  An approach to control tethered wings for airborne wind energy is proposed. A
fixed length of the lines is considered, and the aim of the control system is
to obtain figure-eight crosswind trajectories. The proposed technique is based
on the notion of the wing's "velocity angle" and, in contrast with most
existing approaches, it does not require a measurement of the wind speed or of
the effective wind at the wing's location. Moreover, the proposed approach
features few parameters, whose effects on the system's behavior are very
intuitive, hence simplifying tuning procedures. A simplified model of the
steering dynamics of the wing is derived from first-principle laws, compared
with experimental data and used for the control design. The control algorithm
is divided into a low-level loop for the velocity angle and a high-level
guidance strategy to achieve the desired flight patterns. The robustness of the
inner loop is verified analytically, and the overall control system is tested
experimentally on a small-scale prototype, with varying wind conditions and
using different wings.
