Research On Vehicle Cornering Brake Control Strategy

Cornering Brake Control or CBC is an automotive safety system developed by BMW. It is a further development and expansion of the Anti-lock Braking System, designed to distribute braking force during braking whilst cornering. During braking whilst cornering, because load of rear axle transfers to forward axle and load of inside wheels transfers to outside wheels, vehicles exhibit aturn-inward tendency. When the vehicular velocity is very high, the driver can not correct vehicle performance immediately; even perform wrong operation which aggravates the vehicles instability. Traditional ABS and EBD can not allocate brake pressure to avoid oversteering tendency when the vehicle is braking on a curve.This dissertation focuses on CBC, based on the achievements in this field made both at home and abroad, we report our investigation on CBC.First of all, the working principles of CBC and its realation with ABS are introduced; the domestic and foreign development status is discussed; the contents of the dissertation are also put forward in this part.And then the methods of modeling of vehicle dynamics are described. There are three main types: graphical modeling, computer automatic modeling and artificial modeling. Based on the advantages and disadvantages of the three modeling methods, Matlab software is used for graphical modeling. Vehicle coordinate system, tire coordinate system and geodetic coordinate system are discussed. Vehicle kinematics equations based on the Newton's mechanics is used to establish a seven-degree-of-freedom vehicle model, including brake model, tire model, as well as side slip sngle and dynamic load of tire and other auxiliary modules. In this paper, in order to study the stability of the vehicle braking while turing, considering the complexity of the actual situation and reducing the complexity of the model, the actual vehicle model is simplified. The modle can ensure the accuracy of the system; the various affecting factors of the vehicle handling and stability are simplified and properly disposaled.Thirdly, based on the achievements in this field made both at home and abroad, the investigation on the stability of vehicle braking while conering is performed. Acoording to this the control method and the state variables that influence stability are disscussed. The reasons of oversteer when the vehicle brakes on a curve are analyzed in detail, as well as the means to improve the vehicle stability, the feasibility by changing the lateral force and the longitudinal force to improve the vehicle stability. Based on the analysis, the control of the vehicle stability is achieved through changing braking pressure of inside wheel of the car to. Through the previous theoretical analysis, CBC technical program is determined, and we take the difference between the real yaw rate and the desired yaw rate as the control state variable. For the desired yaw rate, firstly we use the two degrees of freedom linear vehicle model to calculate the desired yaw rate, and finally the desired yaw rate is amended, while the real yaw rate is from the measured yaw rate sensor signals.The CBC activates only when the vehicle is braking and turning, so conering identification must be done in advance. In this thesis, the steering angle sensor signal is used to identify whether the vehicle is steering, and determine the specific conering direction of the vehicle and corresponding inside wheels. If CBC is activated, and the difference between the real yaw rate and the desired yaw rate is greater than the difference threshold, then the vehicle is in the state of oversteer, at this time, mantaining the braking force of the outside wheels, and reducing the braking force of the inside wheels, would cause yaw moment which counteracts oversteer tendency, the vehicle can resume stability. If the vehicle is not in the state of oversteer, according to the previous analysis, this time there is oversteer tendency although the vehicle is in a stable state, in order to prevent the tendency of oversteer, the braking force of the inside wheels need to be controlled, which is determined by the the different logic threshold. Finally, based on the previous theoretical analysis, the simulation model is built up in the Matlab/Simulink environment, which includes the vehicle dynamics model and control algorithm Simulink module. The control strategy in this thesis is researched using open-loop simulation on high adhesion coefficient road and low adhesion coefficient road, respectively. The simulation results show that the control strategy analyzed in this thesis has good control effect on the homogeneous surface, improves vehicle stability when braking and turning, and is also validated effective.The control strategy analyzed in this thesis can improve the stability of vehicle during braking whilst cornering, and the next key point of research is how CBC works with ABS harmoniously, to improve the braking performance of vehicle during braking whilst cornering.