Research On Integrated Control Of Vehicle Yaw And Rollowver Stability Based On Differential Braking

The safety of driving vehicle is mainly dependent on yaw and rollover stability. Sport utility vehicle (SUV) with higher gravity center, popular in recent years, has a lower rollover threshold, consequently prone to rollover while driving with obstacle avoidance or sharp turn on high adhesion surface. With the development of automotive electronic stability control (ESC) and its wide application on vehicle, especially on passenger car, the vehicle active safety is improved largely. However, the main control objective of ESC presently is to achieve vehicle yaw stability. In order to keep the vehicle prone to rollover stable further during driving, we should enhance the function of ESC and make rollover stability as its another important control objective. To coordinate yaw stability control (YSC) and rollover stability control (RSC) effectively and realize both yaw and rollover stability to the utmost, it needs to realize integrated control of YSC and RSC.To better understand the yaw and rollover stability control processes, first we do research on YSC and RSC algorithm separately, and then based on which we do exploitation and research on the integrated control algorithm of yaw and rollover stability. A SUV model is chose as the object vehicle. There are six chapters in this dissertation, and the main contents are as follows:In Chapter One, the causes of vehicle losing yaw and rollover stability are described in detail; the history and academic status of YSC and RSC are elaborated; the main contents of this dissertation are presented.In Chapter Two, a 2-DOF vehicle model as the reference model of YSC and a 3-DOF vehicle model as the prediction model of RSC are built. In the linear 2-DOF vehicle model, the equivalent tire cornering stiffness of front and rear axles are identified offline through fminsearch function in MATLAB Toolbox. On this basis a 3-DOF vehicle model with an increased freedom of roll motion is built. Through dividing the tire cornering characteristics of both front and rear axles into several linear parts the linear 3-DOF vehicle model is changed a nonlinear one. Discretion of 3-DOF vehicle model makes it a true rollover prediction model. The simulation results validate that the 2-DOF vehicle model built can describe the target vehicle's dynamic properties in tire linear region, and that the 3-DOF vehicle model built can describe the target vehicle's dynamic properties in tire linear and nonlinear regions. These provide a reliable foundation for following research on the control algorithms of yaw and rollover stability.In Chapter Three, a yaw stability control algorithm based on differential braking is developed. The control algorithm makes the error of yaw rate and the vehicle side slip rate as control objectives, decides the required yaw moment by threshold and PID control methods, selects the efficient wheel to brake and fulfills the brake pressure demand by adjusting the hydraulic brake pressure, and ultimately achieves vehicle yaw stability. The simulation results of both the maneuvers under low adhesion conditions based on GM's evaluation of ESP and the maneuver under high adhesion conditions based on NHTSA FMVSS126 regulation's evaluation of ESP validate that the yaw stability control algorithm built in this dissertation can improve vehicle yaw stability effectively.In Chapter Four, a rollover prediction system based on TTR(Time-To-Rollover) is developed firstly. The rollover prediction system using 3-DOF vehicle model as the rollover prediction model and transient lateral load transfer rate as the rollover index, considers that the vehicle has rollover risk in some time if the rollover index exceeds its threshold in prediction process and outputs TTR. By comparing the predicted and actual lateral load transfer rate, the simulation results show that the rollover prediction system can accurately predict the rollover risks. Then a rollover stability control algorithm based on rollover prediction is developed. The control algorithm makes TTR the rollover prediction system outputs as control parameter, decides the required yaw moment by PID control method, selects the outside front wheel to brake and fulfills the brake pressure demand by adjusting the hydraulic brake pressure, and ultimately achieves vehicle rollover stability. The simulation results of human-in-the-loop and human-off-the-loop maneuvers under high adhesion surface validate that the rollover stability control algorithm based on rollover prediction built in this dissertation can improve vehicle rollover stability effectively.In Chapter Five, a integrated control algorithm of yaw and rollover stability based on differential braking is developed. The integrated control algorithm, based on the idea that independently decision-making firstly and integrating by weight then, makes YSC and RSC decide the required yaw moment and brake torque independently firstly, then unifies the yaw moment and brake torque by way of weight according to vehicle yaw and rollover states, selects single efficient braking wheel to achieve yaw moment and four braking wheels with varying brake pressure to achieve brake torque, and ultimately calculates the required brake force of each wheel. In order to prevent the wheels from locking, the integrated control algorithm also combines ABS control algorithm. The simulation results of human-in-the- loop maneuvers validate that the integrated control algorithm of yaw and rollover stability built in this dissertation is effective, which can improve vehicle rollover stability and simultaneously make vehicle follow drivers'driving intension well.In Chapter Six, summarizes and conclusions of this dissertation are presented, and prospects of future research is pointed out.