Research On Integrated Control Of Vehicle Yaw And Roll Stability

In recent years, increasingly heavy traffic conditions have caused frequent occurrence of accidents, among which vehicle rollover and sideslip accidents have become the major threats to human life and property safety. Considering the strong coupling with two motions, this paper will control vehicle yaw and roll motion at the same time while with a coordinated manner of active front steering and differential braking.First of all, for vehicle yaw stability performance requirements, this paper takes advantage of Lyapunov’s second method to define the reference yaw rate, and determines side-slip angle stable boundary map curve in different vehicle speeds and friction coefficients. Furthermore, in order to meet the needs of vehicle roll stability, this paper selects Lateral-load-transfer-ratio and Time-to-rollover as two dynamic rollover indexes. Under the stability demands of vehicle yaw and roll motions, model predictive control method is chosen, additional front wheel steering angle and braking force of each wheel are selected as optimization variables. By the means of choosing different objective functions and constraints, a switching control system of vehicle yaw and roll stability is designed, which contains four control modes, that are yaw control mode, roll control mode, no control mode, and integrated control mode. According to vehicle current feedback status information, the corresponding control mode is firstly estimated and then the optimal control variables could be obtained by solving nonlinear optimal issue. At last, the additional front wheel steering angle should be applied to the steering actuator, each tire braking force should be converted to the corresponding wheel brake cylinder pressure and then be applied to the braking actuator. In order to verify the effect of the designed switching control system, a Hongqi HQ430 vehicle dynamics model is selected to take place of the actual vehicle to do off-line simulations in given conditions. The results have shown that although the switching controller is capable to ensure vehicle yaw and roll stability, while in extreme conditions, vehicle status jitter will be aggravated, which not only increases the wear of actuator, also affects vehicle comfort.To avoid the system jitter issue caused by mode switching, an integrated control system is designed while using model predictive control method. Based on the basis of vehicle stability and security performance constraints, the objective function is proposed while integrating vehicle yaw stability and roll stability requirements, for the purpose of satisfying its respective demands in different conditions, a fuzzy control strategy is used to adjust objective function weighting coefficients. By solving the above-mentioned non-linear optimization problem, optimal control variables-additional front wheel steering angle and braking force of each wheel could be obtained and acted on vehicle system.Through the limiting condition experiment, double lane change experiment, fishhook experiment and other off-line simulations, the validity of the integrated controller could be verified. By comparing the effects of the two controllers, it can be concluded that vehicle yaw and roll stability can be both guaranteed, but the integrated control system could avoid vehicle jitter phenomenon effectively, and has certain controlling smoothness.