In this article, we propose a mutual-inductance-dynamic-predicted constant current (CC) control to realize the secondary-feedback-free output current adjustment for drone wireless in-flight charging systems. In practical systems, the challenge is to keep a CC output for drones under the continuous fluctuations of mutual inductance, the variation of desired charging current, and the carrying weight limits of drones, which has been nearly unexplored in previous studies on wireless power transfer systems. Accordingly, this article proposes a novel mutual-inductance prediction scheme combined with optimized phase shift control to maintain the desired CC charging output, which can be implemented at the transmitting side to address the impact of the above-mentioned challenges. In the article, simulated and experimental results are both given to verify the feasibility of the proposed control scheme, wherein the prediction accuracy is above 92.5%, the CC control accuracy is within 5%, and the average response time is less than 320 ms. It shows that the proposed dynamic-predicted CC control scheme has improved real-time capability and enhanced robustness, which is an ideal technical means for drone wireless in-flight charging.

Though the wireless in-flight charging is an ideal way for the energy supply of drones, it also faces the practical challenges, namely dual-disturbance of continuous fluctuation of mutual inductance and battery load variation, changed expected charging power, and the lightweight and no communication design demands for pickup, which have been nearly unexplored in previous article on wireless power transfer (WPT) system. To address the issue, this article proposes a novel communication-free output power control algorithm based on the load identification scheme with automatic frequency adjust. Only the primary current needs to be measured in the proposed algorithm, which can reduce the system complexity with the enhanced real-time performance for WPT system. Simulated and experimental results validate the feasibility of the proposed control algorithm with the identification accuracy of more than 92%, control accuracy of 95%, and average response time within 100 ms. Furthermore, the follow-up of desired output power and the implementation of lightweight pickup demonstrate the flexibility of the power control algorithm, which ensures the rapid and safe energy supply for drones in the drone wireless in-flight charging system.

To address the special challenges for drone wireless in-flight charging systems, namely the continuous fluctuation of mutual inductance, large charging distance, and limited payload of the pickup, a novel high-order primary-series (S) and secondary-single-inductor-double-capacitor (SLDC) topology for parity-time (PT) symmetric wireless power transfer (WPT) system is proposed in this article. The reduced-order model of the proposed system is first established and analyzed based on the coupled-mode theory. It shows the critical coupling coefficient can be reduced by two additional degrees of freedom (λ and L_r ), thus effectively expanding the constant power (CP) region of PT-WPT system. In addition, the inductor L_r can be placed freely in drones and λ can be flexibly adjusted to meet the demands of various environments, which enhance the system flexibility. Also, the communication link and dc–dc converter are not needed, which improve the real time and reduce the drone payload. Simulated and experimental results prove that the transfer distance can be extended by 102% with λ = 0.6 and L_r = 60.5 μH while keeping the CP of 75 W and efficiency of 87.5% against the misalignment and change of the charging distance. Accordingly, with the proposed S/SLDC topology, drone in-flight charging systems can obtain a larger CP region with high efficiency, thus extending the flight time of drones.

As an emerging research topic, wireless power transfer (WPT) with a single transmitter for multiple pickups (STMP) shows salient advantages of reducing the number of power inverters, increasing the power density, and so on, which has profound implications for various applications, such as wireless charging for electric vehicles (EVs) and smart homes. First, this article introduces the concept of STMP-WPT and three critical issues, namely, misaligned transmission, resonance shifting, and energy security. Then, the corresponding solutions are elaborated from the perspectives of the magnetic and electric fields, respectively. Finally, the technical challenges for STMP-WPT systems are discussed.

The special challenge for drone wireless in-flight charging is to keep a constant charging power with a large air gap under the variation of mutual inductance and the limited payload of pickup. To solve the issue, this article proposes a flexible constant-power (CP) range extension scheme with the self-oscillating wireless power transfer system, which is achieved on the primary side. By changing the duty cycle, the proposed autonomous on–off keying modulation (AOKM) scheme can effectively expand and regulate the CP region with high flexibility, avoiding the usual pickup-side-only implementation. With the AOKM scheme, the operating frequency is autonomously adjusted to keep the CP output, where only the primary current needs to be sensed without the dc–dc converter or wireless communication. Thus, it ensures the lightweight pickup of drones. The experimental results verify that the transfer distance of the CP region can be extended by 62.4%, while keeping a constant power of 73.8 W and efficiency of 88.3% regardless of the changed air gap and misalignment. Hence, based on the proposed flexible CP range extension (AOKM) scheme, the large-range CP charging region with high efficiency can be obtained for the drone wireless in-flight charging system, thus prolonging the flight time of drones.

For the drone wireless in-flight charging system, the challenge is to keep a constant charging power under the large range of continuous fluctuation of mutual inductance ( M ), the variation of battery load ( R ), and the limited payload of drones, which has been rarely studied in previous research. To address the issue, this paper presents a primary-series and secondary-series-inductor-series-parallel-capacitor (SLSPC) high-order topology for the parity-time (PT) symmetric wireless power transfer (WPT) system, which can greatly expand the exact PT-symmetric region (constant power range) by the reduction of critical coupling coefficient in comparison with the traditional series-series topology. Besides, the constant power control scheme is proposed based on the series-SLSPC high-order topology to overcome the change of M and R . With the proposed control method, only the primary current needs to be detected, which enhances the real-time performance of the WPT system and reduces the payload of drones. Simulated and experimental results validate the expansion capability of the PT-symmetric range with the proposed series-SLSPC PT-WPT system and the feasibility of the proposed control scheme.