Unmanned Systems Technology 036

62 Digest | Flybotix ASIO “In our case, what we have is effectively an electromagnetic swashplate,” Tschudi explains. “A traditional helicopter swashplate is mechanically too complex and fragile, and requires a lot of maintenance. Take that risk, complexity and work, then put two small versions of that into a UAV, and you’re making its flight mechanics even more complicated. That UAV will break for sure, and sooner rather than later.” Using a pair of variable-pitch propellers at the small sizes necessary for this UAV would also have introduced too much mechanical complexity. As a result, the system in the ASIO uses two fixed-pitch dual-blade propellers, meaning the blades are not individually actuated. With its two blades linked, each propeller contains a comparatively simple system for tilting the entire prop about its x axis (perpendicular to its axis of rotation), using just a permanent magnet and electromagnetic coil for actuation. “We have one of those in each propeller, and the autopilot alters the angle and rotational speed of each prop, with the differential between these values on the upper and lower rotor determining the movement direction and speed of the ASIO,” Bouabdallah explains. “Mechanically it’s very simple, and is essential to being able to control the ASIO’s flight using just two 12 in [30.48 cm] propellers, instead of four small propellers like you typically see on other inspection UAVs.” That increase in propeller diameter and hence area is key to the ASIO’s efficiency, in aerodynamic as well as energy terms. It enables an endurance of up to 24 minutes, compared with the 10-12 minutes typical of battery-electric quadrotors, particularly when they have to hover repeatedly during each flight to take photos and videos of potential damage to buildings and equipment. Extensive CAD and CFD simulations have been carried out to optimise the shape of the propeller blades, for maximising their aerodynamics and their contribution to flight stability, as well as reducing the vibration produced. Also, the propellers can be run at a slower speed than in conventional quadrotors, which in dusty environments causes far less abrasion on them. This slower prop speed, combined with the vibration optimisation, also means the ASIO’s rotors produce far less noise than most multi-copters: about 80 dB when 1 m away. “That lower noise makes it much safer for inspectors and pilots working alongside the UAV,” Tschudi adds. “Factories sometimes have emergency alarms, and if you need to wear ear protection because you’ve chosen an overly noisy UAV, you could miss those alarms and fail to take the appropriate safety actions.” The two motors are custom-built to spec by maxon motor, which also provided considerable technical advice and assistance in engineering the powertrain design. Although a single coaxial motor can lift the ASIO, two separate motors are needed in order to run them at different speeds and torques to produce the desired yaw. “If one motor or prop failed, the ASIO would spin owing to loss of counter- torque, but our algorithm can calculate a safe, controlled auto-landing procedure, so long as it’s no higher than 2 m from the ground,” Bouabdallah says. Power comes from COTS battery cells, although they are integrated in a custom pack and controlled by a custom BMS board, both of which Flybotix produces in-house. The pack is fitted with two plastic quick-release clips to allow fresh batteries to be snapped into place. “It is more expensive to make those ourselves than go with COTS systems, but we’ve created a lighter and much more convenient system for swapping- out batteries without having to use February/March 2021 | Unmanned Systems Technology The inspection gimbal incorporates a Sony 4K EO camera and a FLIR 160 x 120 thermal camera Less noise makes it safer for pilots working alongside the UAV. If you need ear protection you could miss a safety alarm