Fail-Safe Mobility Management and Collision Prevention Platform for Cooperative Mobile Robots with...
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Transcript of Fail-Safe Mobility Management and Collision Prevention Platform for Cooperative Mobile Robots with...
Fail-Safe Mobility Management and Collision Prevention
Platform for Cooperative Mobile Robots with Asynchronous
Communications
Rami YaredSchool of Information Science
Japan Advanced Institute of Science and Technology (JAIST)
Supervised by:Prof. Xavier Défago
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Context
• Group of mobile robots
• Asynchronous communication (No upper bound on communication delays)
• No upper bounds on robots speeds
• No central control
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Research Objective
• Mobility management platform
• Fail-safe mobile robotic system
• Prevent robots collisions.
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Outline
• Related work and motivation
• System architecture
• System model and problem specification
• Fail-safe platform
• Collision prevention for a closed group model
• Collision prevention for a dynamic group model
• Conclusion
• Future directions
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Motion planning
•Find a route from an initial position to a final position in presence of obstacles.
Related work
• Avoid collision between a robot and Fixed obstacles
• Sensing during the motion in dynamic or unknown environments
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Minguez et al 2004. [22]Montano et al 1997. [23]
Motion planning
RT guarantees
Related work
•Upper bound on communication delays.
•Upper bound on processing speeds.
• Wireless LAN, Access point central router
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Synchronous systemsNett et al 2003 [25]
Related work
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Synchronous systemsNett et al 2003 [25]
Collisions between mobile robots
Violation of timeliness properties
Related work
Time elastic: Time bounds can be increased or decreased dynamically
Fail safe: exhibits correct behavior, or put the system in a fail-safe state.
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Martins et al 2005 [21]
•Wireless Communications retransmission ⇒mechanisms.
•Arbitrary sized messages ⇒ unknown delays, not anticipated, ...
⇒ Time free approach is important
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Contribution
Time free mobility management platform
Fail-Safe mobile robotic system.
• Collision prevention protocols:
• Closed group of robots.
• Dynamic group of robots.
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Outline
• Related work and motivation
• System architecture
• System model and problem specification
• Fail-safe platform
• Collision prevention for a closed group model
• Collision prevention for a dynamic group model
• Conclusion
• Future directions
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Motion planning
•Find a route from an initial position to a final position in presence of obstacles.
Outline
• Related work and motivation
• System architecture
• System model and problem specification
• Fail-safe platform
• Collision prevention for a closed group model
• Collision prevention for a dynamic group model
• Conclusion
• Future directions
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System model
• Asynchronous communications
• Retransmission reliable channels⇒
• Positioning system with bounded errors.
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Specification
• Safety
A given zone can be owned by only one robot.
Zonei ∩ Zonej ≠ ⇒ (R⇒ i owns Zonei) XOR (Rj owns Zonej)
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Specification
• Liveness
If Ri requests Zonei then eventually (Ri owns Zonei or an Exception is raised)
Ri requests Zonei (R⇒♢ i owns Zonei or Exception)
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Specification
Raising exceptions occurs only in specified situations.
•Non triviality
Exception is raised only if a deadlock situation occurs.
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Deadlock situation
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Deadlock situation
•Robot Ri requests a resource owned by Rj
•Robot Rj requests a resource owned by Ri
Starvation situation
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Starvation situation
•If robot Rj owns Zonej then Ri is blocked (starvation)
Pathological situation
Outline
• Related work and motivation
• System architecture
• System model and problem specification
• Fail-safe platform
• Collision prevention for a closed group model
• Collision prevention for a dynamic group model
• Conclusion
• Future directions
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Collision prevention protocol
• Requests ordering
• wait-for relations between robots
• Consistency
• All robots agrees on the same wait-for relations.
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Protocol
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•When Request()
•Compute the requested zone
•TO-broadcast(Request, Zone, Release previous zone)
•When TO-deliver(Request, Z, Release previous zone)
•update the wait-for graph Dagwait
•When vertex becomes a sink (no outgoing edges)
•Reserve zone
Fault-tolerant collision prevention
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Robots fail by crash
•Communication part
•Total Order Broadcast
•Problem: If a robot has crashed
•A robot waiting for a crashed robot is blocked
•The number of blocked robots increases Snowball⇒ effect
•A robot cannot distinguish a crashed robot from a very slow one (asynchronous system)
Zoned
Zonej
Zoneb
Zonei
Zonea
Fault-tolerant collision prevention
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Robots fail by crash
•with a failure detector class P
•with a failure detector class P♢
•with a failure detector class S ♢
Solution:
Zoned
Zonej
Zoneb
ZoneiZonea
Fault-tolerant collision prevention
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Robots fail by crash
•with a failure detector class P
•Perfect failure detector
•The suspected robot is considered as an inert obstacle
•A waiting robot becomes unblocked.
Solution:
Zoned
Zonej
Zoneb
ZoneiZonea
Fault-tolerant collision prevention
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Robots fail by crash
•with a failure detector class P♢
•Eventually perfect failure detector
•Preemptive protocol
Solution:
Zoned
Zonej
Zoneb
ZoneiZonea
Fault-tolerant collision prevention
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Preemptive protocol
•If a robot Rd is suspected then
•Zoned is “blocked”
•Requests of Ra and Rj are preempted (alternative zones)
•Other robots Ri and Rb are not blocked.
Zoned
Zonej
Zoneb
ZoneiZonea
Fault-tolerant collision prevention
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Preemptive protocol
•If a robot Ri is suspected and has not owned Zonei then
•Request of Ri is preempted (restarts its request of Zonei)
•Robot Rb is not blocked.
Zoneb
Zonei
Fault-tolerant collision prevention
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•with a failure detector class S♢
Non preemptive protocol
•If Ri suspects Rj and Zonei intersects with Zonej then
•Ri cancels its request of Zonei
(alternative zone)
Zonej
Zonei
Fault-tolerant collision prevention
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•Failure detector class P♢
•Liveness property for the preemptive protocol, because eventually a correct robot is not suspected by any correct robot.
•Failure detector class S♢
•Liveness property for the non preemptive protocol.
•Requires more alternative zones.
Outline
• Related work and motivation
• System architecture
• System model and problem specification
• Fail-safe platform
• Collision prevention for a closed group model
• Collision prevention for a dynamic group model
• Conclusion
• Future directions
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Dynamic group model
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•limited transmission range, No routing is required
•Communication graph is not connected
•Input of Neighborhood Discovery: (x,y) coordinates of the caller.
•Output of Neighborhood Discovery: the set of robots that potentially conflict with the caller.
Neighborhood discovery
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Nghi = {Ra, Rb, Rd, Re, Rj}
Gi = {Rb, Rj}
(G1)i = {Rb}
(G2)i = {Rj}
WLAfteri = {Rk}
Collision prevention protocol
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Performance Analysis
• Robots are active executing the protocol
• reservation range (Dch)
• density of robots (s)
• Average effective speed vs reservation range
• Average effective speed vs density of robots
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Performance Analysis
• Average communication delays Tcom
• Delay of the neighborhood discovery primitive Tnd
• Physical speed of robots Vmot
• Average effective speed V
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Outline
• Related work and motivation
• System architecture
• System model and problem specification
• Fail-safe platform
• Collision prevention for a closed group model
• Collision prevention for a dynamic group model
• Conclusion
• Future directions
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Conclusion
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Closed group Dynamic group
group of robots Static Dynamicgroup knowledge Complete partial
Scalability (design) Low very high
Fault-tolerance ♢S
Closed group Dynamic groupmessages loss Safety violation
Imprecision positioning
systemSafety violation
Neighborhood discovery Safety violation
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Conclusion
Vulnerability with respect to system model assumptions
Outline
• Related work and motivation
• System architecture
• System model and problem specification
• Fail-safe platform
• Collision prevention for a closed group model
• Collision prevention for a dynamic group model
• Conclusion
• Future directions
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