E. G. Hernandez-Martinez

Non-Collision Conditions in Multi-Agent Virtual Leader-Based Formation Control

Formation control is one of the most important issues of group coordination for multi-agent robots systems. Some schemes are based on the leader-followers approach where some robots are considered as group leaders which influence the group behaviour. In this work, we address a formation strategy using a virtual leader which has communication with the rest of the follower robots, considered as omnidirectional robots. The virtual leader approach presents advantages such as analysis simplification and fewer sensing requirements in the control law implementation. The formation control is based on attractive potential functions only. The control law guarantees the convergence to the desired formation but, in principle, does not avoid inter-agent collisions. A set of necessary and sufficient non-collision conditions based on the explicit solution of the closed-loop system is derived. The conditions allow concluding from the initial conditions whether or not the agents will collide. The results are extended to the case of unicycle-type robots.

Collision Avoidance in Formation Control Using Discontinuous Vector Fields

Abstract This paper presents a novel collision avoidance approach in formation control for Multi-agent Robots. The control strategy consists in the mix of attractive vector fields and repulsive vector fields based on a scaled unstable focus centered at the position of another robot, instead of the vector fields obtained from the negative gradient of repulsive potential functions. The analysis of the closed-loop system is presented for the case of two point robots. After that, a modification of the composite vector field is proposed adding a discontinuity in order to avoid the undesired equilibria of the system. Real-time experiments using unicycle-type robots show that the control strategy exhibits good performance.

Distance-based Formation Control Using Angular Information Between Robots

Distance-based formation of groups of mobile robots provides an alternative focus for motion coordination strategies respect to the standard consensus-based formation strategies. However, the setup formulation introduces non rigidity problems, multiple formation patterns that verify the distance constraints or local minima appeared when collision avoidance strategies are added to the control laws. This paper proposes a novel combined distance-based potential functions with attractive-repulsive behavior in order to simplify the navigation problem as well as the use of angular information between robots to reduce the likelihood of unwanted formation patterns. Moreover, this approach eliminates the local minima generated by the control laws to reach the desired formation configuration in the case of three robots. The analysis addresses the case of omnidirectional robots and is extended to the case of unicycle-type robots with numerical simulations and real-time experiments.

Formation Tracking Based on Approximate Velocities

This paper analyses the formation tracking of groups of mobile robots moving on the plane. A leader robot is chosen to follow a prescribed trajectory whilst the rest, considered as followers, are f...

Formation Tracking with Orientation Convergence for Groups of Unicycles

This paper presents three trajectory tracking control strategies for unicycle-type robots based on a leader-followers scheme. The leader robot converges asymptotically to a smooth trajectory, while the follower robots form an undirected open-chain configuration at the same time. It is also shown that the orientation angles of all the robots converge to the same value. The control laws are based on a dynamic extension of the kinematic model of each robot. The output function to be controlled is the midpoint of the wheel axis of every robot. This choice leads to an ill-defined control law when the robot is at rest. To avoid such singularities, a complementary control law is enabled momentarily when the linear velocity of the unicycles is close to zero. Finally, numerical simulations and real-time experiments show the performance of the control strategies.

Multi-robot formation control using distance and orientation

Formation control analyses the convergence of a group of mobile agents to predefined geometric patterns. In traditional approaches, it is assumed that each agent knows the exact position of certain members of the group with respect to a reference frame and the associated control laws are designed according to inter-robot relative positions. Designing a more decentralized scheme, this paper proposes a formation scheme, using Lyapunov techniques, considering that the local controllers of the agents can be equipped with distance and orientation sensors. The main result of the paper applies to certain distance-based potential functions with inter-robot collision avoidance and an arbitrary undirected formation graph. Also, the control law includes an integral-type control that eliminates the effects of the dead-zone of actuators in order to avoid the standard techniques of normalization. The control approach is analyzed for omnidirectional robots with numerical simulations and extended for unicycle-type robots with real-time experiments. Graphical Abstract

Hybrid Architecture for Coordination of AGVs in FMS

This paper presents a hybrid control architecture that coordinates the motion of groups of automated guided vehicles in flexible manufacturing systems. The high-level control is based on a Petri net model, using the industrial standard ISA-95, obtaining a task-based coordination of equipment and storage considering process restrictions, logical precedences, shared resources and the assignment of robots to move workpieces individually or in subgroups. On the other hand, in the low-level control, three basic control laws are designed for unicycle-type robots in order to achieve desired formation patterns and marching behaviours, avoiding inter-robot collisions. The control scheme combines the task assignment for the robots obtained from the discrete-event model and the implementation of formation and marching continuous control laws applied to the motion of the mobile robots. The hybrid architecture is implemented and validated for the case of a flexible manufacturing system and four mobile robots using a virtual reality platform.

Decentralized Discrete-Time Formation Control for Multirobot Systems

Inspired from the collective behavior of biological entities for the group motion coordination, this paper analyzes the formation control of mobile robots in discrete time where each robot can sense only the position of certain team members and the group behavior is achieved through the local interactions of robots. The main contribution is an original formal proof about the global convergence to the formation pattern represented by an arbitrary Formation Graph using attractive potential functions. The analysis is addressed for the case of omnidirectional robots with numerical simulations.

Formation Tracking of Heterogeneous Mobile Agents Using Distance and Area Constraints

This paper presents two formation tracking control strategies for a combined set of single and double integrator agents with an arbitrary undirected communication topology. The first approach is based on the design of distance-based potential functions with interagent collision avoidance using local information about the distance and orientation between agents and the desired trajectory. The second approach adds signed area constraints to the desired formation specification and a control strategy that uses distance as well as area terms is designed to achieve tracking convergence. Numerical simulations show the performance from both control laws.

Formation control of heterogeneous robots using distance and orientation

This paper proposes a planar formation control scheme for a group of mobile robots modeled as single or double integrators. The approach is based on the design of distance-based potential functions with inter-robot collision avoidance using the information about the distance with respect to other robots in polar coordinates. The result applies to an arbitrary undirected communication topology. The formation scheme becomes a decentralized control setup in robots equipped with local sensors of distance and orientation. The control approach can be extended to the case of kinematic and dynamical models of nonholonomic robots applying an appropriated input-output linearization. It enables the possibility to combine heterogeneous robots as shown by numerical simulations using the projection in the 2D plane of multi-rotor Unmanned Aerial Vehicles (UAVs), unicycle-type and omnidirectional wheeled mobile robots. Also, an experiment with these real wheeled mobile robots is presented.