  Current factory adaptive cruise control (ACC) systems consist of an
upper-level planner that computes the optimal trajectory of the vehicle and a
low-level controller in charge of executing it. The literature on the string
stability (SS) of ACC systems is mostly concerned with the upper-level planner.
In this paper, we show that the low-level controller can have a significant
impact on SS. We find that i) a fast controller improves the SS, ii) a slow
controller can result from either insufficient control gains or from an
undershooting gas/brake system or both, and iii) although the integral gain is
helpful for effective control, the integral windup can cause overshooting, and
in turn, undermines the SS of the ACC. Accordingly, we suggest tuning up the
proportional/feedforward gain and ensuring the gas/brake is not to undershoot.
An Anti-windup design is also needed to improve SS. The results of this paper
are validated both numerically and empirically on commercial cars.
