Memory storage to minimize string stable time gaps of cooperative ACC with realistic communication delays

Abstract

Cooperative adaptive cruise control (CACC) employs wireless communications to safely drive at short inter-vehicle distances, which improves traffic efficiency. The communication performance of the direct PC5 technology, such as communication delay should be carefully analyzed. Both the safety and efficiency benefits require string stability, which refers as the attenuation of the effects of disturbances in upstream direction. However, communication delay significantly compromises string stability, since CACC is heavily relying on communicated information. There is a minimum time gap requirement to guarantee string stability. To take full advantages in view of road throughput, a memory strategy with respect to the radar-measured preceding vehicle velocity is integrated to a well-developed CACC algorithm. The constant time gap policy is adapted to follow the past preceding vehicle position. By choosing the memorized time period exactly the same as communication delay, the effect of communication delay on string stability can be perfectly compensated. As a cost, there is an added inter-vehicle time gap, which is due to the actual distance of the preceding having driven in the time window of communication delay. As a result, the final minimum string-stable time gap equals to the value of communication delay. The feasibility of the memory strategy on CACC is explicitly analyzed. With the proposed control scheme, the minimum string-stable time gap can be significantly decreased. The results are validated using simulations with time varying communication delays.