Extreme Conditions steering instability and active control of nonlinear dynamic characteristics of

With the development of senior freeway and the increasing pace of our society,the increasing vehicle speed brings about great threat to Our life safety．It is a crucial issue for the vehicle industry and academic research to control vehicle cornering stability in critical situations．It is has been recognized that to increase vehicle handling and stability by AFS(Active Front Steering)，4WS(Four Wheel Steering)，ESP(Electronic Stability Program)etc．is the direction of the development of vehicle chassis contr01．This thesis focuses on the issue of increasing vehicle cornering stability in critical situations．111e vehicle cornering dynamics in critical situations where the tyre force approaches saturation has great difference、析tll that of in the general situations．It is a bad need to investigate vehicle cornering performance in critical situations througth nonlinear dynamics theory in order to provide more precise information to vehicle stability control system． 111e motivation of this thesis is to investigate the nonlinear vehicle cornering dynamics in critical situations．This research consists of the bifurcation in steady—state cornering，the dynamic mechanism elaboration of vehicle comering destabilization based on state manifold theory,the forecast of the vehicle steady-state cornering destabilization and the application in vehicle cornering stability control，the robust control of vehicle cornering stability facing great uncertainty parameters． The fundaments of vehicle cornering dynamics have been studied broadly． concepts and the principles of understeer gradient handling diagram used to analyze and evaluate vehicle handling and stability performance have been discussed．The vehicle cornering stability is analyzed based on linear tyre model and conventional control theory,during which the yaw stability and damp properties have been compared and analyzed by root·locus theory．Vehicle handling and stability field test has been conducted to understand the vehicle cornering dynamics in advance． The static bifurcation of steady·state cornering destabilization has been studied．Considering the need of nonlinear analysis and numerical computation，a 4·D nonlinear vehicle cornering dynamics system is formulated comprises of the roll and planar motions and a quadratic tyre model．Then the high dimensional system is simplified by central manifold theory to 1-D．It has been proved analytically that not Hopf bifurcation but saddle··node bifurcation phenomena may be appear during the steady··state cornering
destabilization,which will le：ad the vehicle out of control ultimately．nle bifurcation stabilization through state feedback control based on active front steering
is studied．111e results demonstrate that saddle-node bifurcation can be delayed to印pear which enhances the vehicle steady-state cornering stability．
Based on the state manifold theory,the dynamic mechanism of vehicle steady-state and non-steady-state cornering has been explored and elaborated．111e cornering property versus steering frequency has also been studied through numerical method． The state manifold diagrams which can describe the dynamic mechanism have been provided．A new ideal which divides vehicle destabilization into two stages，that is developing stage and out of control stage has been put forward in order to assist the elaboration of the issue．The results demonstrate that the state moves slowly at the developing stage but much rapidly the the state entres the out of control stage which will lead to infinity in limited time and bring the vehicle out of contr01．In the non-steady-state cornering operation，even if the steering angle goes beyond the bifurcation value，if the state is still in the developing stage，the vehicle is still in control which can be steered to
keep stable by AFS．On the contrary,once the state entres the out of control stage，it is very difficulty to control by AFS． The forecast of steady—state cornering destabilization methods and the application of which is studied．Two forecast methods consisting of singular value decomposition and tracking the closest bifurcation point in u一6，parametric space is analyzed and compared．A new cornering stability control scheme is put forward，which is based on the forecast of steady-state cornering destabilization．Ⅲs idea joints the bifurcation analysis and the cornering stability control directly．The influence of critical steering angle set based on the forecast result on the control efficiency of AFS is discussed．111e results demonstrate that little difference is found when the critical steering angle is set smaller than the forecast value，however，the control efficiency declines as the increasing set of the critical steering angle when it goes beyond the forecast value． robust control of vehicle cornering stability considering the parametric uncertainties is studied．9·DOF nonlinear vehicle model which has been validated by vehicle field test is constructed as the test platform of the proposed control scheme．A robust gain·scheduling yaw moment control scheme is proposed in this paper based on LPV(Linear Parameter-Varying)modeling method．Numerical simulation on the 9-DOF nonlinear vehicle model demonstrates that the proposed control scheme has much better adaptability to the variation of the operating conditions．

Keywords：Vehicle；Vehicle dynamics；Nonlinear dynamics；Bifurcation；Robust control