Algorithms for flight control and path planning
Due to their comparatively low energy demand, fixed-wing UAVs are superior to multirotor drones for applications which require a vehicle to stay airborne for a long time or cover large distances, e.g., search and rescue, or surveillance applications. In order to safely and efficiently operate a fixed-wing UAV in challenging environments, however, accurate planning and precise tracking becomes an essential requirement. These requirements are usually not fulfilled by off-the-shelf autopilots for waypoint tracking.
The iFR has developed a control framework for the precise planning and tracking of flyable, spline-based 3D trajectories. An extended Dubins path method guarantees C2-continuity and allows to integrate performance limitations. At the same time, a cubic spline parameterization and flat system formulation are used to derive feedforward commands for increased path following accuracy. The approach therefore allows for an efficient planning of flyable 3D trajectores as well as highly accurate tracking. Nonlinear dynamic inversion is used for the guidance law to achieve global stability.
Instead of cascaded single-input-single-output (SISO) PID rate and attitude control, a multiple-input-multiple-output (MIMO) state feedback control law was designed. Flight tests demonstrate the high tracking accuracy of the complete framework (Figure above).
 Fichter, W. and Stephan, J., Flugregelung-Theoretische Grundlagen für die Lenkung und Regelung von Flächenflugzeugen, https://link.springer.com/book/10.1007/978-3-662-60907-1
Pinchetti, A. Joos and W. Fichter, „Efficient Continuous Curvature Path Generation with Pseudo-Parametrized Algebraic Splines” in CEAS Aeronautical Journal, Vol. 9, No. 4, pp. 557–570, 2018
Stephan, O. Pfeifle, S. Notter, F. Pinchetti and W. Fichter, „Precise Tracking of Extended Three-Dimensional Dubins Paths for Fixed-Wing Aircraft“ in Journal of Guidance, Control, and Dynamics, Vol. 43, No. 12, pp. 2399-2405, 2020