String stability of interconnected systems: An application to platooning in automated highway systems

Automated Highway System (AHS) is primarily aimed at improving the traffic flow capacity of the highways while ensuring safety. Central to successful deployment of AHS is the development of Automated Vehicle Control Systems (AVCS). The longitudinal control aspect of AVCS deals with automatically controlling the intervehicular spacing of close-vehicle formations called platoons. This dissertation investigates various platooning strategies and their impact on the performance of the platoon.;String stability of a vehicle platoon is the primary performance parameter. Intuitively, string stability of a vehicle platoon ensures that the intervehicular spacing errors of all the vehicles are bounded uniformly in time provided the initial spacing errors of all the vehicles are bounded. In this dissertation, we design various decentralized control algorithms and characterize their performance in terms of the minimum attenuation of the maximum spacing errors that can be guaranteed from vehicle to vehicle in the platoon.;Parametric uncertainties degrade the platoon performance. In order to improve the robustness of a string stable control algorithm, a direct adaptive control algorithm that guarantees improved performance is designed.;The concept of string stability is extended to general nonlinear dynamical systems. We derive sufficient conditions for ensuring stability for a countably infinite interconnection of exponentially stable nonlinear systems. We also show that under the same conditions, string stability is preserved for structural and singular perturbations. Then, we present a decentralized adaptive controller to improve the robustness in the presence of parametric uncertainties for the same class of systems.;The contributions of this dissertation are twofold: From an application point of view, this dissertation proposes "practical" platooning strategies. From a theoretical point of view, this study extends the concepts of stability to a countably infinite interconnection of general nonlinear dynamical systems and introduces techniques for analysis and design of decentralized control laws for them.