Phase Transitions of Oscillating Robots with Contact-based Interactions

Groups of decentralized actuation units are known to exhibit emergent collective dynamics. In robotics, this behavior is advantageous since it enables phenomena such as the ability to reconfigure and perform locomotion that is robust to obstacles and damage. Here, we focus on the role of contact interactions for the development of useful collective robotic systems. Biological collectives that rely on forceful contact interactions often exhibit remarkable capabilities, such as resistance to environmental perturbations and functionality without long-range communication. However, many of these capabilities have yet to be realized in the areas of robotic locomotion and reconfiguration. In this study, we present a robotic system composed of oscillatory, nonlinearly interacting elements in both inertially relevant and negligible regimes. Through experiment we study the force-displacement properties of disordered collectives under varying constant load. We show an inverse relationship between the applied force and the displacement of a piston driven by the collective. At low force, the collective expands from high to low density, but at higher force, it becomes "frozen". By adjusting parameters such as density and activity levels, we develop a phase diagram for this active system and compare the results with models such as those based on statistical gas mechanics. Our study opens the possibility for multibody distributed robotic systems that outperform counterparts in nature.