This work discusses a trajectory tracking task accomplished by a formation of two heterogeneous robots, namely a Pioneer 3-DX (UGV) and an AR.Drone 2.0 quadcopter (UAV). The control paradigm considers the formation as a virtual structure, here represented by a 3D straight line linking the two agents. The formation is characterized by the distance between the two robots and two angles, measured between the linking-line and the YZ and XZ planes. In this approach, the formation position coincides with the UGV position. The transformation between the robot to formation variables allows characterizing the motion and shape of the formation starting from the movement of the robots, or vice-versa. Then, a controller guides the formation and manages two conflicting subtasks (reach the desired robot position or keep the formation shape), whose priority is defined by the null space-based approach. Finally, real experiments validate the proposed approach during trajectory-tracking tasks for three scenarios, one assigning the highest priority to the formation position, another to the formation shape, and a last one, without any priority. In addition, we evaluated the formation behavior in failure situations in one of the robots.
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