String Stability Analysis of Bidirectional Adaptive Cruise Control

Automatic vehicle following control is an interesting topic for research. This is because it is a solution to increase the traffic throughput and therefore decrease traffic congestions. This is accomplished by decreasing the inter-vehicle distances at the highest possible vehicle velocity. Human drivers are not able to drive safely at such short distances. Therefore, an automatic distance controller is required. Currently used adaptive cruise control (ACC) systems are mainly focussed on driving comfortably. It is expected that a massive use of these systems results in even more traffic jams. The reason for this is that ACC cannot cope with shockwaves that disrupt the traffic throughput. In literature, the attenuation of shockwaves is denoted with "String Stability". A more advanced ACC, cooperative adaptive cruise control (CACC) is especially designed to increase the traffic throughput and reducing the shockwave effect. This vehicle following system not only depends on a distance sensor, but uses a wireless network to communicate with other CACC instrumented vehicles. The focus of this thesis is to analyse the performance of bidirectional adaptive cruise control (BiACC) using simple vehicle models. The performance is analyzed on the basis of two criteria. The first (main) criterium is string stability as a function of the headway time. The second (additional) criterium is the transient response in terms of rise time, settling time and overshoot. BiACC does not use wireless communication, but uses an additional distance sensor to monitor the intervehicle distance to the follower vehicle. BiACC has two main advantages: Firstly, the bidirectional structure can result in a better attenuation of the shockwave effect with respect to ACC, i.e. improved string stability performance. Secondly, the string coherence property of a bidirectional vehicle string can prevent the string of falling apart. BiACC must not been seen as a direct competitor to CACC. The string stability performance of BiACC cannot compete against the string stability performance of CACC, because it lacks wireless communication. It must merely be seen as a base for more advance bidirectional CACC (BiCACC) that combines the string coherence of BiACC with the string stability performance of CACC. The design of a BiACC consists of a forward ACC and a backward ACC. The influence of the backward term is varied by a weighting factor. A headway time policy is used to define the desired distances to the preceding vehicle and to the follower vehicle. This feedback control design is ideal to show the strength of backward control in addition, because is has great similarities to ACC. A string stability definition in the z-domain is used to determine string stability of BiACC. This definition defines string stability of an infinite vehicle string by using a bilateral z-transformation. From this, it can be concluded that the minimum headway time for string stability decreases with increasing weighting factor. The minimum headway time can be decreased with 30% with respect to an ACC vehicle string if a weighting factor is used near one. Since the z-transformation is only valid for infinite vehicle strings, the string stability of a finite string is determined by using a string stability definition in the frequency domain. This is only possible because the bidirectional vehicle string can be rewritten to an unidirectional vehicle string. The frequency domain analysis shows that the minimum headway time for string stability decreases with 25% if a vehicle string of ten vehicles is used. For even longer vehicle strings it approaches the minimum headway time for infinite vehicle strings. The performance of BiACC is analyzed by studying the transient velocity response. The overshoot, rise time and settling time are compared to ACC and CACC. By choosing an appropriate combination of a headway time and a weighting factor comparable performance to CACC can be achieved. This is only possible for a headway time that is larger than the minimum required headway time for string stability.