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 presents foreign object detection (FOD) methods for MHz wireless power transfer (WPT) systems. Unlike current FOD implementations, the presented methods can operate without requiring a feedback loop from the wireless power receiver to the transmitter. This allows complete decoupling of the transmitter and receiver and therefore reduces the design complexity and cost of the system. The developed FOD methods were implemented on a 13.56 MHz WPT and experimental results are presented showing successful detection of a wide range of objects regardless of the loading condition of the system.

This paper reports on the design and development of a wireless charging solution for a DJI Matrice 100 quadcopter drone. The system is based on a high frequency inductive power transfer system built with lightweight copper pipe air-core coils at both ends and lightweight electronics at the receive side. The developed system is capable of delivering power to the drone at the same rate as the original wired charger (100 W) when landed at any position on the charging pad, regardless of the lateral misalignment or angular orientation. The charging pad is circular with a one-metre diameter, therefore allowing for a lateral misalignment of up to 25 cm. The system has an average mains-to-battery efficiency of 70 % and enables the drone missions to be completely autonomous as it eliminates the need for human interference for battery recharging or swapping.

One of the advantages of high-frequency inductive power transfer systems is the high tolerance to misalignment and large air-gaps. However, the inherently large magnetic field volumes can lead to coupling of additional foreign objects with the primary, causing possible detuning of the system and heating of the objects. These foreign objects and the conditions of the local environment can load the transmitter, which changes the induced voltage on the primary side. Unfortunately, the induced voltage is not directly measurable in an operating transmitter and the most straightforward way of calculating this variable, through a measurement of primary coil current and voltage, can cause a significant decrease in quality factor which reduces system performance. An integrated solution capable of estimating the induced voltage through other less invasive measurements in the primary is needed to ensure safety of operation through foreign object detection. Knowledge of the induced voltage can also be used to correct tuning mismatches where both sides of the link are active (i.e., in synchronous rectification and bidirectional systems). In this article, multiple candidate variables for estimating the induced voltage are assessed based on factors such as measurement practicality and estimation accuracy. It is demonstrated for the first time that a solution which is based on the measurement of only two variables, the amplitude of the fundamental frequency of the switching waveform and input current, can achieve state-of-the-art induced voltage estimation accuracy. These two variables, which can be obtained using simple cost-effective analogue circuitry, are used in a Gaussian process to generate a regression model. This is used to estimate induced voltages at any angle in an approximate magnitude range of 0-20 V with a normalized root-mean-square error of 1% for the real part and 1.5% for the imaginary part. This corresponds to detecting a plastic container with 1 kg of saline solution (0.4% concentration, to emulate the electromagnetic profile of muscle) at a distance of 15 cm (1.5 coil radii). The results Manuscript

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