Abstract
Area coverage and collective exploration are key challenges for swarm robotics. Previous research in this field has drawn inspiration from ant colonies, with real, or more commonly virtual, pheromones deposited into a shared environment to coordinate behaviour through stigmergy. Repellent pheromones can facilitate rapid dispersal of robotic agents, yet this has been demonstrated only for relatively small swarm sizes (N < 30). Here, we report findings from swarms of real robots (Kilobots) an order of magnitude larger (N > 300) and from realistic simulation experiments up to N = 400. We identify limitations to stigmergy in a spatially constrained, high-density environment—a free but bounded two-dimensional workspace—using repellent binary pheromone. At larger N and higher densities, a simple stigmergic avoidance algorithm becomes first no better, then inferior to, the area coverage of non-interacting random walkers. Thus, the assumption of robustness and scalability for such approaches may need to be re-examined when they are working at a high density caused by ever-increasing swarm sizes. Instead, subcellular biology, and diffusive processes, may prove a better source of inspiration at large N in high agent density environments.
Highlights
Exploration and area coverage are classic challenges for cooperating robot teams: agents need to coordinate effectively such that an unknown territory is rapidly reconnoitred and monitored on an ongoing basis
Stigmergy, allows robots to communicate through the environment, for example through depositing 2 chemical markers known as pheromones that facilitate indirect communication and coordination [3]
Social insects like ants are the archetypal inspiration for swarm robotics research: in particular, their use of pheromones facilitates highly effective, low-cost coordination of many individuals
Summary
Exploration and area coverage are classic challenges for cooperating robot teams: agents need to coordinate effectively such that an unknown territory is rapidly reconnoitred and monitored on an ongoing basis. Stigmergy, allows robots to communicate through the environment, for example through depositing 2 chemical markers known as pheromones that facilitate indirect communication and coordination [3]. Just as in nature where stigmergy has limits to its usefulness, for example where army ants form futile circular mills through trail following [4], so too in robotics limitations may arise when the environment becomes saturated with pheromone information. Stigmergy is a likely precursor to the development of internal memory [18], and has been observed as such in very simple organisms, including slime moulds [19] Through this simple organizing principle, swarms of organisms or robots can exhibit remarkable, emergent ‘swarm intelligence’, with respect to tasks such as foraging or exploration [20]. An alternative form of stigmergy to deploying chemicals is depositing building material, as seen in the termites; using this concept to 3 coordinate a group of robots was previously found to show worsening task performance at larger group sizes (four or five versus three) owing to time-consuming robot interactions [23]
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