Virtual Structure Approach Research Articles

Mobile robots path planning and mobile multirobots control: A review

AbstractMobile robots and multimobile robotic system usage for task achievement have been an emerging research area since the last decades. This article presents a review about mobile robot navigation problem and multimobile robotic systems control. The main focus is made on path planning strategies and algorithms in static and dynamic environments. A classification on mobile robots path planning has been defined in the literature and divided to classical and heuristic approaches. Each of them has its own advantages and drawbacks. On the other hand, the control of multimobile robots is presented and the control approaches for a fleet of robots are presented. Scientists found that using more than one robot as opposed to a single one presents many advantages when considering redundant task, dangerous tasks, or a task that scales up or down in time or that requires flexibility. They have defined three main approaches of multiple robots control: behavior-based approach, leader–follower approach, and virtual structure approach. This article addresses these approaches and provides examples from the literature.

Multiple Spacecraft Formation and Station-Keeping Control in Presence of Static Attitude Constraint via Decentralized Virtual Structure Approach

AbstractIn this paper, the problem of spacecraft formation attitude control in the presence of state constraints has been investigated. Multiple constraints such as a spacecraft’s forbidden-attitud...

Methods of Formation Control for a Group of Mobile Robots (a Review)

The multi-robot formation control is an essential issue in robotics. This review focuses on important lines of research on current contr ol is s ues and s tr ateg ies on a g r oup of unmanned autonomous vehicles/r obots for mation. In this paper, we pr ovide a br ief description of each method characterizing its key benefits and drawbacks. A multilayered classification of both centralized and decentralized formation control methods is proposed. We consider the classification of robot communication topologies in terms of centralized control. Seminal works dedicated to the practical application of centralized approach are briefly discussed. The majority of centralized methodsare represented by a " leader-follower" approach, taking into account the robot’s dynamics models. Furthermore, the most common models of vehicle dynamics are mentioned. In the framework of decentralized approach, behaviour-based algorithms, as well as swarm algorithms, are discussed. Then, we present an outlook of both centralized and decentralized virtual structure methods used in robot formation control. The described modifications of these methods allow tracing the evolution of the virtual structure approach to hybrid algorithms used for cooperative movement of a group of robots. This paper deals with formation control approach considering communication delays and low carrying capacity in an inter-vehicular communication network as very few works discussed this issue despite its relevance. We pointed out the main development trends of formation control approaches. The most effective approach is the integration of various methods of the formation control so that their disadvantages are nullified. As the same time, the most common disadvantage of discussed formation control methods is their weak conceptual framework in terms of kinematic and dynamic constraints of robots.

Cooperative Load Transportation With Two Quadrotors Using Adaptive Control

The problem of carrying a bar-shaped payload suspended by flexible cables attached to two quadrotors is analyzed in this work. The aerial vehicles and the load are dealt with as a single system, whose kinematics is described as a multi-robot formation using the virtual structure approach. The dynamic effects caused by the tethered load over the quadrotors, as well as those caused by each quadrotor over the other, are treated by an adaptive dynamic compensator. To validate the proposal, experiments were run testing the system in adverse conditions: transportation far from quasi-static motion, high payload-to-quadrotor weight ratio, 20% of error in the robot model parameters, transportation under wind disturbances, and payload weight changes during flight. The good performance of the proposed control system in all these tests allows concluding that the proposed system is able to accomplish payload positioning, orientation, and trajectory tracking under adverse conditions, with accelerations up to 1.6 m/s2.

Discrete time multiple spacecraft formation flying attitude optimal control in presence of relative state constraints

This paper addresses the challenge of synchronized multiple spacecraft attitude reorientation in presence of pointing and boundary constraints with limited inter-spacecraft communication link. Relative attitude pointing constraint among the fleet of spacecraft has also been modeled and considered during the attitude maneuvers toward the desired states. Formation fling control structure that consists of decentralized path planners based on virtual structure approach joint with discrete time optimal local controller is designed to achieve the mission’s goals. Due to digital computing of spacecraft’s onboard computer, local optimal controller based on discrete time prediction and correction algorithm has been utilized. The time step of local optimal algorithm execution is designed so that the spacecraft track their desired attitudes with appropriate error bound. The convergence of the proposed architecture and stability of local controller’s tracking error within appropriate upper bound are proved. Finally, a numerical simulation of a stereo imaging scenario is presented to verify the performance of the proposed architecture and the effectiveness of the algorithm.

A Multi-UAV Formation Maintaining Method Based on Formation Reference Point

In recent years, Unmanned Aerial Vehicles (UAV), have been increasingly utilized by scholars. Efficient control of UAV swarms opens a set of new challenges. There are several formation control methods, however, artificial potential field (APF) method has imperfections in formation maintaining and virtual structure is insufficient in collision avoidance. In this paper, we proposed a multi-UAV formation maintaining method combined artificial potential field method and virtual structure approach. The method uses the concepts of attraction and repulsion in the artificial potential field method, combined with formation reference points inspired by the virtual structure points in the virtual structure approach to control the formation. On the other hand, we express the leaders, the followers, and the formation reference points uniformly. When the formation of UAVs is determined, after the leader arrives at target point and hovers, the followers can track the formation reference points. The method can avoid collision between UAVs and achieve the purpose of formation maintaining. Finally, a UAV swarm consists of four UAVs is used for demonstration and verification, and the results of experiment prove the validity of the algorithm.

Robust formation control of multiagent systems on the Lie group SE(3)

SummaryThis paper addresses the robust formation control problem of multiple rigid bodies whose kinematics and dynamics evolve on the Lie group SE(3). First, it is assumed that all followers have access to the state information of a virtual leader. Then, a novel adaptive super‐twisting sliding mode control with an intrinsic proportional‐integral‐derivative sliding surface is proposed for the formation control problem of multiagent system using a virtual structure (VS) approach. The advantages of this control scheme are twofold: elimination of the chattering phenomenon without affecting the control performance and no requirement of prior knowledge about the upper bound of uncertainty/disturbance due to adaptive‐tuning law. Since the VS method is suffering from the disadvantages of centralized control, in the second step, considering a network as an undirected connected graph, we assume that only a few agents have access to the state information of the leader. Afterward, using the gradient of modified error function, a distributed adaptive velocity‐free consensus‐based formation control law is proposed where reduced‐order observers are introduced to remove the requirements of velocity measurements. Furthermore, to relax the requirement that all agents have access to the states of the leader, a distributed finite‐time super‐twisting sliding mode estimator is proposed to obtain an accurate estimation of the leader's states in a finite time for each agent. In both steps, the proposed control schemes are directly developed on the Lie group SE(3) to avoid singularity and ambiguities associated with the attitude representations. Numerical simulation results illustrated the effectiveness of the proposed control schemes.

A Stable Control Algorithm for Multi Robot Formation

This paper presents the developed trajectory tracking controller for a formation of nonholonomic robots, which combines features from the leader-follower and virtual-structure approaches. The implemented decentralized control strategy allows the robots to be relatively independent and to switch easily between the executed individual tasks and the collective tasks. Convergence is thoroughly analyzed and guarantied using the Lyapunov approach.

Extension of Leader‐Follower Behaviours for Wheeled Mobile Robots in Multirobot Coordination

This paper presents the extension of leader‐follower behaviours, for the case of a combined set of kinematic models of omnidirectional and differential‐drive wheeled mobile robots. The control strategies are based on the decentralized measurements of distance and heading angles. Combining the kinematic models, the control strategies produce the standard and new mechanical behaviours related to rigid body or n‐trailer approaches. The analysis is given in pairs of robots and extended to the case of multiple robots with a directed tree‐shaped communication topology. Combining these behaviours, it is possible to make platoons of robots, as obtained from cluster space or virtual structure approaches, but now defined by local measurements and communication of robots. Numerical simulations and real‐time experiments show the performance of the approach and the possibilities to be applied in multirobot tasks.

Reconfigurable Formation Control of Multi-Agents Using Virtual Linkage Approach

Formation control is an important problem in cooperative robotics due to its broad applications. To address this problem, the concept of a virtual linkage is introduced. Using this idea, a group of robots is designed and controlled to behave as particles embedded in a mechanical linkage instead of as a single rigid body as with the virtual structure approach. As compared to the virtual structure approach, the method proposed here can reconfigure the group of robots into different formation patterns by coordinating the joint angles in the corresponding mechanical linkage. Meanwhile, there is no need to transmit all the robots’ state information to a single location and implement all of the computation on it, due to virtual linkage’s hierarchical architecture. Finally, the effectiveness of the proposed method is demonstrated using two simulations with nine robots: moving around a circle in line formation, and moving through a gallery with varying formation patterns.

Formation control of multirotor aerial vehicles using decentralized MPC

In this work, the authors propose a formation control strategy of a group of three multirotor aerial vehicles being able to avoid multiple obstacles and collisions. To deal with this problem, a decentralized architecture is proposed which has one model predictive controller per vehicle including a set of convex constraints on the vehicle’s position to prevent collisions with other agents and different shapes of obstacles. The resulting decentralized scheme controls the formation based on a virtual structure approach. For the purpose of avoiding collisions, each local controller considers the predicted position of every neighbor vehicles. The effectiveness of the developed scheme is demonstrated through numerical simulations considering a “figure-of-eight” as the reference trajectory, and the results show its capability to handle thrust force, obstacle and collision avoidance constraints.

Sliding surface-based obstacle avoidance for second order multi-agent systems

This paper studies formation keeping along with obstacle avoidance for second-order multi-agent systems. First, the flexible virtual structure (FVS) approach, used to model the communication topology between the agents of the formation, is recalled and relationship with graph theory-based communication exchange to achieve consensus is established. Second, using the regulation function (RF) which permits to change the behaviour of the system's solution without affecting convergence, obstacle avoidance is investigated for one agent (leader/co-leader) of the formation. Particularly, RF was used to ensure obstacle avoidance for first order agents and is extended in this work for second order agents. Relationship between the dynamic agent and a sliding surface with first order kinematics is established and condition on the sliding surface parameter is developed to ensure obstacle avoidance. Also, new shapes of the obstacle are considered namely square and rhombus. Finally, in order to perform a coordinated obstacle avoidance of a multi-agent system when only a subset of agents namely co-leaders, selected from the boarder, has obstacle information, one defines control laws which permit to control motions of the remaining formation agents.

Vibration signal–based fault diagnosis in complex structures: A beam-like structure approach

Sensor network–based data-driven fault diagnosis in complex structures using limited prior knowledge is an interesting and hot topic in the literature. In this study, an integrated feature characterization and fuzzy decision method is developed based on a novel beam-like structure approach for fault diagnosis using available limited prior knowledge. Complex structures embedded with vibration sensors can be regarded as some virtual beam-like structures by considering the vibration transmission path from vibration sources to each sensor. The dynamic response of the structures in this vibration transmission path can demonstrate obvious fault features if there is a fault (typically, for example, cracks in connecting rods or around bolts, and bolt-loosening). These fault features can be effectively characterized and efficiently captured and utilized for fault diagnosis with a fuzzy decision method based on the virtual beam-like structure concept. This novel virtual beam-like structure approach needs only limited prior knowledge of the faults and does not require stationary response data. It is also not with respect to a specific structure design and is easy to implement within a sensor network. The effectiveness of the proposed method is well validated in the experiments for fault diagnosis including loosening bolts or cracks around bolts of complex structures such as bolted-base hanging structures in satellites.

Distributed Formation Flight Control of Multirotor Helicopters

This paper treats the problem of position formation flight control of a group of three multirotor aerial vehicles under obstacle and collision avoidance constraints. In order to solve the problem, a distributed architecture with model predictive controllers for each vehicle includes a set of convex constraints on the vehicles’s position to prevent collisions with other vehicles and obstacles. The resulting distributed scheme controls the formation based on a virtual structure approach where the computers of the architecture exchange position data through diagrams in Simulink. The performance of the method is assessed through simulations considering that the vehicles are subject to disturbance forces and the results show the effectiveness and the ability of the control architecture to handle the obstacle and collision avoidance constraints.

Merging Strategy for Vehicles by Applying Cooperative Tracking Control

This work presents a merging strategy for cooperative vehicles. In the proposed strategy, each vehicle possesses a tracking controller that considerers both tracking and coupling error signals, where all vehicles in the group know their positions. The trajectories of the cooperative vehicles are developed, to guarantee that each vehicle keeps a certain distance from the rest of the vehicles in the group, and are designed by using the virtual structure approach. The merging strategy is successfully validated in an experimental setup consisting of a group of nonholonomic mobile vehicles that represents the simplest bicycle model for cars with a zero vehicle length. It is shown using Lyapunov theory that this strategy is global asymptotically stable for the group.

A sensor network based virtual beam-like structure method for fault diagnosis and monitoring of complex structures with Improved Bacterial Optimization

This paper proposes a novel method for the fault diagnosis of complex structures based on an optimized virtual beam-like structure approach. A complex structure can be regarded as a combination of numerous virtual beam-like structures considering the vibration transmission path from vibration sources to each sensor. The structural ‘virtual beam’ consists of a sensor chain automatically obtained by an Improved Bacterial Optimization Algorithm (IBOA). The biologically inspired optimization method (i.e. IBOA) is proposed for solving the discrete optimization problem associated with the selection of the optimal virtual beam for fault diagnosis. This novel virtual beam-like-structure approach needs less or little prior knowledge. Neither does it require stationary response data, nor is it confined to a specific structure design. It is easy to implement within a sensor network attached to the monitored structure. The proposed fault diagnosis method has been tested on the detection of loosening screws located at varying positions in a real satellite-like model. Compared with empirical methods, the proposed virtual beam-like structure method has proved to be very effective and more reliable for fault localization.

Formation Control Strategy for Nonholonomic Intelligent Vehicles Based on Virtual Structure and Consensus Approach

This paper proposes a novel formation control strategy for nonholonomic intelligent vehicles based on virtual structure and consensus approach. The formation model is obtained based on the coordinate transformation and the virtual structure technique. The controllers are designed by using nonholonomic target tracking technique and leader-following consensus protocol. Depending on virtual structure approach and coordinate transformation, the formation control problem of multiple nonholonomic intelligent vehicles is converted into the target tracking and state consensus stabilization problem. Simulation and real-world experimental results show the correctness and effectiveness of the strategy.

Model Predictive Lateral Control of a UAV Formation Using a Virtual Structure Approach

Model Predictive Lateral Control of a UAV Formation Using a Virtual Structure Approach

Coordinated Path Following for Mobile Robots Using a Virtual Structure Strategy with Model Predictive Control

In this paper, we propose a novel coordinated path following controller based on model predictive control (MPC) for mobile robots. The strategy is based on a virtual structure approach where the entire formation is considered as a rigid body and the control laws for a virtual leader vehicle and for actual follower robots are optimized by considering the dynamics of the virtual structure and the desired motion of each vehicle. Besides, we also fulfill time convergence for trajectory tracking by integrating an additional penalty term into our model predictive control scheme. However, the major concern in the use of model predictive control is whether such an open-loop control scheme can guarantee system stability. In this case, we apply the idea of a contractive constraint to guarantee the stability of our MPC framework. Although our approach is centralized, numerous simulation scenarios have been conducted to illustrate its effectiveness and its superior performance for a small group of mobile robots. Furthermore, we show that path following control can offer a number of advantages over its trajectory tracking counterpart.

Formation Flight and Collision Avoidance for Multiple UAVs using Concept of Elastic Weighting Factor

In this paper, the guidance law for formation flight and collision avoidance of multiple Unmanned Aerial Vehicle (UAV)s is proposed. To construct the physically comprehensible guidance law for formation flight, the virtual structure approach is used. To develop a guidance law for collision avoidance considering both other UAVs and unknown static obstacles, a geometric approach using information such as a relative position vector is utilized. Through the Lyapunov theorem, the stability of the proposed guidance law is proved. To combine guidance commands, the concept of the elastic weighting factor inspired by the elastic behavior of shape memory polymer, which tends to regain its original shape after deformation, is introduced. By using the concept of elastic weighting factor, multiple UAVs are able to cope actively with the situation of a collision between both UAVs and static obstacles during the formation flight. To verify the performance of the proposed method, numerical simulations are performed.