Dynamics and Control of Single-Line Maneuverable Kites

Background:  High-altitude winds (200 to 800m) are faster and more consistent around the world, but inaccessible to conventional wind turbine technology. Controlled kites can be used to harness this energy and produce less of a footprint than a wind tower. Wind tower height is limited by structural strength. The higher they are built, the larger bending moment from the wind. A tethered system only needs to withstand a tension force. Current kite power systems rely on precision GPS tracking and other on-kite instrumentation to provide feedback to the controller. An alternative is to use a maneuverable, single-line, tension-controlled kite. These kites, unlike common multiple line kites, have stable and unstable states that are toggled between by changing the tether tension. A flyer constantly changes the length of the tether to control the kite.  Above critical tension the kite flexes and flies straight. Below critical tension the kite is unstable and spins about the tether. The principal goal of our research is to understand the dynamics and achieve automatic control of such a kite. We use a combination of physics-based numerical models and experimental testing. The single-line maneuverable kite could reduce overall complexity and cost of a power generating system. The removal of on-kite sensors makes the kites expendable and eliminates the difficulties of data transmission and powering the on-kite sensors.

Methods:        A numerical simulation of a simplified three-dimensional, three degree-of-freedom system has been created to predict the motion of the kite under applied control of the tether. The simulation uses explicit numeric integration routines to approximate the solution of the set of coupled non-linear governing equations of motion. Control algorithms are initially tested using the simulation before testing on the experimental test bed.

Experimental validation is currently underway. A three axis load cell test bed is under construction and will provide the controller with the magnitude and direction of the tether tension. The encoder on the drive motor will determine tether length. Velocity feedback will be used to control the kite.

Results:           Initial simulation shows that the kite tracks to a stable position while in the high tension state, which has also been validated by manual flight of the existing kites in extreme wind conditions. Manually flying the kite, and subsequent video analysis, has provided guidance in improving simulation parameters such as spin rate and direction.

Conclusions:  The goal of this research is to achieve automated, sustained flight of a naturally unstable kite system and to better understand the system as a whole. This base knowledge can be built on to better understand, control, and design kite power systems. Experienced human flyers can expertly maneuver these kites with precision from hundreds of feet solely by changing the tension in the tether. The potential exists to produce a low cost, renewable energy source that can be implemented around the world.