Ultrasonic Power Transfer (UPT) has been developed as an alternative solution for achieving wireless power transfer. This paper proposes a new model describing UPT systems with tightly coupled piezoelectric transducers firmly bound to solid media. The model is derived from the short-circuit admittance of the system measured from the primary transducer. The mechanical characteristics of the system are modeled with parallel LCR branches, which reveal the fundamental relationships between the power transfer characteristics of the tightly coupled UPT system and system parameters. The loading conditions for achieving the maximum power transfer are identified, and the operating frequencies corresponding to the peak power transfer points for variable loads are determined. A practical UPT system is built with two 28 kHz Langevin-type piezoelectric transducers connected to a 5 mm-thick aluminum plate, and the practical results have verified the accuracy of the proposed model.

In light of the increased interest in e-mobility, comfortable, and safe charging systems, such as inductive charging systems, are gaining importance. Several standardization bodies develop guidelines and specifications for inductive power transfer systems in order to ensure a good interoperability between different coil architectures from the various car manufacturers, wireless power transfer suppliers, and infrastructure companies. A combination of a bipolar magnetic coil design on the primary side with a secondary solenoidal coil promises a good magnetic coupling and a high-transmitted power with small dimensions at the same time. In order to get a profound knowledge of the influence and behavior of the main variables on the coil system, a detailed parameter study is conducted in this paper. Based on these findings, a solenoid was designed for a specific case of application. Further, this design is optimized. The dimensions of the system could be reduced by 50% with a constant coupling factor at the same time. Besides the reduction of the dimensions and subsequently the costs of the systems, the stray field could be reduced significantly.

This paper deals with the numerical evaluation of the magnetic field emitted by a wireless power system (WPT) in an electric vehicle (EV). The numerical investigation is carried out using a finite element method (FEM) code with a transition boundary condition (TBC) to model conductive materials. First, the TBC has been validated by comparison with the exact solution in simple computational domains with conductive panels at frequencies used in WPT automotive. Then, the FEM with TBC has been used to predict the field in an electric car assuming the chassis made by three different materials: steel, aluminum, and fiber composite. The magnetic field source is given by a WPT system with 7.7 kW power level operating at frequencies of 85 or 150 kHz. The calculated magnetic field has been compared with the International Commission on Non-Ionizing Radiation Protection (ICNIRP) reference level demonstrating compliance for an EV with metallic (steel or aluminum) chassis. On the contrary, a fiber composite chassis is much more penetrable by magnetic fields and the reference level is exceeded.

The performance of wireless power transfer (WPT) systems is a function of many parameters such as resonance matching, coil quality factor, system impedance match, and others. When designing and testing WPT systems, reliable measurement of system performance is essential. In our application, we use WPT to power biomedical implants for telemetry acquisition from small rodents, where rodent behavior data is used to study disease models. Such an application employs a large primary coil and a much smaller moving secondary coil, which can be defined as a loosely coupled WPT (LCWPT) system. This paper presents a novel wireless measurement system (WMS) that is used to collect real-time performance data from the secondary circuit (implant), while testing LCWPT systems. Presently, measuring the performance of the secondary side of LCWPT systems while they are in operation can be problematic. The literature reports various measurement errors when using voltage/current probes, or coaxial cables placed directly into the primary magnetic field. We have designed the WMS to greatly reduce such measurement errors, where the WMS measures the induced voltage (and hence received power) and relays this information by radio. Experiments were done to test the WMS, as well as comparison with cable-based measurements.

As battery capacities become suitable for the mass market, there is an increasing demand on technologies to charge electric vehicles. Wireless charging is regarded as the most promising technique for automatic and convenient charging. Especially in publicly accessible parking spaces, foreign objects are able to enter the large air gap between the charging coils easily. Since the evoked magnetic field does not meet regulations, wireless charging systems are demanded to take further precautions related to the protection of endangered objects. Thus, additional sensors are required to protect primarily living objects by preventing them from being exposed to the magnetic field. In this paper, we propose a new approach for monitoring the air gap under the vehicle underbody using an automotive radar sensor on the vehicle side. The concept feasibility is evaluated with the help of a prototypical implementation. Further, two-dimensional signal processing techniques are applied to meet the requirements of inductive charging systems. Consequently, this paper provides measurement data for relevant use cases frequently discussed in the community of inductive charging.

This paper presents a resonant inductive link for power and data transfer to a pulse generator implanted in the chest. The proposed link consists of two planar resonators and has been optimized for operating in the MedRadio band centered at 403 MHz. The wireless power/data link occurs between an external resonator operating in direct contact with the skin and a receiving resonator integrated in the silicone header of a pulse generator implanted in the chest. Numerical and experimental results are presented and discussed. From measurements performed by using minced pork to simulate the presence of human tissues, an efficiency of about 51% is demonstrated. The feasibility of using the proposed link for recharging the battery of the medical device in compliance with safety regulations is also verified and discussed.

Radio-frequency identification (RFID) technology is a consolidated example of wireless power transfer system in which passive electromagnetic labels called tags are able to harvest electromagnetic energy from the reader antennas, power-up their internal circuitry and provide the automatic identification of objects. Being fully passive, the performance of RFID tags is strongly dependent on the context, so that the selection of the most suitable tag for the specific application becomes a key point. In this work, a cost-effective but accurate system for the over-the-air electromagnetic characterization of assembled UHF RFID tags is firstly presented and then validated through comparison with a consolidated and diffused measurement systems. Moreover, challenging use-cases demonstrating the usefulness of the proposed systems in analyzing the electromagnetic performance of label-type tags also when applied on different material or embedded into concrete structures have been carried out.

This work proposes a tunable sidelobe reduction method based on a GaN active-antenna technique, in which the output radio frequency power is controlled by the DC drain voltage of the amplifiers. In this study, a 1 × 4 array of active antenna with GaN amplifiers is designed and fabricated. GaN amplifiers capable of up to 10 W-class power output are fabricated and arranged for a four-way active-array antenna. The fabricated single-stage GaN amplifier offers a maximum power-added efficiency of 59.6% and a maximum output power of 39.3 dBm. The maximum output power is decreased to 36.5 dBm upon decreasing the operating drain voltage from 55 to 35 V. In this study, a 4.5 dB sidelobe reduction is demonstrated in a 1 × 4 active antenna based on this output power difference for each amplifier.

Output voltage regulation is an essential technology for achieving stable wireless power supply. A receiver-side switching voltage regulator is useful for realizing output voltage regulation. However, this paper shows that the switching voltage regulator degrades the transfer efficiency to below 50% in a wireless power transfer system that consists of a class-D power inverter and series-resonant transmitting and receiving resonators. Such efficiency degradation is caused by the instability of an operating point where the efficiency is >50%. The input resistance value of the switching voltage regulator at a stable operating point is much higher than the optimum value for maximizing the efficiency. To stabilize the high-efficiency operating points, this paper formulates a stability condition and derives its sufficient condition. The sufficient condition facilitates a system design method using a K-impedance inverter that allows for the optimum input resistance value to lie in the range of allowable input resistance values. In addition, we introduce an input-voltage-based efficiency maximization method for the system with the receiver-side switching voltage regulator. By combining these two methods, efficiency maximization is realized with the receiver-side switching voltage regulator. The proposed methods were verified by both simulations and measurements.

We address transmission between array antennas in the Fresnel region, where there is a difference between the theoretical and actual transmission efficiencies. In particular, we focus on the effect of synthesis loss in the receiving antenna's power combiner circuit caused by amplitude and phase differences among the signals received by the elements. We designed 24 GHz array antennas and investigated the effect of synthesis loss on transmission efficiency via simulation. The synthesis loss was found to increase for smaller transmitting antenna sizes and larger receiving antenna sizes. In addition, to clarify the origin of the discrepancy between the theoretical and actual efficiency values and accurately estimate the efficiency, we defined four other loss factors and calculated them via simulation. Based on the results obtained, we propose an approximate equation for transmission efficiency in terms of synthesis loss and aperture efficiency. Finally, we calculate the efficiency with the effect of the loss factors included and confirm that the calculated and measured efficiencies are almost identical.

This paper describes the design, realization, and application of a custom temperature sensor devoted to the monitoring of the temperature differential between the leaf and the air. This difference is strictly related to the plant water stress and can be used as an input information for an intelligent and flexible irrigation system. A wireless temperature sensor network can be thought as a decision support system used to start irrigation when effectively needed by the cultivation, thus saving water, pump fuel oil, and preventing plant illness caused by over-watering.

A terrestrial microwave power transmission system is considered as an effective method for collecting natural energy. In this system, frequency dependence of the refractive index and multipath propagation due to the ground surface poses problems. In the study, a single-frequency retrodirective system was proposed with a beam pilot signal using dual-mode dielectric resonator antenna elements. The results confirmed that the beam pilot signal radiated from the entire surface of the receiving antenna and accurately used the same propagation space as that of microwave power by beam propagation method simulation. A dual-mode dielectric resonator antenna was proposed as a common array antenna element for the beam pilot signal and microwave power. This involves a cross-shaped hemispherical dielectric resonator structure, and an isolation level exceeding 60 dB was experimentally measured between the orthogonal ports of the fabricated antenna. The dual-mode dielectric resonator antenna with an isolation exceeding 60 dB was successfully developed as an enabling device to realize a single-frequency retrodirective system with a beam pilot signal.

In this work, a shorting control (SC) scheme is integrated into a complementary metal-oxide-semiconductor (CMOS) synchronous rectifier for the output voltage regulation of a wireless power supply. The rectifier is designed to operate in a parallel tuned pickup with a 500 mW output power capability for biomedical implants. Without any additional components, the proposed SC method enables the power pickup to operate with high efficiency under variable coupling conditions while maintaining the required load power to keep the output voltage constant. Desired operating conditions are achieved with increased power transfer capability at weak magnetic coupling conditions and higher power efficiency at strong coupling. A novel derivation describes the change in efficiency with SC duty ratio. Experimental validation is completed with an original custom CMOS integrated rectifier with embedded SC. It is demonstrated that the proposed SC method can increase the overall secondary pickup power transfer efficiency by 14% as the magnetic coupling increases to the stronger end.

This paper proposes a Voltage-controlled Variable Capacitor Structure (VVCS) to adjust the frequency of an autonomous push–pull converter. Unlike traditional switch mode capacitors or inductors where active switches are used, the equivalent capacitance of the VVCS varies with the on/off periods of a diode controlled by a DC voltage. The frequency of the autonomous push–pull converter can be controlled by this DC voltage when the VVCS is used as a variable resonant capacitor. As no active switching is involved in the VVCS, the circuit operates more smoothly than its switch mode counterpart so as to provide a simple way to adjust the operating frequency of the autonomous push–pull converter for high frequency and low electro magnetic interference operations. Mathematical model is developed for the relationship between the equivalent capacitance of the VVCS and the DC control voltage, and is verified by experimental results at more than 900 kHz with an approximately 12 W inductive power transfer system.

New small unmanned air vehicles designated as micro air vehicles (MAVs) are increasingly attractive for research, environmental observation, and commercial purposes. As described herein, the feasibility of a system for wireless power transmission via microwaves for MAVs was investigated. For its light weight and flexibility, a textile-based rectenna was proposed for microwave wireless power transmission of MAVs. To investigate bending effects on radiation performance, a microstrip patch antenna with a 5.8 GHz left-hand circular polarization was developed on a textile substrate. The antenna return loss, 20 dB, increased slightly with the antenna bending angle. An axial ratio <3 dB was maintained when the antenna bend angle was <30°. A rectification circuit was formed on the back side felt with sandwiched copper foil as a ground plate. Its weight per unit area was 0.08 g/m2, with maximum rectification efficiency of 58% with 100 Ω load at 63 mW input power. The average and maximum total transmission efficiency using the 5.8 GHz multiple rectenna with a 2.45 GHz retrodirective system were, respectively, 0.44 and 0.60%. The possibility and feasibility of microwave power transmission system using the textile-based rectenna were evaluated.

In this contribution, the authors perform the design and show the experimental results relative to a prototype of a combined wireless power transfer (WPT)–power line communications (PLC) system, in which the WPT channel is interfaced to a PLC environment to allow data transfer when the cabled connection is no longer available. The main rationale behind this idea stays in the fact that PLC communication is now a popular choice to enable communications, for instance, in smart grids and in home automation, while WPT devices start to be available in the market (i.e. for mobile phones) and soon they will be a reality also for higher power (i.e. vehicle battery charging). In particular, theoretical insights about the requirements of the system are given; a two coils system has been implemented and a measurement campaign, together with simulations, show that the system is of great potentiality and could be used in applications where both wireless power and data transfer are needed (such as vehicles battery charging), achieving maximum power transfer and good data rate in order to transmit high-speed signals.