New paradigms for the integration of yaw stability and rollover prevention functions in vehicle stability control

The integration of rollover prevention and yaw stability control objectives in electronic stability control (ESC) has traditionally been done based on a priority calculation. The control system nominally focuses on yaw stability control until a danger of rollover is detected. When a danger of rollover is detected, the control system switches from yaw stability control to rollover prevention. This paper focuses on an integrated ESC system wherein the objectives of yaw stability and rollover prevention are addressed simultaneously, rather than one at a time. First, we show that staying on a desired planar trajectory at a specified speed results in an invariant rollover index. This implies that rollover prevention can be achieved whenever there is a danger of rollover only by reducing vehicle speed, since changing the desired vehicle trajectory is not a desirable option. In this regard, it is shown that a vehicle that reduces its speed before entering a sharp curve performs significantly better than a vehicle that uses differential braking during the turn for yaw stability control. Second, this paper explores how the use of steer-by-wire technology can address the tradeoff between yaw stability, speed, and rollover prevention performance. It is shown that the use of traditional steer-by-wire simply as an additional actuator cannot by itself ameliorate the tradeoff. However, this tradeoff can be eliminated if steer-by-wire is used to invert the direction of the roll angle of the vehicle. A new steer-by-wire algorithm that uses transient countersteering is shown to change the location of the rollover dynamics from the neighborhood of an unstable to a stable equilibrium. In this case, a desired trajectory can indeed be achieved by the vehicle at the same speed with a much smaller danger of rollover. This is a novel and viable approach to integrating the yaw stability and rollover prevention functions and eliminating the inherent tradeoffs in the performance of both.