  Airborne wind energy systems aim to generate renewable energy by means of the
aerodynamic lift produced by a wing tethered to the ground and controlled to
fly crosswind paths. The problem of maximizing the average power developed by
the generator, in presence of limited information on wind speed and direction,
is considered. At constant tether speed operation, the power is related to the
traction force generated by the wing. First, a study of the traction force is
presented for a general path parametrization. In particular, the sensitivity of
the traction force on the path parameters is analyzed. Then, the results of
this analysis are exploited to design an algorithm to maximize the force, hence
the power, in real-time. The algorithm uses only the measured traction force on
the tether and it is able to adapt the system's operation to maximize the
average force with uncertain and time-varying wind. The influence of inaccurate
sensor readings and turbulent wind are also discussed. The presented algorithm
is not dependent on a specific hardware setup and can act as an extension of
existing control structures. Both numerical simulations and experimental
results are presented to highlight the effectiveness of the approach.
