String stability of relative-motion PID vehicle control systems

An important performance criterion for moving vehicle longitudinal control systems is string stability in the face of perturbations in the motion of individual vehicles. Systems employing a moving-cell reference for each vehicle always exhibit string stability since there is no vehicle interaction within the string. This correspondence investigates the string stability for a class of relative-motion systems where a moving-cell position reference is not available. It is shown that string stability can be achieved only by using both forward and rearward intervehicle separation measurements.