  Robust cooperative formation control is investigated in this paper for
fixed-wing unmanned aerial vehicles in close formation flight to save energy. A
novel cooperative control method is developed. The concept of virtual structure
is employed to resolve the difficulty in designing virtual leaders for a large
number of UAVs in formation flight. To improve the transient performance,
desired trajectories are passed through a group of cooperative filters to
generate smooth reference signals, namely the states of the virtual leaders.
Model uncertainties due to aerodynamic couplings among UAVs are estimated and
compensated using uncertainty and disturbance observers. The entire design,
therefore, contains three major components: cooperative filters for motion
planning, baseline cooperative control, and uncertainty and disturbance
observation. The proposed formation controller could at least secure ultimate
bounded control performance for formation tracking. If certain conditions are
satisfied, asymptotic formation tracking control could be obtained. Major
contributions of this paper lie in two aspects: 1) the difficulty in designing
virtual leaders is resolved in terms of the virtual structure concept; 2) a
robust cooperative controller is proposed for close formation flight of a large
number of UAVs suffering from aerodynamic couplings in between. The efficiency
of the proposed design will be demonstrated using numerical simulations of five
UAVs in close formation flight.
