Showing papers on "Consensus published in 2016"

Consensus of Linear Multi-Agent Systems by Distributed Event-Triggered Strategy

Abstract: This paper studies the consensus problem of multi-agent systems with general linear dynamics. We propose a novel event-triggered control scheme with some desirable features, namely, distributed, asynchronous, and independent. It is shown that consensus of the controlled multi-agent system can be reached asymptotically. The feasibility of the event-triggered strategy is further verified by the exclusion of both singular triggering and Zeno behavior. Moreover, a self-triggered algorithm is developed, where the next triggering time instant for each agent is determined based on its local information at the previous triggering time instant. Continuous monitoring of measurement errors is thus avoided. The effectiveness of the proposed control schemes is demonstrated by two examples.

Distributed robust finite-time nonlinear consensus protocols for multi-agent systems

Abstract: This paper investigates the robust finite-time consensus problem of multi-agent systems in networks with undirected topology. Global nonlinear consensus protocols augmented with a variable structure are constructed with the aid of Lyapunov functions for each single-integrator agent dynamics in the presence of external disturbances. In particular, it is shown that the finite settling time of the proposed general framework for robust consensus design is upper bounded for any initial condition. This makes it possible for network consensus problems to design and estimate the convergence time offline for a multi-agent team with a given undirected information flow. Finally, simulation results are presented to demonstrate the performance and effectiveness of our finite-time protocols.

Tendermint: Byzantine Fault Tolerance in the Age of Blockchains

Abstract: TENDERMINT: BYZANTINE FAULT TOLERANCE IN THE AGE OF BLOCKCHAINS Ethan Buchman Advisor: University of Guelph, 2016 Professor Graham Taylor Tendermint is a new protocol for ordering events in a distributed network under adversarial conditions. More commonly known as consensus or atomic broadcast, the problem has attracted significant attention recently due to the widespread success of digital currencies, such as Bitcoin and Ethereum, which successfully solve the problem in public settings without a central authority. Tendermint modernizes classic academic work on the subject to provide a secure consensus protocol with accountability guarantees, as well as an interface for building arbitrary applications above the consensus. Tendermint is high performance, achieving thousands of transactions per second on dozens of nodes distributed around the globe, with latencies of about one second, and performance degrading moderately in the face of adversarial attacks.

Distributed Consensus of Second-Order Multi-Agent Systems With Heterogeneous Unknown Inertias and Control Gains Under a Directed Graph

Abstract: In this paper, we study the consensus problem for second-order multi-agent systems with heterogeneous unknown inertias and control gains under a general directed graph. Unlike the existing consensus algorithms for second-order multi-agent systems in which all agents are assumed to have common unit inertias or share common control gains, we allow the inertias and the control gains to be heterogeneous and time-varying for each agent. We propose fully distributed consensus algorithms over a general directed graph when there exist, respectively, absolute velocity damping and relative velocity damping. Novel integral-type Lyapunov functions are proposed to study the consensus convergence. Moreover, the adaptive $\sigma$ - modification schemes for the gain adaptation are proposed, which renders smaller control gains and thus requires smaller amplitude on the control input without sacrificing consensus convergence. Furthermore, we show that one proposed algorithm also works for consensus of agents with intrinsic Lipschitz nonlinear dynamics. The control gains are varying and updated by distributed adaptive laws. As a result, the proposed algorithms require no global information and thus can be implemented in a fully distributed manner.

Distributed consensus tracking for multi‐agent systems under two types of attacks

Abstract: Summary This paper studies a distributed coordinated control problem for a class of linear multi-agent systems subject to two types of attacks. The problem boils down to how to achieve secure consensus tracking for multi-agent systems with connected and disconnected (paralyzed) directed switching topologies caused by two types of attacks. The attacks on the edges instead of nodes lead to the loss of security performance. Two cases are studied in this paper. First, under only a class of connectivity-maintained attacks, sufficient conditions are derived to achieve secure consensus tracking in mean-square. Second, when the multi-agent systems are further subject to a class of connectivity-broken attacks, novel sufficient conditions are further obtained to ensure secure consensus tracking with a specified convergence rate by virtue of the idea of average dwell time switching between some stable and unstable subsystems. Three numerical simulations are finally given to illustrate the theoretical analysis. Copyright © 2015 John Wiley & Sons, Ltd.

Prescribed Performance Consensus of Uncertain Nonlinear Strict-Feedback Systems With Unknown Control Directions

Abstract: In this paper, a leader-following consensus scheme is presented for networked uncertain nonlinear strict-feedback systems with unknown control directions under directed graphs, which can achieve predefined synchronization error bounds. Fuzzy logic systems are employed to approximate system uncertainties. A specific Nussbaum-type function is introduced to solve the problem of unknown control directions. Using a dynamic surface control technique, distributed consensus controllers are developed to guarantee that the outputs of all followers synchronize with that of the leader with prescribed performance. Based on Lyapunov stability theory, it is proved that all signals in closed-loop systems are uniformly ultimately bounded and the outputs of all followers ultimately synchronize with that of the leader with bounded tracking errors. Simulation results are provided to demonstrate the effectiveness of the proposed consensus scheme.

Asynchronous Distributed ADMM for Large-Scale Optimization—Part I: Algorithm and Convergence Analysis

Abstract: Aiming at solving large-scale optimization problems, this paper studies distributed optimization methods based on the alternating direction method of multipliers (ADMM). By formulating the optimization problem as a consensus problem, the ADMM can be used to solve the consensus problem in a fully parallel fashion over a computer network with a star topology. However, traditional synchronized computation does not scale well with the problem size, as the speed of the algorithm is limited by the slowest workers. This is particularly true in a heterogeneous network where the computing nodes experience different computation and communication delays. In this paper, we propose an asynchronous distributed ADMM (AD-ADMM), which can effectively improve the time efficiency of distributed optimization. Our main interest lies in analyzing the convergence conditions of the AD-ADMM, under the popular partially asynchronous model, which is defined based on a maximum tolerable delay of the network. Specifically, by considering general and possibly non-convex cost functions, we show that the AD-ADMM is guaranteed to converge to the set of Karush–Kuhn–Tucker (KKT) points as long as the algorithm parameters are chosen appropriately according to the network delay. We further illustrate that the asynchrony of the ADMM has to be handled with care, as slightly modifying the implementation of the AD-ADMM can jeopardize the algorithm convergence, even under the standard convex setting.

The Leader-Following Consensus for Multiple Uncertain Euler-Lagrange Systems With an Adaptive Distributed Observer

Abstract: In this note, we first propose an adaptive distributed observer for a linear leader system. This observer only assumes those followers who are the children of the leader know the system matrix of the leader system. Then, we further develop a distributed adaptive controller utilizing this adaptive distributed observer to solve the leader-following consensus problem for multiple uncertain Euler-Lagrange systems without assuming that all the followers need to know the system matrix of the leader system.

Distributed Finite-Time Voltage and Frequency Restoration in Islanded AC Microgrids

Abstract: This paper investigates the distributed finite-time restoration of inverter voltage and frequency terms in an islanded ac microgrid. Formulating networked inverters of ac microgrids as a cooperative multiagent system, the voltage and frequency restoration can be cast as synchronization problems, while the active power sharing can be viewed as a consensus problem. One popular distributed control approach is the neighbor-based linear consensus protocol, where the consensus at the frequency and voltage set points is achieved over an infinite-time horizon with an exponential convergence. To achieve accelerated convergence and better disturbance rejection properties, a distributed finite-time control protocol, based on feedback linearization approach, is proposed for voltage restoration, which synchronizes the voltage term at each inverter to the reference value in finite time period. Then, a finite-time control protocol for both frequency restoration and active power sharing problems is proposed to synchronize the microgrid frequency to the nominal value, and share the active power among inverters based on their ratings in a finite time. Rigorous Lyapunov proofs are provided to establish the upper bounds on the convergence times. Numerical simulation studies verify the effectiveness of the proposed control protocols.

Consensus of Switched Multiagent Systems

Abstract: In this brief, we consider the consensus problem of a switched multiagent system composed of continuous-time (CT) and discrete-time (DT) subsystems. By combining the classical consensus protocols of CT and DT multiagent systems, we propose a linear consensus protocol for switched multiagent system. Based on the graph theory and the Lyapunov theory, we prove that the consensus of switched multiagent system is solvable under arbitrary switching with undirected connected graph, directed graph, and switching topologies, respectively. Simulation examples are also provided to demonstrate the effectiveness of the theoretical results.

Design, Analysis, and Experimental Validation of a Distributed Protocol for Platooning in the Presence of Time-Varying Heterogeneous Delays

Abstract: This paper presents a novel control design framework for vehicle platooning together with its experimental validation. The problem of controlling the vehicles within a platoon, so that they converge to their desired velocities and intervehicle distances, is formulated as a high-order network consensus problem. By means of Lyapunov–Razumikhin functions, convergence is proven of the platoon to the desired consensus speed and intervehicle spacing under both fixed and switching communication network topologies, thus confirming the capability of the proposed approach to cope with maneuvers where vehicles join or leave the platoon and communication failures. Tuning criteria for the control gains are provided to guarantee string stability under the proposed control law. Finally, results of numerical simulations and in-vehicle experiments demonstrate the effectiveness of the proposed approach in a three-vehicle platoon.

Use of blockchain based distributed consensus control

Abstract: A system for the cryptographically-secure, autonomous control of devices comprising, connected to or remotely operating devices in an electrically powered network and the transaction of the benefits, costs or value created by or transacted through the devices in this electrically powered network.

Distributed Consensus of Stochastic Delayed Multi-agent Systems Under Asynchronous Switching

Abstract: In this paper, the distributed exponential consensus of stochastic delayed multi-agent systems with nonlinear dynamics is investigated under asynchronous switching. The asynchronous switching considered here is to account for the time of identifying the active modes of multi-agent systems. After receipt of confirmation of mode’s switching, the matched controller can be applied, which means that the switching time of the matched controller in each node usually lags behind that of system switching. In order to handle the coexistence of switched signals and stochastic disturbances, a comparison principle of stochastic switched delayed systems is first proved. By means of this extended comparison principle, several easy to verified conditions for the existence of an asynchronously switched distributed controller are derived such that stochastic delayed multi-agent systems with asynchronous switching and nonlinear dynamics can achieve global exponential consensus. Two examples are given to illustrate the effectiveness of the proposed method.

Distributed Consensus Control of Multi-Agent Systems With Higher Order Agent Dynamics and Dynamically Changing Directed Interaction Topologies

Abstract: In 2005, Ren and Beard, under the network connectivity assumption that the union of the directed interaction graphs contains a spanning tree frequently enough, proposed a control protocol to solve the distributed consensus control problem for a multi-agent system with first order integrator agent dynamics. The consensus control protocol for second order systems proposed by the same authors, Ren and Beard, requires that the switching directed interaction graph has a spanning tree at every time instant to guarantee consensus. In this technical note, we propose a distributed consensus control protocol which extends the first order system results of Ren and Beard to agents represented by a general higher order controllable linear system, under the same network connectivity assumption that the union of the directed interaction graphs contains a spanning tree frequently enough. We show that the consensus can be achieved asymptotically by the group of agents under our proposed distributed control protocol. Simulation study illustrates the effectiveness of our proposed method.

Reverse Group Consensus of Multi-Agent Systems in the Cooperation-Competition Network

Abstract: In this paper, a reverse group consensus problem is investigated for the dynamic agents with the inputs in the cooperation-competition network which can be divided into two sub-networks. The weights between the agents in the same sub-network are positive, while the weights between the agents among different sub-networks are negative. Then, the reverse group consensus is firstly studied without the in-degree balance condition. By defining the mirror graph and establishing the solution of the multi-agent system, it is found that the reverse group consensus problem can be achieved if the mirror graph is strongly connected. The explicit expression of the error level is also derived, which would be vanished for multi-agent systems with some special kinds of inputs. Furthermore, as an extension, the decomposing of the cooperation-competition network is discussed, where the concept of the condensation undirected graph and the path balance condition are defined, and several effective sufficient conditions are obtained. Finally, numerical simulation demonstrates the effectiveness of the theoretical analysis.

Decentralized Consensus Control of a Rigid-Body Spacecraft Formation with Communication Delay

Abstract: The decentralized consensus control of a formation of rigid-body spacecraft is studied in the framework of geometric mechanics while accounting for a constant communication time delay between spacecraft. The relative position and attitude (relative pose) are represented on the Lie group SE(3) and the communication topology is modeled as a digraph. The consensus problem is converted into a local stabilization problem of the error dynamics associated with the Lie algebra se(3) in the form of linear time-invariant delay differential equations with a single discrete delay in the case of a circular orbit, whereas it is in the form of linear time-periodic delay differential equations in the case of an elliptic orbit, in which the stability may be assessed using infinite-dimensional Floquet theory. The proposed technique is applied to the consensus control of four spacecraft in the vicinity of a Molniya orbit.

Distributed consensus tracking for non-linear multi-agent systems with input saturation: a command filtered backstepping approach

Abstract: This study deals with the distributed consensus tracking problem for non-linear multi-agent systems under a fixed directed graph. The dynamics of the followers are taken as strict-feedback structures with unknown non-linearities and input saturation. Neural networks are utilised to identify a certain scalar related to the unknown non-linear functions, and an auxiliary system is introduced into the control design to compensate the effect of input saturation. By incorporating the command filtered technique into the backstepping design framework, a distributed consensus control scheme is constructed recursively. Using the Lyapunov stability theory, it is proved that all signals in the closed-loop systems are cooperatively semi-globally uniformly ultimately bounded and the consensus tracking errors converge to a small neighbourhood of origin by tuning the design parameters. Finally, simulation result demonstrates the effectiveness of the proposed control approach.

Distributed consensus-based formation control for nonholonomic wheeled mobile robots using adaptive neural network

Abstract: This paper investigates the distributed formation control problem for multiple nonholonomic wheeled mobile robots. A variable transformation is first proposed to convert the formation control problem into a state consensus problem. Then, when the dynamics of the mobile robots are considered, the distributed kinematic controllers and neural network torque controllers are derived for each robot such that a group of nonholonomic mobile robots asymptotically converge to a desired geometric pattern along the specified reference trajectory. The specified reference trajectory is assumed to be the trajectory of a virtual leader whose information is available to only a subset of the followers. Also the followers are assumed to have only local interaction. Moreover, the neural network torque controllers proposed in this work can tackle the dynamics of robots with unmodeled bounded disturbances and unstructured unmodeled dynamics. Some sufficient conditions are derived for accomplish the asymptotically stability of the systems based on algebraic graph theory, matrix theory, and Lyapunov control approach. Finally, simulation examples illustrate the effectiveness of the proposed controllers.