Abstract
Co-operative Adaptive Cruise Control (CACC) uses wireless communication between vehicles to form a vehicle platoon. It has been proven that CACC aids in the stability and efficiency of traffic flow, and also reduces the energy consumption of the vehicles by lowering air resistance of the following vehicles. However, communication delay is a natural uncertainty for a platoon under CACC mode. When the delay gets worse to a particular level, the string stability of the platoon will be destroyed, meaning spacing or speed errors will be amplified upstream from vehicle to vehicle in the platoon. Therefore, in this study, a numerical method is proposed to identify the communication delay boundary according to the vehicle dynamics and the parameter settings of the CACC controller. First, in the Laplace domain, a series of string stable bodies under different disturbance frequencies are created based on the string stability criteria. Considering the frequency characteristics of traffic flows, the string stable bodies are then combined into a uniform one. Lastly, with the increase of communication delay, its upper bound will be obtained until the area of string stability totally disappears. Two simulations were conducted to verify the correctness and accuracy of the proposed method. The results indicate that the string stability cannot be guaranteed once the communication delay just exceeds the upper bound. This study can provide an objective boundary for practitioners or engineers, thereby allowing them to determine whether wireless inter-vehicle communication is capable of maintaining a string-stable platoon.
Highlights
In the last decade, the e-commerce industry worldwide has developed rapidly and the logistics industry is becoming increasingly prosperous
A homogenous platoon working on the Co-operative Adaptive Cruise Control (CACC) mode is shown in Fig. 1, where di is the gap between vehicle i and its preceding vehicle i-1; di+1 is the gap between vehicle i+1 and its preceding vehicle i; vi+1, vi, and vi−1 are the speeds of vehicle i+1, i, and i-1, respectively; and vi · td + Gmin is the desired gap between vehicle i and its preceding vehicle i-1
A 3D stable body was established according to the current controller settings and vehicle dynamics
Summary
The e-commerce industry worldwide has developed rapidly and the logistics industry is becoming increasingly prosperous. Using inter-vehicle wireless communication, ACC has evolved into cooperative adaptive cruise control (CACC). The performance of the platoon will decrease dramatically At this time, the control model of the CACC should be shifted to an ACC model to avoid instability and inter-vehicle collisions. The major contribution of this work is the proposal of a numerical method to identify the communication delay boundary that guarantees string stability of a platoon. We verified the proposed numerical analysis results using realistic simulations, in which the string stability of the vehicle platoons was tested under different levels of communication delay by applying various metrics, including the speed, speed error, spacing error, and headway time. The remainder of this paper is organized as follows: Section II presents related studies on the impact of a communication delay on CACC longitudinal control.
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