Title: Fail-Safe Mobility Management and Collision Prevention Platform for Cooperative Mobile Robots with Asynchronous Communications
1Fail-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
1
2(No Transcript)
3Context
- Group of mobile robots
- Asynchronous communication (No upper bound on
communication delays) - No upper bounds on robots speeds
- No central control
3
4(No Transcript)
5Research Objective
- Mobility management platform
- Fail-safe mobile robotic system
- Prevent robots collisions.
5
6Outline
- 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
7Motion planning
- Find a route from an initial position to a final
position in presence of obstacles.
7
8Related 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
8
9Related work
Synchronous systems
Nett et al 2003 25
- Upper bound on communication delays.
- Upper bound on processing speeds.
- Wireless LAN, Access point central router
9
10(No Transcript)
11Related 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.
11
12(No Transcript)
13- Wireless Communications ? retransmission
mechanisms. - Arbitrary sized messages ? unknown delays, not
anticipated, ... - ? Time free approach is important
13
14Contribution
- Time free mobility management platform
- Fail-Safe mobile robotic system.
- Collision prevention protocols
- Closed group of robots.
- Dynamic group of robots.
14
15Outline
- 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
15
16Motion planning
- Find a route from an initial position to a final
position in presence of obstacles.
16
17System architecture
17
18Outline
- 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
18
19System model
- Asynchronous communications
- Retransmission ? reliable channels
- Positioning system with bounded errors.
19
20Approach
- Distributed path reservation system.
- Primitives
- Request
- Reserve
- Release
20
21Reserve / Release
21
22Specification
- Safety
- A given zone can be owned by only one robot.
-
- Zonei n Zonej ? Ø ? (Ri owns Zonei) XOR (Rj owns
Zonej)
22
23Specification
- Liveness
- If Ri requests Zonei then eventually (Ri owns
Zonei or an Exception is raised) - Ri requests Zonei ? ? (Ri owns Zonei or
Exception)
23
24Specification
- Raising exceptions occurs only in specified
situations. - Non triviality
- Exception is raised only if a deadlock situation
occurs.
24
25Reserved Zone
25
26Request / Released zone
26
27Deadlock situation
- Robot Ri requests a resource owned by Rj
- Robot Rj requests a resource owned by Ri
Deadlock situation
27
28Starvation situation
- If robot Rj owns Zonej then Ri is blocked
(starvation)
Starvation situation
Pathological situation
28
29Ri
29
30Deadlock situation
30
31Outline
- 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
31
32- Part 1
- Collision prevention protocol for a closed group
of mobile robots.
32
33Closed group model
- Composition known to all robots
- Communication graph is fully connected
33
34Collision prevention protocol
- Requests ordering
- wait-for relations between robots
- Consistency
- All robots agrees on the same wait-for relations.
34
35Total Order Broadcast
TO-broadcast
TO-deliver
35
36Protocol
- 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
36
37Example
37
38Fault-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
38
39Fault-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
39
40Fault-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
40
41Fault-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
41
42Fault-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
42
43Fault-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
43
44Fault-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
44
45Fault-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.
45
46Outline
- 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
46
47- Part 2
- Collision prevention protocol for a dynamic group
of mobile robots.
47
48Dynamic group model
- limited transmission range, No routing is
required - Communication graph is not connected
48
49(No Transcript)
50Neighborhood 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.
50
51(No Transcript)
52(No Transcript)
53Performance 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
53
54Performance Analysis
- Average communication delays Tcom
- Delay of the neighborhood discovery primitive Tnd
- Physical speed of robots Vmot
- Average effective speed V
54
55Performance Analysis
55
56Performance Analysis
Effective speed vs reservation range. range
56
57Performance Analysis
- Effective speed vs density of robots
57
58Outline
- 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
58
59Conclusion
59
60Conclusion
Vulnerability with respect to system model
assumptions
60
61Outline
- 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
61
62Future directions
62
63Thank you for your attention
63