Unmanned Systems Technology 036

8 Platform one Researchers have used a new type of image sensor to help keep a multi-rotor UAV flying when a motor fails (writes Nick Flaherty). The joint team, from the University of Zurich in Switzerland and the Delft University of Technology in the Netherlands, used an event-driven sensor alongside a conventional camera sensor to stabilise the UAV and keep it flying autonomously if one rotor suddenly gives out. “When a rotor fails, the UAV begins to spin on itself like a ballerina,” said Davide Scaramuzza, head of the Robotics and Perception Group at UZH. “This high- speed rotational motion causes standard controllers to fail unless the UAV has access to very accurate position measurements.” One way to solve the problem, called visual inertial odometry (VIO), is to provide the UAV with a reference position through a GNSS satellite navigation system. However, that may not be available in urban environments with tall buildings, where safe operation needs to be guaranteed. That is particularly important with delivery services now certified to fly BVLOS. Instead, the researchers combined a standard image sensor that records images at a fixed rate, usually 50 Hz, with an event-driven sensor. This is based on independent pixels that are only activated when they detect a change in the light that reaches them (see UST 35, December/January 2021, page 84). The team developed VIO state estimation algorithms that combine information from the two sensors to track the quadrotor’s position relative to its surroundings. This enables an onboard computer to control the UAV as it flies, and spins, with only three rotors. This would allow the UAV to steer to a safe location. The researchers found that both types of sensors perform well in normal light conditions, but that the event-driven one has an advantage in low light. “When the illumination decreases, however, standard sensors begin to experience motion blur that ultimately disorientates the UAV and crashes it, whereas event- driven sensors work well in very low light,” added Sihao Sun, a postdoctoral researcher in Scaramuzza’s lab. Airborne vehicles February/March 2021 | Unmanned Systems Technology Spin sensor for stability The sensor combines a standard image sensor with one that is event-driven