Abstract
This letter presents the modeling, system identification and nonlinear model predictive control (NMPC) design for longitudinal, full envelope velocity control of a small tiltwing hybrid unmanned aerial vehicle (H-UAV). A first-principles based dynamics model is derived and identified from flight data. It captures important aerodynamic effects including propeller-wing interaction and stalled airfoils, but is still simple enough for on-board online trajectory optimization. Based on this model, a high-level NMPC is formulated which optimizes throttle, tilt-rate and pitch-angle setpoints in order to track longitudinal velocity trajectories. We propose and investigate different references suitable to regularize the optimization problem, including both offline generated trims as well as preceding NMPC solutions. In simulation, we compare the NMPC with a frequently reported dynamic inversion approach for H-UAV velocity control. Finally, the NMPC is validated in flight experiments through a series of transition maneuvers, demonstrating good tracking capabilities in the full flight envelope.
Highlights
H HYBRID unmanned aerial vehicles (H-UAVs) are versatile aircraft that can convert between fixed-wing and rotary-wing flight modes and feature both efficient and fast cruise flight as well vertical take-off and landing (VTOL) capabilities
We focus on control strategies for tiltwing H-UAVs, see Fig. 1
The nonlinear model predictive control (NMPC) interfaces with the low-level attitude-controlled system and, apart from the wing-tilt, not directly with the aircraft actuators. This abstraction reduces the aerodynamic modeling required for the NMPC to the translational dynamics
Summary
H HYBRID unmanned aerial vehicles (H-UAVs) are versatile aircraft that can convert between fixed-wing and rotary-wing flight modes and feature both efficient and fast cruise flight as well vertical take-off and landing (VTOL) capabilities. 1) Related Work: The task of velocity-, or cruise-control on UAVs is most commonly studied for fixed-wing UAVs, where non-linear, velocity-dependent aircraft trims and the dominant interrelations between potential and kinetic energy require particular attention Such control systems need to respect flight envelope boundaries to protect the aircraft from entering stall. The few existing velocity control systems for tiltwing and tailsitter UAVs are almost exclusively based on NDI [1], [3], [4], [7] where control inputs are calculated to follow a desired model reference While those systems are shown to work well in general, careful tuning and heuristics in the reference generation are required to prevent, e.g., actuator saturation. We conclude with a set of flight experiments to evaluate the NMPC performance on a real tiltwing H-UAV
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