Null Space Based Formation Control for a UAV Landing on a UGV

摘要

This paper proposes a controller to autonomously guide a formation of two mobile robots, a differential drive wheeled ground vehicle and an unmanned aerial vehicle, in tasks of positioning. The controller is designed considering the paradigm of virtual structure, in which the virtual structure is the tridimensional straight line linking the two robots, and adopting the technique of null space, to split the whole control task in two sub-tasks, which are to keep the formation and to move the formation. To do that the formation is described by a set of variables, the formation variables, which are derived from the individual robot positions. To apply the null-space technique, the formation variables are grouped into two sets, one associated to the shape of the virtual structure and the other associated to the position of the virtual structure. The use of the null space technique allows to give higher priority to the sub-task of keeping the formation shape, leaving the sub-task of moving the virtual structure to a new position with a lower priority, thus generating a rigid formation. Although the reverse situation, to give higher priority to the movement of the formation to a new position, generating a flexible formation, is also possible, simulated and experimental results have shown that to give higher priority to keep the shape of the formation is the best solution. The reason is that we are aiming at applications demanding that the position of the aerial robot with respect to the ground one be well known along time, allowing the aerial vehicle to land over the terrestrial one.