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

1

Application

Garden Cultivation by cooperative mobile robots.

2

Context

• Group of mobile robots

• Asynchronous communication (No upper bound on communication delays)

• No upper bounds on robots speeds

• No central control

3

Problem

• Prevent collisions between mobile robots.

4

Research Objective

• Mobility management platform

• Fail-safe mobile robotic system

• Prevent robots collisions.

5

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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7

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

8

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

9

Synchronous systemsNett et al 2003 [25]

Related work

10

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.

11

Martins et al 2005 [21]

Related work

12

Martins et al 2005 [21]

Collisions between mobile robots

•Wireless Communications retransmission ⇒mechanisms.

•Arbitrary sized messages ⇒ unknown delays, not anticipated, ...

⇒ Time free approach is important

13

Contribution

Time free mobility management platform

Fail-Safe mobile robotic system.

• Collision prevention protocols:

• Closed group of robots.

• Dynamic group of robots.

14

Outline







• Conclusion


15

16

Motion planning

•Find a route from an initial position to a final position in presence of obstacles.

System architecture

17

•Fail-safe

•Time free

Outline







• Conclusion


18

System model

• Asynchronous communications

• Retransmission reliable channels⇒

• Positioning system with bounded errors.

19

Approach

• Distributed path reservation system.

• Primitives:

• Request

• Reserve

• Release

20

Reserve / Release

21

Specification

• Safety

A given zone can be owned by only one robot.

Zonei ∩ Zonej ≠ ⇒ (R⇒ i owns Zonei) XOR (Rj owns Zonej)

22

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)

23

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

•εgps : Positioning system

•εtr : translation movement

•εθ : rotation movement

Request / Released zone

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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

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•Next Zonej

Ri

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•Next Zonej

Deadlock situation

Outline







• Conclusion


31

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-broadcastTO-broadcast TO-deliverTO-deliver

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

Example

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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


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•with a failure detector class P

•with a failure detector class P♢

•with a failure detector class S ♢

Solution:

Zoned

Zonej

Zoneb

ZoneiZonea


40


•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


41


•with a failure detector class P♢

•Eventually perfect failure detector

•Preemptive protocol

Solution:

Zoned

Zonej

Zoneb

ZoneiZonea


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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


43

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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•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


45

•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







• Conclusion


46

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

47

Dynamic group model

48

•limited transmission range, No routing is required

•Communication graph is not connected

Reservation range

49

Reservation range ≤ Transmission range / 2

Dch ≤ Dtr / 2

•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}


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52

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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• Average communication delays Tcom

• Delay of the neighborhood discovery primitive Tnd

• Physical speed of robots Vmot

• Average effective speed V

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55

Performance AnalysisEffective speed vs reservation

range. range

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•Effective speed vs density of robots


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Outline







• Conclusion


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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







• Conclusion


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Future directions

62

•Simulation

•Optimizations

Thank you for your attention

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