Stefan Schemmer

Reliable real-time communication in cooperative mobile applications

Embedded systems are expected to provide increasingly complex and safety-critical services that will, sooner or later, require the cooperation of several such systems for their fulfillment. In particular, coordinating the access to shared physical and information technological resources will become a general problem. Examples are mobile robots in industrial automation or car-to-car coordination for future traffic control applications. In such applications, cooperation is subject to strong real-time and reliability requirements. In this paper, we present an architecture that allows autonomous mobile systems to schedule shared resources in real-time using their own wireless distributed infrastructure. In this architecture, there is a clear separation between the application-specific scheduling part and the application independent communication part that constitutes the real-time and reliability hardcore of the system. The latter provides clock synchronization, real-time atomic multicast, and real-time group membership based on an IEEE 802.11 standard wireless LAN. An application prototype shows how the architecture can be used in future mobile cooperative applications.

Efficient Reliable Real-Time Group Communication for Wireless Local Area Networks

We consider teams of mobile autonomous robot systems that coordinate their work via communication over a wireless local area network. In such a scenario, timely delivery and group support are the predominant requirements on the communication protocol. As the mobile robot systems are communicating via standard hardware, we base our work on the IEEE 802.11 standard for wireless local area networks. In this paper, we present a reliable real-time group communication protocol that enhances the IEEE 802.11 standard. The reliability and real-time properties of the protocol are based on time-bounded dynamic time redundancy. By this, network bandwidth is used much more efficiently than in a message diffusion approach based on static redundancy. The fault detection mechanism needed to implement the time-bounded dynamic redundancy concept is achieved by an implicit acknowledgement scheme that consumes no additional bandwidth for acknowledgement messages.

Automatic WLAN localization for industrial automation

The increasing WLAN deployment in industrial environments opens possibilities for location based services such as asset-tracking and workflow optimization. This paper presents our current research to develop a WLAN-localization system that fulfils the requirements of industrial automation: scalability and 5-10 m localization accuracy. Our envisaged system produces the training data automatically using automatic measurements, and model calibration. The suggested approach avoids the effort for walk-around and extensive manual measurements and is stable under environmental changes, while still achieving an accuracy equivalent to state-of-the art technology. Preliminary evaluation shows an average accuracy of 3.7 m (12ft).

Reliable real-time cooperation of mobile autonomous systems

Autonomous systems are expected to provide increasingly complex and safety-critical services that will, sooner or later, require the cooperation of several autonomous systems for their fulfillment. In particular, coordinating the access to shared physical and information technological resources will become a general problem. Scheduling these resources is subject to strong real-time and reliability requirements. In this paper, we present an architecture that allows autonomous mobile systems to schedule shared resources in real-time using their own wireless distributed infrastructure. In our architecture, there is a clear separation between the application-specific scheduling part that is modeled as a function of the global state and the communication part that is used to provide the global state. By isolating the more error-prone communication part within a communication hardcore, the reliability of the overall system is increased and the locally executed scheduling function can be designed with primary focus on the application-specific real-time requirements.

PLANNING AVAILABLE WLAN IN DYNAMIC PRODUCTION ENVIRONMENTS

Abstract What is still missing in the convergence of IT and automation technologies is the integration of wireless communication. In this paper we consider the problem of planning available Wireless LAN (WLAN) in production environments with dynamic radio-propagation properties. Our approach is an autonomic control loop with feedback simulation and optimization. We present in detail the simulation and calibration module which automatically keeps the knowledge about the current radio coverage up-to-date. Evaluation showed that the constrained least squares calibration method results in a more accurate model compared to other methods. The results are general to planning WLAN availability in production environments. Copyright

Robust scheduling in team-robotics

Mobile robots interact with a dynamically changing, physical environment. All tasks controlling such interactions must be performed reliably and in real-time. Information from the local sensors often is incomplete or inconsistent. Distributed sensor fusion is a technique that enables a team to get a more complete view of the world with a better quality of the provided information. In this paper we address the problem of scheduling the local processing tasks that are part of the overall fusion process. The particular problem to be addressed lies in the unpredictable execution times of these tasks, which do not allow for scheduling using worst-case execution times. The Time-Aware Fault-Tolerant (TAFT) scheduler allows working with expected-case execution times instead, and still achieves a predictable timing behavior. The paper details an efficient scheduling strategy for TAFT based on Earliest Deadline algorithms, formalizing the adopted task model and the underlying scheduling mechanism. Results are presented showing the achieved real-time behavior with an increased acceptance rate, a higher throughput, and a graceful degradation in transient overload situations compared to standard schedulers. Additionally, it describes the implementation of TAFT in the real-time platform that is embedded in our robot team.

An architecture to support cooperating mobile embedded systems

There is a sustained trend to embed computer systems in all kinds of intelligent products. Increasing emphasis is given to enhance the functionality of such systems beyond the provision of easy-of-use and comfort to more safety-critical tasks where they exert direct control over the intelligent product. Examples of such systems can be exploited in many domains like team robotics, factory automation, transport systems, and intelligent traffic control. To master the inherent complexity, we present a layered system architecture that partly integrates already existing components. It has to cope with two major challenges imposed by the considered type of applications. First, the architecture has to resolve the conflict between the autonomy of the individual systems and the demand to cooperate. We think that coordination is the key to the required effective cooperation. Coordination means that the individual systems can take actions under local control, based on a sufficiently consistent view of the common system environment and on the application of a set of common rules. To enable this, the proposed architecture provides the necessary communication services. Secondly, system control must be prepared to cope with a dynamic and unpredictable system environment. Still guaranteeing QoS properties, especially real-time behavior, necessitates innovative solution concepts. Our approach is to exploit system -- and application -- inherent redundancies across different layers of the architecture.

Real-Time Mesh Networks for Industrial Applications

Wireless LANs (IEEE 802.11) are increasingly used in industrial applications. They reduce cabling costs, increase flexibility and enable mobile applications for maintenance or logistics tasks. Mesh networks provide a self-configuring and -healing wireless backbone for large scale deployments (e.g. in process automation). This paper presents a routing algorithm which provides QoS in wireless mesh networks, thus leveraging their use in industrial applications. It allows reserving bandwidth for real-time flows based on measurements of the physically available bandwidth. Thus it fully utilizes the bandwidth while still preventing congestion. Simulation results demonstrate the reliability of the algorithm and its advantage over previous works.

Managing dynamic groups of mobile systems

Since road networks are the backbone of transportation and personal mobility, congestions are a severe problem. Improving the utilization of existing roads by scheduling shared resources, e.g. crossroads, more efficiently is a promising approach to alleviate the problem. We have developed an architecture allowing mobile systems to schedule shared resource in real-time based on wireless communication. In this paper, we present two membership protocols, which are at the core of this architecture. The protocols allow mobile systems to join and leave a group with predictable delay. A synchronous system model with omission failures describes the dynamic properties of the medium. The protocols exploit application knowledge to allocate bandwidth to joining stations dynamically yet predictably. Nevertheless, they can be converted to a general solution if predictable join delays are not a requirement.

Evaluating a wireless real-time communication protocol on Windows NT and WaveLAN

One of the main reasons explaining the success of field busses in industrial automation was the drastic reduction of wiring, thus allowing for flexible reconfigurations at low cost. A possible next step, which allows even more flexibility, would be to abandon wires altogether and to rely on a wireless medium for communication. However, the question of the reliability and real time guarantees arises. The authors present and evaluate a reliable real time communication protocol that enhances the IEEE 802.11 standard. The reliability and real time properties of the protocol rely on determining parameters that model basic assumptions on the occurrences of message losses and bound of message delays. We determine realistic parameters by fine grained measurements of message delays and message losses and measure the protocol performance resulting from the so determined parameters.