Infrastructure for Autonomous Mobile Robots Communication and Coordination

Transcription

1 90 Work in Progress Session Infrastructure for Autonomous Mobile Robots Communication and Coordination Marcelo M. Sobral, Leandro B. Becker Dept of Automation and Systems Universidade Federal de Santa Catarina (UFSC) Florianópolis SC Brazil Abstract. Teams of mobile robots need to communicate using wireless medium to perform common tasks in a coordinated manner. Communications normally involve control, status, and possibly stream messages containing real-time and QoS requirements. It is often the case that mobile robots are deployed in unknown environments, requiring them to form an ad-hoc wireless network to establish connectivity. Moreover, a dynamic topology is also required due to the mobility of nodes. This work proposes a communication infrastructure to be used by mobile robots operating under the described conditions. It provides a convenient communication model to abstract from topology changes caused by movements of robots and assures, whenever possible, that messages are delivered in a timely manner, with application required QoS. 1. Introduction Applications using teams of mobile robots are currently used to perform tasks which are dangerous or impossible to be performed by human beings. Some application examples are pipe maintenance, search and rescue robot teams, and spatial exploration. Teams of mobile robots can be seen as sets of autonomous systems that communicate using wireless medium to perform coordinated tasks. They normally form an ad-hoc wireless network due to the fact that they are deployed in unknown environments, differently from infra-structured wireless networks that have access-points. Normally such applications exchange messages with real-time requirements, such as deadlines to deliver messages, temporal validity of data, and synchronization requirements (delay variation limits). Also, nodes mobility may establish and disrupt links dynamically, often changing the network topology, therefore demanding frequent routes updates. To make the problem more difficult, wireless networks are prone to transmission errors, making it potentially hard to guarantee the correctness of protocols. The adopted communication model must be appropriate to enable nodes to share sensor/status data and control information, making it transparent for them the location of the mobile nodes. Kaiser et al (2005) points out that it must reflect the requirements of a control system composed of such autonomous systems, what includes the spontaneous generation and dissemination of data and many-to-many communications to efficiently disseminate them. Another important point is that the communication infrastructure for such systems must be able to support messages delivery in a timely manner, with application required quality of service (QoS), and also to support the modifications of topology caused by movements of the robots. Moreover, the communication model is also an important issue to be considered. In this paper authors present a proposal to handle the mentioned aspects. To be

2 IX Workshop on Real-Time Systems 91 more specific, section 2 describes the fundamental problems under consideration and some related works. Section 3 explains the proposed infrastructure, including details concerning its foreseen mechanisms. Section 4 discusses preliminary results and presents the next steps of this research. 2. Fundamental Problems and Related Works A search and rescue application is used in our work to provide an overview of the existing communication problems on teams of autonomous mobile robots. To perform the exploration of an environment, according to Reich and Sklar (2006), a robot team faces some major difficulties related to: (i) their location in the environment, (ii) the acquiring of information about this environment, and (iii) the communication between the robots. Our work concentrates in item (iii), that is, in the communication infrastructure that must provide communication services to support robots coordination. A set of requirements for such infrastructure is described bellow: Routing: the routing function must forward messages through the network, to deliver them to all destination nodes, even if they change their locations. For the search and rescue application, the communication infrastructure must either keep the connectivity of the robots or deal with the disruption of links. The dynamic topology due to the mobility of nodes make it difficult, and even not feasible, to keep track of each node individually. Carzaniga and Wolf (2001) points out contentbased routing as a basic requirement for mobile devices networks. End-to-end timely delivery: messages must be delivered to all target nodes, wherever they are, respecting their time limits. This influences the routing, since paths are chosen according to transmission delay and temporal characteristics of the messages. As described by Hughes and Cahill (2004), it is clear that a resource reservation mechanism is needed to accomplish this, and that hop-to-hop timely delivery is a basic requirement. Also, some questions arise about what to do when it is not possible to reach some interested nodes whithin the time limit. Possible semantics are: simply exclude them from the delivery; refuse to deliver the message; or deliver it even if the message deadline is exceeded. Hop-to-hop timely delivery: the network layer needs a worst case time to transmit a message through the medium, as further described by Kaiser et al (2005) and Hughes and Cahill (2003). In wireless networks the medium is shared, requiring a medium access control function (MAC) to coordinate transmissions and avoid collisions. A time division of the medium is usually imposed to accomplish this requirement, allocating time slots to the nodes. The dynamic allocation of time slots needs a consensus of all involved nodes, what can be hard to obtain with a distributed approach, as shown by Facchinetti et al (2004). Anonymous communications: as the nodes are mobile and links can disrupt, it is better to deliver messages based on its contents rather than on node address. This is naturally associated with publisher-subscriber (P/S) protocols, as described by Kaiser et al (2005). The adoption of anonymous communications on the network layer could make it easier the implementation of P/S protocols in the upper layer. But for stream data to be transmitted between two nodes, some selection mechanism that does not conflict with the anonymous communication model must be implemented, like peer-to-peer (P2P).

3 92 Work in Progress Session Communication model: the communication model should be adequate for both many-to-many and point-to-point communications, due to both the dissemination of sensor/status data and streams transmission. Moreover, it must be compatible with anonymous communications. A P/S protocol, as already mentioned, matches smoothly these requirements, except for stream data, where the communication model needs to provide an appropriate way to establish a pipe between producer and consumer. In this case, a P2P protocol seems more appropriated. 3. The Proposed Communication Infrastructure To satisfy the requirements identified in section II, it is proposed a communication infrastructure with a three layer structure. On the top, the transport layer includes P/S and P2P protocols, providing communication models suited to uncoupled, anonymous, content based, and peer-to-peer communications. The network layer provides the timely delivery of messages to every location on the network, and deals with the mobility of the nodes. Finally, in the bottom, the link layer provides hop-to-hop timely delivery, implementing a deterministic MAC protocol. The Link Layer must satisfy the hop-to-hop timely delivery requirement - a worst case transmission time for a packet, what implies a deterministic MAC. It is not in the scope of this work to propose such MAC, but to adapt an existing one. Therefore, it will be investigated the adoption of a scheduling approach based on the IEEE e standard, as described by Demarch and Becker (2007). Their work defines a scheduler for guaranteed firm deadlines using the Hybrid Coordination Controlled Channel Access (HCCA). However, HCCA relies on a central coordinator, which is normally the access point (AP). Therefore it must be investigated if some or all robots could play this role of coordinator in the absence of an AP. The main purpose of the Network Layer is to perform routing. In our work some restrictions are imposed to the topology of the network and the roles of the robots. To reduce the complexity of a multi-hop network, robots are organized within clusters, which contains one responsible node called coordinator. The network layer also implements a key abstraction named Virtual Channel (VC) that resembles the concept of a virtual circuit, but allowing multiple senders and receivers. The VC abstraction intentionally brings anonymous communication to the network layer: it requires no node addresses, with all delivery based on the existence of active VCs in the clusters. In every coordinator a VC, if existing, can be active or pending. When a packet from a VC arrives in the coordinator, it is active it than forwards (repeats) this packet. An important issue on how to create a VC relates to how the coordinator becomes aware of the existing VCs. This is defined as responsibility of the producers and consumers of the VC: producers announce the creation of VCs to all the network, and consumers announce its acceptance. coordinators should be aware of that to activate the VC whenever one of its nodes is interested on that clusterthis provides a natural multipoint-to-multipoint communication service. This mechanism aims to create routes with smaller number of hops, just like a distance vector routing strategy. When there are more than one producer in a VC, which is a very probable situation, the question is how to define if a coordinator is part of the route for a producer. Here, two ideas are being considered: (i) each coordinator keeps a list of neighbor clusters for each producer, or (ii) each VC producer generates a sequence number for its messages, so a coordinator can repeat only the first occurrence of a message. Finally, to achieve end-to-end timely delivery, a QoS descriptor must be associated to a VC, and messages must be scheduled

4 IX Workshop on Real-Time Systems 93 according to their time limits. This is still an open issue. The transport layer should support both P/S and P2P protocol, providing. For the P/S, messages are identified by subject or content, with channel numbers or predicates. Channel numbers is the simplest case, and can be mapped directly to VC numbers in the network layer. Predicates are useful when the condition for reception of messages varies according to operational variables of the nodes, like geographical position. Here, predicates are thought as a composition of attributes in the content of the message, and attributes of target nodes. For the P2P protocol, some selection mechanism that does not conflict with the anonymous communication model must be developed. This looks contradictory, because it only makes sense to create a pipe between two known nodes. But it can be achieved without the need of addressing, being enough to maintain an identity or condition in the P2P layer. 4. Preliminary Results and Further Work The proposed infrastructure has a prototype implementation covering mainly the network layer and a subject based P/S protocol in the transport layer. It emphasizes the routing, more specifically the protocol for VCs management, and the transport layer. Currently, it is able to create and activate VCs and forward VC data. The mapping of channels in the P/S protocol and VCs in the network layer is defined, exploring the natural association between them: the creation of a VC is similar to a publishing, and the acceptance of a VC maps to a subscribing. It will be further investigated the integration of this proposal with an existing middleware named Cosmic, an event oriented middleware developed by Kaiser et al (2005). Cosmic has interesting features, such as a well defined P/S protocol, real-time event classes, a clock synchronization protocol and CAN network and TCP/IP support. This proposal could enable Cosmic to work with wireless networks, in scenarios composed by mobile devices. References Reich J. and Sklar, E. (2006) Robot Sensor Networks for Search and Rescue, In: IEEE International Workshop on Safety, Security and Rescue Robotics, USA. Kaiser, J., Brudna, C. and Mitidieri, C. (2005) COSMIC: A Real-Time Event-Based Middleware for the CAN Bus. In: The Journal of Systems and Softwares vol. 77, pp Hughes, B. and Cahill, V. (2003) Achieving Real-Time Guarantees in Mobile Ad Hoc Wireless Networks, In: Work in progress session, IEEE Real Time Systems Symposium, Mexico. Facchinetti, T. et al (2004) Real-Time Resource Reservation Protocol for Wireless Mobile Ad Hoc Networks, In: Proceedings of the 25th IEEE Real-Time Systems Symposium (RTSS 2004), Portugal. Carzaniga, A. and Wolf, A. (2001) Content-based networking: A New Communication Infrastructure, In: NSF Workshop on an Infrastructure for Mobile and Wireless Systems, USA. Demarch, D.D. and Becker, L. B. (2007) A Centralized Scheduling and Retransmission Proposal for Firm Real-time Traffic in IEEE e, In 25 th Simpósio Brasileiro de Redes de Computadores (SBRC). Belém, Brazil.

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