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.

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.

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.

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.

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.

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.