Analysis and design of controllers for cooperative and automated driving (2014)

Analysis and design of controllers for cooperative and automated drivingThe limited capacity of the road network has become an important factor in meeting the road transport demand.In addition, an increasing societal demand exists to reduce fuel consumption and emissions, and to improve traffic safety.Road capacity can be increased by improving the traffic efficiency, as an overall measure for throughput and time of travel, which is determined by the interaction between vehicles rather than by the characteristics of the individual vehicle and its driver.Although fuel efficiency and traffic safety can still be enhanced by optimizing the individual vehicle, acknowledgement of the fact that this vehicle is part of a traffic system creates new possibilities for further improvement of these aspects.To address this traffic-system approach, the field of Intelligent Transportation Systems (ITS) emerged in the past decade.A promising ITS application is provided by Cooperative Adaptive Cruise Control (CACC), which allows for automatic short-distance vehicle following using intervehicle wireless communication in addition to onboard sensors.The CACC system is subject to performance, safety, and comfort requirements.To meet these requirements, a CACC-equipped vehicle platoon needs to exhibit string-stable behavior, such that the effect of disturbances is attenuated along the vehicle string, thereby avoiding congestion due to so-called ghost traffic jams.The notion of string stability is, however, not unambiguous in the literature, since both stabilitybased and performance-based interpretations for string stability exist.Therefore, in this thesis, a novel string stability definition of nonlinear cascaded systems is proposed, based on the notion of input-output stability.This definition is shown to characterize well-known string stability conditions for linear cascaded systems as a special case.Employing these conditions, the string stability properties of a CACC system using information of the directly preceding vehicle are analyzed.Motivated by the proposed conditions for string stability of linear systems, a controller synthesis approach is developed that allows for explicit inclusion of the string stability requirement in the design specifications, thus preventing a v vi SUMMARY posteriori controller tuning to obtain string-stable CACC behavior.The potential of this approach is illustrated by its application to the design of controllers for CACC for a one-vehicle and a two-vehicle look-ahead communication topology.As a result, string-stable platooning strategies are obtained in both cases, also revealing that the two-vehicle look-ahead topology is particularly effective at a larger communication delay.To validate the theoretical analysis, a prototype CACC system has been developed and installed in a platoon of six passenger vehicles.Experiments performed with this setup clearly show that the practical results match the theoretical analysis, thereby illustrating the practical feasibility for automatic short-distance vehicle following.At the same time, however, the experiments clearly indicate the need for graceful degradation mechanisms, due to the fact that wireless communication is subject to impairments such as packet loss.To address this need, a control strategy for graceful degradation is proposed to partially maintain the string stability properties of CACC in case of a failure of the wireless link.The development of driver assistance systems, among which CACC, is supported by hardware-in-the-loop experiments.In such experiments, a test vehicle is placed on a roller bench, whereas wheeled mobile robots (WMRs) represent other traffic participants.These WMRs are overactuated, due to the fact that they have independently driven and steered wheels.Consequently, the WMRs can be regarded as automated vehicles, albeit with features far beyond those of nowadays road vehicles.To achieve a high degree of experiment reproducibility, in this thesis, focus is put on the design of an accurate position control system for the overactuated WMRs, taking the tire slip into account.A position controller based on input-output linearization by static state feedback is presented, using the so-called multicycle approach, which regards the robot as a set of identical unicycles.The controller thus aims for motion coordination of the four driven and steered wheels, such that a shared objective, i.e, trajectory tracking of the WMR body, is satisfied.In this sense, the control problem is conceptually similar to the aforementioned platoon control problem, in which also coordinated behavior of multiple systems is pursued.Practical experiments with the designed controller indicate that the WMR is capable of accurately following a desired spatial trajectory, thus allowing reproducible testing of intelligent vehicles in a controlled environment.Summarizing, this thesis focusses on the analysis and the design of controllers for cooperative and automated driving, both theoretically and experimentally.As an important result, it can be concluded that short-distance vehicle following by means of CACC is technically feasible, due to, firstly, the availability of lowlatency wireless communication technologies, and, secondly, fundamental insight into the mechanism of disturbance propagation in an interconnected vehicle string.A prerequisite, however, is that graceful degradation strategies are implemented to cope with wireless communication impairments such as packet loss.Consequently, safety-critical cooperative driving applications require a thorough development process, to which end advanced hardware-in-the-loop test facilities are currently available.