Fail-Safe Mobility Management and Collision Prevention Platform for Cooperative Mobile Robots with Asynchronous Communications - PowerPoint PPT Presentation

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Fail-Safe Mobility Management and Collision Prevention Platform for Cooperative Mobile Robots with Asynchronous Communications

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Title: Fail-Safe Mobility Management and Collision Prevention Platform for Cooperative Mobile Robots with Asynchronous Communications


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Fail-Safe Mobility Management and Collision
Prevention Platform for Cooperative Mobile Robots
with Asynchronous Communications
  • Rami Yared
  • School 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.

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Related work
Motion planning
  • Avoid collision between a robot and Fixed
    obstacles
  • Sensing during the motion in dynamic or unknown
    environments

RT guarantees
Minguez et al 2004. 22
Montano et al 1997. 23
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Related work
Synchronous systems
Nett et al 2003 25
  • Upper bound on communication delays.
  • Upper bound on processing speeds.
  • Wireless LAN, Access point central router

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Related work
Martins et al 2005 21
  • 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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  • 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.

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System architecture
  • Fail-safe
  • Time free

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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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System model
  • Asynchronous communications
  • Retransmission ? reliable channels
  • Positioning system with bounded errors.

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Approach
  • Distributed path reservation system.
  • Primitives
  • Request
  • Reserve
  • Release

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Reserve / Release
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Specification
  • Safety
  • A given zone can be owned by only one robot.
  • Zonei n Zonej ? Ø ? (Ri 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 ? ? (Ri 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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Reserved Zone
  • egps Positioning system
  • etr translation movement
  • e? rotation movement

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Request / Released zone
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Deadlock situation
  • Robot Ri requests a resource owned by Rj
  • Robot Rj requests a resource owned by Ri

Deadlock situation
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Starvation situation
  • If robot Rj owns Zonej then Ri is blocked
    (starvation)

Starvation situation
Pathological situation
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  • Next Zonej

Ri
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  • Next Zonej

Deadlock situation
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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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  • Part 1
  • Collision prevention protocol for a closed group
    of mobile robots.

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Closed group model
  • Composition known to all robots
  • Communication graph is fully connected

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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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Total Order Broadcast
TO-broadcast
TO-deliver
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Protocol
  • 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

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Example
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Fault-tolerant collision prevention
Zoneb
Robots fail by crash
Zonei
  • 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)

Zonea
Zoned
Zonej
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Fault-tolerant collision prevention
Robots fail by crash
Zoneb
Solution
Zonei
Zonea
  • with a failure detector class P
  • with a failure detector class ?P
  • with a failure detector class ?S

Zoned
Zonej
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Fault-tolerant collision prevention
Robots fail by crash
Zoneb
Solution
Zonei
Zonea
  • with a failure detector class P
  • Perfect failure detector
  • The suspected robot is considered as an inert
    obstacle
  • A waiting robot becomes unblocked.

Zoned
Zonej
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Fault-tolerant collision prevention
Robots fail by crash
Zoneb
Solution
Zonei
Zonea
  • with a failure detector class ?P
  • Eventually perfect failure detector
  • Preemptive protocol

Zoned
Zonej
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Fault-tolerant collision prevention
Zoneb
  • 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.

Zonei
Zonea
Zoned
Zonej
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Fault-tolerant collision prevention
  • 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
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Fault-tolerant collision prevention
  • 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)

Zonei
Zonej
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Fault-tolerant collision prevention
  • 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.

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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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  • Part 2
  • Collision prevention protocol for a dynamic group
    of mobile robots.

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Dynamic group model
  • limited transmission range, No routing is
    required
  • Communication graph is not connected

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Neighborhood discovery
  • Input of Neighborhood Discovery (x,y)
    coordinates of the caller.
  • Output of Neighborhood Discovery the set of
    robots that potentially conflict with the caller.

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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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Performance Analysis
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Performance Analysis
Effective speed vs reservation range. range
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Performance Analysis
  • Effective speed vs density 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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Conclusion
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Conclusion
Vulnerability with respect to system model
assumptions
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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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Future directions
  • Simulation
  • Optimizations

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Thank you for your attention
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