Dimitra Panagou

Cooperative Visibility Maintenance for Leader–Follower Formations in Obstacle Environments

Vision-based formation control of multiple agents, such as mobile robots or fully autonomous cars, has recently received great interest due to its application in robotic networks and automated highways. This paper addresses the cooperative motion coordination of leader-follower formations of nonholonomic mobile robots, under visibility and communication constraints in known polygonal obstacle environments. We initially consider the case of N = 2 agents moving in L-F fashion and propose a feedback control strategy under which L ensures obstacle avoidance for both robots, while F ensures visibility maintenance with L and intervehicle collision avoidance. The derived algorithms are based on set-theoretic methods to guarantee visibility maintenance, dipolar vector fields to maintain the formation shape, and the consideration of the formation as a tractor-trailer system to ensure obstacle avoidance. We furthermore show how the coordination and control design extends to the case of N > 2 agents, and provide simulation results, which demonstrate the efficacy of the control solutions. The proposed algorithms do not require information exchange among robots, but are instead based on information locally available to each agent. In this way, the desired tasks are executed and achieved in a decentralized manner, with each robot taking care of converging to a desired configuration, while maintaining visibility with its target.

Distributed Coordination Control for Multi-Robot Networks Using Lyapunov-Like Barrier Functions

This paper addresses the problem of multi-agent coordination and control under multiple objectives, and presents a set-theoretic formulation amenable to Lyapunov-based analysis and control design. A novel class of Lyapunov-like barrier functions is introduced and used to encode multiple, non-trivial control objectives, such as collision avoidance, proximity maintenance and convergence to desired destinations. The construction is based on recentered barrier functions and on maximum approximation functions. Thus, a single Lyapunov-like function is used to encode the constrained set of each agent, yielding simple, gradient-based control solutions. The derived control strategies are distributed, i.e., based on information locally available to each agent, which is dictated by sensing and communication limitations. Furthermore, the proposed coordination protocol dictates semi-cooperative conflict resolution among agents, which can be also thought as prioritization, as well as conflict resolution with respect to an agent (the leader) which is not actively participating in collision avoidance, except when necessary. The considered scenario is pertinent to surveillance tasks and involves nonholonomic vehicles. The efficacy of the approach is demonstrated through simulation results.

Multi-objective control for multi-agent systems using Lyapunov-like barrier functions

This paper addresses the problem of multi-agent coordination and control under multiple objectives, and presents a set-theoretic formulation which is amenable to Lyapunov-based analysis and control design. A novel class of Lyapunov-like barrier functions is introduced and used to encode multiple, non-trivial control objectives, such as collision avoidance, proximity maintenance and convergence to desired destinations. The construction is based on the concept of recentered barrier functions and on approximation functions. A single Lyapunov-like function encodes the constrained set of each agent, yielding simple, closed-form control solutions. The proposed construction allows also for distributed control design based on information locally available to each agent. The scenario considered here involves nonholonomic vehicles, while simulation results demonstrate the efficacy of the approach.

Viability control for a class of underactuated systems

This paper addresses the feedback control design for a class of nonholonomic systems which are subject to inequality state constraints defining a constrained (viability) set K. Based on concepts from viability theory, the necessary conditions for selecting viable controls for a nonholonomic system are given, so that system trajectories starting in K always remain in K. Furthermore, a class of state feedback control solutions for nonholonomic systems are redesigned by means of switching control, so that system trajectories starting in K converge to a goal set G in K, without ever leaving K. The proposed approach can be applied in various problems, whose objective can be recast as controlling a nonholonomic system so that the resulting trajectories remain forever in a subset K of the state space, until they converge into a goal (target) set G in K. The motion control for an underactuated marine vehicle in a constrained configuration set K is treated as a case study; the set K essentially describes the limited sensing area of a vision-based sensor system, and viable control laws which establish convergence to a goal set G in K are constructed. The robustness of the proposed control approach under a class of bounded external perturbations is also considered. The efficacy of the methodology is demonstrated through simulation results.

Target-referenced Localization of an Underwater Vehicle using a Laser-based Vision System

This paper describes the development of an image-based position tracking system designed for a Remotely Operated Vehicle (ROV). The sensor package consists of two underwater laser pointers and a single CCD camera mounted on the ROV. The proposed system fuses data deriving from the projection of the laser pointers on the image plane and computer vision object tracking algorithms. The results deriving from the data fusion compose the position vector of the vehicle, with respect to the center of the tracked object. This position vector can be used for a closed loop steering control of the vehicle. The integration of the system was proved successful through the experimental procedure. The effective working range of the system is 40-150 cm, at 17 Hz refresh rate and 6% of absolute error. The working range is limited due to the existing hardware. However the implemented software is versatile to future hardware upgrades

Motion planning and collision avoidance using navigation vector fields

This paper presents a novel method on the motion and path planning for unicycle robots in environments with static circular obstacles. The method employs a family of 2-dimensional analytic vector fields, which have singular points of high-order type and whose integral curves exhibit various patterns depending on the value of a parameter λ. More specifically, for a known value of λ the vector field has a unique singular point of dipole type and its integral curves are suitable for steering the unicycle to a goal configuration. Furthermore, for the value of λ that the vector field has a continuum of singular points, the integral curves can be used to define flows around circular obstacles. An almost global feedback motion plan is then constructed by suitably blending attractive and repulsive vector fields in a static obstacle environment. The proposed motion planning and control design is also extended to the multi-agent case, where each agent needs to converge to a desired configuration while avoiding collisions with other agents. The efficacy of the approach is demonstrated via simulation results.

Dynamic positioning for an underactuated marine vehicle using hybrid control

The increasing interest in autonomous marine systems and related applications has motivated, among others, the development of systems and algorithms for the dynamic positioning of underactuated marine vehicles (ships, surface vessels and underwater vehicles) under the influence of unknown environmental disturbances. In this paper, we present a state feedback control solution for the navigation and practical stabilisation of an underactuated marine vehicle under non-vanishing current disturbances, by means of hybrid control. The proposed solution involves a logic-based switching control strategy among simple state feedback controllers, which renders the position trajectories of the vehicle practically stable to a goal set around a desired position. The control scheme consists of three control laws; the first one is active out of the goal set and drives the system trajectories into this set, based on a novel dipolar vector field. The other two control laws are active in the goal set and alternately regulate the position and the orientation of the vehicle, so that the switched system is practically stable around the desired position. The overall system is shown to be robust, in the sense that the vehicle enters and remains into the goal set even if the external current disturbance is unknown, varying and only its maximum bound (magnitude) is given. The efficacy of the proposed solution is demonstrated through simulation results.

A viability approach for the stabilization of an underactuated underwater vehicle in the presence of current disturbances

In this paper we present a viability-based formulation for the stabilization of an underactuated underwater vehicle under the influence of a known, constant current and state constraints. The stabilization problem is described by three problems in terms of viability theory. We present a solution to the first problem which addresses the safety of the system, i.e. guarantees that there exists a control law such that the vehicle always remains into the safe set of state constraints. In order to overcome the computational limitations due to the high dimension of the system we develop a two-stage approach, based on forward reachability and game theory. The control law is thus the safety controller when the system viability is at stake, i.e. close to the boundary of the safe set. The viability kernel and the control law are numerically computed.

Maintaining visibility for leader-follower formations in obstacle environments

This paper addresses the problem of controlling a leader-follower (L - F) formation of two unicycle mobile robots moving under visibility constraints in a known obstacle environment. Visibility constraints are realized as inequality state constraints that determine a visibility set K. Maintaining visibility is translated into controlling the robots so that system trajectories starting in K always remain in K. We provide the conditions under which visibility is maintained, as well as a feedback control scheme that forces F to converge and remain into a set of desired configurations w.r.t. L while maintaining visibility. We also propose a cooperative control scheme for the motion of the formation in a known obstacle environment, so that both collision avoidance and maintaining visibility are ensured. The proposed control schemes are decentralized, in the sense that there is no direct communication between the robots. The efficacy of our algorithms is evaluated through simulations.

Towards the stabilization of an underactuated underwater vehicle in the presence of unknown disturbances

In this paper a dynamic model based control scheme is proposed for the stabilization of an underactuated underwater vehicle, in the presence of slowly varying, unknown disturbances. An unscented Kalman filter (UKF), based on the vehicles dynamic model, is applied for the sensor fusion process to provide an estimation of the full state vector of the system. External disturbances and unmodeled phenomena are included in the dynamic model as zero-mean Gaussian white noise processes. The estimation of the state vector is used as feedback for the proposed control scheme, which stabilizes the vehicle to the desired position and orientation. The proposed methodology is experimentally implemented using a 3-DOF underactuated vehicle. The efficiency of the methodology under various environmental conditions is demonstrated by simulation results. The overall system can be used for inspection tasks of sea platforms, e.g. during the inspection of a ship hull for possible damage tracking.