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

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

6

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.

24

25

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

37

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


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•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


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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


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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

50

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

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