Recently, unmanned aerial vehicles (UAVs) have shown immense potential in military and civilian applications. However, the limited endurance of UAVs poses a crucial challenge to their widespread adoption and development. Fortunately, wireless power transfer (WPT) technology has emerged as a promising solution to address this issue. In contrast to the previous papers on the topic, this review aims to provide a holistic top-down view of WPT technology for UAVs. More specifically, it introduces the fundamental principles of WPT technology firstly. Then, the design and optimization methods of coupling mechanisms, especially for UAVs, are introduced and compared. Numerous different and significant designs are reviewed and discussed. Furthermore, the compensation topology and control strategies are elaborated in detail subsequently. Finally, the key technical challenges faced by WPT for UAVs are put forward and future directions for development is proposed. Therefore, this review can be considered as an expedient reference for researchers conducting studies in the field of WPT technology for UAVs. Keeping this in mind, we present meaningful perspective for each of the analyzed key aspects, providing inspiration for what follows.

Mid-range wireless power transfer (WPT) system for consumer electronics has been hamstrung by modest energy efficiencies and low misalignment tolerance. As the increase in transfer distance, the increasing leakage magnetic flux between transmitting (Tx) coil and receiving (Rx) will deteriorate the system performance. In this paper, a novel optimization-based design method of passive array coils (PACs) is presented in order to maximize the power transfer efficiency (PTE) and improve the misalignment tolerance in WPT system. A modified multiobjective genetic algorithm is implemented to optimize parameters of PACs in consideration of PTE, displacement flexibility (DF) and volume of PACs. Moreover, a parameter design scheme for compensation network, namely S-Ss-S topology for co-planar multiple coils is investigated, which can achieve zero phase angle (ZPA) in primary side and enhance power factor. Finally, the reasonable prototype of the proposed WPT system with PACs is designed, fabricated and tested. The experimental results show that the proposed structure can increase the efficiency by almost 50% over the system without PACs at 400 mm distance and deliver almost 40 W of output power. Moreover, the PTE and output power can still remain stable under large lateral misalignment with PACs.

In this article, electromagnetic metamaterials with mu-near-zero (MNZ-MM) have been presented to explain the shielding effect in the wireless power transfer systems. First, the optimized design of the metamaterials unit cell has been carried on to achieve the near-zero permeability at the working frequency of 13.56 MHz. The equivalent circuit model of the MNZ-MM unit cell has been derived, which illustrates metamaterials with near-zero permeability have the great electromagnetic field (EMF) shielding effect. Additionally, the self- and mutual inductances associated with the resonance coils and MNZ-MM unit cells are deduced in detail. Moreover, the simplified human head model is established to investigate the magnetic field cancellation by MNZ-MM on the coil side. To evaluate the performance of the proposed MNZ-MM, the magnetic field strengths of the receiving terminal with different shielding materials have been compared in simulation. Finally, a series of experiments in real-environment are implemented to discuss the shielding effect of the MNZ-MM. The measurement results show the MNZ-MM has better performance in the swine brain sample than the ferrite and aluminum plate on the whole. With the proposed MNZ-MM, we demonstrate a maximum reduction of 17.52 dB in the leakage EMF and basically no effect on transfer efficiency.

For the coil misalignment problem in the charging process of automatic guidance vehicles (AGVs). This paper proposes an optimization method for the bilateral LCC compensation network parameters based on quadruple-D quadrature (4DQ) coils. By introducing two coefficients K _T and K _f1 , the compensation capacitors C _T and C _f1 are optimized, achieving smaller output power fluctuations. Firstly, the capacitor values C _T and C _f1 at resonance in the SS and PS equivalent circuit topology are obtained, meanwhile, the relationship between the output power P _out and the coupling coefficient k , represented by the coefficients K _T and K _f1 , is obtained. Furthermore, with the transmission efficiency as the limiting condition, the output power fluctuation is effectively suppressed by adjusting K _T and K _f1 . Finally, the experimental results show that after optimization the output power is 427.08W, the transmission efficiency is 93.506% when the coupling coil are aligned; The system can still maintain a power output of at least 382.36W with a coil offset of 0-153mm in the X/Y axis directions, and the transmission efficiency is always above 89.472%. At the same time, the maximum deviation value Δ P _{abs_max} decreased from 213.1W before optimization to 111.0W after optimization, a decrease of 47.91%, effectively suppressing the power fluctuation and improving anti-offset performance.

Wireless power transfer (WPT) systems have been hamstrung by modest energy efficiencies and electromagnetic safety concerns. Conventional electromagnetic shielding devices can reduce the electromagnetic interference (EMI) to the ambient environment but at the same time deteriorate the transfer performance. In this article, a novel hybrid resonance optimization-based passive shielding magnetic structure and the corresponding circuit are proposed. The shielding structure and topological network have the advantage of minimizing electromagnetic fields (EMFs) around the system while guaranteeing transfer performance (PTE, G_V, P_OUT, phase relation) constant. Moreover, a multiobjective genetic algorithm is utilized to optimize the shielded directional coils (SDCs) parameters in consideration of the transfer efficiency, EMF, and shielding structure volume. Finally, a reasonable prototype of the proposed WPT system is designed, fabricated, and tested. Experimental results show that the maximum leakage EMF level in the evaluated area is reduced by 4.18 dBmV (nearly 39.5 µT). Additionally, the PTE changes by only 1.5%, the G_V changes by just 9.5%, and the phase relationship remains unaffected under the 80 W output power condition. The presented solution merges design approach, compensation network, and optimization plan. It can be further extended to analyze arrays of arbitrary order and shielding in different orientations, providing useful insights for smart homes.

The application of metasurface in wireless power transfer (WPT) system has attracted great attention due to its excellent properties. However, most studies focus on singlefrequency transmission of energy (generally MHz-band frequency), while the operating frequency of most WPT systems is kHz band. In this letter, a cross-frequency (both kHz and MHz bands) metasurface cell for WPT system is designed and equivalent permeability extraction method is proposed for the first time. Firstly, a double-layer nested structure with two different helical coils is utilized to enable the metasurface cell to obtain two different resonant frequencies. Secondly, the equivalent circuit of this metasurface cell is established and detailed theoretical analysis of equivalent relative permeability is derived for cross-frequency metasurface unit cell. Finally, the precise prototype of unit cell is designed. The experimental results agree well with the theoretical ones. This work paves the way to dual-frequency band WPT system based on metasurface. Particularly, it provides a good preparation for the application of cross-frequency metasurface to realize simultaneous wireless power and data transfer in WPT system