As Internet of Things (IoT) technology continues to become more commonplace, demand for self-sustainable and low-power networking schemes has increased. Future IoT devices will require a ubiquitous energy source and will need to be capable of low power communication. RF energy can be harvested through ambient or dedicated RF sources to satisfy this energy demand. In addition, these RF signals can be modified to convey information. This thesis surveys a variety of RF energy harvesting methods. A new low complexity energy harvesting system (circuit and antenna) is proposed. Low power communication schemes are examined, and low complexity and efficient transmitter designs are developed that utilize RF backscattering, harmonics, and intermodulation products. These communication schemes operate with minimal power consumption and can be powered solely from harvested RF energy. The RF energy harvester and RF-powered transmitters designs are validated through simulation, prototyping, and measurements. The results are compared to the performance of state-of-the-art devices described in the literature. / Master of Science / Future devices are expected to feature high levels of interconnectivity and have long lifetimes. RF energy from dedicated power beacons or ambient sources, such as Wi-Fi, cellular, DTV, or radio stations can be used to power these devices allowing them to be battery-less. These devices that harvest the RF energy can use that energy to transmit information. This thesis develops various methods to harvest RF energy and use this energy to transmit information as efficiently as possible. The designs are verified through simulation and experimental results.

International Telemetering Conference Proceedings / October 25-28, 1999 / Riviera Hotel and Convention Center, Las Vegas, Nevada / Comparison of short-range telemetry and radio frequency identification (RFID) systems reveals that they are based on very similar operating principles. Combining the identification and measurement functions into one transponder sensor offers added value for both RFID and telemetry systems. The presence of a memory (e.g. FRAM) in the transponder, required for ID information, can also be utilized for storing measurement results. For passive transponders low power consumption is one of the main objectives. Wireless power transfer for passive transponder sensors together with above aspects concerning a combined telemetry and identification system are discussed.

The emergence of microwave energy harvesting systems, commonly referred to as rectenna or Wireless Power Transfer (WPT) systems, has enabled numerous applications in many areas since their primary goal is to recycle the ambient microwave energy. In such systems, microstrip antennas are used as the main source for collecting the electromagnetic energy. In this work, a novel collector based on metamaterial particles, in what is known as a Split Ring Resonator (SRR), to harvest electromagnetic energy is presented. Such collectors are much smaller in size and more efficient than existing collectors (antennas). A feasibility study of SRRs to harvest electromagnetic energy is conducted using a full wave simulator (HFSS). To prove the concept, a 5.8 GHz SRR is designed and fabricated and then tested using a power source, an Infiniium oscilloscope and a commercially available patch antenna array. When excited by a plane wave with an H-field normal to the structure, a voltage build up of 611 mV is measured across a surface mount resistive load inserted in the gap of a single loop SRR. In addition, a new efficiency concept is introduced, taking into account the microwave-to-AC conversion efficiency which is missing from earlier work. Finally, a 9X9 SRR array is compared with a 2X2 patch antenna array, both placed in a fixed footprint. The simulation results show that the array of SRRs can harvest electromagnetic energy more efficiently and over a wider bandwidth range.

Simulation, optimization, and implementation of a capacitor-loaded wireless power transfer structure at 6.78 MHz for a target transfer distance of one meter are presented. First, an investigation into the operating principles behind a capacitor-loaded coupled loop structure is carried out via simulation. By adjusting the structural design parameters, it is found that an optimal configuration for this structure is coplanar. A prototype constructed using thin 18 AWG wire for the loops and a variable capacitor for tuning is used to verify simulation. To reduce losses in the wire, thick 9 AWG wire is implemented and measured. Thick wire is necessary for high efficiency yet undesirable for planarization. Since current flows only on the surface of the wire, ‘unwrapping’ that portion yields copper strips that reduce loss by increasing only the width. Thus, by replacing thick wires with copper strips, a planarized structure can be obtained that can reduce ohmic losses without sacrificing its form factor. Next, additional advantages of a capacitor-loaded system, which include reduced electric near-field and the possibility of resonant frequency tuning, are investigated. It is shown by simulation that the capacitor-loaded structure is not strongly affected by nearby dielectric materials since the stored electric energy is significantly lower than the stored magnetic energy in air at resonance. Finally, further optimizations of the structure are considered along with the analytical expressions for maximum efficiency. / text

The emergence of microwave energy harvesting systems, commonly referred to as rectenna or Wireless Power Transfer (WPT) systems, has enabled numerous applications in many areas since their primary goal is to recycle the ambient microwave energy. In such systems, microstrip antennas are used as the main source for collecting the electromagnetic energy. In this work, a novel collector based on metamaterial particles, in what is known as a Split Ring Resonator (SRR), to harvest electromagnetic energy is presented. Such collectors are much smaller in size and more efficient than existing collectors (antennas). A feasibility study of SRRs to harvest electromagnetic energy is conducted using a full wave simulator (HFSS). To prove the concept, a 5.8 GHz SRR is designed and fabricated and then tested using a power source, an Infiniium oscilloscope and a commercially available patch antenna array. When excited by a plane wave with an H-field normal to the structure, a voltage build up of 611 mV is measured across a surface mount resistive load inserted in the gap of a single loop SRR. In addition, a new efficiency concept is introduced, taking into account the microwave-to-AC conversion efficiency which is missing from earlier work. Finally, a 9X9 SRR array is compared with a 2X2 patch antenna array, both placed in a fixed footprint. The simulation results show that the array of SRRs can harvest electromagnetic energy more efficiently and over a wider bandwidth range.

Wireless power transfer is becoming relevant today because of its effectiveness and convenience. It has been employed into consumer electronics such as cellular charging and electric vehicle charging. In general, inductive wireless power transfer (IPT) is mostly used for WPT. IPT requires coils and power transfer enhancing material such as ferrite to transfer power. However, Capacitive wireless Power Transfer (CPT) appears as an alternative because it requires cost effective and light metal plate couplers. Among CPT couplers, Vertical (stacked) Four-Plate Coupler (V4PC) structure offers the advantage of higher input and output self-capacitances, rotational misalignment. Safety is one of the most important aspect of wireless power transfer. This thesis proposes a solution to minimize the leakage electric field of Vertical 4-Plate Couplers (V4PCs). It does so by finding the optimum value of circuit parameters. The effectiveness of the proposed solution is shown by experimental results.

Wireless charging of the EVs offers a convenient, reliable, and automatic charging of the autonomous vehicles without user interference. The focus of this thesis is the design and optimization of new structures for stationary EV wireless charging applications. The fundamentals of the Wireless Power Transfer (WPT) system and its main components including the magnetic couplers, transmitter and receiver power converters, and control methods are studied in depth. The requirements of the EV wireless charging application and design criteria are discussed in detail. The advantages and disadvantages of each topology are highlighted, and possible candidates for EV wireless charging applications are selected. Optimization of the resonant networks in terms of maximum efficiency and misalignment tolerance is studied. Different resonant topologies are studied in detail and their sensitivity functions are extracted. For each topology, an efficiency model is presented that includes the inverter, resonant capacitors, resonant inductor, diode-bridge, and core and conduction losses. Each topology is optimized with two different objective functions and the results are compared through the simulation and experiments. According to the optimization results, suitable topologies for the EV wireless charging application are selected. In order to increase the power density of the wireless charging system, and save ferrite material, integrated inductors into the magnetic couplers are proposed. In this structure, the DC-DC inductor is integrated into the receiver main coil and the resonant inductor is integrated into the transmitter coil. This integration introduces new challenges to the design of the resonant network and magnetic coupler due to the unwanted cross-coupling effect. To address this issue, the fully integrated magnetic structure is optimally designed to have minimum cross-coupling. Moreover, the resonant network is designed based on an optimization problem that includes the cross-coupling into the system equations to ensure maximum efficiency. The proposed fully-integrated magnetic structure is built and experimental tests are presented to validate the performance of the proposed magnetic structure and its optimization method. To reduce the implementation cost, size and weight a PCB-based magnetic coupler is proposed to replace the Litz wire in the magnetic coupler of the WPT system. Moreover, the proposed PCB-magnetic coupler increases the repeatability of the design and reduces manufacturing errors. The PCB-based magnetic coupler is studied through Finite Element Analysis (FEA) to minimize the AC resistance of the coil. Different parameters such including the number of the PCB layers, copper cross-section, and layer thickness are studied in detail to evaluate their effect on the coil resistance. Thermal analysis is performed to ensure the feasibility of the design under different loading conditions. A 3.3 kW/85 kHz wireless charging system is built and experimental tests are presented. A novel modular resonant topology for fast wireless charging is proposed. A modular structure offers reliability, scalability, and better thermal management. The proposed topology is made by multi-parallel inverter legs connected to an LCC resonant network. The outputs of the resonant networks are connected in parallel to feed the transmitter coil with a high excitation current. The proposed modular system is compared with a conventional system and it showed superior performance in different aspects. / Thesis / Doctor of Philosophy (PhD)

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24232号 / 工博第5060号 / 新制||工||1790(附属図書館) / 京都大学大学院工学研究科電気工学専攻 / (主査)教授 篠原 真毅, 教授 小嶋 浩嗣, 教授 山本 衛 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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Due to the loose coupling nature and separated primary/secondary side, achieving tight load regulation or implementing closed-loop control of output voltage/current is nontrivial in a wireless power transfer (WPT) system. Previously presented methods for regulating or controlling the output of a WPT system include incorporating either post-regulator stage, wireless communication from secondary to primary side, primary side sensing and modulation scheme, or dual active bridge type of topology. However, all existing methods have limitations and disadvantages in terms of increased size/cost, control complexity, or reliability in electrically noisy environments. This dissertation proposes a single switch control and regulation mechanism based on the secondary side of the WPT system. Specifically, the duty cycle of the secondary side synchronous rectifier (SR) switch is modulated to control the output voltage or current. By modulating the SR duty cycle, output of the WPT system can be controlled without requiring additional regulator stages/power devices, a primary side sensing mechanism, or secondary to primary communication. The proposed control method lowers cost and simplifies the design of WPT systems while improving reliability in noisy environments. The proposed control and modulation mechanism maintains zero voltage switching of all power semiconductor switches so efficiency of the WPT system would not be compromised by implementing the proposed control scheme. The proposed secondary side SR based control method can be applied to dc-dc WPT systems to control output voltage or current, or it could be used in a dc-ac WPT system to generate and regulate ac output if combined with an unfolding stage. When used in dc-ac WPT systems, the bulky output filter stage usually required in conventional dc-ac inverters is eliminated. The proposed control scheme is evaluated with computer simulation as well as hardware implementation and testing. / Doctor of Philosophy / Wireless power transfer (WPT) is an emerging technology that supplies electric power to loads without using wires or electrical contacts. WPT technology has many promising uses in consumer, industrial, transportation, biomedical, and other applications. However, unlike controlling the output voltage of a conventional power supply or power converter, controlling the output of a WPT system is not a simple task due to the physical separation between the transmitting and receiving sides. State-of-the-art methods for controlling the output of a WPT system include adding another power regulator stage to regulate output, incorporating secondary side (power receiver) to primary side (power transmitter) communication so that output information can be passed back to the primary side where that information is used to monitor and regulate output. In some systems, output information may also be estimated indirectly from primary side voltage/current information. However, all these methods have significant disadvantages. Adding another power converter stage increases cost and efficiency loss of the WPT system. Incorporating secondary to primary communication for output control is detrimental to the reliability of the PWT system because communication may be impacted by external noise. The reliability of primary side sensing and regulation is also severely impacted by component parameter variations in the WPT system. This dissertation proposes a new mechanism that controls output of a WPT system at the receiver or secondary side without needing another power conversion stage, communication or any cooperation from primary side. The proposed control mechanism controls the turn on duration of the synchronous rectifier (SR) switch at the receiver side to modulate output voltage or current. Since SR technology is already prevalently used in power electronics systems, including WPT systems, to efficiently convert high frequency ac to dc before delivering power to the load, implementing the proposed control mechanism does not increase complexity or cost of the WPT system. The proposed control mechanism is useful in both dc-dc and dc-ac WPT systems. In a dc-dc WPT system, the proposed mechanism can control or regulate output voltage or current independently from the primary side, while in a dc-ac WPT system the proposed mechanism can generate and regulate ac output. If used in a dc-ac WPT system an unfolding stage needs to be added, but the bulky output filter stage required in conventional pulse width modulation (PWM) dc-ac inverters for suppressing switching ripple is not needed. The proposed mechanism is verified with computer simulation as well as hardware prototyping in this dissertation.