# A Comprehensive Review of Micro UAV Charging Techniques
## 4 Wireless Power Transfer (WPT)  
UAVs have gained significant attention due to their diverse applications in the electronic industry with several key benefits in terms of safety, convenience, reliability and a fully automated charging mechanism. These benefits can be attained through different WPT techniques. Another key feature of WPT is that it is very essential in different environments where wired power transfer techniques are dangerous, difficult, or impossible, such as the underwater environment and high voltage power applications. The research fraternity has been exploring trade-offs and evaluating different WPT techniques. These techniques are categorized as: radiative electromagnetic (EM) and non-EM techniques. In non-EM, power is transferred using acoustics or optical sources such as lasers. EM is further classified into radiative far-field, non-radiative and mid-field radiative and non-radiative. In radiative power transmission, RF is used, while in non-radiative power transmission, capacitive coupling, inductive coupling and magnetic resonance coupling are used. Currently, these WPT techniques are commercially available for different applications such as smart phone charging and charging implantable medical devices, etc. and are still being developed for different applications. The aforementioned WPT techniques ensure efficient and reliable wire power transmission between UAV and base station. The application of WPT technology to UAVs should take into account some critical issues such as misalignment, interference and payload. UAV-based WPT system must be lightweight to avoid payload reduction. Furthermore, the WPT techniques must ensure misalignment tolerance between coils, high landing precision and efficient power transfer. Among these challenges, misalignment is a dominant issue as landing accuracy is low in the case of UAVs. Consequently, it affects coupling factor, transfer efficiency and power transfer. Several research studies have reported these major issues concerning recharging UAV batteries through WPT techniques [78; 79]. UAV charging can be achieved through inductive coupling WPT to enhance both range and flight time for inspection, monitoring and surveillance tasks. UAV-empowered inspection techniques can overcome several limitations of existing inspection techniques, such as expensive task and hazardous operation through manned helicopters. In [80], the authors demonstrated magnetic resonance coupling (MRC) WPT to recharge drones and tested the proposed system with different distances and misalignment topologies. The authors reported 90% transfer efficiency at 10 cm distance. Moreover, [PERSON] et al. [81] presented a vision-based, closed-loop target detection through UAV for outdoor applications. The authors designed a charging station which prolonged the flight time and enhanced the endurance of a UAV. In [82], [PERSON] proposed a novel mid-air inductive charging mechanism to charge multiple drones at the same time without any need for landing using global energy transmission (GET). Most studies on charge scheduling of UAVs consider a centralized architecture. Only a few works have been reported through the peer-to-peer network on UAVs charging through blockchain technology [83; 84]. Next, we discuss laser power transfer (LPT) in detail.