This paper deals with the navigation in formation of a group of mobile robots. A set of virtual targets (points) forms a virtual structure of the same shape as the desired formation. Hence, to join and to remain in this formation, each robot has only to track one of these targets. In order to track the chosen target, it has to be attainable by the robot despite its kinematic constraints. This paper studies then the maximum allowed dynamic of the virtual structure according to the kinematic constraints of the robots. Both linear and angular velocities of the targets are constrained. Moreover, depending on these velocities, some relative positions (targets) in the formation become unattainable. These positions are also defined. A stable control law allows us to attain the generated set-points. Simulation and experimental results validate the proposed contributions.

This paper considers the navigation in formation of a mobile Multi-Robot System (MRS) in presence of obstacles. In such areas, the collision avoidance between the robots themselves and with other obstacles (static and dynamic) is a challenging issue. To deal with it, a reactive and a distributed control architecture is built. The navigation in formation of the MRS is ensured while tracking a global virtual structure (first controller). Limit-cycle principle is used to compute the setpoint of the obstacle avoidance task (second controller). In this paper, kinematic constraints of the robot are taken into account in order to generate an attainable set-point. The objective is to guarantee safety of the mobile robots with respect to their maximum velocities. Simulation and experimental results validate the proposed contributions.

—This paper presents an online method for a cooperative mobile robots navigation using local robot's reference frame. The group of robots must navigate while keeping a geometric shape. To achieve with distributed and reactive way the Multi-Robot Formation (MRF), a cooperative object detection using range data is presented. This information will permits to detect the position of the leader robot in the formation. Indeed, in this work it is proposed that the leader can be surrounded by an ellipse and its parameters are obtained online using the sequential range data from all the mobile robots. An appropriate method is used to identify the enclosed ellipse. To perform the MRF, it is proposed to use a combination between behavior-based, dynamical virtual structure (already presented in [1] which use global reference frame) and leader-follower approach. Simulations will permit to show the efficiency of the proposed reactive and distributed cooperative navigation.

— This paper deals with the navigation of a multi-robot system (MRS). The latter must reach and maintain a specific formation in dynamic environment. In such areas, the collision avoidance between the robots themselves and with other obstacles (static and dynamic) is a challenging issue. To deal with it, a reactive and a distributed control architecture is proposed. The navigation in formation of the MRS is insured while tracking a global virtual structure. In addition, according to the robots' perception context (e.g., static or dynamic obstacle), the most suitable obstacle avoidance strategy is activated. These approaches use mainly the limit-cycle principle and a penalty function to obtain linear and angular robots' velocities. The proposed control law guarantees the stability (using Lyapunov function) and the safety of the MRS. The robustness and the efficiency of the proposed control architecture is demonstrated through a multitude of experiments which shows the MRS in different configuration of avoidance.

This paper deals with keeping the formation of a group of mobile robots. A set of virtual targets (points) form a virtual structure of the same shape as the desired formation. Hence, to join and to keep this formation, each robot has only to track one of these targets. The objective of the paper is to propose a cooperative strategy between the robots in order to rapidly join the virtual structure: instead of assigning ahead the targets to the robots, this strategy consists of making each one able to negotiate the closest target. If the latter is desired by a lot of robots, it is left to the robot which meets more difficulties to find an other target. Negotiation is based on a minimalist communication of relative cost coefficients between the robots. Simulation and experimental results validate the proposed contributions.

Reactive navigation in very cluttered environment while insuring maximum safety and task efficiency is a challenging subject. This paper proposes online and adaptive elliptic trajectories to perform smooth and safe mobile robot navigation. These trajectories use limit-cycle principle already applied in the literature but with the difference that the applied limit-cycles are now elliptic (not circular) and are more generic and flexible to perform navigation in environments with different kinds of obstacles shape. The set points given to the robot are generated while following reactive obstacle avoidance algorithm embedded in a multi-controller architecture (Obstacle avoidance and Attraction to the target controllers). This algorithm uses specific reference frame which gives accurate indication of robot situation. The robot knows thus if it must avoid the obstacle in clockwise or counterclockwise direction and prevent robot from local minima, dead ends and oscillations. The stability of the proposed bottom-up control architecture is proved according to Lyapunov synthesis. Simulations and experiments in different environments are performed to demonstrate the efficiency and the reliability of the proposed control architecture.

In this paper, the control problem for a group of mobile robots keeping a geometric formation is considered. The proposed architecture of control allows to each robot to avoid obstacles and to rejoin the desired formation. To not complicate the control of such a system, it is proposed to divide the overall complex task into two basic tasks: attraction to a dynamical target, and obstacle avoidance. Thus, a desired geometric shape is defined and each robot has to track one node of this mobile shape. Each robot has to be autonomously able to avoid disturbing obstacles and to rejoin the formation in a reactive manner. Moreover, it chooses the optimal avoidance side thanks to limit-cycle method in order to reach as rapidly as possible its virtual target. The proposed control architecture is implemented in a distributed manner. In addition, this architecture uses the same control law (Lyapunov stable) for the two elementary tasks, and the switching from one task to another occurs only by changing the set-points. Experimental results validate the proposed control architecture.

— This paper deals with the navigation of a mobile robot in unknown environment. The robot has to reach a final target while avoiding obstacles. It is proposed to break the task complexity by dividing it into a set of basic tasks: Attraction to a target and obstacle avoidance. Each basic task is accomplished through the corresponding elementary controller. The activation of one controller for another is done according to the priority task. To ensure the overall stability of the control system, especially at the switch moments, properties of hybrid systems are used. Hybrid systems allow switching between continuous states in presence of discrete events. In this paper, it is proposed to act on the gain of the proposed control law. The aim is to ensure the convergence of a common Lyapunov function to all the controllers. This ensures the stability of the overall control. Simulation results confirm the theoretical study.

— Ce papier traite la navigation d'un robot mobile dans un environnement en presence d'obstacles. Le robot doit alors atteindre une cible finale tout enevitantenevitant des obstacles. Il est propose de briser la complexite de la tâchetâche a realiser en la decomposant en un ensemble de tâches tâcheselementaires : Attraction vers une cible etevitementetevitement d'obs-tacles. Chacune de ces tâches est accomplie grâce a un controleurcontroleurelementaire dedie. L'activation d'un controleur controleura la faveur d'un autre se fera en fonction de la tâche priori-taire a realiser. Pour assurer la stabilite globale du systeme notamment aux moments des transitions entre controleurs, les proprietes des systemes hybrides permettant le passage d'un systeme continu a un autre en presence d'´ evenements discrets sont exploitees. Ici, il est propose d'agir sur le gain de la loi de commande elaboree afin d'assurer la stabilite de l'architecture globale, et ce meme aux instants des com-mutations. La stabilite estetudieeestetudiee grâce a une fonction de Lyapunov commune a tous les controleurs. Des resultats de simulations et experimentaux appuieront les resultats theoriques. Mots-cles— Robot mobile, architecture de controle hybride, stabilite au sens de Lyapunov.

Hybrid architecture for autonomous navigation robots is presented in this paper. The principal future aim of this architecture is to achieve multi-robot convoy which must navigate in urban background, where autonomous vehicles have to track the desired trajectory while avoiding possible obstacles (walkers, other robots, etc.). Currently, we consider only the case of fixed obstacles. We propose to coordinate two continuous controllers (trajectory tracking and obstacle avoidance) by discrete events. However, hard switch from obstacle avoidance to trajectory following controller, may cause collision or undesired effects due to nonholonomic constraint or switching effects. Therefore, we introduce a third controller to overcome these drawbacks.