A Message Passing Strategy for Decentralized Connectivity Maintenance in Agent Removal

In a multi-agent system, agents coordinate to achieve global tasks through local communications. Coordination usually requires sufficient information flow, which is usually depicted by the connectivity of the communication network. In a networked system, removal of some agents may cause a disconnection. In order to maintain connectivity in agent removal, one can design a robust network topology that tolerates a finite number of agent losses, and/or develop a control strategy that recovers connectivity. This paper proposes a decentralized control scheme based on a sequence of replacements, each of which occurs between an agent and one of its immediate neighbors. The replacements always end with an agent, whose relocation does not cause a disconnection. We show that such an agent can be reached by a local rule utilizing only some local information available in agents' immediate neighborhoods. As such, the proposed message passing strategy guarantees the connectivity maintenance in arbitrary agent removal. Furthermore, we significantly improve the optimality of the proposed scheme by incorporating $\delta$-criticality (i.e. the criticality of an agent in its $\delta$-neighborhood).Comment: 9 pages, 9 figure

Automatic coordination and deployment of multi-robot systems

We present automatic tools for configuring and deploying multi-robot networks of decentralized, mobile robots. These methods are tailored to the decentralized nature of the multi-robot network and the limited information available to each robot. We present methods for determining if user-defined network tasks are feasible or infeasible for the network, considering the limited range of its sensors. To this end, we define rigid and persistent feasibility and present necessary and sufficient conditions (along with corresponding algorithms) for determining the feasibility of arbitrary, user-defined deployments. Control laws for moving multi-robot networks in acyclic, persistent formations are defined. We also present novel Embedded Graph Grammar Systems (EGGs) for coordinating and deploying the network. These methods exploit graph representations of the network, as well as graph-based rules that dictate how robots coordinate their control. Automatic systems are defined that allow the robots to assemble arbitrary, user-defined formations without any reliance on localization. Further, this system is augmented to deploy these formations at the user-defined, global location in the environment, despite limited localization of the network. The culmination of this research is an intuitive software program with a Graphical User Interface (GUI) and a satellite image map which allows users to enter the desired locations of sensors. The automatic tools presented here automatically configure an actual multi-robot network to deploy and execute user-defined network tasks.Ph.D.Committee Chair: Dr. Magnus Egerstedt; Committee Co-Chair: Dr. Ayanna Howard; Committee Member: Dr. David Taylor; Committee Member: Dr. Frank Dellaert; Committee Member: Dr. Ian Akyildiz; Committee Member: Dr. Jeff Shamm

Algorithmic and combinatorial problems on multi-UAV systems

Mathematics has always been a fundamental piece in robotics and, research in robotics has played an important role in the development of mathematics. This thesis is motivated by the growing interest on problems that appear in aerial robotics applications, specifically, on cooperative systems of multiple aerial robots or drones. Most of the research works in multi-robot systems have focused primarily on construction and validation of working systems, rather than more general and formal analysis of problems and solutions. By contrast, this thesis focuses on formally solving problems of aerial multi-robot systems from a discrete and combinatorial optimization perspective. Inspired on problems of this area, the thesis introduces some new theoretical models and problems of interest for mathematicians and computer scientists. The following topics are covered in this thesis: (1) synchronization: design of a coordination strategy to allow periodical communication between the members of a cooperative team while performing a task along fixed trajectories in a scenario with limited communication range, (2) robustness: analysis of the detrimental effects in the performance of a synchronized system when one or more robots fail, (3) stochastic strategies: performance analysis of a synchronized system using drones with stochastic decision making, and (4) task allocation: decentralized coordination to perform periodical task allocation in order to maintain a balanced work load for all members of a team with limited communication range. In the first part of the thesis, we study the synchronization problem giving a theoretical characterization of the solutions and, we present an algorithm to build a synchronized system for a given set of covering trajectories. The second part focuses on the study of the robustness in a synchronized system regarding to two key aspects: covering of the working area and communication between the members of the team. We rigorously study several combinatorial problems to measure how robust a system is to deal with drones failures. Connections of theseproblemswithnumbertheory, graphtheory, circulantgraphsandpolynomial multiplication are shown. The third part is devoted to an analysis of synchronized systems using random aerial robots. This topic is closely related to the random walk theory. It is shown that stochastic strategies increase the robustness of a synchronized system. Finally, this thesis introduces the block sharing strategy to addresstheproblemofmaintainingabalancedtaskallocationamongtherobotsby using periodical communications. A proof on the convergence to an optimal task allocation is given and, a case study for structure construction using a cooperative team of aerial robots is presented. All algorithms developed in this thesis have been implemented and extensive experiments have been conducted to analyze and validate the proposed methods.Las matemáticas siempre han sido una pieza fundamental en el desarrollo de la robótica, así como los problemas de robótica han jugado un importante papel en el desarrollo de las matemáticas. Esta tesis está motivada por el creciente interés en problemas que aparecen en aplicaciones de robótica aérea, específicamente, está enfocada en sistemas cooperativos de múltiples robots aéreos o drones. La mayoría de los trabajos de investigación en sistemas de robots se han centrado en la construcción y validación de arquitecturas desde un enfoque empírico. Por el contrario, esta tesis enfoca el estudio de problemas relacionados con tareas para equipos de robots aéreos desde el punto de vista de la optimización discreta y combinatoria. Inspirada en problemas de este campo, esta memoria plantea nuevos modelos teóricos y problemas de interés para las matemáticas aplicadas y la ciencia computacional. Enestatesisse abordanlostemassiguientes: (1) sincronización: diseñodeuna estrategia de coordinación que permita comunicación periódica entre los miembros de un equipo cooperativo mientras ejecutan una tarea sobre trayectorias fijadas, (2) robustez: análisis del efecto que produce el fallo de los agentes en un sistema sincronizado, (3)estrategias estocásticas: análisisdelfuncionamientodeunsistema sincronizado cuando se utilizan drones con toma de decisiones aleatorias, y (4) asignación de tareas: coordinación no centralizada usando asignación periódica de tareas que permita mantener una carga de trabajo balanceada. En la primera parte, se estudia teóricamente el problema de la sincronización, dando condiciones necesarias y suficientes para la existencia de solución y se presenta un algoritmo que construye un sistema sincronizado para un conjunto fijado de trayectorias de vuelo. La segunda parte de la tesis estudia la robustez de un sistema sincronizado teniendo en cuenta dos aspectos fundamentales: el cubrimiento del terreno y la comunicación entre los miembros del equipo. Se estudian de forma rigurosa problemas combinatorios que surgen cuando se requiere saber cómo de robusto es un sistema con respecto a fallos. Se muestran conexiones con áreas matemáticas como la teoría de números, la teoría de grafos, los grafos circulantes o multiplicación de polinomios. En la tercera parte de la tesis, se estudia la robustez del sistema cuando se introducen decisiones aleatorias de los drones. Se prueba la relación de este problema con la teoría de caminatas aleatorias y se muestra que el uso de estrategias estocásticas supone una mejora de la robustez del sistema sincronizado. Por último, se propone la estrategia de coordinación por bloques para la asignación balanceada de tareas. Se prueba la convergencia del método a una asignación óptima y se realiza un estudio de caso para la construcción de una estructura mediante un equipo cooperativo de drones. Todos los algoritmos desarrollados en esta tesis han sido implementados y se han llevado a cabo diversos experimentosque demuestran la validez de los métodos propuestos

Recent Advances in Multi Robot Systems

To design a team of robots which is able to perform given tasks is a great concern of many members of robotics community. There are many problems left to be solved in order to have the fully functional robot team. Robotics community is trying hard to solve such problems (navigation, task allocation, communication, adaptation, control, ...). This book represents the contributions of the top researchers in this field and will serve as a valuable tool for professionals in this interdisciplinary field. It is focused on the challenging issues of team architectures, vehicle learning and adaptation, heterogeneous group control and cooperation, task selection, dynamic autonomy, mixed initiative, and human and robot team interaction. The book consists of 16 chapters introducing both basic research and advanced developments. Topics covered include kinematics, dynamic analysis, accuracy, optimization design, modelling, simulation and control of multi robot systems

Distributed bees algorithm for task allocation in swarm of robots

In this paper, we propose the distributed bees algorithm (DBA) for task allocation in a swarm of robots. In the proposed scenario, task allocation consists in assigning the robots to the found targets in a 2-D arena. The expected distribution is obtained from the targets' qualities that are represented as scalar values. Decision-making mechanism is distributed and robots autonomously choose their assignments taking into account targets' qualities and distances. We tested the scalability of the proposed DBA algorithm in terms of number of robots and number of targets. For that, the experiments were performed in the simulator for various sets of parameters, including number of robots, number of targets, and targets' utilities. Control parameters inherent to DBA were tuned to test how they affect the final robot distribution. The simulation results show that by increasing the robot swarm size, the distribution error decreased

Robust String Stability of Vehicle Platoons with Communication

This work investigates longitudinal spacing policies and vehicular communication strategies that can reduce inter-vehicular spacing between the vehicles of automated highway platoons, in the presence of parasitic actuation lags. Currently employed platooning technologies rely on the vehicle’s onboard sensors for information of the neighboring vehicles, due to this they may require large spacing between the vehicles to ensure string stability in the presence of uncertainties, such as parasitic actuation lags. More precisely, they require that the minimum employable time headway (hmin) must be lower bounded by 2τ₀ for string stability, where τ₀ is the maximum parasitic actuation lag. Recent studies have demonstrated that using vehicular communication one may be able to employ smaller spacing between vehicles while ensuring robustness to parasitic lags. However, precise results on the extent of such reduction are sparse in the literature. In this work, platoon string stability is used as a metric to study controllers that require vehicular communication, and find the amount of reduction in spacing such controllers can offer. First, the effects of multiple vehicle look ahead in vehicle platoons that employ a Constant Spacing Policy (CSP) based controller without lead vehicle information in the presence of parasitic lags is studied and string instability of such platoons is demonstrated. A robustly string stable CSP controller that employs information from the leader and the immediate predecessor is considered to determine an upper bound on the allowable parasitic lag; for this CSP controller, a design procedure for the selection of controller gains for a given parasitic lag is also provided. For a string of vehicles adopting a Constant Time Headway Policy (CTHP), it is demonstrated that the minimum employable time headway can be further decreased via vehicular communication in the following manner: (1) if the position, velocity and acceleration of the immediate predecessor vehicle is used, then the ii minimum employable time headway hmin can be reduced to τ₀; (2) if the position and velocity information of r immediately preceding vehicles is used, then hmin can be reduced to 4τ₀/(1 + r); (3) furthermore, if the acceleration of ‘r’ immediately preceding vehicles is used, then hmin can be reduced to 2τ₀/(1 + r); and (4) if the position, velocity and acceleration of the immediate and the r-th predecessors are used, then hmin = 2τ₀/(1 + r). Note that cases (3) and (4) provide the same lower bound on the minimum employable time headway; however, case (4) requires much less communicated information. Representative numerical simulations that are conducted to corroborate the above results are discussed. Vehicle formations employing ring structured communication strategies are also studied in this work and a combinatorial approach for developing ring graphs for vehicle formations is proposed. Stability properties of the platoons with ring graphs, limitations of using ring graphs in platoons, and methods to overcome such limitations are explored. In addition, with ring communication structure, it is possible to devise simple ways to recon- figure the graph when vehicles are added to or removed from the platoon or formation, which is also discussed in this work. Further, experimental results using mobile robots for platooning and two-dimensional formations using ring graphs are discussed

A novel coordination framework for multi-robot systems

Having made great progress tackling the basic problems concerning single-robot systems, many researchers shifted their focus towards the study of multi-robot systems (MRS). MRS were shortly found to be a perfect t for tasks considered to be hard, complex or even impossible for a single robot to perform, e.g. spatially separate tasks. One core research problem of MRS is robots' coordinated motion planning and control. Arti cial potential elds (APFs) and virtual spring-damper bonds are among the most commonly used models to attack the trajectory planning problem of MRS coordination. However, although mathematically sound, these approaches fail to guarantee inter-robot collision-free path generation. This is particularly the case when robots' dynamics, nonholonomic constraints and complex geometry are taken into account. In this thesis, a novel bio-inspired collision avoidance framework via virtual shells is proposed and augmented into the high-level trajectory planner. Safe trajectories can hence be generated for the low-level controllers to track. Motion control is handled by the design of hierarchical controllers which utilize virtual inputs. Several distinct coordinated task scenarios for 2D and 3D environments are presented as a proof of concept. Simulations are conducted with groups of three, four, ve and ten nonholonomic mobile robots as well as groups of three and ve quadrotor UAVs. The performance of the overall improved coordination structure is veri ed with very promising result