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.

To satisfy independent and controllable power supply requirements for loads with different frequencies and power levels, a multifrequency and multiload (MFML) magnetic coupling resonant wireless power transfer (MCR-WPT) system using hybrid modulation waves sinusoidal pulsewidth modulation (HMW-SPWM) control method is proposed in this article. Based on SPWM, loading a hybrid modulation wave formed by superimposing multifrequency modulation waves to drive the inverter, so the multifrequency hybrid current on the primary side can be obtained. According to the principle of mutual inductance coupling, inductive power with different frequencies is obtained and then separated by multiresonant networks on the secondary side for loads with different frequencies. First, the structure and working principle of a MFML MCR-WPT system controlled by HMW-SPWM are introduced. Then, taking a dual-frequency and dual-load MCR-WPT system as an example, the system is mathematically modeled. Besides, parameters design criteria for reducing interfrequency interference is studied. After that, load characteristics and power factor are analyzed to select appropriate parameters. Furthermore, dynamic characteristics are analyzed. Finally, theoretical results are validated by experiments. The experimental results show that the MFML MCR-WPT system controlled by HMW-SPWM realizes independent and controllable wireless power supply for loads with different frequencies and power levels.

For a general multi-coil magnetic coupling wireless power transfer (MC-WPT) system with different coil properties and non-negligible cross couplings, there is no analytical solution which connects the resonant frequencies of the system to its electric parameters. Establishing such a generalized analytical solution not only permits quantitative analysis of resonant points but also provides a generic platform for the physical interpretation of resonance. For this purpose, a novel modeling and analysis method of integrating time-domain and frequency-domain is proposed in this paper, with that all possible resonant points of arbitrary coil MC-WPT systems can be quickly and accurately determined. Firstly, an accurate state space model of a general MC-WPT system with arbitrary number of transmitters, repeaters and receivers is established and the analytical current response on each circuit is derived. Then several key system concepts about resonance are elucidated, based on which the generalized analytical expressions of resonant points of MC-WPT systems, including resonant frequencies and corresponding powers and efficiencies, are derived. Further, the physical interpretation along with the mathematic relationships of several key easy-confusing concepts are deduced, which provides systematic theoretical criteria for the selection of operating frequency to achieve the resonant states for MC-WPT systems.

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.

To reveal the general characteristic of the maximum power transfer points of MC-WPT systems, and to quickly and accurately determine all possible maximum power transfer points for arbitrary coil MC-WPT systems, a novel theoretical analysis method is proposed in this paper. Based on the modal theory, an accurate time-domain model of an n-coil MC-WPT system is established and the analytical current response on each coil is derived. According to the current response of the load, a detailed power spectrum distribution is presented, based on which the mathematic expressions of all possible maximum power transfer points, including the maximum output powers and corresponding operating frequencies, are deduced. The theoretical results are verified in experiment prototypes practically. The experimental results show that: 1) the output power reaches its maximum value at the modal frequencies rather than the designed natural frequency of the resonators in MC-WPT systems; 2) there are n modal frequencies of an n-coil MC-WPT system. The analysis results provide useful information to choose the operating frequency for practical MC-WPT systems with arbitrary number of coils.

The existing simultaneous wireless power and information transmission (SWPIT) technology used for wireless power transfer (WPT) systems is seriously plagued by the interference between the power and information transmission and the complex design of the information transmission and receiving circuits. To combat these problems, this article proposes a novel SWPIT method, which does not need additional information transmitting or receiving coils. The stroboscopic mapping model of the WPT system is established first, and the soft-switching points of the dc–ac inverter are derived and compared for better utilizing deviation frequency enlargement effect (DFEE) to transfer power and information simultaneously. Subsequently, the information detection circuit is designed based on DFEE. The impacts caused by the detection circuit on the power and information transmission are analyzed in detail. Then, the parameters of the detection circuit are optimized for minimizing the mutual interference between the power and information transmission. Finally, a 50 W experimental platform is established to verify the correctness and effectiveness of the proposed SWPIT strategy. The experimental results show that the information transmission rate can reach up to 16 kbps, the load voltage fluctuation can be controlled below 3%, and the efficiency of the whole system can be maintained at 90%.