String stability of homogenous vehicle platoons based on cooperative extended state observers

We study platoon control of homogeneous vehicles with linear third-order longitudinal dynamics with the constant time headway (CTH) policy. In order to ensure string stability with a small time headway, a distributed control law based on extended state observers is proposed. The controller of each follower vehicle only depends on its own velocity, acceleration, inter-vehicle distance and velocity difference with respect to its immediate predecessor. First, a dynamic model based on velocity differences between adjacent vehicles is established. Then cooperative extended state observers are designed to estimate the acceleration differences between adjacent vehicles, based on which distributed cooperative controllers are designed. By analyzing the transfer function of inter-vehicle distances errors, the sufficient conditions to ensure string stability are presented. It is shown that for any given positive time headway, the parameters of distributed cooperative state observers and controllers can be properly designed so that the inter-vehicle distance errors are not amplified during the backward propagation along the platoon. The effectiveness of the cooperative control law is demonstrated by simulations.