Platoon String Stability: A Passivity Perspective

Abstract

String stability is a vital property of vehicle platoons which ensures disturbances of system states are not amplified along the string of vehicles. In this paper, we discuss string stability in the frequency domain showing the platoon as a cascade of linear time-invariant subsystems. We show that a sufficient condition for string stability is that the Nyquist plot of the transfer function of each subsystem is within the unit circle. Also, an energy-based notion of string stability based on passivity is derived by showing output strict passivity as a bridge between passivity and string stability of a vehicle platoon. By constraining the output strict passivity index of each subsystem (vehicle) in a string to one or greater than one through proper control strategy, the condition for string stability is met. This is validated through numerical simulations.