Biological Inspirations for Distributed Roboticsftang/courses/CS599-DI/notes... · Biological...
Transcript of Biological Inspirations for Distributed Roboticsftang/courses/CS599-DI/notes... · Biological...
Outline
� Biological inspirations
� Understand two types of biological parallels
� Understand key ideas for distributed robotics obtained from study of biological systems
� Understand concept of stigmergy
� Understand use of stigmergy for tasks in collective robotics
Movies of Some Animal Collectives
� School of fish� http://www.youtube.com/watch?v=_tGOKngtkt4&feature=related
� Flock of birds
� http://www.youtube.com/watch?v=TL8diH-I9EQ
� Etc.
Why Biological Systems?
� Key reasons:
� Animal behavior defines intelligence
� Animal behavior provides existence proof that intelligence is achievable
� Typical objects of study:
� Ants
� Bees
� Birds
� Fish
� Herding animals
Societies that Differentiate
� Innate differentiation of blood relatives
� Strict division of work and social interaction
� Individuals:
� Exist for the good of society
� Are totally dependent on society
� Examples:
� Bees
� Ants, termites
Stay Together
Societies that Integrate
� Depend on the attraction of individual animals
� Exhibit loose division of labor
� Individuals:
� Integrate ways of behavior
� Thrive on support provided by society
� Are motivated by selfish interests
� Examples:
� Wolf, hunting dogs, etc.
� Bird colonies
Come Together
Which Approach To Choose?
� Differentiating approach:
� For tasks that require numerous repetitions of same activity over a fairly large area
� Examples:
� Waxing floor
� Removing barnacles off ships
� Collecting rock samples on Mars
� Integrating approach:
� For tasks that require several distinct subtasks
� Examples:
� Search and rescue
� Security, surveillance, or reconnaissance
Key Ideas from Biological Inspiration
� Communication
� Auditory, chemical, tactile, electrical
� Direct, indirect, explicit, implicit
� Roles
� Strict division vs. loose “assignments”
� Hierarchies
� Absolute linear ordering, partial ordering, relative ordering
� Purpose: reduction in fighting, efficiency
� Territoriality
� Reduces fighting, disperses group, simplifies interactions
� Imitation
� Complex mechanism for learning
Our Distributed Robotics Studies
� First: low-level, homogeneous, swarm robots
� Swarming, dispersion, homing, etc.
� Search/coverage
� Etc.
� Then: higher-level strategies, heterogeneous robots
� Multi-robot path planning, traffic management
� Task allocation
� Etc.
Key Concept in “Swarm” Distributed Robotics: Stigmergy
� Stigmergy:
� Term used by some biologists to describe influence on behavior due to persisting environmental effects of previous behavior
� Originally used by French biologist Pierre-Paul Grasse to describe behavior of nest-building termites and trails
� Equivalent concept: implicit communication by means of modifying the environment
� A mechanism for binding task state information to local features of a task site, and for communicating (implicitly) by modifying those features
� Stigmergy is a powerful tool for coordination in a loosely coupled system
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
Ants Finding The Shortest Path
� Ants follow random paths, influenced by presence of pheromones
� Ants returning with food leave stronger trails
� Pheromones evaporate, causing frequent trails to dominate
� Shortcuts result in higher traffic (more trips per ant per unit time) and thus are selected with greater probability
� http://www.youtube.com/watch?v=kN0M49iqFRc
Termites Building An Arch
� Termites make mud balls with pheromones
� Termites deposit mud balls near existing pheromone concentrations
� As columns get taller pheromones on the bottom evaporate
� Pheromones on neighboring columns cause the top to be built together to form an arch
� http://www.youtube.com/watch?v=0m7odGafpQU&feature=PlayList&p=598428DDC4E49D85&index=0&playnext=1
Ant Corpse-Gathering
� Scattered corpses are picked up and dropped
� Small piles form
� Gradually the piles are aggregated into a single large pile
How Does Stigmergy Produce Complex Patterns?
� The state of the environment, and the current distribution of agents within it, determine how the environment and the distribution of agents will change in the future
� Any structure emerging is a result of self-organization
� Self-organization:� A set of dynamic mechanisms whereby structures appear at
global level of a system resulting from interactions among lower-level components
� Rules specifying interaction are executed purely based on local information, without reference to global pattern
Minimal Qualities of Agent and Environment to Support Stigmergy
� Agent has 2 key abilities:
� It can move through environment
� It can act on environment
� To enable stigmergy:
� Environment must be changed locally by agents
� Changes must persist long enough to affect the choice, parameters, or consequences of agents’ behavior
Two Ways to Structure Behavioral Sequences
� In solitary species� Execution of first movement in sequences sets
internal state
� With external cue, internal state initiates second movement, etc.
� In solitary and social insects� No internal state required (in many, but not all,
cases)
� External cue alone is sufficient to invoke subsequent actions
� Sets the stage for stigmergy
Compare Stigmergy to Direct Communication
� Direct communication requires:
� Sending robot to encode and transmit message about what is to be done, and where it is to be done
� Implies knowledge of location
� Message is local in time and space, therefore only robots close enough and not otherwise engaged will be free to receive the message
� Robots must decode received messages, and remember them long enough to get to the place and carry out the action, or even longer if they are currently carrying out a more important task
� Stigmergic communication:
� Requires no encoding or decoding
� Requires no knowledge of place
� Requires no memory
� Is not transient
Example Use of Stigmergy in Collective Robotics
� Paper references:
� “Stigmergy, Self-Organization, and Sorting in Collective Robotics”, by Holland and Melhuish, Artificial Life 5: 173-202, 1999.
� “From Local Actions to Global Tasks: Stigmergy and Collective Robotics”, by Beckers, Holland and Deneubourg in Brooks and Maes (eds.), Artificial Live IV: 181-189, Cambridge, MA: MIT Press, 1994.
Collective Pile Formation Task
� The robots� ~20cm square base with two
wheels and a gripper
� Battery powered
� Infrared (IR) sensors for obstacle detection
� Gripper force sensor
� Environment� Square arena, about
2.5x2.5m
� 81 circular pucks (4cm) arranged on a 25cm grid
Beckers’ approach
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 single cluster
Robot Behaviors
� Very simple set of 3 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
Back to Experiment (Becker)
� 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
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
� Large piles have a larger ratio of areas in the middle to those on the edge. Therefore probability of hitting tangent to pile decreases with increasing pile size
� Thus larger piles have a larger probability of increasing as a result of this process
Where is the Stigmergy?
� By adding pucks to a pile, a robot makes the pile larger, and “votes” (implicitly) 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”
Experimental Results (Beckers)
� The experiment was performed with varying numbers of robots
� Adding robots sped convergence, up to 3 robots
� Why?
� More than three robots got in each others’ way(i.e., interference)
� 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
� Interference is a function of robot density
Experimental Results (Cont.)
1. Over time, size of the biggest cluster grows
2. More robots � faster cluster
growth up to a point of robot interference
1. Over time, # of clusters shrinks
2. More robots � faster
reduction, up to a point of robot interference
Experimental Results (Cont.)
� For these experiments, 3 robots was optimal
� Number of interactions increased significantly with number of robots
� Robot efficiency for these experiments was optimized at 3 robots
Summary of Stigmergy
� Stigmergy piggybacks communication on top of robot’s existing sensing and actuation� Allows system to scale to additional robots with 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, pheromones
� In robotics, variety of sensors
� Stigmergy can be regarded as the general exploitation of the environment as external memory resource
Second Case Study
� Title
� Multi-robot System Based on Model of Wolf Hunting Behavior to Emulate Wolf and Elk Interactions
� Authors:
� John D. Madden, Ronald C. Arkin and Daniel R. MacNulty
� IEEE International Conference on Robotics and Biomimetics, 2010
Goal of the Project
� Models of behavior from biology are used to develop heterogeneous unmanned network teams (HUNT)
� The ability to reduce communication and planning requirements for robot groups, while still achieving missions
� Mission: pursuit-evasion tasks
Wolf Behavior from Nature
� No obvious pattern of coordinated hunting behavior
� Rules of thumb:
� Attack while minimizing risk of injury with no overall had behavioral constraints on actions
Coordination or Lack Thereof
� Wolves show no signs of planned strategies and no noticeable communication while hunting
� They do not make transitions together
� Coordination is a byproduct where each individual is maximizing its own utility
� Seeing elk being chased signals a sign of weakness of the prey, so they join the pursuit
List of Releasers and Transitions
Weighted roulette wheel was used to decide which transition to take
Conclusion
� High fidelity bio models can provide utility for a range of multi-robot applications
� Byproduct mutualism can provide robust results for bio groups
� The ability to reduce communication and planning for robot groups
Summary of Biological Inspirations
� Study social biological systems either to:
� Obtain inspiration for how to build multi-robot systems
� Validate theoretical models for how biological systems work
� Two types of biological parallels: differentiating and integrative
� Many possible sources of inspiration from biology
� Stigmergy is important concept for swarm cooperation “through the world”