencies, which have been verified with numerical simulations and measurements. Our results show that the proposed hybrid-feed, single-layered microstrip antennas, although having a compact size and a low profile, can achieve descent realized gain (1.2 – 3.5 dB), good impedance matching (return loss \u3c -15 dB), high isolation (\u3c-20 dB), and favorable co/cross-polarization properties. The proposed microstrip antennas may benefit various size-restricted harmonic transponders used for harmonic radars, harmonic sensors, medical implants, passive radio-frequency identification (RFID), and internet-of-things (IoT) applications

Wireless sensor system for infrastructure health monitoring

In this thesis, radio frequency identification (RFID)-based wireless sensor system for infrastructure health monitoring (IHM) is designed and developed. It includes mountable semi-passive tag antenna integrated sensors capable of measuring critical responses of infrastructure such as dynamic acceleration and strain. Furthermore, the system is capable of measuring structural displacement. One of the most important parts of this system is the relatively small, tunable, construction material mountable RFID tag antenna. The tag antenna is electronically integrated with the sensors. Leading to the process of developing tag antenna integrated sensors having satisfactory wireless performance (sensitivity and read range) when mounted on concrete and metal structural members, the electromagnetic performance of the tag antenna is analyzed and optimized using both numerical and experimental procedures. Subsequently, it is shown that both the simulation and the experimental measurement results are in good agreement. The semi-passive RFID-based system is implemented in a wireless IHM system with multiple sensor points to measure dynamic acceleration and strain. The developed system can determine the natural frequencies of infrastructure and identify any state changes of infrastructure by measuring natural frequency shifts. Enhancement of the spectral bandwidth of the system has been performed under the constraints of the RFID hardware. The influence of the orientation and shape of the structural members on wireless power flow in the vicinity of those members is also investigated with the RFID reader-tag antenna system in both simulation and experiments. The antenna system simulations with a full-scale structural member have shown that both the orientation and the shape of the structural member influence the wireless power flow towards and in the vicinity of the member, respectively. The measurement results of the conducted laboratory experiments using the RFID antenna system in passive mode have shown good agreement with simulation results. Furthermore, the system’s ability to measure structural displacement is also investigated by conducting phase angle of arrival measurements. It is shown that the system in its passive mode is capable of measuring small structural displacements within a short wireless distance. The benchmarking of the developed system with independent, commercial, wired and wireless measurement systems has confirmed the ability of the RFID-based system to measure dynamic acceleration and strain. Furthermore, it has confirmed the system’s ability to determine the natural frequency of an infrastructure accurately. Therefore, the developed system with wireless sensors that do not consume battery power in data transmission and with the capability of dynamic response measurement is highly applicable in IHM

Design of a Chipless RFID Tag to Monitor the Performance of Organic Coatings on Architectural Cladding

Coating degradation is a critical issue when steel surfaces are subject to weathering. This paper presents a chipless, passive antenna tag, which can be applied onto organically coated steel. Simulations indicated that changes associated with organic coating degradation, such as the formation of defects and electrolyte uptake, produced changes in the backscattered radar cross section tag response. This may be used to determine the condition of the organic coating. Simulating multiple aging effects simultaneously produced a linear reduction in tag resonant frequency, suggesting coating monitoring and lifetime estimation may be possible via this method. For coatings thinner than calculations would suggest to be optimum, it was found that the simulated response could be improved by the use of a thin substrate between the coated sample and the antenna without vastly affecting results. Experimental results showed that changes to the dielectric properties of the coating through both the uptake of water and chemical degradation were detected through changes in the resonant frequency

The effect of bending on laser-cut electro-textile inductors and capacitors attached on denim as wearable structures

In this paper we present the design, fabrication and characterization of electro-textile inductor and capacitor patterns on denim fabric as a basis for the development of wearable e-textiles. Planar coil inductors have been harnessed as antenna structures for the development of Near Field Communication (NFC) tags with temperature sensing capability, while interdigitated electrode (IDE) capacitors have been used as humidity sensors for wearable applications. The effect of bending in the electrical performance of such structures was evaluated, showing variations below 5% in both inductance and capacitance values for bending angles in the range of interest, i.e. those fitting to human limbs. In the case of the fabricated NFC tags, a shift in the resonance frequency below 1.7% was found, meaning that the e-textile tag would still be readable by an NFC- enabled smartphone. In respect of the capacitive humidity sensor, we obtained a minimum capacitance variation of 40% for a relative humidity range from 10% to 90%. Measured thermal shift was below 5% in the range from 10 to 40oC. When compared to the 4% variation due to bending, it can be concluded that this capacitive structure can be harnessed as humidity sensor even under bending strain conditions and moderate temperature variations. The development and characterization of such structures on denim fabrics, which is one of the most popular fabrics for everyday clothing, combined with the additional advantage of affordable and easy fabrication methodologies, means a further step towards the next generation of smart e-textile products