A Study in the Computational Complexity of Temporal Reasoning

Reasoning about temporal and spatial information is a common task in computer science, especially in the field of artificial intelligence. The topic of this thesis is the study of such reasoning from a computational perspective. We study a number of different qualitative point based formalisms for temporal reasoning and provide a complete classification of computational tractability for different time models. We also develop more general methods which can be used for proving tractability and intractability of other relational algebras. Even though most of the thesis pertains to qualitative reasoning the methods employed here can also be used for quantitative reasoning. For instance, we introduce a tractable and useful extension to the quantitative point based formalism STP. This extension gives the algebra an expressibility which subsumes the largest tractable fragment of the augmented interval algebra and has a faster and simpler algorithm for deciding consistency. The use of disjunctions in temporal formalisms is of great interest not only since disjunctions are a key element in different logics but also since the expressibility can be greatly enhanced in this way. If we allow arbitrary disjunctions, the problems under consideration typically become intractable and methods to identify tractable fragments of disjunctive formalisms are therefore useful. One such method is to use the independence property. We present an automatic method for deciding this property for many relational algebras. Furthermore, we show how this concept can not only be used for deciding tractability of sets of relations but also to demonstrate intractability of relations not having this property. Together with other methods for making total classifications of tractability this goes a long way towards easing the task of classifying and understanding relational algebras. The tractable fragments of relational algebras are sometimes not expressive enough to model real-world problems and a backtracking solver is needed. For these cases we identify another property among relations which can be used to aid general backtracking based solvers to finnd solutions faster.Article I is a revised and extended version of the following three papers: 1. Mathias Broxvall and Peter Jonsson. Towards a Complete Classification of Tractability in Point Algebras for Nonlinear Time. In Proceedings of the 5th International Conference on Principles and Practice of Constraint Programming (CP-99), pp. 129-143, Alexandria, VA, USA, Oct, 1999. 2. Mathias Broxvall and Peter Jonsson. Disjunctive Temporal Reasoning in Partially Ordered Time Structures. In Proceedings of the Seventeenth National Conference on Artificial Intelligence (AAAI-2000), pp. 464-469, Austin, Texas, USA, Aug, 2000. 3. Mathias Broxvall. The Point Algebra for Branching Time Revisited. In Proceedings of the Joint German/Austrian Conference on Artificial Intelligence (KI-2001), pp. 106-121, Vienna, Austria, Sep, 2001. --- Article II is a revised and extended version of the following paper: Mathias Broxvall, Peter Jonsson and Jochen Renz: Refinements and Independence: A Simple Method for Identifying Tractable Disjunctive Constraints. In Proceedings of the 6th International Conference on Principles and Practice of Constraint Programming (CP-2000), pp. 114-127, Singapore, Sep, 2000.</p

A Study in the Computational Complexity of Temporal Reasoning

Reasoning about temporal and spatial information is a common task in computer science, especially in the field of artificial intelligence. The topic of this thesis is the study of such reasoning from a computational perspective. We study a number of different qualitative point based formalisms for temporal reasoning and provide a complete classification of computational tractability for different time models. We also develop more general methods which can be used for proving tractability and intractability of other relational algebras. Even though most of the thesis pertains to qualitative reasoning the methods employed here can also be used for quantitative reasoning. For instance, we introduce a tractable and useful extension to the quantitative point based formalism STP. This extension gives the algebra an expressibility which subsumes the largest tractable fragment of the augmented interval algebra and has a faster and simpler algorithm for deciding consistency. The use of disjunctions in temporal formalisms is of great interest not only since disjunctions are a key element in different logics but also since the expressibility can be greatly enhanced in this way. If we allow arbitrary disjunctions, the problems under consideration typically become intractable and methods to identify tractable fragments of disjunctive formalisms are therefore useful. One such method is to use the independence property. We present an automatic method for deciding this property for many relational algebras. Furthermore, we show how this concept can not only be used for deciding tractability of sets of relations but also to demonstrate intractability of relations not having this property. Together with other methods for making total classifications of tractability this goes a long way towards easing the task of classifying and understanding relational algebras. The tractable fragments of relational algebras are sometimes not expressive enough to model real-world problems and a backtracking solver is needed. For these cases we identify another property among relations which can be used to aid general backtracking based solvers to finnd solutions faster.Article I is a revised and extended version of the following three papers: 1. Mathias Broxvall and Peter Jonsson. Towards a Complete Classification of Tractability in Point Algebras for Nonlinear Time. In Proceedings of the 5th International Conference on Principles and Practice of Constraint Programming (CP-99), pp. 129-143, Alexandria, VA, USA, Oct, 1999. 2. Mathias Broxvall and Peter Jonsson. Disjunctive Temporal Reasoning in Partially Ordered Time Structures. In Proceedings of the Seventeenth National Conference on Artificial Intelligence (AAAI-2000), pp. 464-469, Austin, Texas, USA, Aug, 2000. 3. Mathias Broxvall. The Point Algebra for Branching Time Revisited. In Proceedings of the Joint German/Austrian Conference on Artificial Intelligence (KI-2001), pp. 106-121, Vienna, Austria, Sep, 2001. --- Article II is a revised and extended version of the following paper: Mathias Broxvall, Peter Jonsson and Jochen Renz: Refinements and Independence: A Simple Method for Identifying Tractable Disjunctive Constraints. In Proceedings of the 6th International Conference on Principles and Practice of Constraint Programming (CP-2000), pp. 114-127, Singapore, Sep, 2000.</p

The peis kernel: A middleware for ubiquitous robotics

Abstract—The fields of autonomous robotics and ambient intelligence are converging toward the vision of smart robotic environments, or ubiquitous robotics, in which tasks are performed via the cooperation of many simple networked robotic devices. The concept of Ecology of Physically Embedded Intelligent Systems, or PEIS-Ecology, combines insights from these fields to provide a new solution to building intelligent robots in the service of people. To enable this vision, we need a common communication and cooperation model that allows dynamically assembled adhoc networks of robotic devices, a flexible introspection and configuration model allowing automatic (re)configuration and that can be shared between robotic devices at different scales, ranging from standard mobile robots to tiny networked embedded devices. In this paper we discuss the development of a middleware suitable for ubiquitous robotics in general and PEIS-Ecologies in specific. Our middleware is suitable for building truly ubiquitous robotics applications, in which devices of very different scales and capabilities can cooperate in a uniform way. We discuss the principles and implementation of our middleware, and also point to experimental results that show the viability of this concept. I

A middleware for ecologies of robotic devices

Abstract—The fields of autonomous robotics and ambient intelligence are converging toward the vision of smart robotic environments, in which tasks are performed via the cooperation of many simple networked robotic devices. The concept of Ecology of Physically Embedded Intelligent Systems, or PEIS-Ecology, combines insights from these fields to provide a new solution to building intelligent robots in the service of people. To enable this vision, we need a common communication and cooperation model that allows dynamically assembled ad-hoc networks of robotic devices, a flexible introspection and configuration model and that can be shared between robotic devices at different scales, ranging from standard mobile robots to tiny networked embedded devices. In this paper we discuss the development of a middleware suitable for ubiquitous robotics in general and PEIS-Ecologies in specific. Our middleware is suitable for building truly ubiquitous robotics applications, in which devices of very different scales and capabilities can cooperate in a uniform way. We discuss the principles and implementation of our middleware, and also point to experimental results that show the viability of this concept. I

PEIS Ecologies: Ambient intelligence meets autonomous robotics

www.aass.oru.se/˜peis A common vision in the field of autonomous robotics is to create a skilled robot companion that is able to live in our homes and perform physical tasks to help us in our everyday life. Another vision, coming from the field of ambient intelligence, is to create a network of intelligent home devices that provide us with information, communication, and entertainment. We propose to combine these two visions into the new concept of an ecology of networked Physically Embedded Intelligent Systems (PEIS). In this paper, we define this concept, and illustrate it by describing an experimental system that involves real robotic devices.

The PEIS-ecology project: a progress report

fields of ubiquitous robotics and ambient intelligence to provide a new solution to building intelligent robots in the service of people. While this concept provides great potential, it also presents a number of new scientific challenges. The PEIS-Ecology project is an ongoing collaborative project between Swedish and Korean researchers which addresses these challenges. In this paper we introduce the concept of PEIS-Ecology, discuss its potential and its challenges, and present our current steps toward its realization. We also point to experimental results that show the viability of this concept

Steps toward an ecology of physically embedded intelligent systems

The concept of Ecology of Physically Embedded Intelligent Systems, or PEIS-Ecology, combines insights from the fields of ubiquitous robotics and ambient intelligence to provide a new solution to building intelligent robots in the service of people. While this concept provides great potential, it also presents a number of new scientific challenges. In this paper we introduce this concept, discuss its potential and its challenges, and present our current steps toward its realization. We also point to experimental results that show the viability of this concept. The discussion in this paper is also relevant to any type of ubiquitous robot or network robotic system.