Effects of Communication Delay on String Stability in Vehicle Platoons

The throughput of vehicles on highways can be greatly increased by forming vehicle platoons with small inter-vehicle spacings. The control law that maintains stable operation of a platoon is dependent on the lead and preceding vehicle’s position, velocity and acceleration profiles. These profiles guarantee string stability of a platoon and are transmitted via wireless communication networks. The safety critical nature of the automated highway system requires the information exchange over the communication networks to be reliable and timely. However, communication networks generally introduce delays and drop packets. These communication faults are not typically taken into account in controller designs. In this paper, we examine the robustness of current longitudinal controller designs to communication delays. The results show that string stability is seriously compromised by communication delays introduced by the network when the controllers are triggered by the receipt of either the lead vehicle information or the preceding vehicle information. We find that when all the vehicles are synchronized to update their controllers at the same time, string stability can be maintained if the delay in preceding vehicle information is small. An upper bound on the preceding vehicle information delay is derived through a simple partial fraction expansion approach. Our results indicate a need for controller designs that are robust to communication delays and packet drops.