Unmanned Systems Technology 016 | Hydromea Vertex AUV | Power management systems | Unmanned Space Vehicles | Continental CD-155 turbodiesel | Swift 020 UAV | ECUs | DSEI 2017 Show report

82 October/November 2017 | Unmanned Systems Technology PS | Wireless charging D espite rapid advances in battery and solar cell technology, the limit on endurance for electrically powered UAVs is the need to recharge their batteries (writes Peter Donaldson). Another layer of difficulty emerges when the vehicle is autonomous, and is expected to find its way back to a charging station. The basic options are to enable the vehicle to come to the charger, have the charger come to the vehicle after it has landed nearby or, more radically, have the charger target the vehicle with a beam of RF or laser energy to charge it while it remains airborne. For small UAVs, the need to plug into a connector has been eliminated by wireless charging devices that use the same magnetic induction coupling principles now commonly used to charge smartphones. But the UAV still has to make contact very accurately in terms of orientation as well as position, so it requires precise position feedback that is unavailable outdoors without differential GNSS, and the necessary receivers might be too heavy and bulky for smaller drones. Vision-based systems that exploit the UAV’s camera can be useful, but external disturbances such as wind gusts mean that achieving the required accuracy in the touchdown remains elusive. A team from Kingston University in London and the Khalifa University of Science and Technology in Abu Dhabi is therefore tackling the problem with the combined use of a GPS receiver, a vision-based closed-loop target detection and tracking system and a multi-coil, omnidirectional magnetic resonant induction wireless power transfer (WPT) system that is said to be effective and efficient over distances of up to 2 m. This allows the Parrot AR Drone used in the study to land on any convenient heading, while the use of multiple charging coils enlarges the WPT surface area, further reducing the need for precise positioning. Meanwhile, a team at Imperial College in London is experimenting with another magnetic induction charging system intended to allow small multicopters to recharge during hover. Back in 2002, a RAND Corporation study declared microwave recharging of airborne mini- and micro-UAVs to be impractical, although the concept it examined put a 95 GHz charger aboard a Global Hawk circling at 60,000 ft. Since then, the concept has been reconsidered by others in more modest form, with Japan’s IHI successfully testing a system consisting of a 10 kW, 5.8 GHz transmitter with a steerable antenna on the ground and a small, fixed-wing UAV with a rectenna system (an antenna with a rectifier to convert its output to DC) built into the underside of the wings that proved able to generate 166 W to charge the battery. The transmitter tracks the UAV visually with an accuracy of ±0.5°, while the UAV flies in a 30 m diameter circle 50 m off the ground. Laser recharging is also advancing. For example, NASA prize-winning company Laser Motive reportedly raised more than $1.5 million in equity funding to continue developing its power beaming technology, having kept a Lockheed Martin Stalker airborne for 48 hours in a 2012 demonstration. The days of an electric UAV having to be grounded for recharging are clearly numbered. Now, here’s a thing “ ” Laser Motive’s power beaming technology kept a Lockheed Martin Stalker airborne for 48 hours in a 2012 demonstration