Robophysical Study of Excavation in the Confined Environments

Nest excavation in social insects is an integral part of a colony’s life cycle. Soil-dwelling colonies build complex systems of narrow underground chambers and tunnels spanning thousands of insect body lengths below the ground. The nest excavation is performed by multiple animals simultaneously and is governed by local interactions of the workers with environment and other workers. The challenges and advantages of such a system are poorly investigated, mainly due to the complexity of the biological system and the lack of experimental control on animal behavior. To address this problem we designed and built groups of robotic excavators, capable of performing days of autonomous tunnel excavation in a model cohesive granular media. The excavator behavior was governed by a simple set of rules triggered by interactions with the surrounding environment. In the experiments we tested the effect of the tunnel width and the size of the excavating group on the rate of the tunnel growth and the energetic costs of excavation for individual workers. An experimentally validated cellular automata model was developed to extend experimental results to systems with larger numbers of robots. The experimental data and simulations showed that in sufficiently wide tunnels the increase in the size of the excavating group had a positive effect on the tunnel excavation rates without significant increase in the energy consumption per robot. A decrease in the tunnel width resulted in a decrease in the tunnel excavation rates and increase in the energetic costs of excavation per robot. We attribute this effect to the emergence of multiple interactions (jams) among excavating robots in the confined spaces. Although the jams were successfully resolved based on local mechanical interactions of the robots in the tunnel, their presence slowed the excavation down appreciably. The duration of jams was longer in the systems with higher number of robots or narrower tunnels. We expect that despite its relative simplicity our robotic system can be used to investigate the behavior of social insects in the confined spaces as well as inspire more sophisticated robotic search-and-rescue teams in crowded environments.