Controlling rigid-wing airborne wind energy systems during circular flight without exact path following

Abstract. We propose a feedback architecture that enables effective flight control of rigid-wing airborne wind energy systems during circular-pattern reel out. The controller performs well with only proportional-integral regulators, thereby presenting a significantly simpler solution compared to the existing literature. The key idea is in tracking the roll angle on a non-static reference frame, effectively reducing the control problem to one degree of freedom. This method of navigation does not require users to define an exact path for the kite to follow, which contributes to stability and robustness. In its minimum viable form, the controller can function with only ailerons while requiring no pitot-tube measurement, although the addition of elevators and rudder enables angle-of-attack and zero-sideslip tracking for more efficient power generation. Simulation-based verification is conducted on an industrial six-degree-of-freedom model with a flexible tether, nonlinear aerodynamics, and realistic wind conditions, showing satisfactory performance. Three expansions to the control law are then presented. The first one reduces angle of attack fluctuation during reel out by adding a proportional pitch angle feedback term to the elevator, resulting in more power. In the second expansion, the reel-out radius is automatically adjusted to enable phase synchronisation of multiple kites in a farm configuration, where minimum separation rules may apply. The third expansion implements a proportional feedback rule that enables figure-of-eight flight. By using proportional-integral architecture, the controller is easy to implement, making it a suitable baseline system for benchmarking more advanced control laws.