Control Allocation for Multicopter Unmanned Aerial Vehicles

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Unmanned aerial vehicles are experiencing a surge in civilian and commercial use as new applications for airborne monitoring are being developed. ARA Robotics is a Montreal-based company that has taken advantage of these developments by providing multi-rotor-based solutions for commercial applications. In particular, ARA currently provides airborne photogrammetry services for construction sites, in which multi-rotor aircraft are programmed to fly through a series of waypoints to collect images of these sites. To reduce downtime of on-site operations during these flights, it is crucial that they be executed as quickly as possible. However, this carries the consequence of pushing the thrusters on the aircraft to their limits, leading to motor commands needing to be clipped to ensure that they operate within these limits. This then leads to unpredictable and potentially unsafe behaviour of the aircraft. To tackle this issue of actuator saturation during unconventional flight, we will work to develop a mixer-based solution that incorporates the saturation and rate limits of the thrusters. This solution will consist of real-time optimization methods that will prioritize particular degrees of freedom (dofs) in order to minimize the impact of actuator saturation on the dofs that have the greatest impact on vehicle stability and safety. A secondary approach will then be investigated to develop a controller which embeds actuator and/or motion limitations. Cascaded control will be considered in which pitch and roll control will be prioritized and performed separately from yaw control, which would be attempted only if actuator limits are not reached. Following this, Model-Predictive Control will be used to explicitly consider actuator limitations during the generation of control commands. Simulations and experimental testing will be performed to evaluate the performance of all these approaches, with the assistance of ARA's engineers. The improved surveying capabilities for drones will broaden the range of applications of these small vehicles and would also have application in other scenarios, such as situation assessment in disaster response and search and rescue, which are key to Canada's humanitarian contribution on the global stage.