Boda Ning

A survey on recent advances in distributed sampled-data cooperative control of multi-agent systems

Abstract Distributed cooperative control of multi-agent systems has been one of the most active research topics in the fields of automatic control and robotics. This paper provides a survey on recent advances in distributed cooperative control under a sampled-data setting, with special emphasis on the published results since 2011. First, some typical sampling mechanisms related to this topic, such as uniform sampling, nonuniform sampling, random sampling, and event-triggered sampling, are summarized in both asynchronous and synchronous paradigms. Then, based on different coordinated tasks, recent results on distributed sampled-data cooperative control of multi-agent systems are categorized into four classes, i.e., sampled-data leaderless consensus, sampled-data leader-following consensus, sampled-data containment control, and sampled-data formation control. For each class, some explicit research lines are identified according to various sampling mechanisms. In particular, depending on definitions of event triggering conditions, some representative event-triggered sampling mechanisms are sorted out and discussed in detail. Finally, several challenging issues for future research are proposed.

Collective Behaviors of Mobile Robots Beyond the Nearest Neighbor Rules With Switching Topology

This paper is concerned with the collective behaviors of robots beyond the nearest neighbor rules, i.e., dispersion and flocking, when robots interact with others by applying an acute angle test (AAT)-based interaction rule. Different from a conventional nearest neighbor rule or its variations, the AAT-based interaction rule allows interactions with some far-neighbors and excludes unnecessary nearest neighbors. The resulting dispersion and flocking hold the advantages of scalability, connectivity, robustness, and effective area coverage. For the dispersion, a spring-like controller is proposed to achieve collision-free coordination. With switching topology, a new fixed-time consensus-based energy function is developed to guarantee the system stability. An upper bound of settling time for energy consensus is obtained, and a uniform time interval is accordingly set so that energy distribution is conducted in a fair manner. For the flocking, based on a class of generalized potential functions taking nonsmooth switching into account, a new controller is proposed to ensure that the same velocity for all robots is eventually reached. A co-optimizing problem is further investigated to accomplish additional tasks, such as enhancing communication performance, while maintaining the collective behaviors of mobile robots. Simulation results are presented to show the effectiveness of the theoretical results.

Distributed optimization for multiagent systems: an edge-based fixed-time consensus approach

This paper deals with the problem of distributed optimization for multiagent systems by using an edge-based fixed-time consensus approach. In the case of time-invariant cost functions, a new distributed protocol is proposed to achieve the state agreement in a fixed time while the sum of local convex functions known to individual agents is minimized. In the case of time-varying cost functions, based on the new distributed protocol in the case of time-invariant cost functions, a distributed protocol is provided by taking the Hessian matrix into account. In both cases, stability conditions are derived to ensure that the distributed optimization problem is solved under both fixed and switching communication topologies. A distinctive feature of the results in this paper is that an upper bound of settling time for consensus can be estimated without dependence on initial states of agents, and thus can be made arbitrarily small through adjusting system parameters. Therefore, the results in this paper can be applicable in an unknown environment such as drone rendezvous within a required time for military purpose while optimizing local objectives. Case studies of a power output agreement for battery packages are provided to demonstrate the effectiveness of the theoretical results.

Fixed-time consensus for multi-agent systems with discontinuous inherent dynamics over switching topology

ABSTRACT This paper is concerned with the fixed-time consensus problem of multi-agent systems. Unlike conventional consensus-based investigations, where nonlinear inherent dynamics satisfying the Lipschitz continuous condition is assumed or simply no inherent dynamics is involved for each agent, we are dealing with a more general case: agents with discontinuous nonlinear inherent dynamics. By using non-smooth analysis and fixed-time stability techniques, distributed protocols are proposed to achieve the fixed-time consensus over fixed and switching topology. Then, for a class of linear multi-agent systems, a new distributed controller is proposed to further reduce the calculation cost while achieving the agreement. A distinctive feature of the work is that the estimation of settling time for consensus is independent of initial states of agents, which provides flexibility for applications implemented in unknown environment. Finally, numerical simulations are given to demonstrate the effectiveness of the theoretical results.

Two-Stage Deployment Strategy for Wireless Robotic Networks via a Class of Interaction Models

Suppose a disaster happens, and several groups of robots are dispatched from distant control stations. To enable rescue staff to make collective decisions, reliable and robust connections need to be established among stations. Motivated by this scenario, a two-stage robot deployment strategy is proposed for wireless robotic networks (WRNs). In the first stage, robots in distant groups are merged into one group that covers a desirable area. Since connectivity alone cannot guarantee a high communication quality, in the second stage, the flow between any two stations is further optimized in terms of expected number of transmissions per successfully delivered packet. In both stages, a distributed collision-free controller is proposed to regulate the interactive force among robots. The stability issues of WRNs, where the proposed controller together with a class of interaction models based on an acute angle test is implemented for robots, are analyzed under both fixed and switching topology. In order to efficiently switch the neighbor set for each robot, a new energy function is constructed taking finite-time consensus into account. To guarantee that energy agreement is achieved before the next topology change, a fixed-time consensus approach is further proposed and an upper bound of the settling time for the energy agreement is obtained. Numerical simulations are provided to demonstrate the effectiveness of the two-stage deployment strategy.

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.

Connectivity control and performance optimization in Wireless Robotic Networks: Issues, approaches and a new framework

In Wireless Robotic Networks (WRNs), robots interact with each other to fulfill complex tasks such as target tracking, area coverage or environment monitoring. When designing the interactive controller with a more realistic consideration, connectivity needs to be maintained instead of simply assumed. In this paper, we give an introduction on how to maintain the connectivity of a communication graph which corresponds to a WRN. Due to the limited communication range of robots, proximity-limited communication models are used for the co-operative control problem and the connectivity maintenance problem in WRNs. Therefore, we review two kinds of proximity-limited communication models in this paper: the disk-based communication model and the hysteresis-based communication model. In addition, we present some network performance optimization work that based on communication metrics of Bit Error Rate (BER) and Signal to Interference plus Noise Ratio (SINR). Finally, we propose a new framework that involves the connectivity control and the network performance optimization, and potential solutions are discussed accordingly.

Minimizing network interference through mobility control in wireless robotic networks

In this paper, we treat mobility as a network control primitive in wireless robotic networks (WRNs), which consist of a number of mobile robots. The aim is to achieve better communication performance by making use of the mobility of the robots. In the literature, it has been shown that for a graph-based connectivity model, an equal-space configuration of robot nodes is optimal in terms of energy efficiency. To be more realistic, we adopt a signal to interference plus noise ratio (SINR)-based physical model in this work. With this model, we show that the equal-space configuration of robots is no longer optimal from the perspective of network interference. Instead, an equal-SINR configuration of robots is demonstrated to be optimal for a unicast topology in WRNs. Based on our network interference reduction method, a distributed mobility control algorithm is proposed to achieve the equal-SINR configuration. Simulation results verify that the proposed algorithm can drive robots to the desired positions where the network interference is minimized.

Distributed fixed-time cooperative tracking control for multi-robot systems

In this paper, we study the fixed-time cooperative tracking control problem for multi-robot systems with doubleintegrator dynamics. First, a novel distributed observer is proposed for each follower to estimate the leader state in a fixed time, then a local tracking controller based on sliding mode technique is proposed such that the estimated leader state is tracked in a fixed time. Both cases of a stationary leader and a dynamic leader are investigated. Since nonholonomic dynamics can better describe the mobile robots in reality, we further extend the results to achieve fixed-time cooperative tracking for multi-robot systems with nonholonomic dynamics. Different from the conventional finite-time cooperative tracking strategies, the fixed-time approach in this work guarantees that an upper bound of settling time can be prescribed without dependence on initial states of robots, which provides additional system information in advance. Finally, numerical simulations are given to demonstrate the effectiveness of the theoretical results.