This paper presents the design of a multi-MHz inductive power transfer (IPT) system showcasing lightweight and energy-efficient solutions for non-radiative wireless power transfer. A proof of concept is developed by powering a drone without a battery that can hover freely in proximity to an IPT transmitter. The most challenging aspect, addressed here for the first time, is the complete system level design to provide uninterrupted power-flow efficiently while allowing for variable power demand and highly variable coupling factor. The proposed solution includes the design of lightweight air-core coils that can achieve sufficient coupling without degrading the aerodynamics of the drone, and designing newly-developed resonant power converters at both ends of the system. At the transmittingend, a load-independent Class EF inverter, which can drive a transmitting-coil with constant current amplitude and achieves zero-voltage switching (ZVS) for the entire range of operation, was developed; and at the receiving-end, a hybrid Class E rectifier, which allows tuning for large changes in coupling and power demand, was used. For the demo, the range of motion of the drone was limited by a 7.5 cm nylon string tether, connected between the centre of the transmitting-coil and the bottom of the drone. The design of the IPT system, including all the power conversion stages and the IPT link, is explained in detail. The results on performance and specific practical considerations required for the physical implementation are provided. An average end-to-end efficiency of 60% was achieved for a coupling range of 23% to 5.8%. Relevant simulations concerning human exposure to electromagnetic fields are also included to assure that the demo is safe according to the relevant guidelines. This paper is accompanied by a video featuring the proposed IPT system. Paul D. Mitcheson (SM12) received the M.Eng. degree in electrical and electronic engineering and the Ph.D. degree in micro-power motion based energy harvesting for wireless sensor networks from the
This paper proposes solutions for an IPT system to operate efficiently when large changes in coupling take place. To achieve high power-efficiency independent of coupling, we utilise inherent regulation properties of resonant converters to avoid losing soft switching for any coupling value, and present the optimal load to the IPT-link at the maximum energy-throughput coupling. A probability-based model is introduced to assess and optimise the IPT system by analysing coupling as a distribution in time, which depends on the dynamic behaviour of the wireless charging system. The proposed circuits are a Class D rectifier with a resistance compression network (RCN) in the receiving-end and a load-independent Class EF inverter in the transmitting-end. Experiments were performed at 6.78 and 13.56 MHz verifying high efficiency for dynamic coupling and variable load resistance. End-to-end efficiencies of up to 88% are achieved at a coil separation larger than one coil-radius for a system capable of supplying 150 W to the load, and the energy-efficiency was measured at 80% when performing a uniformly-distributed linear-misalignment of 0-12.5 cm, corresponding to a receiver moving at constant velocity over a transmitter without power throughput control
Large magnetic field volumes associated with coreless high frequency inductive power transfer (HF-IPT) systems allow multiple receivers to be powered from one transmitter, but also provide greater probability for foreign objects to couple to the system. Knowledge of the types of objects (legitimate receivers or otherwise) that are coupled to the transmitter is critical. Such knowledge on the transmit side would allow the system to be deactivated in the presence of foreign objects, and to determine the exact state of tuning of the receivers so that it may adjust itself accordingly to optimise system performance. This paper introduces a technique to calculate the induced voltage generated by coupled receivers and foreign objects on the transmit coil in real-time. Changes in the position or electrical quantities of the receivers, and foreign objects, alter the induced voltage on the transmit coil, and with it the trajectory of the switching waveforms of the inverter driving the transmit coil. From the shape of these waveforms, information on the phase and amplitude of the induced voltage can be extracted, thus enabling the induced voltage on the primary to be estimated with a single, easy to access, voltage measurement, which is easier than estimating the induced voltage from measurements of coil current and total coil voltage. We used a support-vector-machine (SVM) to perform regression analysis on the drain voltage data. The experimental setup uses a 100 W, 13.56 MHz Class EF inverter, and the model was generated from a large number of samples of the drain voltage waveforms operating at different known loads. These were generated from our in-house HF-IPT test load, which uses a Class EF synchronous rectifier. The results allow the induced voltage on the transmit coil to be estimated in real time from the drain voltage waveform alone, with a normalised root mean square error of 1.1 % for the real part (reflected resistance) and 1.2 % for the imaginary part (reflected reactance).This paper is accompanied by a video file demonstrating the experiments.
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