Copyright (c) 2009 All rights reserved. http://works.bepress.com/vijay_kumar Recent documents in R. Vijay Kumar en-us Sun, 31 May 2009 13:01:12 PDT 3600 http://works.bepress.com/vijay_kumar/74 http://works.bepress.com/vijay_kumar/74 Wed, 16 Jan 2008 15:53:24 PST This paper proposes a new methodology for coordinating multi-robot teams in the execution of cooperative tasks. It is based on a dynamic role assignment mechanism, in which the robots assume and exchange roles during cooperation. We model the role assignment under a hybrid systems framework, using a hybrid automaton to represent roles, transitions and controllers. Using a multi-robot simulator, the methodology is demonstrated in a cooperative transportation task, in which a group of robots must find and cooperatively transport several objects scattered in the environment. Luiz Chaimowicz http://works.bepress.com/vijay_kumar/73 http://works.bepress.com/vijay_kumar/73 Wed, 16 Jan 2008 15:53:21 PST Formation stability is now analyzed under a new prism using input-to-state stability. Formation ISS relates leader input to internal state of the formation and characterizes the way this input affects stability performance. Compared to other notions of stability for interconnected systems, formation ISS does not require attenuation of errors as they propagate, but instead quantifies the amplification and provides worst case bounds. The control interconnections that give rise to the formation are represented by a graph. The formation graphs considered are built from a small number of primitive graphs, the stability properties of which are used to reason about the composite. For the case of linear dynamics, a recursive expression allows the calculation of the bounds using the graph theoretic representation of the formation via the adjacency matrix. Illustrative examples demonstrate how formation ISS can be used as an analysis and a design tool. Herbert G. Tanner http://works.bepress.com/vijay_kumar/72 http://works.bepress.com/vijay_kumar/72 Wed, 16 Jan 2008 15:53:17 PST In this paper, we formulate a semi-implicit time-stepping model for multibody mechanical systems with frictional, distributed compliant contacts. Employing a polyhedral pyramid model for the friction law and a distributed, linear, viscoelastic model for the contact, we obtain mixed linear complementarity formulations for the discrete-time, compliant contact problem. We establish the existence and finite multiplicity of solutions, demonstrating that such solutions can be computed by Lemke's algorithm. In addition, we obtain limiting results of the model as the contact stiffness tends to infinity. The limit analysis elucidates the convergence of the dynamic models with compliance to the corresponding dynamic models with rigid contacts within the computational time-stepping framework. Finally, we report numerical simulation results with an example of a planar mechanical system with a frictional contact that is modeled using a distributed, linear viscoelastic model and Coulomb's frictional law, verifying empirically that the solution trajectories converge to those obtained by the more traditional rigid-body dynamic model. Peng Song http://works.bepress.com/vijay_kumar/71 http://works.bepress.com/vijay_kumar/71 Wed, 16 Jan 2008 15:53:14 PST A new type of stability of leader follower formations is defined, based on input-to-state stability (ISS) properties of cascade interconnections. Formation ISS links leader input to internal state of the formation and characterizes the way this input affects performance. The effect of feedforward and feedback inter-agent communication is then investigated in this framework and it is indicated how the structure of interconnections and the amount of available information can affect stability performance. Herbert G. Tanner http://works.bepress.com/vijay_kumar/70 http://works.bepress.com/vijay_kumar/70 Wed, 16 Jan 2008 15:53:11 PST While there is extensive literature available on parallel manipulators in general, there has been much less attention given to cable-driven parallel manipulators. In this paper, we address the problem of analyzing the reachable workspace using the tools of semi-definite programming. We build on earlier work [1,2] done using similar techniques by deriving limiting conditions that allow us to compute analytic expressions for the boundary of the reachable workspace. We illustrate this computation for a planar parallel manipulator with four actuators. Ethan Stump http://works.bepress.com/vijay_kumar/69 http://works.bepress.com/vijay_kumar/69 Wed, 16 Jan 2008 15:53:07 PST We describe a framework and control algorithms for coordinating multiple mobile robots with manipulators focusing on tasks that require grasping, manipulation and transporting large and possibly flexible objects without special purpose fixtures. Because each robot has an independent controller and is autonomous, the coordination and synergy are realized through sensing and communication. The robots can cooperatively transport objects and march in a tightly controlled formation, while also having the capability to navigate autonomously. We describe the key aspects of the overall hierarchy and the basic algorithms, with specific applications to our experimental testbed consisting of three robots. We describe results from many experiments that demonstrate the ability of the system to carry flexible boards and large boxes as well as the system's robustness to alignment and odometry errors. Thomas G. Sugar http://works.bepress.com/vijay_kumar/68 http://works.bepress.com/vijay_kumar/68 Wed, 16 Jan 2008 15:53:04 PST When working in an unstructurcd environment, a robot has partial or no a priori knowledge of the environment. The purpose of exploratory robotics is to provide robots with the ability to learn and automatically construct models of the environment by exploring and interacting with the environment. This paper describes a two-arm exploratory system whose purpose is to identify movable and removable parts of an object, and the mobility of the parts. The system is implemented by integrating RCI (Robot Control Interface) with Timix (a real-time kernel). The identification is accomplished through exp1oratory procedures which are generated from a numbcr of robust motion primitives. Xiaoping Yan http://works.bepress.com/vijay_kumar/67 http://works.bepress.com/vijay_kumar/67 Wed, 16 Jan 2008 15:53:01 PST When multiple arms are used to manipulate a large object, it is necessary to maintain and control contacts between the object and effector(s) on one or more arms. The contacts are characterized by holonomic as well as nonholonomic constraints. This paper addresses the control of mechanical systems subject to nonholonomic constraints, rolling constraints in particular. It has been shown that such a system is always controllable, but cannot be stabilized to a single equilibrium by smooth feedback [l, 2]. In this paper, we show that the system is not input-state linearizable though input-output linearization is possible with appropriate output equations. Further, if the system is position-controlled (i.e., the output equation is a functions of position variables only), it has a zero dynamics which is Lagrange stable but not asymptotically stable. We discuss the analysis and controller design for planar as well as spatial multi-arm systems and present results from computer simulations to demonstrate the theoretical results. Xiaoping Yun http://works.bepress.com/vijay_kumar/66 http://works.bepress.com/vijay_kumar/66 Wed, 16 Jan 2008 15:52:57 PST This paper addresses the problem of coordinating aerial and ground vehicles in tasks that involve exploration, identification of targets and maintaining a connected communication network. We focus on the problem of localizing vehicles in urban environments where GPS signals are often unreliable or unavailable. We first describe our multi-robot tesbed and control software used to coordinate ground and aerial vehicles. We present the results of experiments in air-ground localization analyzing three complementary approaches to determining the positions of vehicles on the ground. We show that the coordination of aerial vehicles with ground vehicles is necessary to get accurate estimates of the state of the system. Luiz Chaimowicz http://works.bepress.com/vijay_kumar/65 http://works.bepress.com/vijay_kumar/65 Wed, 16 Jan 2008 15:52:53 PST We describe the development and assessment of a computer controlled wheelchair equipped with a suite of sensors and a novel interface, called the SMARTCHAIR. The main focus of this paper is a shared control framework which allows the human operator to interact with the chair while it is performing an autonomous task. At the highest level, the autonomous system is able to plan paths using high level deliberative navigation behaviors depending on destinations or waypoints commanded by the user. The user is able to locally modify or override previously commanded autonomous behaviors or plans. This is possible because of our hierarchical control strategy that combines three independent sources of control inputs: deliberative plans obtained from maps and user commands, reactive behaviors generated by stimuli from the environment, and user-initiated commands that might arise during the execution of a plan or behavior. The framework we describe ensures the user's safety while allowing the user to be in complete control of a potentially autonomous system. Sarangi P. Parikh