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35 range, that couples with an antenna on the unmanned platform. That requires a comms link between the transmitter coil and the receiver to tune the frequency to create the most efficient link. Magnetic inductive systems require accurate positioning to get the coils aligned to achieve a cost-efficient transfer of power, so tight control of the positioning of the unmanned platform is needed to ensure that the coils are aligned correctly. For driverless cars this can be achieved by using a relatively large ground plate for charging, as in the latest BMW systems now entering production. For smaller UAVs, however, this alignment can be a major challenge as the charging distance is only a few millimetres. That works for a UAV landing on a simple, low-cost charging pad, but not if the craft needs to keep flying. Magnetic inductive charging designs can also require large ferrite components, which can be heavy and costly, so developers have looked at capacitive approaches where the receiver can be a thin foil of metal. That is more suited to the weight requirements of UAVs, and allows ‘free space’ charging over distances of metres Contact charging However, the most developed charging system uses a series of contacts. The system consists of a charging platform made of one, four or nine 462 x 462 mm conductive platforms and spring-loaded contacts that are retrofitted onto the legs of a UAV. When a UAV lands on the platform, the contacts on the landing gear physically touch the surface of the platform and establish an electrical connection. As long as it lands on the platform, the system always works at its maximum efficiency regardless of the craft’s position, dimensions and orientation. This provides up to 6 Ah for the indoor system but it can easily reach 10 Ah in the outdoor version and with an efficiency of 92% – comparable to a wired charger. The system can charge lithium-polymer batteries in a few minutes via the battery controller. A hangar has even been developed to protect the UAV while charging, and it has a webcam so that operators can monitor the craft. It is the prospect of far-field charging though that is inspiring several developers. Technology initially developed at the University of Washington for powering medical devices in the body through skin and bone has been adapted for UAVs and ground vehicles. The magnetic resonance technology has been enhanced for range, power level and the ability to adapt to changing conditions. It can provide up to 300 W to a battery system at a distance of up to 5 cm. That allows powered carts in hostile environments such as a steel foundry to eliminate the power cable. This is the leading cause of failures in the system, as the cable sometimes gets caught or run over by a cart. To provide the wireless charging, the field generated by a 20 cm circular trace on a printed circuit board forming the transmitter coil couples to a 10 cm diameter receiver, allowing 5 cm of free movement in any direction. This movement allows the cart to pull up to a wall station and charge quickly, with an end-to-end efficiency of 70-75%. The present 125 W systems allow a 5 Ah, 25 V battery to charge in about an hour, but that would fall to 25 minutes in the 300 W system using a constant Wireless charging | Focus If a base station is simple and low cost enough then a lot of them can be rolled out to provide a chain of charging points for UAVs Unmanned Systems Technology | December/January 2019 Inductive wireless charging uses a magnetic field to transfer energy (Courtesy of Qualcomm/Ricardo)