title: Wireless Power Transfer Systems for Unmanned Aerial Vehicles
Creators: name: Li, Xiaokun
Publisher: name: Griffith University
Year: 2022
Subjects: subject: unmanned aerial vehicles
subject: Wireless power transfer (WPT)
subject: magnetic materials
subject: magnetic flux concentrator (MFC)
subject: magnetic coupler

description: In recent years,unmanned aerial vehicles (UAVUAVs) have been widely used in various fields of military and civilian, given that they are fast, convenient, discrete and multifunctional. HoweveHowever, the short cruising time limits the development of UAVs due to the lack of UAV batteries At present, UAVs are charged using plug-in AC and DC chargers. This charging manner may cause mechanical wear, joint heating and electric spark. In addition, this manner of charging requires manual operation and cannot work outdoors. Wireless power transfer (WPT) can solve these problems and avoid the loss and safety risk caused by the plug-in. Therefore, WPT is a good choice for UAV battery charging to enhance the cruising capability of UAVs. Moreover, because of the inherent nature of the wireless connection, UAVs can be charged using WPT regardless of the environment, which makes UAV charging convenient and safe. However, the applications for UAVs wireless charging warrant future investigation and improvement due to drawbacks, such as transfer power lever, efficiency, stability and safety. This thesis aims to investigate and optimise WPT systems for UAVs and mainly focuses on the key component magnetic coupler in WPT systems. Magnetic materials, magnetic core structures and a magnetic flux concentrator are investigated to obtain UAVs WPT systems with high transmission efficiency, stability, security and practicability. Various simulation models and experimental testing platforms are implemented to verify and analyse the proposed magnetic couplers. The main contributions of this thesis are summarised as follows. The first contribution is the investigation and comparison of magnetic materials. To investigate the influence of different magnetic materials on the performance of magnetic couplers, ferrite, amorphous and nanocrystalline are added to a receiving coil, respectively. Results show that using thinner amorphous and nanocrystalline alloys can obtain a similar coupling coefficient when thicker ferrite is used. To investigate the influence of ferrite thickness on the performance of magnetic couplers, different thicknesses of ferrites are added to the receiving coil, respectively. Results show that the coupling coefficient increases with an increased thickness of ferrites. To investigate the effect of magnetic material layer stacked on the performance of magnetic couplers, different layer numbers of amorphous and nanocrystalline alloys are added to the receiving coil, respectively. Results show that the coupling coefficient increases with an increased number of layers. Therefore, amorphous and nanocrystalline can be a good choice for magnetic couplers in WPT systems, because they can obtain a better magnetic coupling, are thin and light, which will greatly reduce the volume and weight of magnetic couplers. The second contribution is proposing an optimised plug-in magnetic coupler for WPT systems of UAVs with brackets. The transmitting part of the magnetic coupler comprises a vertical magnetic core structure with ferrites and a transmitting coil. Moreover, the receiving coil is installed on the UAVs bracket. Results show that the proposed plug-in magnetic coupler has a high coupling coefficient. The magnetic flux is concentrated in the magnetic coupler, and the low leakage magnetic flux can effectively reduce electromagnetic interference problems. The third contribution is proposing two types of magnetic couplers for WPT systems of small UAVs with flat bottoms. One type is a magnetic coupler comprising vertical spiral coils with ferrite PQI cores. The transmitting part comprises a vertical spiral coil and a ferrite PQ core, and the receiving part comprises a vertical spiral coil and a ferrite I core, which is installed on the abdomen of UAVs. This magnetic coupler can achieve tight coupling when the receiving coil is inserted into the transmitting coil given the small air gap. Results show that this magnetic coupler can provide strong magnetic flux densities, achieve a high coupling coefficient and maintain stable power transfer. Another type of magnetic coupler comprises sandwich coils with ferrite PQI cores. The transmitting part is constructed from two series-connected transmitting coils and a PQ core, and the receiving part comprises two series-connected transmitting coils and an I core. The transmitting and receiving coils are vertical spiral coils, which can concentrate the magnetic flux and reduce leakage magnetic flux. Results show that this kind of magnetic coupler can obtain a strong and stable magnetic field and improve the power transfer of WPT systems. The final contribution is the design of WPT systems using a planar magnetic flux concentrator (MFC). This planar MFC is a conductive metal plate with a centre hole and a slit in the radial direction. A switch connected to both sides of the slit. The MFC can achieve magnetic field concentration or magnetic shielding when the switch is off or on. The simulation results show that in the transmitting coil with an MFC, the magnetic flux density of the transmitting and receiving coils increase around the centre hole (which can increase power transfer) and reduces on the outer surface of the transmitting and receiving coils (which can reduce leakage magnetic field). Magnetic shielding can be obtained by a short-circuited MFC. Meanwhile, the power of the receiving coil increases by using a small size receiving coil in the transmitting coil with an MFC. The equivalent T-circuit for the coupling coils with an MFC is proposed on the basis of the impedance analysis. The MFC design is optimised to achieve an optimal result by using an MFC with a higher thickness and a smaller slit width. Moreover, the investigation in the case of coupling coils adding ferrite cores and with and without an MFC is carried out. Results show that using an MFC can enhance the magnetic field and increase the receiving power.