Abstract
RF energy harvesting (REH) has emerged as a promising technology for battery-less healthcare and infrastructure monitoring systems, offering significant advantages over traditional battery-powered and wired alternatives. While extensive research has been conducted on REH in free space, there has been limited exploration into fully REH-powered sensor systems in specific environments. Developing an efficient REH-powered system for healthcare and infrastructure monitoring presents several challenges. First, the performance of the REH is influenced by varying environmental parameters. Second, implantable REH-powered wireless sensors require flexible, miniaturized, and encapsulated designs. Third, the design of the power management circuit and embedded wireless sensor affect the overall efficiency.In this thesis, I systematically quantify the influence of environmental conditions on REH performance through numerical simulation and experimental tests. I develop optimal designs for rectifiers and antennas to extend the working distance and enhance efficiency. Additionally, I design and characterize low-power wireless sensors to minimize energy consumption while ensuring accurate data measurement. Furthermore, I study the performance of the REH-powered passive wireless sensors at the system level in practical scenarios. This thesis presents three works on REH-powered wireless sensing systems.
In the first work, I developed a battery-less wireless sensor tag (BLWST) for railway condition monitoring based on REH. The BLWST eliminates cables and the need for battery replacement, offering an efficient monitoring solution. I conducted a comprehensive study on the structure of the rectifier. Through this study, I determined the optimal configuration of rectifier stages, ensuring efficient energy harvesting. To further enhance power efficiency, I designed a low-power embedded wireless sensor and developed an efficient program, minimizing power consumption while maintaining reliable data acquisition. The BLWST can be powered by a dedicated RF source at a maximum distance of 2.3 m. Experimental results demonstrated a maximum energy conversion efficiency of 25% and the ability to acquire 500 data packets per second at maximum.
In the second work, I present the design and implementation of a battery-free wireless torque sensor to provide a solution to maintenance-free dynamic torque monitoring. Dynamic torque monitoring is crucial for real-time assessment of rotational forces, ensuring optimal performance and preventing mechanical failures in various industrial and mechanical systems. I design a Koch-Meander dipole antenna with a miniaturization rate of 42.8% for metal-rich environments. Investigations were conducted on the influence of the surrounding metal object on the REH performance, employing numerical simulations and experiments. Various impedance matching strategies were developed and compared to attain the optimal design within the limited space. A sensitive rectifier is designed, allowing successful torque measurement with an input power of -15 dBm (32 µW). The measured operational distance of the sensor in open air reaches 3 m. The REH performance was studied under dynamic conditions, with a particular focus on the impact of rotation speed on the sample rate. Additionally, a successful field test was conducted on a running car, marking the first implementation of REH-powered wireless torque monitoring in an automotive setting.
In the third work, I developed a battery-free wireless orthodontic force sensing system. Maintaining orthodontic forces within an optimal range is crucial to mitigate serious risks like tissue injury. Quantifying in vivo orthodontic force not only signals when treatment adjustments are needed but also acts preventively, reducing potential risks and enhancing patient safety during orthodontic procedures. To harvest the RF energy in the oral cavity, I design a flexible and implantable rectenna and conducted numerical and experimental analyses to study the in vivo characteristics. The simulated specific absorption rate is below the safety standard. An experiment in the oral cavity of a volunteer shows RF energy transfer efficiency at 1.5% for a 5 cm distance. The optimized rectenna can harvest 2 V voltage at 35 cm. I depicted an energy flow diagram to investigate the efficiency and energy loss throughout the entire REH path. The fabricated prototype demonstrated a wireless measurement distance of up to 220 mm. The developed system can perform orthodontic force monitoring in vivo anytime and anywhere, improving clinical treatments and advancing orthodontic research.
In summary, this thesis presents three works on REH-powered wireless sensing systems. The main contribution includes: 1) I designed REH systems for condition monitoring. While extensive research has been conducted on REH in free space, there is limited exploration of fully REH-powered sensor systems in specific environments. I address this research gap by studying the influence of the environment on REH and optimizing the components of the REH system for dynamic condition, metal proximity environments and in vivo conditions. 2) In addition to designing and evaluating the rectenna, I also develop and evaluate the power management circuit and embedded wireless sensors. I thoroughly study and validate the design and performance of the REH-powered wireless sensing systems, demonstrating the reliability of the designed systems. 3) The studies in this thesis contribute to the practical application and advancement of REH technology while broadening the understanding of REH.
| Date of Award | 11 Sept 2024 |
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| Original language | English |
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| Supervisor | Ji-jung KAI (Supervisor) & Zhengbao YANG (External Co-Supervisor) |