Grip optimization in critical braking and cornering maneuvers using semi-active and active road vehicle suspensions

Abstract

It is essential to optimize road holding in passenger cars for improving vehicle safety, especially in emergency situations. There are several factors that affect road holding: The material and texture of the tire contacting area with road; the vertical force pressing the contacting surface; and the friction of the road surface, et cetera. This study focuses on the in uence of tire vertical force variation on the tire grip. Due to the lack of available sensors for tire vertical force, instead suspension system is controlled to affect the vertical tire force. By affecting ride comfort and handling performance, suspension is an important system that can ensure driving comfort by providing a sense of stability. Active and semi-active suspensions are nowadays often used to provide higher road grip since they can actively adjust damping force for different control objectives. This thesis investigates how both active and semi-active suspensions can be controlled to minimize the tire vertical force variation, and accordingly, improve the road grip. The main control objectives here are to minimize the tire vertical force variation and to improve the horizontal tire force generation ability. The tire performance in evasive maneuvers such as ABS braking is also discussed in this thesis. To simplify the problem, a quarter-car model with 2 degrees of freedom is developed and serves as the plant for control design. Three control strategies are developed and discussed: LQG control, active curve fitting control and compression maximization control. The first two controllers are separately tuned and applied to both active and semi-active suspensions, while the third one is only used for the semi-active damper. The performance of the developed controllers are evaluated in simulation and compared with passive suspensions. The overall improvements on the minimization of tire vertical force variation and the reduction of the braking distance are achieved. The tire grip can be considered improved in the case of emergency braking with only longitudinal maneuvers. Validation of the controlled systems performance for lateral tire force generation, and the driving test on real road, as well as further improvement of the control systems remain as future tasks.