Microwave antennas for infrastructure health monitoring

Infrastructure health monitoring (IHM) is a technology that has been developed for the detection and evaluation of changes that affect the performance of built infrastructure systems such as bridges and buildings. One of the employed methods for IHM is wireless sensors method which is based on sensors embedded in concrete or mounted on surface of structure during or after the construction to collect and report valuable monitoring data such as temperature, displacement, pressure, strain and moisture content, and information about defects such as cracks, voids, honeycombs, impact damages and delamination. The data and information can then be used to access the health of a structure during and/or after construction. Wireless embedded sensor technique is also a promising solution for decreasing the high installation and maintenance cost of the conventional wire based monitoring systems. However, several issues should be resolved at research and development stage in order to apply them widely in practice. One of these issues is that wireless sensors cannot operate for a long time due to limited lifetime of batteries. Once the sensors are embedded within a structure, they may not be easily accessible physically without damaging the structure. The main aim of this research is to develop effective antennas for IHM applications such as detection of defects such as gaps representing cracks and delaminations, and wireless powering of embeddable sensors or recharging their batteries. For this purpose, modelling of antennas based on conventional antipodal Vivaldi antennas (CAVA) and parametric studies are performed using a computational tool CST Studio (Studio 2015) including CST Microwave Studio and CST Design Studio, and experimental measurements are conducted using a performance network analyser. Firstly, modified antipodal Vivaldi antenna (MAVA) at frequency range of 0.65 GHz – 6 GHz is designed and applied for numerical and experimental investigations of the reflection and transmission properties of concrete-based samples possessing air gap or rebars. The results of gap detection demonstrate ability of the developed MAVA for detection of air gaps and delivery of power to embeddable antennas and sensors placed at any depth inside 350-mm thick concrete samples. The investigation into the influence of rebars show that the rebar cell can act as a shield for microwaves if mesh period parameter is less than the electrical half wavelength. At higher frequencies of the frequency range, microwaves can penetrate through the reinforced concrete samples. These results are used for the investigating the transmission of microwaves at the single frequency of 2.45 GHz between the MAVA and a microstrip patch antenna embedded inside reinforced concrete samples at the location of the rebar cell. It is shown that -15 dB coupling between the antennas can be achieved for the samples with rebar cell parameters used in practice. Secondly, a relatively small and high-gain resonant antipodal Vivaldi antenna (RAVA) as a transmitting antenna and modified microstrip patch antenna as an embeddable receiving antenna are designed to operate at 2.45 GHz for powering the sensors or recharging their batteries embedded in reinforced concrete members. These members included reinforced dry and saturated concrete slabs and columns with different values of mesh period of rebars and steel ratio, respectively. Parametric study on the most critical parameters, which affect electromagnetic (EM) wave propagation in these members, is performed. It is shown that there is a critical value of mesh period of rebars with respect to reflection and transmission properties of the slabs, which is related to a half wavelength in concrete. The maximum coupling between antennas can be achieved at this value. The investigation into reinforced concrete columns demonstrates that polarisation configuration of the two-antenna setup with respect to rebars and steel ratios as well as losses in concrete are important parameters. It is observed that the coupling between the antennas reduces faster by increasing the value of steel ratio in parallel than in vertical configuration due to the increase of the interaction between electromagnetic waves and the rebars. This effect is more pronounced in the saturated than in dry reinforced concrete columns. Finally, a relatively high gain 4-element RAVA array with a Wilkinson power divider, feeding network and an embeddable rectenna consisting of the microstrip patch antenna and a rectified circuit are developed. Two wireless power transmission systems, one with a single RAVA and another with the RAVA array, are designed for recharging batteries of sensors embedded inside reinforced concrete slabs and columns with different configurations and moisture content. Comparison between these systems shows that the DC output voltage for recharging commonly used batteries can be provided by the systems with the single RAVA and the system with the RAVA array at the distance between the transmitting antenna and the surface of reinforced concrete members of 0.12 m and 0.6 m, respectively, i.e. the distance achieved when the array is 5 times longer that the distance achieved with a single antenna

Design and optimization of a rectangular microstrip patch antenna for dual-band 2.45 GHz/ 5.8 GHz RFID application

This paper introduces a new rectangular slot antenna structure based on a simple rectangular shape with two symmetrical rectangular slots on the radiated element. The aim of this work is to design an antenna and enhance it to function in the band (2.45 GHz and 5.8 GHz). We formulated the dimensions of the antenna using the transmission line model of the analytical methods and then we optimized these parameters using the CST Microwave Studio simulator. We made changes to two important parameters in our design: the position and width of the slots when the other parameters are kept constant. The resulting antenna provides good adaptation, high gain that achieves 5.96 dBi at 2.45 GHz and 6.491 dBi at 5.8 GHz, good return loss values of -49.859 dB and -34.303 dB for the lower and upper operating frequencies respectively. For radio frequency identification (RFID) implementations, the proposed antenna is ideal, and its main advantage is that it has high gain and is simple to design and fabricate

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

Wearable Antennas for Remote Health Care Monitoring Systems

Remote monitoring of the elderly in telehealth applications requires that the monitoring must not affect the elderly's regular habits. To ensure this requirement, the components (i.e., sensor and antenna) necessary to carry out such monitoring should blend in with the elderly's daily routine. To this end, an effective strategy relies on employing wearable antennas that can be fully integrated with clothes and that can be used for remotely transmitting/receiving the sensor data. Starting from these considerations, in this work, two different methods for wearable antenna fabrication are described in detail: The first resorts to the combined use of nonwoven conductive fabrics and of a cutting plotter for shaping the fabric, whereas the second considered fabrication method resorts to the embroidery of conductive threads. To demonstrate the suitability of the considered fabrication techniques and to highlight their pros and cons, numerical and experimental results related to different wearable antennas are also reported and commented on. Results demonstrate that the presented fabrication techniques and strategies are very flexible and can be used to obtain low-cost wearable antennas with performance tailored for the specific application at hand

Chipless RFID sensor systems for structural health monitoring

Ph. D. ThesisDefects in metallic structures such as crack and corrosion are major sources of catastrophic failures, and thus monitoring them is a crucial issue. As periodic inspection using the nondestructive testing and evaluation (NDT&E) techniques is slow, costly, limited in range, and cumbersome, novel methods for in-situ structural health monitoring (SHM) are required. Chipless radio frequency identification (RFID) is an emerging and attractive technology to implement the internet of things (IoT) based SHM. Chipless RFID sensors are not only wireless, passive, and low-cost as the chipped RFID counterpart, but also printable, durable, and allow for multi-parameter sensing. This thesis proposes the design and development of chipless RFID sensor systems for SHM, particularly for defect detection and characterization in metallic structures. Through simulation studies and experimental validations, novel metal-mountable chipless RFID sensors are demonstrated with different reader configurations and methods for feature extraction, selection, and fusion. The first contribution of this thesis is the design of a chipless RFID sensor for crack detection and characterization based on the circular microstrip patch antenna (CMPA). The sensor provides a 4-bit ID and a capability of indicating crack width and orientation simultaneously using the resonance frequency shift. The second contribution is a chipless RFID sensor designed based on the frequency selective surface (FSS) and feature fusion for corrosion characterization. The FSS-based sensor generates multiple resonance frequency features that can reveal corrosion progression, while feature fusion is applied to enhance the sensitivity and reliability of the sensor. The third contribution deals with robust detection and characterization of crack and corrosion in a realistic environment using a portable reader. A multi-resonance chipless RFID sensor is proposed along with the implementation of a portable reader using an ultra-wideband (UWB) radar module. Feature extraction and selection using principal component analysis (PCA) is employed for multi-parameter evaluation. Overall, chipless RFID sensors are small, low-profile, and can be used to quantify and characterize surface crack and corrosion undercoating. Furthermore, the multi-resonance characteristics of chipless RFID sensors are useful for integrating ID encoding and sensing functionalities, enhancing the sensor performance, as well as for performing multi-parameter analysis of defects. The demonstrated system using a portable reader shows the capability of defects characterization from a 15-cm distance. Hence, chipless RFID sensor systems have great potential to be an alternative sensing method for in-situ SHM.Indonesia Endowment Fund for Education (LPDP

Dielectric resonator antennas for wireless powering of sensors embedded in civil structures

Wireless power transmission (WPT) is currently being used in a number of applications, including monitoring the health of concrete infrastructure such as bridges, buildings, tunnels, and dams, where antennas of different types, geometries and sizes have been designed for powering sensors that are embedded in concrete structures. However, dielectric resonator antennas (DRAs), which have shown great potential in free-space for microwave WPT applications, have not been investigated in concrete. In this thesis, different DRAs are designed and their performance in the X-band frequency range for the WPT in concrete is investigated using an electromagnetic computational tool: CST Microwave Studio (MWS). In addition, selected DRAs are manufactured and measurement results are compared to simulation results. Furthermore, CST MWS is utilised to design and investigate surface-mountable electromagnetic sensors that can be used for concrete characterisation, crack location and crack width estimation. Firstly, rectangular DRAs of different lengths operating in the X-band are designed, and a two-antenna setup with an external transmitting antenna and an embedded receiving antenna is used to investigate WPT to embedded sensors in concrete. It is found that, the designed DRAs can be used for WPT to the embedded sensors in concrete. However, it is also found that some of the electromagnetic energy from the external transmitting antenna radiates away from the embedded antenna. Short and long DRAs are fabricated and measured in free-space and a good agreement with simulation results is observed. The short DRA has better WPT performance than long DRA over the entire X-band. To achieve a reliable WPT system, the sensitivity of the WPT to variations in the electrical (loss tangent tan δ and the relative dielectric constant) of concrete is investigated. It is found that WPT ii using the long and short DRAs appear to be more sensitive to the electrical properties of concrete at frequencies that are closer to the lower and upper frequency limits, respectively. Secondly, gratings techniques are used to redirect the radiated electromagnetic energy from the transmitting antenna towards the receiving antenna. For this purpose, rectangular-, hexagon-, and octagon-shaped DRAs with metal loaded dielectric gratings are designed, investigated, and optimised to maximise microwave WPT to embedded sensors in concrete. These antennas are suitable for wireless powering of multiple sensors, which is illustrated by changing the positioning of the embedded/receiving antenna with respect to the transmitting antenna. Furthermore, the sensitivity of WPT using these antennas to variations in the electrical properties of concrete is investigated and the simulation results are recorded. The obtained results show that reliable WPT can be obtained with the octagon-shaped DRA because it is less sensitive to variations in the electrical properties of concrete. Finally, surface-mountable electromagnetic sensors using dual-port rectangular DRA, hexagon-, and octagon-shaped planar DRAs are designed and investigated for characterisation of concrete and crack detection. It is found that the surface-mountable electromagnetic sensor using the rectangular DRA has the highest sensitivity to variations of the electrical properties of concrete and can be used to approximate the loss tangent and the relative dielectric constant of concrete, whereas hexagon-shaped planar DRA is highly sensitive to the crack width at different locations, and can be used to estimate crack width and position remotely

RFID multiantenna systems for wireless communications and sensing

Many scientific, industrial and medical applications require the measurement of different physical parameters in order to collect information about the spatially distributed status of some process. Very often this information needs to be collected remotely, either due to the spatial dispersion of the measurement points or due to their inaccessibility. A wireless embedded self-powered sensor may be a convenient solution to be placed at these inaccessible locations. This thesis is devoted to study the analytical relation governing the electromagnetic coupling between a reader and a embeddable self-powered sensor, based on radio frequency identification (RFID) technology, which is capable of wirelessly retrieving the status of physical parameters at a remote and inaccessible location. The physical parameter to be sensed may be the electromagnetic (EM) field existing at that location (primary measurement) or the indirect measurement of other parameters such as the temperature, humidity, etc. (secondary measurement). Given the simplicity of the RFID solution (highly embeddable properties, scavenging capabilities, penetration and radio coverage characteristics, etc.) the measurement can be done at a single location, or it can be extended to a set of measuring locations (an array or grid of sensors). The analytical relation is based on a reciprocity formulation studying the modulation of the scattered field by the embedded sensor in relation with the incident field, and allows to define a set of quality parameters of interest for the optimum design of the sensors. Particular attention is given to the scavenging circuitry as well as to the antenna design relevant to the sensing objective. In RFID tags, the existence of an RF harvesting section is an improvement with respect to conventional scattering field probes since it removes the need of DC biasing lines or optical fibers to modulate the sensor. However, this harvesting section introduces non-linearities in the response of the sensor, which requires a proper correction to use them as EM-field probes, although the characterization of the non-linearities of the RFID tag cannot be directly done using a conventional vector network analyzer (VNA), due to the requirements of an RFID protocol excitation. Due to this, this thesis proposes an alternative measurement approach that allows to characterize the different scattering states used for the modulation, in particular its non-linear behavior. In addittion, and taking this characterization as the starting point, this thesis proposes a new measurement setup for EM-field measurements based on the use of multiple tones to enlarge the available dynamic range, which is experimentally demonstrated in the measurement of a radiation pattern, as well as in imaging applications. The RFID-based sensor response is electromagnetically sensitive to the dielectric properties of its close environment. However, the governing formulation for the response of the probe mixes together a set of different contributions, the path-loss, the antenna impedance, the loads impedance, etc. As a consequence, it is not possible to isolate each contribution from the others using the information available with a conventional RFID sensor. This thesis mathematically proposes and experimentally develops a modification of the modulation scheme to introduce a new set of multi-load scattering states that increases the information available in the response and properly isolate each term. Moreover, this thesis goes a step forward and introduces a new scattering state of the probe sensitive to temperature variations that do not depend on the environment characteristics. This new configuration enables robust environmental sensing in addition to EM-field measurements, and sensing variations of the dielectric properties of the environment

Wireless Sensors and Actuators for Structural Health Monitoring of Fiber Composite Materials

This work evaluates and investigates the wireless generation and detection of Lamb-waves on fiber-reinforced materials using surface applied or embedded piezo elements. The general target is to achieve wireless systems or sensor networks for Structural Health Monitoring (SHM), a type of Non-Destructive-Evaluation (NDE). In this sense, a fully wireless measurement system that achieves power transmission implementing inductive coils is reported. This system allows a reduction of total system weight as well as better integration in the structure. A great concern is the characteristics of the material, in which the system is integrated, because the properties can have a direct impact on the strength of the magnetic field. Carbon-Fiber-Reinforced-Polymer (CFRP) is known to behave as an electrical conductor, shielding radio waves with increasing worse effects at higher frequencies. Due to the need of high power and voltage, interest is raised to evaluate the operation of piezo as actuators at the lower frequency ranges. To this end, actuating occurs at the International Scientific and Medical (ISM) band of 125 kHz or low-frequency (LF) range. The feasibility of such system is evaluated extensively in this work. Direct excitation, is done by combining the actuator bonded to the surface or embedded in the material with an inductive LF coil and setting the circuit in resonance. A more controlled possibility, also explored, is the use of electronics to generate a Hanning-windowed-sine to excite the PWAS in a narrow spectrum. In this case, only wireless power is transmitted to the actuator node, and this lastly implements a Piezo-driver to independently excite Lamb-waves. Sensing and data transfer, on the other hand, is done using the high-frequency (HF) 13.56 MHz. The HF range covers the requirements of faster sampling rate and lower energy content. A re-tuning of the antenna coils is performed to obtain better transmission qualities when the system is implemented in CFRP. Several quasi-isotropic (QI) CFRP plates with sensor and actuator nodes were made to measure the quality of transmission and the necessary energy to stimulate the actuator-sensor system. In order to produce baselines, measurements are prepared from a healthy plate under specific temperature and humidity conditions. The signals are evaluated to verify the functionality in the presence of defects. The measurements demonstrate that it is possible to wirelessly generate Lamb-waves while early results show the feasibility to determine the presence of structural failure. For instance, progress has been achieved detecting the presence of a failure in the form of drilled holes introduced to the structure. This work shows a complete set of experimental results of different sensor/-actuator nodes

Advanced microwave embedded sensors for infrastructure health monitoring

Microwave sensor systems have been widely investigated for many applications due to their ability to provide non-destructive, noncontact, one-sided and wireless testing. Among these applications infrastructure health monitoring of bridges, building, and dams using microwave sensors, which are mounted on or embedded in composite structures of infrastructure has been attracting an increasing interest. One of the current needs of infrastructure health monitoring includes the detection and monitoring of disbonding and gaps in concrete-based structures, which are also required for simultaneous characterization of concrete. A recently proposed microwave sensor technique exploiting a relatively simple waveguide sensor embedded in a concrete-metal structure such as a concrete-filled steel tube exhibited great potential. However, it suffers from a few drawbacks that need to be solved. This thesis aims to develop and investigate advanced microwave embedded sensors to solve main problems in the current microwave sensory technique including characterization of concrete in concrete-based structures at different stage of its life, size of the interface under inspection, detection and monitoring of a small gap between concrete and dielectric material surfaces and sensitivity to gaps. To achieve this aim the following five research contributions have been made: The first contribution is the methodology for the determination of the complex dielectric permittivity of concrete using both measurement data and simulation results at different stages (fresh, early-aged and dry) of its life. Firstly, it is developed and tested for a single flanged open-ended waveguide sensor with a hardened concrete specimen, and then the methodology is modified for the developed sensors embedded in concrete-based composite structures with fresh, early-age and dry concrete. Modern computational tool CST Microwave Studio and a performance network analyser are used for simulation and measurement, respectively, throughout this research work. The second contribution is a dual waveguide sensor, which is proposed, designed and applied for the detection and monitoring of a small gap in concrete-metal composite structures. It consists of two waveguide sections and a metal plate and uses the transmission of electromagnetic waves along gap when it occurs between the metal plate and concrete surfaces. It provides more measurement data than the single waveguide sensor for characterising concrete-metal structures such as transmission properties of guided waves along the gap and reflection properties of the metal–concrete interface at two different places at the same stage of concrete. As a result, the proposed sensor increases the size of the interface under inspection and sensitivity to the gap using the magnitude of reflection coefficient and magnitude of transmission coefficient together and/or independently. The third contribution is the design and application of a dual waveguide sensor with rectangular dielectric insertions that is proposed and tested for the characterisation of concrete–metal structures at different stages of the concrete life including its fresh stage. The dielectric insertions are designed and implanted in the waveguide sections in such a way that they create the resonant response of the sensor and prevent water and concrete entering the sections. The resonant properties of the sensor allow long-term monitoring of the concrete hydration, including the detection of the transition from fresh to hardened concrete on its first day. The proposed sensor along with the modified algorithm provides the determination of the complex dielectric permittivity of fresh concrete. The fourth contribution is a dual waveguide sensor with tapered dielectric insertions. Each tapered dielectric insertion is designed with a tapered part and rectangular part to reduce wave reflection from the insertions over an entire frequency band. The proposed sensor has improved performance at the resonant responses of a quarter-wavelength resonator formed by an open end at the tapered part and shorted end at the rectangular part of each insertion. The last contribution is the development of dual waveguide sensors with attached dielectric layer and their application for the detection and monitoring of gap between dielectric materials and concrete in metal-dielectric layer-concrete composites as well as the determination of complex dielectric permittivity of concrete at different stages of its life. One of the most attractive designs is the sensor with empty waveguide sections due to its simplicity and robustness, and capability of the layer for preventing penetration of the obstacles and water, and for optimization of the sensor. On the other hand, the sensors with dielectric insertions and the layer demonstrate a significantly higher magnitude of transmission coefficient. The proposed DWSs can be applied to characterise fresh concrete in a dielectric mould or on-line, and to investigate the shrinkage of different categories of concrete

AN INVESTIGATION OF NOVEL UHF MICROSTRIP FRACTAL PATCH ANTENNAE FOR AN RFID DOORWAY READER SYSTEM

The applications of Radio Frequency Identification (RFID) technology has expanded drastically after the arrival of data revolution and coming age of human-free industry: Industry 4.0. The focus has been reduction of installation costs by developing plug and play systems when transitioning from traditional manual-scan systems to fully automated systems with improved efficiency. The main contributor to efficiency of an RFID system is the reader antenna. Microstrip patch antennae are found to be most suitable for RFID applications. Miniaturisation of the antenna without compromising its efficiency has been one of the central concerns in the last few decades. For fixed reader RFID antennae, maintaining enough gain while having the ability to read tags moving in any orientation and speed as well as blindspots when clustered together have been major challenges in the industry. The work proposed in this thesis aims to design miniaturised novel modified fractal antennae suitable for an RFID doorway reader system, operating at UK’s RFID UHF band, 870MHz. The work proposed combines novel miniaturising and gain enhancement techniques to meet desired requirements. Fractal patterns are used to increase the electrical length of the antenna while maintaining its physical size and obtain multiband behaviour thus reading tags slightly off-tuned with the help of RFID reader’s Frequency hopping technique. Antennae are made on high dielectric constant substrates, RF60A for further miniaturisation. Several geometry techniques including copper wall construction and 90 degree delayed two port feeders are used for gain enhancement, narrow beamwidth and circular polarisation. CST Microwave Studio Suite simulations demonstrate that a commercially available directivity (5dBi) compared to market research and published research papers have been achieved. RFID testing on manufactured prototypes demonstrated that antenna designs are suitable for a fully automated doorway reader system to obtain 100% detection efficiency with precise manufacturing and fine tuning