183 research outputs found
Fast Computation of Small Cuts via Cycle Space Sampling
We describe a new sampling-based method to determine cuts in an undirected
graph. For a graph (V, E), its cycle space is the family of all subsets of E
that have even degree at each vertex. We prove that with high probability,
sampling the cycle space identifies the cuts of a graph. This leads to simple
new linear-time sequential algorithms for finding all cut edges and cut pairs
(a set of 2 edges that form a cut) of a graph.
In the model of distributed computing in a graph G=(V, E) with O(log V)-bit
messages, our approach yields faster algorithms for several problems. The
diameter of G is denoted by Diam, and the maximum degree by Delta. We obtain
simple O(Diam)-time distributed algorithms to find all cut edges,
2-edge-connected components, and cut pairs, matching or improving upon previous
time bounds. Under natural conditions these new algorithms are universally
optimal --- i.e. a Omega(Diam)-time lower bound holds on every graph. We obtain
a O(Diam+Delta/log V)-time distributed algorithm for finding cut vertices; this
is faster than the best previous algorithm when Delta, Diam = O(sqrt(V)). A
simple extension of our work yields the first distributed algorithm with
sub-linear time for 3-edge-connected components. The basic distributed
algorithms are Monte Carlo, but they can be made Las Vegas without increasing
the asymptotic complexity.
In the model of parallel computing on the EREW PRAM our approach yields a
simple algorithm with optimal time complexity O(log V) for finding cut pairs
and 3-edge-connected components.Comment: Previous version appeared in Proc. 35th ICALP, pages 145--160, 200
Design of testbed and emulation tools
The research summarized was concerned with the design of testbed and emulation tools suitable to assist in projecting, with reasonable accuracy, the expected performance of highly concurrent computing systems on large, complete applications. Such testbed and emulation tools are intended for the eventual use of those exploring new concurrent system architectures and organizations, either as users or as designers of such systems. While a range of alternatives was considered, a software based set of hierarchical tools was chosen to provide maximum flexibility, to ease in moving to new computers as technology improves and to take advantage of the inherent reliability and availability of commercially available computing systems
Map building, localization and exploration for multi-robot systems
The idea of having robots performing the task for which they have been designed completely autonomously and interacting with the environment has been the main objective since the beginning of mobile robotics. In order to achieve such a degree of autonomy, it is indispensable for the robot to have a map of the environment and to know its location in it, in addition to being able to solve other problems such as motion control and path planning towards its goal. During the fulfillment of certain missions without a prior knowledge of its environment, the robot must use the inaccurate information provided by its on-board sensors to build a map at the same time it is located in it, arising the problem of Simultaneous Localization and Mapping (SLAM) extensively studied in mobile robotics. In recent years, there has been a growing interest in the use of robot teams due to their multiple benefits with respect to single-robot systems such as higher robustness, accuracy, efficiency and the possibility to cooperate to perform a task or to cover larger environments in less time. Robot formations also belongs to this field of cooperative robots, where they have to maintain a predefined structure while navigating in the environment. Despite their advantages, the complexity of autonomous multi-robot systems increases with the number of robots as a consequence of the larger amount of information available that must be handled, stored and transmitted through the communications network. Therefore, the development of these systems presents new difficulties when solving the aforementioned problems which, instead of being addressed individually for each robot, must be solved cooperatively to efficiently exploit all the information collected by the team. The design of algorithms in this multi-robot context should be directed to obtain greater scalability and performance to allow their online execution. This thesis is developed in the field of multi-robot systems and proposes solutions to the navigation, localization, mapping and path planning processes which form an autonomous system. The first part of contributions presented in this thesis is developed in the context of robot formations, which require greater team cooperation and synchronization, although they can be extended to systems without this navigation constraint. We propose localization, map refinement and exploration techniques under the assumption that the formation is provided with a map of the environment, possibly partial and inaccurate, wherein it has to carry out its commanded mission. In a second part, we propose a multi-robot SLAM approach without any assumption about the prior knowledge of a map nor the relationships between robots in which we make use of state of the art methodologies to efficiently manage the resources available in the system. The performance and efficiency of the proposed robot formation and multi-robot SLAM systems have been demonstrated through their implementation and testing both in simulations and with real robots
Development and Testing of Hardware Simulator for Satellite Proximity Maneuvers and Formation Flying
Satellite Formation Flying (SFF) and Proximity Operations are applications that have increasingly gained interest over the years. These applications foresee the substitution of a single spacecraft with a system of multiple satellites that perform coordinated position and attitude control maneuvers, which in turn results in higher accuracy of payload measurement, higher flexibility, robustness to failure, and reduction of development costs. These systems present however higher difficulties in their design since they have not only absolute but also relative state requirements, which make them also liable to higher control action expense with respect to (wrt) the single satellite systems. Moreover, applications like Automated Rendez-Vous and Docking (RVD) and in general close proximity maneuvers present a high risk of impact between the satellites, which must be treated with an appropriate design of the on board Guidance Navigation and Control (GNC) system. These aspects justify the development and employment of a ground hardware simulator representative of two or more satellites performing coordinate maneuvers, allowing the investigation of these problems with an easily accessible system.
The aim of my Ph.D. Activities has consisted in the development and testing of the cooperating SPAcecRaft Testbed for Autonomous proximity operatioNs experimentS (SPARTANS) hardware simulator, which is under development since 2010 at the Center of Studies and Activities for Space (CISAS) of the University of Padova. This ground simulator presents robotic units that allow the reproduction of the relative position and attitude motions of satellites in proximity or in formation, and can be therefore employed for the extensive study of control algorithms and strategies for these types of applications, allowing dedicated hardware in the loop to be tested in a controlled environment. At the beginning of my Ph.D., the testbed consisted in the first prototype of Attitude Module (AM), a platform with three rotational Degrees of Freedom (DOF) of Yaw, Pitch and Roll, controllable through a GNC system based on incremental encoders and air thrusters. A small contribution was initially given in support of the execution of a series of 3 DOF attitude control maneuvers tests with the AM. Subsequently, the first activity consisted in the design and development of the air suspension system that enables a low friction translational motion of the a whole Unit of the testbed over the test table, with the characterization of air skids available in laboratory. The subsequent activity consisted in the design and development of the Translation Module (TM), the lower section of the whole Unit, as modular structure supporting the air suspension system, the AM, and the on board localization system. After this activity the on board localization system for position and Azimuth estimation, based on Optical Flow Sensors (OFS), was developed and tested. The system was installed on a TM base prototype and it was calibrated and tested with the imposition of known motions through rotational and translational motorized stages wich were used in conjunction, presenting max deviations at the level of 0.1° for a total rotational range of 40°, and max deviations of 1 mm for a total translational range of 100 mm. Combined maneuvers, i.e. translational and rotational motions imposed in sequence, were subsequently performed, showing a drift trend, up to approximately 1 cm for a 90° rotation. Subsequently the OFS system was assembled in the TM and integrated with an external vision system, under development in parallel in the context of the SPARTANS project. Results showed a good general concordance between the two systems, but combined maneuvers with extended rotational range showed again a drift trend in the OFS system solution, not only in position but also in Azimuth.
A parallel activity consisted in the design and development of the levellable test table for the Units with a modular structure. Another activity consisted in the development of a Matlab Software Simulator for Units tests planning. A series of preliminary standard and optimal control maneuvers were planned with the software simulator.
The last activity of my Ph.D. consisted in the analysis of an inspection scenario for satellite removal purposes, with the goal of reproducing the relative dynamics in scale with the SPARTANS simulator. The chosen scenario foresaw the inspection, through a vision system on board an inspection satellite, of the currently freely tumbling Envisat spacecraft . The analysis performed with a Matlab software simulator was focused on the acquisition and maintainance of a circular relative orbit at close range starting from a flyaround orbit, through the employment of Model Predictive Control (MPC) and Linear Quadratic Regulator (LQR) optimal controllers. Simulations results showed a lower tracking error in position with the MPC controller wrt to the LQR controller, but with a higher control action expense: for a 6 hours inspection on a 41 m radius circular relative orbit, the max total delta-v component resulted of 3.3 m/s for MPC, while it resulted of 0.7 m/s for LQR.
In the present configuration the SPARTANS testbed presents a first complete Unit and test table to be assembled in the immediate future for the execution of the first position and attitude control maneuvers. The final configuration of the testbed will present a minimum of two Units allowing to perform coordinate control maneuvers for the investigation and study of problems and strategies related to SFF, Automated Rendez-Vous and Docking, and in general proximity manevuers
Second Generation General System Theory: Perspectives in Philosophy and Approaches in Complex Systems
Following the classical work of Norbert Wiener, Ross Ashby, Ludwig von Bertalanffy and many others, the concept of System has been elaborated in different disciplinary fields, allowing interdisciplinary approaches in areas such as Physics, Biology, Chemistry, Cognitive Science, Economics, Engineering, Social Sciences, Mathematics, Medicine, Artificial Intelligence, and Philosophy. The new challenge of Complexity and Emergence has made the concept of System even more relevant to the study of problems with high contextuality. This Special Issue focuses on the nature of new problems arising from the study and modelling of complexity, their eventual common aspects, properties and approaches—already partially considered by different disciplines—as well as focusing on new, possibly unitary, theoretical frameworks. This Special Issue aims to introduce fresh impetus into systems research when the possible detection and correction of mistakes require the development of new knowledge. This book contains contributions presenting new approaches and results, problems and proposals. The context is an interdisciplinary framework dealing, in order, with electronic engineering problems; the problem of the observer; transdisciplinarity; problems of organised complexity; theoretical incompleteness; design of digital systems in a user-centred way; reaction networks as a framework for systems modelling; emergence of a stable system in reaction networks; emergence at the fundamental systems level; behavioural realization of memoryless functions
Networks in cognitive science
Networks of interconnected nodes have long played a key role in Cognitive Science, from artificial neural networks to spreading activation models of semantic memory. Recently, however, a new Network Science has been developed, providing insights into the emergence of global, system-scale properties in contexts as diverse as the Internet, metabolic reactions, and collaborations among scientists. Today, the inclusion of network theory into Cognitive Sciences, and the expansion of complex-systems science, promises to significantly change the way in which the organization and dynamics of cognitive and behavioral processes are understood. In this paper, we review recent contributions of network theory at different levels and domains within the Cognitive Sciences.Postprint (author's final draft
Working notes of the KI \u2796 Workshop on Agent Oriented Programming and Distributed Systems
Agent-oriented techniques are likely to be the next significant breakthrough in software development process. They provide a uniform approach throughout the analysis, design and implementation phases in the development life cycle. Agent-oriented techniques are a natural extension to object-oriented techniques, but while there is a whole pIethora of analysis and design methods in the object-oriented paradigm, very little work has been reported on design and analysis methods in the agent-oriented community. After surveying and examining a number of well-known object-oriented design and analysis methods, we argue that none of these methods, provide the adequate model for the design and analysis of multi-agent systems. Therefore, we propose a new agent-specific methodology that is based on and builds upon object-oriented methods. We identify three major models that need to be build during the development of multi-agent applications and describe the process of building these models
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