Collaboration through the exploitation of local interactions in autonomouscollective robotics: The stick pulling experiment

This article presents an experiment which investigates how collaboration in a group of simple reactive robots can be obtained through the exploitation of local interactions. A test-bed experiment is proposed in which the task of the robots is to pull sticks out of the ground-an action which requires the collaboration of two robots to be successful. The experiment is implem ented in a physical setup composed of groups of 2 to 6 Khepera robots, and in Webots, a 3D simulator of Khepera robots. The results using these two implementations are compared with the predictio ns of a probabilistic modeling methodology (A. Martinoli, A. Ijspeert, and F. Mondada, 1999, Robotics and Autonomous Systems, 29:51-63, 1999; A. Marti noli, A. Ijspeert, and L. Gambardella, 1999, in Proceedings of Fifth Europe an Conference on Artificial Life, ECAL99, Lecture Notes in Computer Science , Springer Verlag: Berlin, pp. 575-584) which is here extended for the char acterization and the prediction of a collaborative manipulation experiment. Instead of computing trajectories and sensory information, the probabilist ic model represents the collaboration dynamics as a set of stochastic event s based on simple geometrical considerations. It is shown that the probabil istic model qualitatively and quantitatively predicts the collaboration dyn amics. It is significantly faster than a traditional sensor-based simulator such as Webots, and its minimal set of parameters allows the experimenter to better identify the effect of characteristics of individual robots on th e team performance. Using these three implementations (the real robots, Webots and the probabil istic model), we make a quantitative investigation of the influence of the number of workers (i.e., robots) and of the primary parameter of the robots ' controller-the gripping time parameter-on the collaboration rate, i.e., t he number of sticks successfully taken out of the ground over time. It is f ound that the experiment presents two significantly different dynamics depe nding on the ratio between the amount of work (the number of sticks) and th e number of robots, and that there is a super-linear increase of the collab oration rate with the number of robots. Furthermore, we investigate the use fulness of heterogeneity in the controllers' parameters and of a simple sig nalling scheme among the robots. Results show that, compared to homogeneous groups of robots without communication, heterogeneity and signalling can s ignificantly increase the collaboration rate when there are fewer robots th an sticks, while presenting a less noticeable or even negative effect other wise.