A small-scale and autonomous testbed for three-line delta kites applied to airborne wind energy

Abstract. A mechanical control system and the guidance and control modules of a small-scale and autonomous testbed for three-line kites applied to airborne wind energy are presented. It extends the capabilities of a previously developed infrastructure by (i) changing the actuation system to add a third tether to control the kite pitch angle, (ii) adding running-line tensiometers to measure the three tether tensions while allowing tether reel-in and reel-out and onboard load cells to measure the bridle tensions, and (iii) providing a real-time control system to operate the kite autonomously in figure-eight trajectories. A controller based on a hybrid guidance scheme for figure-eight flight paths, which combines attractor points for the straight segments and a continuous heading angle tracking for the turns, was implemented and validated in an experimental campaign. Two flights of the campaign were used to illustrate the performance of the controller and its capability to adjust the lateral amplitude, elevation, and radius of the turns by varying a few parameters of the guidance module. The proposed control scheme was proven effective in achieving satisfactory and repeatable figure-eight paths. The experimental data collected during the autonomous flight were used to investigate the dynamics and control of the kite and the tethers. A correlation between the heading and roll angles of the kite was identified and modeled with a simple analytical law with empirical coefficients. Similarly to previous works on airborne wind energy, a linear relation between the derivative of the course angle and the steering input was found. The analysis of the on-ground tensiometers and the onboard load cells revealed a variable time delay of up to 0.2 s between both measurements. The work shows that the testbed and its instruments are suitable for investigating the effect of tether sagging and for developing and testing controllers for airborne wind energy systems.