Rodney Swee Huat Teo

Distributed Adaptive Integrated-Sliding-Mode Controller Synthesis for String Stability of Vehicle Platoons

This paper presents a distributed finite-time adaptive integral-sliding-mode (ISM) control approach for a platoon of vehicles consisting of a leader and multiple followers subjected to bounded unknown disturbances. In order to avoid collisions among the vehicles, control protocols have to be designed to ensure string stability of the whole vehicle platoon. First, the constant time headway (CTH) policy known to improve string stability is applied to the case of zero initial spacing errors. Contrary to requiring zero initial spacing and zero initial velocity errors simultaneously in existing methods based on constant spacing (CS) policy, initial velocity errors here are not required to be zero. Then, since string stability condition can fail at the initial conditions, a modified CTH policy is constructed to overcome string instability caused by nonzero initial spacing errors. Moreover, the proposed adaptive ISM control schemes can be implemented without the requirement that the bounds of the disturbances be known in advance. In addition, one effective method is proposed to reduce the chattering phenomenon caused by the indicator function. Finally, simulation results are included to demonstrate its effectiveness and advantages over existing methods.

Distributed Adaptive Sliding Mode Control Strategy for Vehicle-Following Systems With Nonlinear Acceleration Uncertainties

This paper investigates the distributed adaptive control problems for nonlinear vehicle-following systems subject to nonlinear acceleration uncertainties involving vehicle acceleration disturbances, wind gusts, and parameter uncertainties. It is worth mentioning that the acceleration of the leader in most existing studies is always assumed to be zero or constant; the evolution of the leader in this paper may be subject to some unknown bounded input. Distributed adaptive control strategies based on an integral sliding mode control (ISMC) technique are proposed to maintain a rigid formation for a string of vehicle platoon in one dimension. First, distributed adaptive control based on traditional constant time headway (TCTH) policy under the assumption that the initial spacing and velocity errors are zero is developed to guarantee that all spacing errors are uniformly ultimately bounded and that the string stability of the whole vehicle platoon is also satisfied. Then, a modified constant time headway (MCTH) policy is proposed to remove the assumption of zero initial spacing and velocity errors and simultaneously effectively decrease the intervehicle spacing (i.e., increase the traffic density), making them nearly equal to those by using the constant spacing (CS) policy. Adaptive compensation terms without requiring a prior knowledge of upper bounds of the uncertainties are constructed to compensate for the time-variant effects caused by nonlinear acceleration uncertainties. Finally, numerical simulation results show the validity and advantages of the proposed policy up to a significant higher traffic density.

A survey on recent progress in control of swarm systems

It has been witnessed that swarm systems are superior to individual agents in performing complicated tasks. In recent years, new results in some branches of control for swarm systems have developed and investigated with respect to various objectives and scenarios. This survey is to take a glimpse into some newly developed control techniques for swarm systems, especially those presented after 2013. The covered topics include some up-to-date progress in the areas of consensus, formation, flocking, containment, optimal coverage/mission planning, and sensor networks. Contributions and connections of the mentioned references are discussed briefly. Based on the new results in control of swarm systems, some possible new future research topics are suggested.

Distributed Formation and Reconfiguration Control of VTOL UAVs

In this brief, a novel distributed cascade robust feedback control approach is proposed for formation and reconfiguration control of a team of vertical takeoff and landing (VTOL) unmanned air vehicles (UAVs). This approach is based on dynamic communication network. It guarantees intervehicle collision avoidance and considers dynamic constraints of UAVs. In the outer loop of the cascade formation control, a potential field approach is used to generate a desired velocity for each UAV, which ensures that the team of UAVs can perform formation flying, formation rotating and reconfiguration, avoid intervehicle collision, as well as track a specified virtual leader. In the inner loop of the cascade formation control, the velocity of each UAV is designed to track its desired velocity generated by the outer loop, subject to dynamic constraints. The proposed approach is demonstrated via both simulation and flight test.

Decentralized Multi-UAV Flight Autonomy for Moving Convoys Search and Track

This brief is concerned with integrated autonomous takeoff, target search, task assignment, and tracking using multiple fixed-wing unmanned aerial vehicles (UAVs) in urban environments. The problem is to design flight autonomy that is embedded onboard each UAV to enable autonomous flight coordination and distributed tasking. Control logic design based on a finite state automaton model, integrating four modes of operations, namely, the takeoff mode, the fly-to-area of operation mode, the search mode, and the tracking mode, is presented. Different from the state-of-the-art of recent research, this brief provides a preliminary research on the autonomous cooperative takeoff for miniature fixed-wing UAVs, by considering collision avoidance, communication failure, etc. To make UAVs autonomously and cooperatively search roads in the urban environments, an efficient improved search algorithm is proposed based on recent research on the coverage search in the literature. For the target tracking, using geometric relations (relative position, orientations, speed ratio, and minimal turning radius), a systematic algorithm is developed to generate an optimal online path. All the algorithms in this work are developed based on realistic miniature fixed-wing UAV dynamic models. The main focus of the brief is to test the developed control logic and also the algorithms. The proposed framework is evaluated by our 3-D multi-UAV test bed.

Decentralized search, tasking and tracking using multiple fixed-wing miniature UAVs

This paper is concerned with integrated target search, tasking and tracking using multiple fixed-wing UAVs. The problem is to design control logic and plan the flight paths for UAVs. The fixed-wing UAVs are required to cooperatively search the potential targets and keep monitoring the found targets according to a predefined minimal revisit time. Each UAV can only communicate with its neighbors and also flight autonomy is designed for individual UAVs. Decentralized target search, task assignment and target tracking algorithms are developed and evaluated by simulations using a real miniature fixed-wing UAV model.

Decentralized Control of Multi-UAVs for Target Search, Tasking and Tracking

Abstract This paper considers integrated target search, tasking and tracking using multiple fixed-wing UAVs in urban environments. The problem is to design autonomy for each individual UAV autonomous and distributed tasking. Control logic design based on finite state automaton (FSA) model, integrating the four modes of operations, i.e., takeoff mode, fly-to-AO (area of operation) mode, search mode and tracking mode, is developed. An efficient distributed multi-UAV target search algorithm is also presented. UAV guidance and control is built based on combined urban road map and target detection probability map information. For target tracking, by using geometric relations (relative position, orientations, speed ratio, and minimal turning radius), a systematic algorithm is developed to generate an optimal path online for a fixed-wing UAV to track a moving target. In addition, control method for a group of UAVs to keep track a target convoy is also addressed. Finally, the proposed decentralized algorithms are evaluated by simulations adopting a real UAV model.

Ultra-wideband based cooperative relative localization algorithm and experiments for multiple unmanned aerial vehicles in GPS denied environments:

This article puts forward an indirect cooperative relative localization method to estimate the position of unmanned aerial vehicles (UAVs) relative to their neighbors based solely on distance and self-displacement measurements in GPS denied environments. Our method consists of two stages. Initially, assuming no knowledge about its own and neighbors’ states and limited by the environment or task constraints, each unmanned aerial vehicle (UAV) solves an active 2D relative localization problem to obtain an estimate of its initial position relative to a static hovering quadcopter (a.k.a. beacon), which is subsequently refined by the extended Kalman filter to account for the noise in distance and displacement measurements. Starting with the refined initial relative localization guess, the second stage generalizes the extended Kalman filter strategy to the case where all unmanned aerial vehicles (UAV) move simultaneously. In this stage, each unmanned aerial vehicle (UAV) carries out cooperative localization through the inter-unmanned aerial vehicle distance given by ultra-wideband and exchanging the self-displacements of neighboring unmanned aerial vehicles (UAV). Extensive simulations and flight experiments are presented to corroborate the effectiveness of our proposed relative localization initialization strategy and algorithm.

String stability of heterogeneous leader-following vehicle platoons based on constant spacing policy

This paper is concerned with a leader-follower problem for a heterogeneous vehicle platoon subject to external bounded unknown acceleration disturbances. Distributed controller based on sliding mode control (SMC) approach are designed for the second-order follower-vehicles under the common assumption that the initial spacing and velocity errors are zero. The constant spacing policy known to have high traffic density and thus have high traffic flow is applied to design distributed controller. In addition, adaptive compensation technique is applied to compensate the time-varying effect of external disturbances. It is worth mentioning that the upper and lower bounds of the disturbances are not required to be known in advance. Furthermore, with the help of an explicitly constructed Lyapunov function, it is proved that the string stability of the vehicle platoon can be guaranteed. At the same time, the reduction of the chattering in sliding mode is achieved by introducing continuous function in control. Finally, a numerical example is given for illustration.

ROS+unity: An efficient high-fidelity 3D multi-UAV navigation and control simulator in GPS-denied environments

In this paper, we will introduce our newly developed 3D simulation system for miniature unmanned aerial vehicles (UAVs) navigation and control in GPS-denied environments. As we know, simulation technologies can verify the algorithms and identify potential problems before the actual flight test and to make the physical implementation smoothly and successfully. To enhance the capability of state-of-the-art of research-oriented UAV simulation system, we develop a 3D simulator based on robot operation system (ROS) and a game engine, Unity3D. Unity3D has powerful graphics and can support high-fidelity 3D environments and sensor modeling which is important when we simulate sensing technologies in cluttered and harsh environments. On the other hand, ROS can provide clear software structure and simultaneous operation between hardware devices for actual UAVs. By developing data transmitting interface and necessary sensor modeling techniques, we have successfully glued ROS and Unity together. The integrated simulator can handle real-time multi-UAV navigation and control algorithms, including online processing of a large number of sensor data.