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This paper proposes a bounded continuous control law to steer a mobile robot to approach and maintain a predefined distance from an unknown stationary target in a GPS-denied environment where the positions of the robot and the target are not available. The control algorithm utilizes only the derivative of the distance between the mobile robot and the target, that is, range rate measurement. We demonstrate that the controller can achieve global convergence regardless of the initial states of the mobile robot. Compared with the existing works, the proposed control algorithm is the simplest form (derivative-type controller) and easy to implement. Numerical simulations and real-time experiments are both performed to confirm the effectiveness of the proposed control scheme.

This paper presents a closed-form solution for the global path following control of the planar vertical takeoff and landing (PVTOL) aircraft. The proposed control scheme consists of two steps. In the first step, a reference model is well constructed to generate a reference trajectory and a reference velocity satisfying some necessary constraints. In the second step, the backstepping control technique is utilized to develop a tracking controller that enables the aircraft to track the generated reference commands. Consequently, the obtained controller allows the aircraft to converge to the desired path at the desired speed under arbitrary initial conditions. The desired path can be a general geometric curve, and the desired speed can be time-varying. The main novelty of the proposed control architecture lies in the design of the reference model, which serves as a command filter and resolves the conflict between the global convergence to the desired path and the velocity assignment. The numerical simulation results verify the effectiveness of the control scheme.

This paper studies the robust practical tracking control problem of an underactuated hovercraft with unknown external disturbances. Controlling such a system is difficult and challenging d...

This paper investigates the leaderless consensus control problem of high-order nonholonomic chained systems. Based on the relationship between state and consensus error, a novel Lyapunov function i...

This paper focuses on characterizing edge dynamics of signed networks subject to both cooperative and antagonistic interactions and copes with the state convergence problems of the resulting edge systems. To represent the two competitive classes of interactions that emerge in signed networks, signed digraphs are adopted and the relevant edge Laplacian matrices are introduced, with which an edge-based distributed protocol is presented. The relation between the edge Laplacian matrix and the structural balance (or unbalance) of a signed digraph is disclosed by taking advantage of properties of undirected cycles. Further, it is shown that for a signed network, the state of its edge system converges to a constant vector, regardless of whether its associated signed digraph is structurally balanced or unbalanced. This result does not need to impose the assumption upon the digon sign-symmetry of the signed digraph that is generally required by the node-based distributed protocols. In particular, the state convergence results of edges can be exploited to handle traditional bipartite consensus problems for the nodes of signed networks. Simulation examples are given to illustrate the effectiveness of the edge-based analysis method proposed for signed networks.

This paper copes with distributed control problems for signed networks that consist of a group of nonlinear agents. A distributed control algorithm is designed by using the nearest neighbor rule. For Lipschitz-type nonlinear dynamics, this algorithm guarantees structurally balanced signed networks to achieve bipartite consensus and structurally unbalanced signed networks to reach state stability, respectively. When bounded nonlinear dynamics are considered, all agents exponentially converge to a definite bound within a finite time, regardless of whether the signed networks are structurally balanced or structurally unbalanced. A Lyapunov approach is simultaneously exploited to carry out the dynamic behaviors analysis of signed networks. Four examples are provided to demonstrate the validity of the developed theoretical results.

This paper deals with the problem on how to algebraically identify structure properties and characterise dynamic behaviours of signed networks with cooperative and antagonistic interactions...

The work addresses the trajectory tracking control problem of surface vessel without yaw angle measurement. A new adaptive output feedback controller is proposed ensuring that position error converges to zero asymptotically. The new proposed control law can guarantee the independence of position tracking error on convergence of yaw angle estimation error with bounded sway velocity and system inputes. The design of proposed controller is based on unknown yaw angle, while previous controller design was based on unknown speed. Simulations are presented to verify the effectiveness of the proposed controller.

This note presents a distributed formation tracking scheme for multiple nonholonomic unicycle robots with any smooth reference trajectory, either feasible or non-feasible. The formation tracking problem is solved via a local tracking control law and a distributed virtual reference trajectory, converting the cooperative formation problem into a local tracking case. Several state transformations, dynamic oscillator approach, graph theory, and Lyapunov's method are combined to design formation tracking control law. We prove that the closed-loop formation tracking errors are globally uniformly ultimately convergent to an arbitrarily small neighborhood of the origin. This convergence does not depend on any persistent excitation (PE) condition of the reference trajectory. The simulation results validate the proposed formation tracking design.

Constrained shortest distance (CSD) querying is one of the fundamental graph query primitives, which finds the shortest distance from an origin to a destination in a graph with a constraint that the total cost does not exceed a given threshold. CSD querying has a wide range of applications, such as routing in telecommunications and transportation. With an increasing prevalence of cloud computing paradigm, graph owners desire to outsource their graphs to cloud servers. In order to protect sensitive information, these graphs are usually encrypted before being outsourced to the cloud. This, however, imposes a great challenge to CSD querying over encrypted graphs. Since performing constraint filtering is an intractable task, existing work mainly focuses on unconstrained shortest distance queries. CSD querying over encrypted graphs remains an open research problem. In this paper, we propose Connor , a novel graph encryption scheme that enables approximate CSD querying. Connor is built based on an efficient, tree-based ciphertext comparison protocol, and makes use of symmetric-key primitives and the somewhat homomorphic encryption, making it computationally efficient. Using Connor , a graph owner can first encrypt privacy-sensitive graphs and then outsource them to the cloud server, achieving the necessary privacy without losing the ability of querying. Extensive experiments with real-world data sets demonstrate the effectiveness and efficiency of the proposed graph encryption scheme.

This paper investigates the tracking control problem of underactuated surface vessels with non-diagonal inertia and damping matrices. Based on the inherent cascaded structure of vessel dynamics and the cascaded system theory, the tacking control problem of vessels is converted to the stabilization control problem of two cascaded subsystems. Two trajectory tracking control laws are designed respectively for known and unknown model parameters. The first feedback control law is derived with the aid of Lyapunov method, realizing the global $${\mathscr {K}}$$ -exponential trajectory tracking of vessels with accurate model parameters. As all the state variables in the first law are independent of model parameters, the sliding mode technique is applied to extend the first control scheme to the case of unknown model parameters, resulting into an another tracking control law, which ensures the global $${\mathscr {K}}$$ -exponential stability of the closed-loop error system despite unknown model parameters. Effectiveness of the proposed controllers is demonstrated by numerical simulations.

In this paper, a novel fractional-order global sliding-mode control scheme is presented. It is first used to stabilise a coupled second-order nonlinear system, and then it is generalised to control...

Summary Global asymptotic stabilization of underactuated surface vessels is generally achieved only by designing complicated controllers. This paper proposes a very simple control law that globally asymptotically stabilizes the position of underactuated surface vessel to a desired constant location and its velocities to zero. The proposed controller is independent of velocity signals and is robust to model parameters. It neither includes an observer nor an adaptive/sliding-mode law. Controller development and stability analysis rely on a novel Lyapunov function and LaSalle's theorem. Furthermore, by extending the proposed control strategy, a saturated control law is also obtained ensuring the semiglobal asymptotic stability of position error system. Effectiveness of the proposed control schemes is demonstrated by simulation examples. Copyright © 2017 John Wiley & Sons, Ltd.

This work investigates a new formation control problem of multiple underactuated surface vessels. The controller design is based on input-output linearisation technique, graph theory, consensus ide...

This study investigates the periodic event-triggered control for the hand position centroid consensus/formation problems of multiple non-holonomic wheeled mobile robots. By constructing invertible input transformation, the dynamics of the hand positions are formulated as two groups of first-order integrators. Then, the event-triggered control laws and the event conditions are proposed. On the basis of Lyapunov theory and the algebraic graph theory, permissible value ranges of the sampling period and the event condition parameters are established, guaranteeing that all the hand positions asymptotically tend to the common initial centroid location or the desired formation shape centred at the initial centroid. In addition, the orientations and the velocity inputs of each robot are ensured to reach some constants and zeros, respectively. Finally, simulations illustrate the efficiency of the proposed theoretical results.

This paper present a new obstacle avoidance algorithm based on 2D-lidar. The algorithm is applied to the problem of reactive navigation for onmi-directional mobile robots in the face of concave obstacles. Two-layer structures are involved in the proposed algorithm. The first layer aims at determining the current exit point. In the second layer, a basic planner is adopted, steering the robot move to the current exit. The core idea of the algorithm is to determine the exit point at each time step. To verify the effectiveness of the proposed algorithm, numerical simulations are presented. The algorithm is compared with potential field method. Results show that the proposed algorithm has good performance especially facing with concave obstacles.

This paper aims to study the distributed robust control problems of signed networks subject to external disturbances. We perform a model transformation of signed networks, by which the consensus problems can be transformed into the stability problems. With robust \(H_\infty \) control theory, we can develop the sufficient conditions for signed networks achieving bipartite consensus (respectively, state stability) with the desired resilient performance when its associated signed digraphs are structurally balanced (respectively, unbalanced). In addition, two simulation examples are proposed to demonstrate the correctness of the theoretical results.

The edge convergence problems have been explored for directed signed networks recently in 2019 by Du, Ma, and Meng, of which the analysis results, however, depend heavily on the strong connectivity of the network topologies. The question asked in this article is: whether and how can the edge convergence be achieved when the strong connectivity is not satisfied? The answer for the case of spanning tree is given. It is shown that if a signed network is either structurally balanced or r-structurally unbalanced, then the edge state can be ensured to converge to a constant vector. In contrast, if a signed network is both structurally unbalanced and r-structurally balanced, then its edge state does not converge to a constant vector any longer, but to a time-varying vector trajectory with a constant speed. Further, the dynamic behavior results of edges can be derived to address the node convergence problems of signed networks. The simulation examples are provided to illustrate the validity of the established edge convergence results.

This paper mainly addresses the problem of tracking a moving target for a nonholonomic mobile robot equipped with an on-board camera. A visual tracking control system consisting of a robust visual tracking algorithm and a robust visual tracking controller is developed. The proposed visual tracking algorithm, namely the depth-based Camshift, possesses robustness against partial occlusion, image noise, deformation and similar-colored objects. Supplied with direct measurements from visual tacking, a robust controller is developed based on the idea of feedback linearization and sliding mode techniques to decouple relative motion equation in kinematic level and overcome unknown motion of the target. Simulation studies and experiments are conducted then to verify the performance of proposed controller and the whole mobile robot tracking scheme.

This paper designs a smooth time-invariant distributed static control law to solve the asymptotic distributed leaderless consensus control problem of nonholonomic chained systems over a communication graph. A novel Lyapunov function is firstly constructed, then a control law is constructed to ensure the Lyapunov function non-increasing. The Lyapunov method, the LaSalle invariance principle, the graph theory, technical lemmas and matrix computations are used to analyze the system stability, overcoming the difficulties caused by high dimension and nonholonomic constraint. The proposed control law is able to guarantee the state vector of the whole systems globally asymptotically converge to a common configuration, under undirected connected communication scenarios. A numerical simulation example is implemented to demonstrate the effectiveness of the control schemes.

This paper proposes a path following controller for the uncoupled planar vertical take off and landing (PVTOL) aircraft via the backstepping procedure. The idea of the path following controller design is finding a virtual target point to be tracked on the desired path and designing control inputs to guarantee the errors between the actual aircraft and virtual point converge to zero. The proposed path following controller enjoys the crucial properties of the path attractiveness and the path invariance (i.e. if the mass center of the PVTOL is initialized on the path and its initial velocity is tangent to the path, then the PVTOL aircraft remains on the path at all future time). In the end of this article, several numerical simulations are shown to verify the effectiveness of the proposed nonlinear state feedback controller.

This study is concerned with the stabilisation problem for a class of Caputo type fractional-order interval systems with perturbation. Employing Riemann–Liouville fractional integral sliding surface, a novel robust sliding mode control law is established to drive the dynamics of the system to the manifold s=0 in finite time. Based on linear matrix inequality criterions and stability theorems, the closed-loop system will asymptotically converge to the origin as time progresses. Meanwhile, the unknown perturbation is well adjusted on-line by the designed adaptive law. Furthermore, a new reaching law is introduced to reduce the chattering which is caused by the discontinuity of the switching function, and to improve the robustness and the stability of system. Besides, some results about the control and stabilisation of fractional-order interval systems are illustrated in this study; several comparisons with the related works are given to reveal the potential advantages of the proposed controller over the previous results. Finally, an example with numerical simulations is provided to show the validity and feasibility of the proposed method.

BACKGROUND: Men who have sex with men (MSM) are highly vulnerable to human immunodeficiency virus (HIV) infection and more likely to migrate due to widespread stigma and discrimination in China. Their mobility complicates estimation of local MSM population sizes and the provision of HIV services and may also contribute to the spread of HIV. METHODS: Between 1 January 2008 and 31 December 2012 the visits of all individuals to the largest Chinese MSM dating website were recorded. After a predesigned de-identification procedure by the website we analyzed Internet Protocol addresses for migration patterns. Migrants were defined as individuals who were away from their registered residence for >6 months in the last 12 months. RESULTS: The website contained data on 794 912 MSM eligible for the study of which 34.5% were migrants. The median age was 26 years (range 18-61 years) and 85.5% were unmarried. Compared with nonmigrant MSM migrants were less likely to be married to a woman (8.6% vs 13.5%; P < .001). The 5 provinces with the highest migrant inflow ratios were Guangdong Shanghai Beijing Tianjin and Zhejiang. Eastern coastal cities were the primary destination of MSM from southwestern China. CONCLUSIONS: Preferential MSM migration may influence MSM population sizes in both originating and destination provinces particularly for provinces with uneven inflow and outflow. MSM migration from southwestern China which has the highest HIV prevalence in this population to coastal cities with lower prevalence may have implications for the spread of the HIV epidemic as well as HIV care services. (c) The Author 2016. Published by Oxford University Press for the Infectious Diseases Society of America. All rights reserved. For permissions e-mail journals.permissions@oup.com.