This video shows three quadrocopters cooperatively play with a ball.
The quadrocopters are attached to an elastic net. The net allows them to catch the ball by moving such that they span the net under the falling ball. Since the net is elastic, the quadrocopters can not just catch but also toss the ball back into the air by rapidly accelerating outwards, thereby stretching the net and launching the ball from it. As can be seen in the video, the three flying robots are then forcefully pulled back together by the elastics, requiring them to rapidly stabilize their position to avoid a collision.
To achieve this task, novel approaches are required. Unlike previous algorithms used to perform very rapid flips, juggle balls individually, or move in synchrony with each other, this task requires cooperation between physically coupled flying vehicles that fly at the limits of their capabilities, exploiting the dynamical potential that these vehicles provide.
The algorithms that are used include an optimality-based real-time trajectory generation for the catching maneuver, a time-varying trajectory following control strategy to control the individual vehicles under consideration of the forces induced by the net, and learning algorithms that accommodate model inaccuracies in the aiming of the ball.
By Robin Ritz, Mark W. Müller, Markus Hehn, and Raffaello D'Andrea.
IDSC, ETH Zürich, Switzerland