Applied Tracking Control for Kite Power Systems

This paper presents a tracking controller applicable to tethered flying objects, such as kites for power generation or towing purposes. A kinematic framework is introduced, employing definitions and terminology known from aerospace engineering, and is used for both modeling and control design. Derived from measurement data, an empirical steering-law correlation is presented, establishing a highly reliable connection between the steering inputs and the kite's yaw rate, and thus providing an essential part of the cascaded controller. The target trajectory is projected onto a unit sphere centered at the tether anchor point, and based on geometrical considerations on curved surfaces, a tracking-control law is derived, with the objective to reduce the kite's spacial displacement smoothly to zero. The cascaded controller is implemented and integrated into the software and hardware framework of a 20 kW technology demonstrator. Because of the lack of a suitable simulation environment, its performance is assessed in various field tests employing a 25 m(2) kite, and the results are presented and discussed. The results, on the one hand, confirm that autonomous operation of the traction kite in periodic pumping cycles is feasible; yet, on the other, that the control performance is severely affected by time delays and actuator constraints.