Abstract
Active Front Steering (AFS) systems are known to yield enhanced vehicle handling performance through application of a corrective steer angle to realize a target vehicle response over a wide range of forward speed. An AFS system, however, may yield limited performance under severe steering maneuvers involving substantial lateral load shift and saturation of the inside tire-road adhesion. The adhesion available at the outer tire, on the other hand, would remain under-utilized. This study explores effectiveness of an Active Independent Front Steering (AIFS) system that could introduce a corrective measure at each wheel in an independent manner. Unlike the AFS, the AIFS system can thus permit maximum utilization of the tire-road adhesion at both the steered wheels without approaching saturation.
The effectiveness of the AIFS system is investigated through simulation of a nonlinear yaw plane model of a two-axle truck with limited roll degree-of-freedom (DOF) under a range of steering maneuvers. A simple PI controller is synthesized to track the reference response based on the neutral steer system. Considering instantaneous load shift between the steered wheels, controller applies relatively higher steering correction to the outer wheel with higher normal load compared to the inside wheel subjected to lower normal load. The steering corrections, however, are limited such that none of the tires approach saturation. For this purpose, a tire saturation zone is identified considering the normalized cornering stiffness property of the tire. The responses are evaluated in terms of work-load of the inside tire and the controller strategy is formulated so as to limit the work-load magnitude at a pre-determined level to ensure sufficient tire-road adhesion reserve to meet the braking demand, when exists.
Simulations results are obtained for the truck models integrating AFS and AIFS systems subjected to a range of steering maneuvers, namely: a J-turn maneuver on uniform as well as split-μ road conditions, and path change and obstacle avoidance maneuvers. The directional responses in terms of yaw rate, path trajectory, steer angles and tire work-load are discussed to illustrate the effectiveness of AIFS relative to the AFS system. The responses are also compared with those of the model without an active steering control. It is shown that both AFS and AIFS can effectively track the target yaw rate of the vehicle, while the AFS exhibits limited performance due to saturation of adhesion at the inner tire particularly under severe maneuvers. The AIFS, however, helps limit saturation of the inside tire by reducing its steer angle and permits maximum utilization of the available tire-road adhesion of the outside tire by increasing its steering correction. The AIFS can thus be designed to provide target response while maintain a desired level of tire-work load over the entire range of speed up to the vehicle rollover limit.
KEYWORDS – active steering, active independent front steering (AIFS) system, tire work-load, tire saturation, handling performance