Grip optimization in critical braking and cornering maneuvers using semi-active and active road vehicle suspensions
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Examensarbete för masterexamen
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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.
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Ämne/nyckelord
semi-active suspension, fully-active suspension, damper, LQG control, grip, braking