Cooperative Adaptive Cruise Control: Network-Aware Analysis of String Stability

Oncu, Sinan; Ploeg, Jeroen; van de Wouw, Nathan; Nijmeijer, Henk · 2014 · OpenAlex-citations

DOI: 10.1109/tits.2014.2302816

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Summary

This paper addresses the challenge of maintaining string stability in Cooperative Adaptive Cruise Control (CACC) systems, where vehicles in a platoon communicate wirelessly to regulate intervehicle distances. While CACC improves traffic throughput and safety over traditional Adaptive Cruise Control (ACC), wireless communication introduces network-induced imperfections, such as transmission delays, sampling effects, and zero-order hold artifacts, due to limited bandwidth and shared media. The authors argue that these network effects are critical to system performance but are often overlooked in existing literature. The research aims to provide a network-aware analysis framework to study the tradeoffs between CACC performance and network specifications, specifically focusing on how communication delays impact the attenuation of disturbances along a vehicle string. The study employs a Networked Control System (NCS) perspective to model the interconnected vehicle string. The authors develop a discrete-time interconnected system model that incorporates longitudinal vehicle dynamics, the CACC controller, and network-induced effects. The vehicle dynamics are represented by a linearized third-order state-space model including actuator delays, which are approximated using Padé approximations to maintain finite-dimensional models. The control structure combines a local proportional-derivative feedback controller (ACC) using sensed data with a feedforward controller (CACC) that utilizes wirelessly transmitted acceleration data from the preceding vehicle. The analysis focuses on a homogeneous vehicle string with a constant time headway spacing policy. String stability is evaluated using frequency-response-based analysis, quantified by the H-infinity norm of the string stability transfer function, which measures the amplification of disturbances (such as distance error or acceleration) as they propagate through the platoon. The main findings demonstrate that network imperfections, particularly time delays, significantly affect string stability. The authors derive conditions for string stability and determine maximum allowable time delays for various controller and network parameters. The analysis reveals that while CACC can improve performance, the benefits are constrained by the reliability and latency of the wireless communication link. The theoretical framework is validated through experiments conducted on a test track using CACC-equipped prototype vehicles. These experiments confirm the validity of the discrete-time NCS model and the derived stability conditions, showing that the predicted tradeoffs between control performance and network delays hold in practical scenarios. The significance of this work lies in its contribution to the multidisciplinary design of CACC systems. By explicitly modeling network effects, the paper provides designers with guidelines for balancing control specifications with communication constraints. This approach ensures that the implementation of CACC in real-world traffic conditions remains robust against the inevitable imperfections of wireless networks. The results underscore the necessity of considering network-aware modeling in the development of Intelligent Transportation Systems to prevent the amplification of disturbances that can lead to traffic shockwaves and jams.

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• v2x connected vehicle