Showing papers on "Consensus published in 2018"

Fixed-Time Consensus Tracking for Multiagent Systems With High-Order Integrator Dynamics

Abstract: This paper addresses the fixed-time leader–follower consensus problem for high-order integrator multiagent systems subject to matched external disturbances. A new cascade control structure, based on a fixed-time distributed observer, is developed to achieve the fixed-time consensus tracking control. A simulation example is included to show the efficacy and the performance of the proposed control structure with respect to different initial conditions.

Event-Triggered Control for Consensus of Multiagent Systems With Fixed/Switching Topologies

Abstract: In this paper, the leader-following consensus problem of high-order multiagent systems via event-triggered control is discussed. A novel distributed event-triggered communication protocol based on state estimates of neighboring agents is proposed to solve the consensus problem of the leader-following systems. We first investigate the consensus problem in a fixed topology, and then extend to the switching topologies. State estimates in fixed topology are only updated when the trigger condition is satisfied. However, state estimates in switching topologies are renewed with two cases: 1) the communication topology is switched or 2) the trigger condition is satisfied. Clearly, compared to continuous-time interaction, this protocol can greatly reduce the communication load of multiagent networks. Besides, the event-triggering function is constructed based on the local information and a new event-triggered rule is given. Moreover, “Zeno behavior” can be excluded. Finally, we give two examples to validate the feasibility and efficiency of our approach.

Consensus of Hybrid Multi-Agent Systems

Abstract: In this brief, we consider the consensus problem of hybrid multiagent systems. First, the hybrid multiagent system is proposed, which is composed of continuous-time and discrete-time dynamic agents. Then, three kinds of consensus protocols are presented for the hybrid multiagent system. The analysis tool developed in this brief is based on the matrix theory and graph theory. With different restrictions of the sampling period, some necessary and sufficient conditions are established for solving the consensus of the hybrid multiagent system. The consensus states are also obtained under different protocols. Finally, simulation examples are provided to demonstrate the effectiveness of our theoretical results.

Event-Triggered Control for Consensus Problem in Multi-Agent Systems With Quantized Relative State Measurements and External Disturbance

Abstract: For decreasing communication load and overcoming network constrains, such as the limited bandwidth and data loss in multi-agent networks, this paper integrates the two control strategies to investigate the bounded consensus problem of multi-agent systems (MASs) with external disturbance on the basis of an undirected graph, namely, the quantized control and the event-triggered control. In the existence of the external disturbance, two types of the high-gain control laws with the uniform quantized relative state measurements for the bounded consensus problem of MASs are first discussed, respectively. Then, in order to save the limited network resources in a multi-agent network, the event-triggered quantized communication protocols are designed based on the first case to obtain the bounded consensus in multi-agent systems. Moreover, it is shown that “Zeno behavior” phenomenon can be excluded under the two event-triggered quantized control mechanisms, and the boundness of the relative state error can be adjusted by selecting the different parameters. Finally, two examples are shown to validate the feasibility and efficiency of our theoretical analysis.

Bipartite Tracking Consensus of Linear Multi-Agent Systems With a Dynamic Leader

Abstract: In this brief, the distributed bipartite tracking consensus problem for linear multi-agent systems (MASs) in the presence of a single leader is investigated. Compared with some related works on this topic, the leader’s control inputs in the present MAS model are allowed to be nonzero and unknown to each follower. To guarantee bipartite tracking consensus, a new kind of distributed non-smooth protocols based on the relative state information of neighboring agents are proposed and analyzed. With the help of tools from Lyapunov stability theory and graph theory, it is shown that bipartite tracking consensus in the close-loop MAS can be achieved if the gain matrix of protocol is suitably constructed and the control parameters of protocol are, respectively, larger than some positive quantities depending on global information of the considered MAS. To provide some efficient criteria for consensus seeking without involving any global information, some fully distributed protocols with adaptive control parameters are further designed and discussed. Finally, numerical simulations are given for illustration.

Finite-Time Bipartite Consensus for Multi-Agent Systems on Directed Signed Networks

Abstract: This paper addresses the finite-time bipartite consensus problem for multi-agent systems (MASs) on a directed signed network. Some properties for signed digraphs are first investigated and two nonlinear control protocols are then designed for first- and second-order MASs, respectively, where agents may be influenced by bounded disturbances. Particularly, for MASs with second-order dynamics, a new estimation technique is developed to estimate the settling time. Simulation results are finally presented to verify the effectiveness of the proposed control protocols.

Bipartite Consensus in Networks of Agents With Antagonistic Interactions and Quantization

Abstract: This brief deals with the consensus problem in a network of agents with cooperative and antagonistic interactions subject to quantization. By employing the techniques from nonsmooth analysis, we prove that all agents can be guaranteed to asymptotically reach bipartite consensus for any logarithmic quantizer accuracy under connected and structurally balanced topology and the states of all agents asymptotically converge to zero under connected and structurally unbalanced topology. In addition, finite-time bipartite consensus is considered for single-integrator agents with binary quantized information. The simulation results are given to demonstrate the effectiveness of the theoretical results.

A Better Method to Analyze Blockchain Consistency

Abstract: The celebrated Nakamoto consensus protocol [16] ushered in several new consensus applications including cryptocurrencies A few recent works [7, 17] have analyzed important properties of blockchains, including most significantly, consistency, which is a guarantee that all honest parties output the same sequence of blocks throughout the execution of the protocol To establish consistency, the prior analysis of Pass, Seeman and Shelat [17] required a careful counting of certain combinatorial events that was difficult to apply to variations of Nakamoto The work of Garay, Kiayas, and Leonardas [7] provides another method of analyzing the blockchain under the simplifying assumption that the network was synchronous The contribution of this paper is the development of a simple Markov-chain based method for analyzing consistency properties of blockchain protocols The method includes a formal way of stating strong concentration bounds as well as easy ways to concretely compute the bounds We use our new method to answer a number of basic questions about consistency of blockchains: Our new analysis provides a tighter guarantee on the consistency property of Nakamoto's protocol, including for parameter regimes which [17] could not consider; We analyze a family of delaying attacks first presented in [17], and extend them to other protocols; We analyze how long a participant should wait before considering a high-value transaction "confirmed"; We analyze the consistency of CliqueChain, a variation of the Chainweb [14] system; We provide the first rigorous consistency analysis of GHOST [20] and also analyze a folklore "balancing"-attack In each case, we use our framework to experimentally analyze the consensus bounds for various network delay parameters and adversarial computing percentages We hope our techniques enable authors of future blockchain proposals to provide a more rigorous analysis of their schemes

Towards Scaling Blockchain Systems via Sharding

Abstract: Existing blockchain systems scale poorly because of their distributed consensus protocols. Current attempts at improving blockchain scalability are limited to cryptocurrency. Scaling blockchain systems under general workloads (i.e., non-cryptocurrency applications) remains an open question. In this work, we take a principled approach to apply sharding, which is a well-studied and proven technique to scale out databases, to blockchain systems in order to improve their transaction throughput at scale. This is challenging, however, due to the fundamental difference in failure models between databases and blockchain. To achieve our goal, we first enhance the performance of Byzantine consensus protocols, by doing so we improve individual shards' throughput. Next, we design an efficient shard formation protocol that leverages a trusted random beacon to securely assign nodes into shards. We rely on trusted hardware, namely Intel SGX, to achieve high performance for both consensus and shard formation protocol. Third, we design a general distributed transaction protocol that ensures safety and liveness even when transaction coordinators are malicious. Finally, we conduct an extensive evaluation of our design both on a local cluster and on Google Cloud Platform. The results show that our consensus and shard formation protocols outperform state-of-the-art solutions at scale. More importantly, our sharded blockchain reaches a high throughput that can handle Visa-level workloads, and is the largest ever reported in a realistic environment.

Event-Triggered Leader-Following Consensus for Nonlinear Multiagent Systems Subject to Actuator Saturation Using Dynamic Output Feedback Method

Abstract: This paper addresses the distributed event-triggered consensus problem for a class of nonlinear multiagent systems subject to actuator saturation. A new distributed event-based dynamic output feedback controller is put forward via the relative output measurements of neighboring agents. It removes the impractical assumptions that the controllers for agents have to update continuously and the observers embedded in agents have to share information with their neighbors, thus the energy consumptions of agents are significantly reduced. Two event-triggered strategies for the cases with and without continuous communication among neighboring agents are established. Sufficient conditions are derived in terms of matrix inequalities to guarantee the exponential leader-following consensus. The problem of designing a distributed event-triggered control law such that the domain of attraction for consensus errors is enlarged, formulated, and solved as an optimization problem with matrix inequality constraints. Simulation results are given to verify the effectiveness of the theoretical results.

Distributed Consensus of Second-Order Multiagent Systems With Nonconvex Velocity and Control Input Constraints

Abstract: This paper is devoted to a distributed consensus problem for second-order multiagent systems with nonconvex velocity and control input constraints. A distributed control algorithm is introduced using local information. It is shown that consensus can be achieved as long as the union of the communication graphs has directed spanning trees among each time interval of certain bounded length in the presence of arbitrarily bounded communication delays. Finally, a numerical example is included to illustrate the theoretical results.

Distributed Consensus Control of One-Sided Lipschitz Nonlinear Multiagent Systems

Abstract: This paper addresses the distributed consensus controller design approaches for one-sided Lipschitz nonlinear multiagents by employing relative state feedback. A new treatment for one-sided Lipschitz nonlinearity is rendered from the consensus control point of view. By employing the quadratic inner-boundedness and the one-sided Lipschitz constraints, a sufficient condition for asymptotic consensus of nonlinear systems under strongly connected communication topologies is provided. Further, a robust consensus protocol design scheme for the nonlinear multiagents is derived by ensuring the ${L_{2}}$ stability of the consensus error system. Furthermore, a novel robust consensus control scheme against amplitude-bounded perturbations is developed that guarantees asymptotic convergence of the consensus error into a compact set. Extensions to the proposed methodologies for the leader-following consensus for a spanning tree communication topology with the leader as the root are addressed. In contrast to the conventional consensus control methodologies, this paper is less conservative and can be applied to a broader class of nonlinear multiagent systems. Moreover, the proposed consensus control approach is less conservative for robustness against disturbances owing to its ability to handle amplitude-bounded disturbances and due to the relaxation of a balanced communication topology. A numerical simulation study is provided for the consensus control of eight mobile agents.

Positive Edge Consensus of Complex Networks

Abstract: This paper focuses on addressing the consensus problem of edges for both undirected and directed nodal networks whose edges are described by nominal or uncertain positive linear systems. The definition of line graph is given, and the similar properties and connections between the two types of graphs are established. Based on the rigorous theoretical analysis, distributed positive consensus algorithms are proposed, and the main results are given without utilizing the global interaction information. Subsequently, sufficient conditions for consensus with positivity constraint are given via the vertex number and the edge number of the original nodal network. Furthermore, according to the properties of Metzler matrix, we also solve the consensus problem for multiple edges which are modeled by uncertain positive systems. Some simulation studies are finally presented to verify the feasibility and effectiveness of the proposed results.

Consensus Control for Vehicular Platooning With Velocity Constraints

Abstract: In this paper, a distributed consensus control approach for vehicular platooning systems is proposed. In formalizing the underlying consensus problem, a realistic vehicle dynamics model is considered and a velocity-dependent spacing policy between two consecutive vehicles is realized. As a particular case, the approach allows to consider bidirectional vehicle interaction, which improves the cohesion between vehicles in the platoon. Exponential stability of the platoon dynamics is evaluated, also in the challenging scenario in which a limitation on the velocity of one of the vehicles in the platoon is introduced. The theoretical results are experimentally validated using a three-vehicle platoon consisting of (longitudinally) automated vehicles equipped with wireless intervehicle communication and radar-based sensing.

Distributed Adaptive Consensus Disturbance Rejection for Multi-Agent Systems on Directed Graphs

Abstract: This paper considers the distributed consensus disturbance rejection problem for general linear multiagent systems with deterministic disturbances under directed communication graphs. Based on the relative state information of the neighboring agents, the consensus protocols, which consist of two observers, including a state observer and a separate disturbance observer, are designed to guarantee that the consensus error goes to zero with complete disturbance rejection. Furthermore, the state observer is designed in a fully distributed fashion with adaptive coupling gain, which has the advantage that the consensus controller design is independent of the Laplacian matrix associated with the communication network. The distributed observer-based consensus disturbance rejection protocols are further extended to containment control. Finally, an example is provided to demonstrate the effectiveness of the proposed strategies.

Finite-time and fixed-time leader-following consensus for multi-agent systems with discontinuous inherent dynamics

Abstract: This paper is concerned with finite-time and fixed-time consensus of multi-agent systems in a leader-following framework. Different from conventional leader-following tracking approaches where inherent dynamics satisfying the Lipschitz continuous condition is required, a more generalised case is investigated: discontinuous inherent dynamics. By nonsmooth techniques, a nonlinear protocol is first proposed to achieve the finite-time leader-following consensus. Then, based on fixed-time stability strategies, the fixed-time leader-following consensus problem is solved. An upper bound of settling time is obtained by using a new protocol, and such a bound is independent of initial states, thereby providing additional options for designers in practical scenarios where initial conditions are unavailable. Finally, numerical simulations are provided to demonstrate the effectiveness of the theoretical results.

Event-Based Distributed Active Power Sharing Control for Interconnected AC and DC Microgrids

Abstract: This paper investigates the issue of active power sharing among a cluster of microgrids formed by a set of ac and dc microgrids network-interconnected through a set of interlinking converters (ICs). An event-based distributed consensus control approach is proposed to address this issue, in which each IC is assigned to be an agent that needs only the information of its local and neighboring ICs. We first construct the agent system for each IC and design its consensus control protocol. This control mechanism uses a cooperative approach to indirectly adjust the active power load of individual microgrid and, accordingly, navigate the active power flow among them. Thus active power sharing among the interconnected microgrids can be achieved proportionally. Then, an event-based control scheme is utilized to design the consensus protocol, which can dramatically reduce the communication between ICs. Communication time delays and reactive power support are also considered. The proposed distributed control method allows a sparse communication structure and higher reliability and flexibility operation. Simulation results are presented to demonstrate the proposed control method.

Sampled‐data consensus of nonlinear multiagent systems subject to cyber attacks

Abstract: Summary This paper is concerned with the sampled-data consensus problem for a class of nonlinear multiagent systems subject to cyber attacks. The considered attacks are assumed to be recoverable, which destroy the connectivity of the network topology with directed spanning tree. In light of the designed sampled-data control protocol, a more general switched system is proposed to model both the cyber attacks and the sampled-data mechanism within a unified framework. Then, by using the contradiction methods, the solution of a class of differential equations is provided to deal with the technical challenge resulting from the switching and sampling issues. Furthermore, in terms of such a solution combined with the constructed piecewise Lyapunov function, sufficient conditions are derived under which the nonlinear multiagent systems subject to attacks achieve the consensus exponentially. The relationship between the attack frequency and the sampling period is also revealed through the obtained conditions. Finally, simulations are given to show the effectiveness of the proposed results.

Leader-following mean square consensus of stochastic multi-agent systems with input delay via event-triggered control

Abstract: This paper addresses the mean square consensus problem of leader-following stochastic multi-agent systems using a distributed event-triggered control strategy. For each involving agent, generally, the time-varying (or fixed) delay between controller and actuator is unavoidable. The controller is updated only when the event condition is triggered. Based on the Lyapunov function method and Ito formula, three sufficient conditions for leader-following mean square consensus are established, including a delay-independent consensus condition and a delay-dependent criterion for the case with input fixed time delay, and a consensus criterion for the case with input time-varying delay. Furthermore, an inter-event time lower bound between two sampling points is derived. The results are illustrated through several numerical examples.

On Leader–Follower Consensus With Switching Topologies: An Analysis Inspired by Pigeon Hierarchies

Abstract: Inspired by the interesting findings regarding the hierarchy and coordination of pigeons in Nagy et al. [Nature, 2010], we revisit the discrete-time leader–follower consensus problem with switching topologies. The purpose of this paper is to investigate the impact of hierarchical topologies and followers’ self-loops on the convergence performance of leader–follower consensus, including the convergence rate and robustness to switching topologies. We first study the fixed topology case, and show that the followers converge to the leader's state in finite time if and only if each follower has no self-loops and the topology is hierarchical. However, we show via counterexample, that leader–follower consensus may not be achieved when some followers have no self-loops and the topology is switching; even if each interaction graph has a spanning tree rooted at the leader. With the aid of binary relation theory, we further develop a new approach to present a novel sufficient condition for leader–follower consensus with switching topologies and with some followers having no self-loops. We prove that when no followers have self-loops and the same hierarchical organization is kept under the switching topologies, then the fastest rate of convergence in leader–follower consensus can be achieved; even in the presence of complex dynamic topologies. This is consistent with the natural phenomena found in pigeons by Nagy et al.