Robust Adaptive Consensus Research Articles

Robust Adaptive Consensus of Decentralized Large-Scale Bi-Directional Vehicular Platoons With Only Relative Position Measurement

This paper presents a new approach to solve the problem of internal and string stability of decentralized large-scale bi-directional vehicular platoons (DLBVPs) under uncertain dynamics with constant spacing strategy. It is assumed that each following vehicle can only measure the relative position with respect to its predecessor and subsequent vehicles. To estimate the uncertain dynamics of each following vehicle and unknown leader acceleration, a new decentralized robust adaptive consensus protocol is introduced which by using only relative position measurement, guarantees internal and string stability. It will be shown that the control gains are tuned without requiring any knowledge about the platoon size (number of following vehicles) and consequently, the controller is robust against size changing due to common maneuvers. By using Lyapunov theorem, we will prove that under this approach, the position and velocity tracking errors of each vehicle converge to zero asymptotically. Furthermore, it is shown that the adaptation laws have the main role in assuring string stability of DLBVPs with constant spacing strategy. Several simulation and practical results will depict the merits of this algorithm.

A novel approach for adaptive H∞ leader-following consensus of higher-order locally Lipschitz multi-agent systems

This paper addresses the design of fully-distributed edge-based leader-following consensus controller for the higher-order locally Lipschitz nonlinear multi-agents. The proposed fully-distributed consensus approach deals with the local scenario as compared to the traditional consensus control for the globally Lipschitz models. The dynamics of nonlinear systems are considered to satisfy the Lipschitz condition locally and an adaptive fully-distributed relative-state consensus protocol has been established for ensuring a coherent behavior of agents. A local leader-following adaptive consensus controller design condition, for obtaining the adaptive consensus protocol gain, has been formulated using the proposed local treatment, adaptation law for coupling weights, ellipsoidal regional investigation, asymptotic stability consideration, and Lyapunov redesign. The developed results are enriched for attaining a robust adaptive consensus of the locally Lipschitz multi-agents against disturbances by using the local L2 stability approach. This developed technique can be effective for minimizing the effect of external disturbances on the consensus error. From technical point of view, the proposed method can be used for the locally Lipschitz models, like systems having polynomial approximations in their models. Further, the proposed approach adapts (similar to routing in communication) in accordance with the requirement of control gains for a communication topology. Furthermore, our method can be applied to deal with the unwanted disturbance (like change in air flow speed or direction and external perturbations) for attaining a robust design. In contrast to existing studies, the proposed results consider adaptive and H∞ consensus schemes for the higher-order locally Lipschitz multi-agents. The developed results are verified for a network of six vendor pol multi-agent systems and modified Chua's circuits.

Adaptive Consensus and Circuital Implementation of a Class of Faulty Multiagent Systems

This article is concerned with the robust adaptive fault-tolerant consensus control and the circuital implementation problems for a class of homogeneous multiagent systems with external disturbances and actuator faults. A robust adaptive consensus control strategy is developed to automatically eliminate the effects of actuator bias and partial loss-of-control-effectiveness faults, and simultaneously specify the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L_{2}$ </tex-math></inline-formula> performance of systems. The achievement of exponential consensus of the closed-loop disturbed and faulty multiagent system is provided on the basis of the Lyapunov stability theory. Furthermore, a physical implementation method is developed based on circuit theory to translate the proposed adaptive consensus control strategy into analog circuits. By using a professional tool for circuit simulations, effectiveness of the developed circuits is verified via a multiagent system composed by mobile robots with two independent driving wheels.

LPV Scheme for Robust Adaptive Output Feedback Consensus of Lipschitz Multiagents Using Lipschitz Nonlinear Protocol

This article addresses the robust nonlinear adaptive consensus protocol design approach, for nonlinear multiagent (MA) systems under external perturbations with Lipschitz dynamics. Based on the existing results of the adaptive protocol using relative output information, a modified robust nonlinear adaptive consensus protocol for MAs is proposed. The Lipschitz nonlinearity in the agents’ dynamics and the adaptive protocol are reformulated using a composite linear parameter-varying (LPV) approach. Our composite LPV reformulation is less restrictive because it utilizes all the properties of Lipschitz functions. Consensus protocol design conditions based on the proposed reformulation of Lipschitz nonlinearities, Lyapunov theory, and the modified nonlinear consensus protocol are derived to attain the exponential consensus. The proposed consensus protocol approach is further extended by providing necessary and sufficient conditions for the straightforward evaluation of the robust adaptive protocol gains. The proposed fully distributed consensus protocol is robust to ensure the consensus error and adaptive gains bounded by eliminating the effect of disturbances. The resultant consensus control scheme reduces conservatism and is feasible for large values of Lipschitz constants. Finally, simulation results are presented for verifying the potency of the developed control strategies.

Fully distributed robust consensus control of multi-agent systems with heterogeneous unknown fractional-order dynamics

ABSTRACTThis paper investigates the robust consensus control problem of heterogeneous unknown nonlinear fractional-order multi-agent systems (FOMASs) without leader and with multiple leaders of bounded inputs. More specifically, FOMASs with nonidentical unknown coupling nonlinearities and external disturbances are considered in this paper, which takes the first-order MASs as its special case. Based on the σ-modification adaptive control technique, some class of discontinuous robust adaptive control protocols are proposed to solve the leaderless consensus problem and containment consensus problem, respectively. By means of the set-valued maps theory and by artfully choosing Lyapunov function, it is shown that the proposed consensus protocols are user friendly in that they are capable of compensating uncertain coupling nonlinearities, rejecting disturbances, rendering smaller control gains and thus requiring smaller amplitude on the control input while preserving global consensus convergence. All of the proposed robust adaptive consensus protocols are independent of any global and unknown information and thus are fully distributed. Some numerical simulations are provided to validate the correctness of the obtained results.

Robust adaptive consensus of nonstrict-feedback multi-agent systems with quantized input and unmodeled dynamics

This paper deals with the distributed robust adaptive neural network tracking control for a class of nonstrict-feedback multi-agent systems (MASs) with unmodeled dynamics and input quantized by the hysteresis quantizer. A decomposition technique is utilized to handle the problem that only finite and discrete control values are available to control system. Meanwhile, the radial basis function neural networks (RBFNNs) are employed for approximation of the packaged unknown nonlinearities induced by recursive design. By introducing the structural characteristic of Gaussian basis function (see Lemma 5), the effects from the nonstrict-feedback are effectively compensated and the backstepping-based method can be successfully implemented. Then, by combining the dynamic signal with adaptive backstepping control technique, a robust adaptive distributed quantized tracking control scheme is developed for each follower. It is proved that all closed-loop signals are semi-globally uniformly ultimately bounded and the asymptotic consensus tracking is guaranteed. Finally, the effectiveness of the control method is illustrated by two simulation examples.

Consensus disturbance rejection of network-connected dynamic systems with input delay and unknown network connectivity

In this paper, we consider the distributed consensus problem for multi-agent systems with input delay and unknown disturbances. To deal with the input delay and disturbances, a predictor- and disturbance observer-based control strategy is developed for each follower. Then, a distributed robust adaptive consensus protocol with dynamic coupling gains is proposed based on the relative state of the neighbouring agents, guaranteeing that all signals in the closed-loop dynamics are uniformly ultimately bounded and the consensus tracking error converges to an arbitrarily small residual set. The controllers are proposed in a fully distributed way such that the global information related to the eigenvalues of the Laplacian matrix is not needed. Finally, the validity of the proposed controller is demonstrated through a numerical example.

Novel distributed robust adaptive consensus protocols for linear multi-agent systems with directed graphs and external disturbances

ABSTRACTThis paper addresses the distributed consensus protocol design problem for linear multi-agent systems with directed graphs and external unmatched disturbances. Novel distributed adaptive consensus protocols are proposed to achieve leader–follower consensus for any directed graph containing a directed spanning tree with the leader as the root node and leaderless consensus for strongly connected directed graphs. It is pointed out that the adaptive protocols involve undesirable parameter drift phenomenon when bounded external disturbances exist. By using the σ modification technique, distributed robust adaptive consensus protocols are designed to guarantee the ultimate boundedness of both the consensus error and the adaptive coupling weights in the presence of external disturbances. All the adaptive protocols in this paper are fully distributed, relying on only the agent dynamics and the relative states of neighbouring agents.

Robust adaptive consensus tracking for higher‐order multi‐agent uncertain systems with nonlinear dynamics via distributed intermittent communication protocol

SummaryThis paper considers the robust adaptive consensus tracking for higher‐order multi‐agent uncertain systems with nonlinear dynamics via distributed intermittent communication protocol. The main contribution of this work is solving the robust consensus tracking problem without the assumption that the topology among followers is strongly connected and fixed. The focus is the problem of actuator with occasional failure inputs and communication resources constraints. A novel distributed intermittent communication framework is proposed via adaptive approach. In this framework, the underlying communication topologies switch among several directed graphs with a limited directed spanning tree rooted at a leader agent. Furthermore, by introducing a strategy of actuator fault compensation inputs, a combination of robust consensus tracking protocol is designed by the different adaptive feedback controllers. It is proved that the robust adaptive consensus tracking can be achieved by using local states information of neighboring agents if the communication retention rate condition is satisfied. Two examples are presented to demonstrate the effectiveness of the proposed approach. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Adaptive consensus tracking for linear multi‐agent systems with heterogeneous unknown nonlinear dynamics

SummaryThis paper considers the consensus tracking control problem for general linear multi‐agent systems with unknown dynamics in both the leader and all followers. Based on parameterizations of the unknown dynamics of all agents, two decentralized adaptive consensus tracking protocols, respectively, with dynamic and static coupling gains, are proposed to guarantee that the states of all followers converge to the state of the leader. Furthermore, this result is extended to the robust adaptive consensus tracking problem in which there exist parameter uncertainties and Lipschitz‐type disturbances in the network. It is also shown that the parameter estimation errors converge to zero based on contradiction method and Lyapunov function approach. Finally, a simulation example is provided to illustrate the theoretical results. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Adaptive NN consensus tracking control of a class of nonlinear multi-agent systems

This paper proposes a novel robust adaptive consensus tracking control approach for a class of nonlinear multi-agent systems with modeling uncertainties and external disturbances. Radial Basis Function Neural Networks (RBFNNs) are used to approximate the unknown nonlinear function of agent׳s dynamic. Compared with existing NN consensus algorithms of nonlinear multi-agent systems, the proposed consensus control method only needs a small number of adjustable parameters, thus the online computation burden is greatly alleviated. In addition, by online updating the estimation of the NN approximation errors, the proposed consensus control approach can enhance the system robustness against modeling uncertainties. It is proven that all the signals of the multi-agent system are uniformly bounded and the consensus tracking errors converge to a small neighborhood of zero. A simulation is carried out to demonstrate the effectiveness of the proposed control method.

Efficient and robust adaptive consensus services based on oracles

Due to their fundamental role in the design of faulttolerant distributed systems, consensus protocols have been widely studied. Most of the research in this area has focused on providing ways for circumventing the impossibility of reaching consensus on a purely asynchronous system subject to failures. Of particular interest are the indulgent consensus protocols based upon weak failure detection oracles. Following the first works that were more concerned with the correctness of such protocols, performance issues related to them are now a topic that has gained considerable attention. In particular, a few studies have been conducted to analyze the impact that the quality of service of the underlying failure detection oracle has on the performance of consensus protocols. To achieve better performance, adaptive failure detectors have been proposed. Also, slowness oracles have been proposed to allow consensus protocols to adapt themselves to the changing conditions of the environment, enhancing their performance when there are substantial changes on the load to which the system is exposed. In this paper we further investigate the use of these oracles to design efficient consensus services. In particular, we provide efficient and robust implementations of slowness oracles based on techniques that have been previously used to implement adaptive failure detection oracles. Our experiments on a widearea distributed system show that by using a slowness oracle that is well matched with a failure detection oracle, one can achieve performance as much as 53.5% better than the alternative that does not use a slowness oracle.

Efficient and robust adaptive consensus services based on oracles

Abstract Due to their fundamental role in the design of fault-tolerant distributed systems, consensus protocols have been widely studied. Most of the research in this area has focused on providing ways for circumventing the impossibility of reaching consensus on a purely asynchronous system subject to failures. Of particular interest are the indulgent consensus protocols based upon weak failure detection oracles. Following the first works that were more concerned with the correctness of such protocols, performance issues related to them are now a topic that has gained considerable attention. In particular, a few studies have been conducted to analyze the impact that the quality of service of the underlying failure detection oracle has on the performance of consensus protocols. To achieve better performance, adaptive failure detectors have been proposed. Also, slowness oracles have been proposed to allow consensus protocols to adapt themselves to the changing conditions of the environment, enhancing their performance when there are substantial changes on the load to which the system is exposed. In this paper we further investigate the use of these oracles to design efficient consensus services. In particular, we provide efficient and robust implementations of slowness oracles based on techniques that have been previously used to implement adaptive failure detection oracles. Our experiments on a wide-area distributed system show that by using a slowness oracle that is well matched with a failure detection oracle, one can achieve performance as much as 53.5% better than the alternative that does not use a slowness oracle.