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

This paper treats the question of invertibility of input-output maps and th e design of a robust control system for formation flying of multiple unmann ed aerial vehicles (UAVs). In close formation, the wing UAV motion is affec ted by the vortex of the adjacent lead aircraft. The forces produced by the se vortices are complex functions of relative position coordinates of the U AVs. In this paper, these forces are treated as unknown functions. For traj ectory tracking, invertibility of certain input-output maps in the wind axe s system are examined. Interestingly, in the wind axes system, the system i s not invertible, but in a simplified co-ordinate system obtained from the wind axes system for which the velocity roll is zero, inverse control of se paration co-ordinates is possible. Variable structure control laws are deri ved for separation trajectory control of wing aircraft in the simplified wi nd co-ordinate system and for the flight control of the lead aircraft. Simu lation results for two UAVs are presented which show precise separation tra jectory control in spite of the presence of unknown vortex forces, while th e lead aircraft maneuvers. Furthermore, these results confirm that when the wing aircraft is positioned properly in the vortex of the lead aircraft, t here is a reduction in the required flight power. Published in 2000 by John Wiley & Sons, Ltd.