Ioannis Sarras

The state of the art in consensus control for multi-agent Euler-Lagrange (EL) systems relies on one or several of the following assumptions: the graph is undirected or directed and strongly connected, the controller is centralized, knowledge of the inertia and Coriolis matrices is needed, the systems’ trajectories are bounded. In this letter we show that a simple Proportional Derivative controller with gravity compensation, which does not rely on any of the previous hypotheses, achieves consensus for EL systems over connected acyclic directed graphs. The proof follows a cascaded-systems argument. Also, we provide some numerical simulations to illustrate the performance of the controller.

We address the problem of distributed control of a network of cooperative and competitive robot manipulators in end-effector coordinates. We propose a distributed bipartite formation controller that guarantees collision avoidance of the end-effectors. In the considered setting two groups are formed and reach inter-group bipartite consensus or disagreement. On the other hand, the end-effectors achieve intra-group formation. To ensure that the end-effectors do not collide, we design gradient-based control laws using barrier- Lyapunov functions. In addition, the proposed controller ensures that the closed-loop system is robust to external disturbances. The latter are assumed to be generated by an exosystem, so they are effectively rejected by an internal-model-based compensator. More precisely, we establish asymptotic stability of the bipartite formation manifold. Finally, we illustrate our theoretical results via numerical simulations.

We address the distributed bipartite containment tracking-control problem for second-order systems over multi-leader signed networks. In such systems, the networks' interconnections have signed weights; they are positive if the agents are cooperative with each other and negative otherwise. Moreover, as multiple leaders intervene in the network, it is hardly possible to achieve consensus, unless very stringent conditions are met. Instead, all the agents' states converge to a residual compact set resulting from the agents' initial conditions and the network's topology. In this paper, we propose a bipartite-containment controller for second-order linear systems. For the containment set we establish global exponential (hence, robust) stability, and we compute the specific equilibria to which all agents converge inside the containment set. In addition, we provide strict Lyapunov functions for signed-graph networks. Also, we illustrate our theoretical findings via numerical simulations.

We propose a distributed control method to achieve consensus of heterogeneous Euler-Lagrange (EL) systems with bounded time-varying communication delays while simultaneously rejecting periodic external disturbances. Our proposed controller has a simple-to-implement structure of proportional-integral-derivative scheme that employs the internal model approach to reject the disturbance and that it is model-independent (when cancelling the gravity effects). We consider that the network of EL-systems is interconnected through an undirected weighted graph that is static and we assume that the information exchange between any connected nodes is subjected to bounded variable time-delays. In this scenario we provide a sufficient condition for the proportional and the derivative gains of the controller to ensure that the EL-systems globally and asymptotically converge to a consensus position. The efficacy of the proposed method is shown in a numerical simulation using a network of 10 robotic manipulators.

The problem of safe navigation of a human-multi-robot system is addressed in this paper. More precisely, we propose a novel distributed algorithm to control a swarm of unmanned ground robots interacting with human operators in presence of obstacles. Contrary to many existing algorithms that consider formation control, the proposed approach results in non-rigid motion for the swarm, which more easily enables interactions with human operators and navigation in cluttered environments. Each vehicle calculates distributively and dynamically its own safety zone in which it generates a reference point to be tracked. The algorithm relies on purely geometric reasoning through the use of Voronoi partitioning and collision cones, which allows to naturally account for inter-robot, human-robot and robot-obstacle interactions. Different interaction modes have been defined from this common basis to address the following practical problems: autonomous waypoint navigation, velocity-guided motion, and follow a localized operator. The effectiveness of the algorithm is illustrated by outdoor and indoor field experiments.

We propose a solution to the full consensus-based formation problem for torque-controlled nonholonomic mobile robots under time-varying communication delays and without velocity measurements. Under the assumptions of static and undirected communication topologies, and smooth and bounded delays, we propose a distributed smooth, time-varying control law which achieves the goal of acquiring a global formation pattern in a common consensus point for both position and orientation. The controller is a combination of a simple Proportional-Plus-Damping scheme and a simple globally exponentially convergent velocity observer. Consensus is reached independently of all initial conditions. Realistic simulations in the Gazebo-ROS environment illustrate the effectiveness of the proposed algorithm and the robustness with respect to measurement noise, non-smooth delays and uncertain dynamics.

We consider the problem of stabilisation in predefined, finite time of the attitude of a rigid body. Inspired by classical laws in missile guidance, the proposed control law is time-varying but smooth as opposed to classical sliding-mode-based laws that are generally discontinuous, or continuous at best, and suffer from robustness issues. Through a backstepping design, and the explicit construction of a strict Lyapunov function, we are able to ensure stabilisation to a desired attitude even in the presence of uncertainties in the inertia matrix and under the effect of additive disturbances. Numerical simulations show the efficiency of the proposed controller.

We solve output- and state-consensus problems for multi-agent high-order systems in feedback form. We consider systems interconnected over arbitrary (connected) undirected-topology networks as well as directed spanning-trees and directed cycles. We assume that the systems may be subject to multiple restrictions in the form of output or state constraints, such as limited-range measurements, physical limitations, etc. In addition, we suppose that the systems may be subject to external disturbances. Under these conditions, we present a control framework and a formal analysis that establishes robust stability in the input-to-state sense. The former relies on a modified backstepping method and the latter on multi-stability theory. Finally, we apply our approach to a case-study of interest in the aerospace industry: safety-aware rendezvous control of underactuated UAVs subject to connectivity and collision-avoidance constraints.