Input-output invertibility and sliding mode control for close formation flying

This paper treats the question of invertibility of input-output maps and the design of a robust control system for formation flying of multiple unmanned aerial vehicles (UAVs). In close formation, the wing UAV motion is affected by the vortex of the adjacent lead aircraft. In this paper, these forces are treated as unknown functions. For trajectory tracking, invertibility of certain input-output maps in the wind axes system are examined. Interestingly, in the wind axes system, the system is not invertible, but in a simplified coordinate system obtained from the wind axes system for which the velocity roll is zero, inverse control of separation coordinates is possible. Variable structure control laws are derived for separation trajectory control of wing aircraft in the simplified wind coordinate system and for the flight control of the lead aircraft. Simulation results for two UAVs are presented which show precise separation trajectory control in spite of the presence of unknown vortex forces, while the lead aircraft maneuvers. Furthermore, these results confirm that when the wing aircraft is positioned properly in the vortex of the lead aircraft, there is a reduction in the required flight power.