Virtual structure based target-enclosing strategies for nonholonomic agents

In this paper, we discuss a target-enclosing problem for a group of multiple nonholonomic agents in a plane. The proposed strategies guarantee that multiple agents\u27 coordination finally results in a circular formation enclosing the targe-to-bject which moves in the plane. Firstly, virtual agents for the feedback linearization of the real nonholonomic agents are introduced. Secondly, we propose the target-enclosing control laws based on the consensus algorithm to the virtual agents. Algebraic graph theory and consensus algorithm are employed to prove convergence and stability of the enclosing problem. Finally, experiments are provided to demonstrate the effectiveness of the proposed control laws. ©2008 IEEE

Observer-based consensus control strategy for multi-agent system with communication time delay

This paper proposes an observer-based consensus control strategy for multi-agent system (MAS) with communication time delay. The condition of stability for MIMO agents is derived by Lyapunov theorem. It gives systematic design procedure under assumed unidirectional network. Furthermore, new consensus control law using observers is proposed for the networked MAS with communication delays. Experimental results show effectiveness of our proposed output consensus approaches. ©2008 IEEE

Modelling and control of the coordinated motion of a group of autonomous mobile robots

The coordinated motion of a group of autonomous mobile robots for the achievement of a coordinated task has received signifcant research interest in the last decade. Avoiding the collisions of the robots with the obstacles and other members of the group is one of the main problems in the area as previous studies have revealed. Substantial amount of research effort has been concentrated on defning virtual forces that will yield reference trajectories for a group of autonomous mobile robots engaged in coordinated behavior. If the mobile robots are nonholonomic, this approach fails to guarantee coordinated motion since the nonholonomic constraint blocks sideway motions. Two novel approaches to the problem of modeling coordinated motion of a group of autonomous nonholonomic mobile robots inclusive of a new collision avoidance scheme are developed in this thesis. In the first approach, a novel coordination method for a group of autonomous nonholonomic mobile robots is developed by the introduction of a virtual reference system, which in turn implies online collision-free trajectories and consists of virtual mass-spring-damper units. In the latter, online generation of reference trajectories for the robots is enabled in terms of their linear and angular velocities. Moreover, a novel collision avoidance algorithm, that updates the velocities of the robots when a collision is predicted, is developed in both of the proposed models. Along with the presentation of several coordinated task examples, the proposed models are verifed via simulations. Experiments were conducted to verify the performance of the collision avoidance algorithm

Coordinated multi-robot formation control

Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Faculdade de Engenharia. Universidade do Porto. 201

COOPERATIVE TARGET TRACKING IN CONCENTRIC FORMATIONS

This paper considers the problem of coordinating multiple unmanned aerial vehicles (UAVs) in a circular formation around a moving target. The main contribution is allowing for versatile formation patterns on the basis of the following components. Firstly, new uniform spacing control laws are proposed that spread the agents not necessarily over a full circle, but over a circular arc. Uniform spacing formation controllers are proposed, regulating either the separation distances or the separation angles between agents. Secondly, the use of virtual agents is proposed to allow for different radii in agents’ orbits. Thirdly, a hierarchical combination of formation patterns is described. A Lyapunov analysis is conducted to study the stability characteristics. This paper also addresses the practical issue of collision avoidance that arises while UAVs are developing formations. An additional control component is added that repels agents to steer away from each other once they get too close. All UAVs have constant linear velocities. Control of the UAV is via its yaw rate. The proposed extensions to formation on a portion of a circle, circling on different radii for different agents, formation in local geometric shapes, and inter-vehicle collision avoidance, provide more complete solution to cooperative target tracking in concentric formations

Energetic swarm control with application to multiple vehicle systems

Control and coordination of multiple vehicle systems has been a very active area of research in recent years. Recent advancements in computation, communication, and mechatronics have allowed the development of large groups of vehicles, often referred to as swarms, in order to accomplish complex missions over large areas with redundant fault tolerant capabilities. Existing swarm control work has addressed swarm aggregation, foraging swarms, swarm formation, and swarms that track and enclose targets. Energetic swarm control is another significant recent contribution to the swarm control literature. It allows the control of the internal kinetic energy and potential kinetic energy of the swarm system in order to achieve tasks such as sweeping an area, patrolling, and area coverage. This thesis involves the application of energetic swarm control to wheeled mobile robots. A lower level control layer for wheeled mobile robots, based on feedback linearization, is developed and combined with a higher level particle based energetic swarm controller. Furthermore, input saturation constraints are addressed using a suitable control allocation approach. An experimentally verified model of a wheeled mobile robot is developed and used to demonstrate the capabilities of the new energetic swarm control approach for wheeled mobile robots

Coordination of Multirobot Teams and Groups in Constrained Environments: Models, Abstractions, and Control Policies

Robots can augment and even replace humans in dangerous environments, such as search and rescue and reconnaissance missions, yet robots used in these situations are largely tele-operated. In most cases, the robots\u27 performance depends on the operator\u27s ability to control and coordinate the robots, resulting in increased response time and poor situational awareness, and hindering multirobot cooperation. Many factors impede extended autonomy in these situations, including the unique nature of individual tasks, the number of robots needed, the complexity of coordinating heterogeneous robot teams, and the need to operate safely. These factors can be partly addressed by having many inexpensive robots and by control policies that provide guarantees on convergence and safety. In this thesis, we address the problem of synthesizing control policies for navigating teams of robots in constrained environments while providing guarantees on convergence and safety. The approach is as follows. We first model the configuration space of the group (a space in which the robots cannot violate the constraints) as a set of polytopes. For a group with a common goal configuration, we reduce complexity by constructing a configuration space for an abstracted group state. We then construct a discrete representation of the configuration space, on which we search for a path to the goal. Based on this path, we synthesize feedback controllers, decentralized affine controllers for kinematic systems and nonlinear feedback controllers for dynamical systems, on the polytopes, sequentially composing controllers to drive the system to the goal. We demonstrate the use of this method in urban environments and on groups of dynamical systems such as quadrotors. We reduce the complexity of multirobot coordination by using an informed graph search to simultaneously build the configuration space and find a path in its discrete representation to the goal. Furthermore, by using an abstraction on groups of robots we dissociate complexity from the number of robots in the group. Although the controllers are designed for navigation in known environments, they are indeed more versatile, as we demonstrate in a concluding simulation of six robots in a partially unknown environment with evolving communication links, object manipulation, and stigmergic interactions