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38 effects. Also, the rest of the craft can be easily shielded from the effects of the electric field by using lightweight, low- cost copper tape. That works well when the receiver plates are well below the body of the craft. The receiver also does not have to be a particular shape, like a magnetic coil; it can be a mesh around the legs of a UAV, again reducing the weight. Another advantage is that the receiver does not need to be aligned with the transmitter plate, and several receivers can sit in the same electric field to recharge. This technology can struggle with humid environments and rain though, as moisture in the air weakens the electric field. Systems have been developed to provide from 60 W up to 200 W of power at a distance of 10 cm from the transmitter, and although UAVs can be charged effectively up to 20 cm away, developers are finding that the sweet spot is 5-10 cm. That allows a UAV to hover and still recharge its battery. The electric field is modulated at around 13.56 MHz, although flexible system design allows for the use of 27.12 MHz, and both of these are in the industrial, scientific and medical (ISM) unregulated band. The receiver then uses a radio link (usually dependent on the UAV’s radio) to tune a bank of capacitors to the resonant frequency for the power link. For ground vehicles, the technology gives the advantage that a larger plate can be installed on the side of a vehicle operating in a hazardous environment such as a mine, where there is a lot of dust, making the positioning easier. Using cables for charging brings the risk of dust getting into the connectors and causing short-circuits, while the capacitive transmitter can be a sealed unit attached to a power supply, and the receiver can be integrated into the UGV’s body. The vehicle would then approach the charging plate to a distance of 10-20 cm to recharge. Another developer is using capacitive technology for a high-power far-field recharging system that does not need a transmitter plate. Initial tests of the system show that a battery pack on a UAV while it is in flight can be recharged in six to eight minutes. The initial system was able to power a 1 W light bulb with a 200 W system; last year it could provide 2 kW across 2 m. Now the transmitter generates a charging December/January 2019 | Unmanned Systems Technology Systems have been developed to provide power at 10 cm from the transmitter, which allows a UAV to hover and still recharge itself Capacitive resonant charging technology can create a ‘power cloud’ around a transmitter so that a UAV can be charged in flight (Courtesy of GET Corp)