Reliable, Cloud-based Communication for Multi-Robot Systems Ronny Hartanto
Markus Eich
Robotics Innovation Center German Research Center for Artificial Intelligence 28359 Bremen, Germany Email:
[email protected]
Robotics Innovation Center German Research Center for Artificial Intelligence 28359 Bremen, Germany Email:
[email protected]
Abstract-In contrast to single robotic agent, multi-robot systems are highly dependent on reliable communication. Robots have to synchronize tasks or to share poses and sensor readings with other agents, especially for co-operative mapping task where local sensor readings are incorporated into a global map. The drawback of existing communication frameworks is that most are based on a central component which has to be constantly within reach. Additionally, they do not prevent data loss between robots if a failure occurs in the communication link. During a distributed mapping task, loss of data is critical because it will corrupt the global map. In this work, we propose a cloud-based publish/subscribe mechanism which enables reliable communication between agents during a cooperative mission using the Data Distribution Service (DDS) as a transport layer. The usability of our approach is verified by several experiments taking into account complete temporary communication loss.
I.
INTRODUCTION
In many robotic scenarios, robots do not operate as single agents but have to operate in a coordinated way. In a multi robot scenario, communication is the most important aspect which enables the robots to cooperate. In order to achieve this, robots have to exchange data frequently. Because data loss during coordinated tasks, especially during exploration and mapping, is critical, our main goal is to provide a ro bust and reliable communication channel between the robots. In this work, we present the recent results of our project IMPERA (!ntegrated Mission Elanning for H�terogeneous Robotic Agents) [1]. In our scenario, the robots have to explore an area within a lunar scenario which is significantly larger than the communication range of the provided communication infrastructure. The robots have to build a map of the area using a cooperative SLAM (Simultaneous Localization and Mapping) approach. Hence, message loss during this task is critical. Every message sent out from one robot must be received by the other. This feature is used extensively for the exploration and mapping scenario, in which each participating robot will receive messages from other robots without re requesting it. The robots have to locate and to transport several objects within a lunar environment. The robots used for this scenario consist of heterogeneous robots using, legged, hybrid or wheeled locomotion. The robots are described in detail in [2]-[4]. The SLAM approach we use is based on the graph-based SLAM approach described in [5] which we extended to be usable within our multi-robot scenario. Each robot makes use of the pose updates and the laser scans, generated be each individual robot. All local perception data is
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shared with all other robots continuously and integrated into the local graph-based map representation. This assures that every robot has the same global map during the exploration task. Our exploration strategy and the mapping approach are described in detail in [4]. This means that every map on each robot will be corrupted if data from even one system is lost due to communication failure. In our scenario the robots are running ROS as the local communication framework, each robot running a separate roscore using so called topics and service-call based communication. For details on the ROS communication framework, the reader is referred to [6]. This alone cannot assure a reliable communication between the systems. Another important requirement on the communication sys tem is the transparency and robustness. The multi-robot com munication must be transparent to the ROS robotic framework. Transparency in this context means that the overall system has still to work even if the multi-robot conununication link is temporarily lost. It also means that link loss in the multi-robot communication will not affect other software components. There exists a variety of communication paradigms, such as "Remote Procedure Call (RPC)", "message-oriented com munication", "stream-oriented communication" or "software based distributed shared memory (DSM)". Two of these paradigms, namely RPC and message-oriented communica tion have been used in the robotics and multi-agents system broadly. Major differences between these paradigms are the space coupling, time decoupling and synchronization decou pling. Table I shows a comparison between decoupling abilities for some of there aforementioned paradigms. "Raw-sockets", "Message-Passing Interface (MPI)", "Message-Oriented Mid dleware (MOM)", and "Publish-Subscribe c Olmnunication" are some implementations of the message-oriented conununication paradigm. "Common-Object Resource Broker Architecture (CORBA)", "Java Remote Method Invocation (Java RMI)", and "Distributed Conunon Object (DCOM)" are some example of middlewares which are based on the RPC paradigm. In [7] a comparison of decoupling abilities of some of these paradigms is shown. Publish/Subscribe paradigm has all three decoupling abilities. Space decoupling disconnects the inter acting parties from each other in space; hence the subscribers get the events indirectly through the event service without having any references to the publishers. Both, subscribers and publishers, do not know the number of other publishers or subscribers which are participating in this interaction. Time
DECOUPLING ABILITIES OF INTERACTION PARADIGMS [7]
TABLE I.
Space
Abstraction
decou-
Time
Synchronization de-
piing
decoupling
coupling
Message Passing
No
No
Producer-side
RPCfRMI
No
No
Producer-side
Asynchronous
No
No
Yes
Future RPCfRMI
No
No
Yes
Notification
No
No
Yes
Yes
Yes
Producer-side
Yes
Yes
Producer-side
Yes
Yes
Yes
RPCfRMI
reliable even if the communication link is temporary lost. In Section IV we give the details of our implementations. The proposed approach is verified by means of empirical data in Section V. Finally, in Section VI, we conclude our work and give some ideas about our future research directions regarding safe inter-robot communication.
D.
(Observer
II.
Pattern) Tuple Spaces Message
Queuing
(Pull) Publish/Subscribe
Event Service
NotifyO
Space decoupling
Event Service NotifyO -
-hc> (\S (TeP~A)
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