From Local Actions to Global Tasks: Stigmergy and Collective Robotics

1. From Local Actions to Global Tasks: Stigmergy and Collective Robotics R. Beckers, O.E. Holland, J.L. Deneubourg Presenter: Lewis Girod

2. Outline What is Stigmergy? Stigmergy in nature A robot experiment: collective pile formation Analysis and conclusions

3. Stigmergy Stigmergy is communication by means of modifying the environment Originally used to describe behavior of nest-building termites and ant trails Paper demonstrates its power as a tool for coordination in a loosely coupled system

4. What’s it good for? Stigmergy is a mechanism for binding task state information to local features of a task site, and for communicating by modifying those features. Example: Following a trail: Trail markers are easy to detect, unambiguously map to location Each marker indicates the next step in following the trail Example: Termites building an arch: Two phases: column building and arch formation Pheremone deposition and diffusion processes select phase Ants need no state; simply react to pheremone concentrations Example: Referencing / returning to location Eliminate need to determine & communicate location

5. Stigmergy in nature Ant trails Ants find the shortest path to a food source in their vicinity using stigmergy to maintain traffic statistics Termite nest-building Termites build columns and arches using stigmergy to retain state about the building process Ant corpse-gathering Ants pick up dead ants and drop them in piles, preferring larger piles, until there is only one pile left

6. Ants finding the shortest path Ants follow random paths, influenced by presence of pheremones Ants returning with food leave stronger trails Pheremones evaporate, causing frequently used trails to dominate Shortcuts result in higher traffic (more trips per ant per unit time) and thus are selected with greater probability

7. Termites building an arch Termites make mud balls with pheremones Termites deposit mud balls near existing pheremone concentrations As columns get taller pheremones on the bottom evaporate Pheremones on neighboring columns cause the tops to be built together to form an arch

8. Ant corpse-gathering Scattered corpses are picked up and dropped Small piles form Gradually the piles are aggregated into a single large pile This paper describes an experiment with robots that exhibits a similar behavior.

9. Collective pile formation task The robots 21x17 cm base with two wheels and a “gripper” battery powered IR sensors for obstacle detection “gripper” (really a “pusher”) force sensor Environment square arena, about 2.5x2.5 m 81 circular pucks (4 cm) arranged on a 25 cm grid

10. The pile formation experiment The initial task given the robots was to push all the pucks into a single pile. At the start of an experiment, robots are in the center, oriented randomly After each 10 minute interval, the robots are stopped and sizes and positions of clusters noted Experiment ends when all pucks are in a single cluster.

11. Robot behaviors Very simple set of three behaviors If IR sensor active: turn away from obstacle through a random angle If force sensor active: Force sensor triggered when 3 or more pucks are pushed When sensor activates, pucks are dropped Reverse both motors for one second Then turn away to a random angle Default: Move forward until sensor activated

12. How it works Robots move around randomly If they bump into a puck, they will push it along. When they bump into their third puck, they drop. Initially, all piles are of size 1 Robots will pick them up and will not deposit until they have collected 3 pucks A pile of 3 or more tends to get bigger Robots that hit a pile of 3 or more head-on will add their pucks to pile.

13. How do piles aggregate? Initially, a few small clusters form quickly. Then, gradually those clusters are aggregated This occurs when pucks are stripped from the edge of a pile and then deposited elsewhere. Larger piles have a larger ratio of areas in the middle to those on the edge. Therefore probability of hitting tangent to pile decreases with pile size. Thus larger piles have a larger probability of increasing as a result of this process.

14. Where is the stigmergy? By adding pucks to a pile, a robot makes the pile larger, and votes for that pile to be largest This stigmergically encodes a message “this is the largest pile, add more pucks to it” The strongest such message (i.e. the largest pile) wins and eventually accretes all the pucks. Because all state information is encoded in observed pile size, new robots can be added with no “communication overhead”

15. Results The experiment was performed with varying numbers of robots Adding robots sped convergence, up to 3 robots More than three robots got in each others’ way Whenever they turn to avoid each other, they run the risk of scattering a nearby pile as they turn away Because the frequency of interactions increases with more robots, 3 was experimentally determined to be optimal Although not shown in paper, this is likely a function of robot density.

16. Seeding clusters Further experiments were done by seeding the environment with a heavy object This forms an initial pile that cannot be consumed Seed reliably accreted the largest pile When two seeds were used, two piles formed Never stabilized The larger seed captured most of the pucks

17. Discussion Stigmergy piggybacks communication on top of robot’s existing sensing and actuation Allows system to scale to additional robots without additional communication overhead Although high densities can lead to gridlock, etc. Stigmergy stores state in the environment so that it is easily retrieved by specialized sensors In nature, pheremones In robotics, wireless communications channels