Roberto Brega

The need for autonomy and real-time in mobile robotics: a case study of XO/2 and Pygmalion

Starting from a user point of view the paper discusses the requirements of a development environment (operating system and programming language) for mechatronic systems, especially mobile robots. We argue that user require ments from research, education, ergonomics and applications impose a certain functionality on the embedded operating system and programming language, and that a deadline-driven real-time operating system helps to fulfil these requirements. A case study of the operating system XO/2, its programming language Oberon-2 and the mobile robot Pygmalion is presented. XO/2 explicitly addresses issues like scalabilty, safety and abstraction, previously found to be relevant for many user scenarios.

Approximation of Worst-Case Execution Time for Preemptive Multitasking Systems

The control system of many complex mechatronic products requires for each task the Worst Case Execution Time (WCET), which is needed for the schedulers admission tests and subsequently limits a tasks execution time during operation. If a task exceeds the WCET, this situation is detected and either a handler is invoked or control is transferred to a human operator. Such control systems usually support preemptive multitasking, and if an object-oriented programming language (e.g., Java, C++, Oberon) is used, then the system may also provide dynamic loading and unloading of software components (modules). Only modern, state-of-the art microprocessors can provide the necessary compute cycles, but this combination of features (preemption, dynamic un/loading of modules, advanced processors) creates unique challenges when estimating the WCET. Preemption makes it difficult to take the state of the caches and pipelines into account when determining the WCET, yet for modern processors, a WCET based on worst-case assumptions about caches and pipelines is too large to be useful, especially for big and complex real-time products. Since modules can be loaded and unloaded, each task must be analyzed in isolation, without explicit reference to other tasks that may execute concurrently. To obtain a realistic estimate of a tasks execution time, we use static analysis of the source code combined with information about the tasks runtime behavior. Runtime information is gathered by the performance monitor that is included in the processors hardware implementation. Our predictor is able to compute a good estimation of the WCET even for complex tasks that contain a lot of dynamic cache usage, and its requirements are met by todays performance monitoring hardware. The paper includes data to evaluate the effectiveness of the proposed technique for a number of robotics control kernels that are written in an object-oriented programming language and execute on a PowerPC 604e-based system.

A complex mechatronic system: from design to application

Progress in mobile robotics requires the researchers to access and improve all modules that compose the robot, from low-level mechanical components to high-level reasoning systems. The paper presents the development process of the robots built at the Autonomous Systems Lab, EPFL Lausanne, Switzerland. Starting from the mechanical and electrical design up to the application, we show the challenges that needed to be faced as well as the solutions that have been devised. The description covers aspects like the operating system and framework, because of its role in the overall safety and dependability of the whole software system, the research as a precondition for innovative products, and the man-machine interface, which is indispensable for conveying information to the user as well as allowing the user to interact with the robot. The issues that have been faced stem from the hierarchical, layered construction of a complex mechatronic product, where the operation of the machine depends on the smooth cooperation of each layer. In the same way, the overall safety is undermined by the least reliable piece building the system.

The paradox of service robots-how passers-by can contribute in solving non-deterministic exceptional conditions encountered by service robots

Intelligent, autonomous service robots face unique challenges. Unlike autonomous guided vehicles used in manufacturing facilities, service robots cannot rely on clearly laid assumptions about the environment, as they share the work-space-which has not been designed around them-with human beings. At the same time, they cannot be treated as entertainment robots, since they are designed to accomplish a given set of tasks and their fate is measured by success-rate and not by their cuteness. Service robots need to perform reliably even during exceptional situations. The need for reliability strives for predictability, while the presence of a shared environment yields nondeterministic situations. The clash may be described as the paradox of mobile robotics, where reliability and safety are undermined by the same goals we want to reach. Based on the experience collected with the mobile mail distribution system MoPS, this paper describes a novel approach that is aimed at resolving the large number of error situations that can be addressed by untrained individuals, by allowing the robot to interact with passersby, if an exceptional situation is detected.