1. Concurrent Graph Exploration with Multiple Robots
Hui Wang
12/1/2018

2. Outline Introduction Previous work (background) Concurrency concerns
Skeleton of operation model
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3. Outline Introduction Previous work (background) Concurrency concerns
Skeleton of operation model
12/1/2018

4. Introduction Address robotic exploration problem on a graph-like world
Robot(s) given an unknown environment modeled as a graph
Goal is to establish a graph representation of the underlying world (mapping)
Exist single robot exploration algorithm
Extended to multiple robot case in my Master’s studies
Simple nature of synchronization and parallelism modeled
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5. Introduction (cont’)
Extend the multiple robot exploration with concurrency concerns.
Useful framework within which to explore fundamental issues related to concurrent data structure exploration.
E.g., robot (thread) synchronization, contention, and communication
Allows for in-depth investigation of real life exploration challenges, e.g., measuring real execution time.
Subject to refinement in later work.
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6. Outline Introduction Previous work (background) Concurrency concerns
Skeleton of operation model
12/1/2018

7. Previous work (background)
Problem: Given an unknown environment modeled as an embedded graph
Embedding: Explicit specification of the order of edges incident upon each vertex of the graph
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8. Previous work (background)
Unpleasant Constraints
No prior knowledge of the environment at all (except starting place)
No distance and orientation metric (compass)
Edges are featureless, vertices featureless except degree
Cannot label real edges, vertices…..
Still solvable? Generally not
Unless augment robot with disambiguation capability.
E.g., use marker (bread crumbs) to solve the
‘have I been here before?’ problem
`Have I been here before?’
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9. Single robot exploration
Key idea: maintains currently known sub-graph S, which evolves during process
Basic operation: selects an unexplored edge eunknown from currently known graph S, explores to an unknown end (vertex) vunknown of the edge
Critical question: whether the end vunknown was known, i.e., vunknown in S?
Deterministic solution: drops marker (bread crumbs) at vunknown, searches S for the marker. S
eunknown v1 vu v2 v3
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10. Single robot exploration (cont’)
drop marker at vunknown, search S for the marker.
if marker is not found in S // both eunknown and vunknown are unknown
add eunknown and vunknown to S
else if marker is found in S // vunknown is v2 , only eunknown is unknown
add eunknown to S S
eunknown v1 vu v2 v3 S
eunknown v1 v2
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11. Multiple robots exploration
Extended to multiple robots case.
Same word model,
Unique marker
Common starting place, percept each other, and communicate (within vertex)
Expectation: perform tasks faster, more robust than single one
Alternating phases of independent exploration and coordinated merging of partial representations. Merge similar.
common map := initial starting place
while common map has unexplored part
Partition the common map (Task split)
Explore in parallel
Meet at rendezvous place (common) after a certain time steps
Merge the partial maps
Merged map shared by all robots, becoming new common map
End while
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12. Multiple robot exploration example
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13. 12/1/2018

14. Multiple robot exploration on Real system
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15. Outline Introduction Previous work (background) Concurrency concerns
Skeleton of operation model
12/1/2018

16. Concurrency concerns
Pervious work assumes simple nature of parallelism and synchronization
Operation not in full parallel
Sufficient for specific problem solving (evaluate edge traversal)
Not sufficient for other concerns (evaluate real execution time )
Concurrent modeling better reflects real life exploration
Allow for investigation of more challenging problem
e.g., meet in edge (hallway)
e.g., resource contention, real time allocation
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17. Concurrency concerns
Start with the basic algorithm -- modeling, implementing and testing
Incorporating concurrency-related enhancements developed
opportunistic communication
parallelism in the merge phase
Investigating more concurrency-related problems based on the concurrent model of the problem.
encountering each other on edges
evaluating by real resource consumption, e.g., execution time
dealing with large groups (k ≥ 2) of robots.
More new ideas acquired during the progress of the course work.
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18. Outline Introduction Previous work (background) Concurrency concerns
Skeleton of operation model
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19. Skeleton of operation model
We model the operation at each vertex as a sequence of action
( sense, initiate-communication, manipulate, move ).
We assume the first 3 operation
(sense, initiate-communication, manipulate, move)
requires mutual exclusion.
We distinguish two types of operation and communication at a vertex:
at a rendezvous place
at a non-rendezvous place
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20. Skeleton of operation model
Operation at rendezvous place
Challenge: synchronization
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21. Skeleton of operation model
Operation at non-rendezvous place
Challenge: establish communication (synchronous, asynchronous?)
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22. References
[1] G. Dudek, M. Jenkin, E. Milios, and D. Wilkes. Robotic exploration as graph construction. IEEE Transactions on Robotics and Automation, 7: , (wiki)
[2] G. Dudek, M. Jenkin, E. Milios, and D. Wilkes. Topological exploration with multiple robots. In 7th International Symposium on Robotics and Automation (ISORA) (wiki)
[3] H. Wang. Multiple Robot Graph Exploration. Master’s Thesis. York University, (webpage)
Thank you
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23. Extra slide: Two robots vs. single robot
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24. Extra Slide: About isomorphic
Problem (goal) : Given an unknown environment modeled as a graph, formulate an exploration strategy which enable the robot to form a representation of environment that is isomorphic to the finite world it has been assigned to explore.
isomorphic: Two graphs are isomorphic if there is a one-to-one correspondence between their vertices and there is an edge between two vertices of one graph if and only if there is an edge between the two corresponding vertices in the other graph. v2 v1 v1 v2 v3 v4 v4 v3
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