Leader-follower Formation Research Articles

Leader-following Formation Control of Second-order Nonlinear Systems with Time-varying Communication Delay

In this paper, time-varying formation tracking problem for second-order nonlinear multi-agent systems with time-varying communication delays has been investigated, where the followers maintain a predefined time-varying formation while tracking the moving leader. Based on a distributed observer, a formation tracking protocol with time-varying delay is developed using the relative neighboring information. By constructing the Lyapunov-Krasovskii (L-K) function, sufficient conditions have been obtained to guarantee the stability of time-varying formation tracking. In addition, the unknown gain matrix in the proposed protocol is computed out through the technique of variable substitution. Numerical simulation results are presented to validate the effectiveness of our theoretical analysis.

Connectivity-maintaining and collision-avoiding performance function approach for robust leader–follower formation control of multiple uncertain underactuated surface vessels

A robust leader–follower formation tracking design using connectivity-maintaining and collision-avoiding performance functions is developed for multiple uncertain underactuated surface vessels (USVs). It is assumed that the USVs communicate within limited ranges and all nonlinearities of the USV dynamics are completely unknown. Compared with the related literature, two primary contributions of this paper are to derive connectivity-maintaining and collision-avoiding performance functions for designing a robust formation tracking scheme using only relative state information and to establish an obstacle avoidance strategy guaranteeing the connectivity between the leader and the followers while avoiding an obstacle. Based on the Lyapunov stability theorem, the predesignated formation tracking performance and stability of the proposed control system have been analyzed without using potential-like functions and function approximation techniques. Simulation results are provided to show the effectiveness of the proposed theoretical result.

A Low-Cost Solution for Leader-Follower Formation Control of Multi-UAV System Based on Pixhawk

In recent years, cooperative multi-UAV systems have shown great performance in a variety of potential applications. Aiming at the problem of fixed-wing UAVs cooperative formation flight, this paper presents a low-cost system based on Pixhawk autopilot for keeping the shape of the formation. The kinematic relationship of a leader-follower UAV formation is established and a fuzzy PID control law is designed for the follower to maintain the expected position relative to the leader. Wireless data transmission between the leader and followers is achieved by using several radio telemetry modules. The proposed formation control system is validated by a series of successful flight experiments using 3 small-scaled UAV models, which has proven its feasibility and reliability.

Heterogeneous formation control of multiple rotorcrafts with unknown dynamics by reinforcement learning

In this paper, a distributed model-free solution based on reinforcement learning is proposed for the leader–follower formation control problem of heterogeneous multi-agent systems. The multi-agent system consists of multiple rotorcrafts involving a virtual leader and multiple followers, where the dynamics of leaders and followers is unknown. The formation control problem is firstly formulated as an optimal output regulation problem. A discounted performance function is then introduced to guarantee that the tracking error asymptotically converges to zero, and an online off-policy reinforcement learning algorithm is proposed to solve the optimal output problem online using the data generated along the trajectories of the agents. A simulation example is provided to validate the effectiveness of the proposed control method.

Reduced-Order Observer-Based Leader-Following Formation Control for Discrete-Time Linear Multi-Agent Systems

Formation control of discrete-time linear multi-agent systems using directed switching topology is considered in this work via a reduced-order observer, in which a formation control protocol is proposed under the assumption that each directed communication topology has a directed spanning tree. By utilizing the relative outputs of neighboring agents, a reduced-order observer is designed for each following agent. A multi-step control algorithm is established based on the Lyapunov method and the modified discrete-time algebraic Riccati equation. A sufficient condition is given to ensure that the discrete-time linear multiagent system can achieve the expected leader-following formation. Finally, numerical examples are provided so as to demonstrate the effectiveness of the obtained results.

Adaptive neural finite-time formation control for multiple underactuated vessels with actuator faults

This paper proposes a novel neural finite-time formation control algorithm for multiple underactuated surface vessels with actuator faults. In the algorithm, the leader-follower formation problem is formulated as a two-stage tracking problem. First, to address the leader-follower configuration without the information of leader velocity, the virtual vessel is designed to track the reference trajectory of the leader. Second, an adaptive finite-time fault-tolerant control (AFFTC) algorithm is presented for the follower to track the virtual vessel. By fusion of the neural network (NN) and the fractional power, the model uncertainty is approximated and the finite-time convergence is obtained. Furthermore, a concise adaptive law is developed to compensate the upper bounded of NN weights and lump disturbance which include the approximation error of NN, control gain uncertainty, actuator faults and marine environmental disturbance. On the basis of Lyapunov theory, stability analysis proves that all the signals in the closed-loop system are practical finite-time stable. Finally, numerical simulations are performed to demonstrate the performance and superiority of the proposed algorithm.

A Cooperative Guidance Law for UAVs Target Tracking

The problem of automatic tracking of ground targets is one of the important issues that UAVs need to face in task applications. The main concern of cooperative tracking is how to track a ground target and maintain the UAV formation simultaneously. In this paper, a new leader-follower formation is constructed to track a ground target. Firstly, a new leader UAV guidance law is proposed to track the ground target in the standoff mode, the stability is proved using a Lyapunov function. Secondly, new guidance laws for standoff tracking of the leader UAV and controlling of the inter-UAV angle of the circle formation are designed for follower UAVs, respectively, stabilities are also proved using two Lyapunov functions. Numerical simulation experiments show that the new leader-follower formation can track the static and moving targets well and its performance is better than the classic LVFG algorithm

Connectivity-maintaining obstacle avoidance approach for leader-follower formation tracking of uncertain multiple nonholonomic mobile robots

This paper investigates a connectivity-maintaining obstacle avoidance problem for guaranteed-performance-based leader-follower formation tracking of uncertain multiple nonholonomic mobile robots with communication and sensing range constraints. All nonlinearities in the robot dynamics are assumed to be unknown. The desired relative angles among robots for preserving connectivity between the leader and the follower while avoiding obstacles are derived to develop a novel connectivity-maintaining obstacle avoidance strategy. Then, a leader-follower formation tracker using these desired relative angles and connectivity-preserving and collision-avoiding performance functions is designed for accomplishing connectivity maintenance, collision avoidance, and obstacle avoidance among robots. The performance-functions-based avoidance starting range for the connectivity-maintaining obstacle avoidance achievement is induced from the Lyapunov stability analysis. Furthermore, the proposed formation tracking strategy does not require any potential-functions-based approaches and adaptive approximation techniques. Finally, simulation studies clarify and verify the proposed theoretical approach.

Neural-network-based formation control with collision, obstacle avoidance and connectivity maintenance for a class of second-order nonlinear multi-agent systems

In this paper, a formation control strategy with collision, obstacle avoidance and connectivity maintenance is developed for a class of second-order nonlinear multi-agent systems under external disturbances. Firstly, in order to handle the nonlinear dynamics of the multi-agent systems and the unknown disturbances in the environment, neural network (NN) techniques are employed in the proposed control strategy. Then, the distributed formation controller with collision, obstacle avoidance is designed by combining artificial potential field (APF) methods and leader–follower formation methods. Secondly, due to the collision or obstacle avoidance terms within the formation controller may result in large separation distance among agents and each agent’s communication distance is limited by its hardware, the collision or obstacle avoidance terms increase the chance of losing connectivity between agents. In order to guarantee the connectivity of the formation, the connectivity maintenance controller is designed with taking the communication topology into account. Based on Lyapunov stability theory, it is proved that the stability of the closed-loop multi-agent systems can be guaranteed. Finally, the simulation results verify the effectiveness of the proposed approach.

UAVs-UGV Leader Follower Formation Using Adaptive Non-Singular Terminal Super Twisting Sliding Mode Control

Leader follower formation of unmanned aerial vehicles (UAVs) and unmanned ground vehicles (UGVs) has found numerous applications such as surveillance of critical infrastructure, industrial automation and disaster management emergency. For completion of high precision group tasks, the choice of appropriate control mechanism is of utmost importance. In presence of environmental effects,external disturbances and parametric uncertainties in the UAVs and UGV models, the controller design process is a challenging task. In order to address the aforementioned problems and to ensure minimum tracking errors and fast convergence of the states, this article proposes an adaptive robust formation and trajectory tracking control scheme for a leader follower formation of UAVs and UGV using Non-Singular Terminal Super Twisting Sliding Mode Control Method.Adaptive compensators are derived based on Lyapunov function method and stability of the proposed controllers is guaranteed. Two variants of the control schemes namely Adaptive Super Twisting SMC (AST-SMC) and Adaptive Non-Singular Terminal Super Twisting SMC (ANSTS-SMC) are tested using numerical simulations performed in MATLAB/Simulink. From the results presented, and with the proposed ANSTS-SMC control scheme, the measured $[X~Y]$ tracking errors for leader, follower1 and follower2 UAVs are $[{0~0.01}]m$ , $[{0.01~0.02}]m$ and $[{0.01,~0.02}]m$ respectively, While with the AST-SMC method the peak $[X~Y]$ tracking errors for leader, follower1 and follower2 UAVs are $[{0.05~0.05}]m$ , $[{0.1~0.2}]m$ and $[{0.05~0.1}]m$ respectively. The proposed leader follower formation can be effectively used to monitor solar/PV panels and cables in large solar parks.

Three controllers via 2nd-order sliding mode for leader-following formation control of multi-robot systems

The multi-robot formation has always been an attractive field for cooperative control of robots. This paper uses the improved second-order sliding mode method to control the leader-following formation system. The traditional second-order sliding mode control is not perfect because the disturbances and uncertainties of the system may cause the compensation control to overestimate. The adaptive gain super-twisting sliding mode control suppresses the output oscillation and speeds up the system response by improving the gain function and adding a differential link. However, this method is generally applied to single-input single-output systems. This paper attempts to apply this method to a multi-input-multi-output leader-following formation system and improve its switching function and compensation control part. We use the estimators, i.e., the extreme learning machine and the nonlinear disturbance observer. These estimators can observe disturbance changes to replace the adaptive gain function to realise disturbance estimation and compensation control. Then combine the extreme learning machine and the nonlinear disturbance observer with the super-twisting sliding mode control to produce two new sliding mode methods. We use the Lyapunov method to prove the system stability as well. Finally, we illustrate the feasibility of the three control methods.

On the Design of Outdoor Leader-Follower UAV-Formation Controllers From a Practical Point of View

This work presents a control structure designed to autonomously guide two of-the-shelf quadrotors navigating in outdoor environments, as a leader-follower formation. The objective is to show how simple is to design controllers to guide the two vehicles when taking into account practical assumptions and simplifications in their model. This way, the main contribution of the paper is to show how practical knowledge obtained from real experiments using of-the-shelf quadrotors, translated to the assumptions and simplifications applied to its model, can help in the design of a suitable controller. The basic premise is that the quadrotor is driven by high-level commands that excite its degrees of freedom. The control system here designed following such reasoning is implemented in a ground station that allows manual or automatic control of the leader vehicle, with the follower one always automatically controlled. Experimental results in real outdoor scenarios, subject to unpredicted disturbances, validate the assumptions and simplifications applied to the model of the quadrotor, and the proposed control structure as well.

Joint Channel and Link Selection in Formation-Keeping UAV Networks: A Two-Way Consensus Game

This paper is the first to investigate both communication and control in traffic channel (TCH) and control channel (CCH) respectively when considering leader-follower formation keeping in UAV communication networks. In this paper, we analyze the relationship between the mutual interference and information exchange cost, and then formulate the joint channel and link selection problem as a two-way consensus game between CCH and TCH. To characterize the two-way choice of link selection, we creatively propose the generalized two-way consensus equilibrium (GTCE) to capture the stable state. Then, we prove that the formulated game has at least one pure-strategy GTCE which can maximize the UAV communication network utility. A distributed better reply based joint channel and link selection (BRJCLS) algorithm as well as two-dimensional minimum spanning tree (MST) based initialization (TMSTI) algorithm is proposed to achieve the GTCE. Simulation results are presented to show the convergence and effectiveness of the formulated two-way consensus game and proposed algorithms.

Distributed Event-Triggered Adaptive Coordinated Trajectory Tracking Control of Multi-USVs Based on the Aggregate Tracking Error

Aiming to solving the nonlinear coordinated trajectory tracking control problem of multi-USVs under the influence of slowly changing and uncertain environmental disturbances, reducing energy consumption and ensuring tracking performance and stability, a distributed event-triggered adaptive coordinated trajectory tracking controller (DET-ACTTC) for multiple unmanned surface vessels (multi-USVs) is proposed based on the undirected communication topology. By introducing a virtual leader, a leader-follower formation is used to generate the coordinated tracking errors. Based on the expected formation and states, the aggregate tracking errors are defined, and event-triggered measurement errors are constructed to design the event-triggered conditions. An adaptive term is used to compensate for external environment disturbances, and the DET-ACTTC is deduced for each USV in the group. The tracking errors of the event-triggered coordinated trajectory tracking control (ETCTTC) system gradually converge to zero, and Zeno behavior is avoided. The theoretical results are verified by simulations.

Three controllers via 2nd-order sliding mode for leader-following formation control of multi-robot systems

Three controllers via 2nd-order sliding mode for leader-following formation control of multi-robot systems

Distance-based Leader-Follower Formation Control of Mobile Robots Using Relative Target Positions

Distance-based Leader-Follower Formation Control of Mobile Robots Using Relative Target Positions

Impacts of Heat and Mass Flux on MHD Flow and Heat Transfer of Radiative Nanofluid Past an Exponentially Shrinking/stretching Sheet With Viscous and Ohmic Dissipations

In this paper, a distributed formation flight control topology for Leader-Follower formation structure is presented. Such topology depends in the first place on online generation of the trajectories that should be followed by the agents in the formation. The trajectory of each agent is planned during execution depending on its neighbors and considering that the desired reference trajectory is only given to the leader. Simulation using MATLAB/SIMULINK is done on a formation of quadrotor UAVs to illustrate the proposed method. The topology shows very good results in achieving the formation and following the reference trajectory.

Formation Flight Control of Multi-UAV System Using Neighbor-based Trajectory Generation Topology

In this paper, a distributed formation flight control topology for Leader-Follower formation structureis presented. Such topology depends in the first place on online generation of the trajectories that should befollowed by the agents in the formation. The trajectory of each agent is planned during execution depending onits neighbors and considering that the desired reference trajectory is only given to the leader. Simulation usingMATLAB/SIMULINK is done on a formation of quadrotor UAVs to illustrate the proposed method. Thetopology shows very good results in achieving the formation and following the reference trajectory.

Fixed-Time Formation Control of Unicycle-Type Mobile Robots With Visibility and Performance Constraints

This article presents a fixed-time leader-follower formation control protocol for a swarm of unicycle-type mobile robots with visibility and performance constraints. Under a vision-based formation control framework, mobile robots calculate their control signals with only the local relative distance and bearing angle provided by onboard stereo or RGB-D cameras. Since the sensing capability of the fixed onboard cameras is subject to limited range and angle of view, the field-of-view constraints are considered to ensure that each follower can always detect its leader and meanwhile avoid collision with the leader. The time-varying and asymmetric performance constraints on formation tracking errors are applied to guarantee the transient and stead-state performance. Furthermore, the proposed control protocol can be extended to address the control design problem without constraint requirements. Based on the fixed-time control design and Lyapunov analysis, formation tracking errors are shown to converge to a small neighborhood of zero in fixed settling time. Simulation and experiment studies are performed to illustrate the effectiveness of the formation control protocol.

Adaptive Leader–Follower Formation Control of Underactuated Surface Vehicles With Guaranteed Performance

This article studies the formation tracking control problem for a group of underactuated surface vehicles with guaranteed transient properties, including connectivity maintenance, collision avoidance, and tracking performance specifications. The formation is within the leader–follower control framework, in which every follower is controlled to track its leader and maintain a desired relative distance and bearing angle with respect to its leader such that the prescribed formation geometry is achieved based on local sensing capability. The onboard sensor systems are of limited range and angle of view, thus defining a cone of detectable region for every follower. Each follower can detect its leader, if and only if the relative distance and bearing angle keep always inside the predefined detectable region such that the connectivity between the follower and its leader is maintained over time. In addition to the consideration of connectivity maintenance, no collision between the follower and its leader is also considered. A transverse function control approach is introduced to overcome the difficulties caused by the off-diagonal system matrix and underactuation. The barrier Lyapunov function and adaptive backstepping procedure are incorporated into the formation control design to achieve the boundedness of the closed-loop systems with guaranteed transient performance. Collision avoidance and connectivity maintenance between every follower and its leader are also proven mathematically. Simulation studies are performed to show the effectiveness of the proposed control design technique.