| Brake system is one of the most important factors to assure the safety running. Because of hardware limit, traditional brake systems that using oil or air as the force transmission media, are difficult to satisfy the requirement of development of brake system. The difference between Brake-by-Wire (BBW) application and conventional brake systems rests with that the hydraulic or pneumatic pipelines in the middle of the brake pedal and the wheel brakes being replaced by electrical signal paths and components. Commands from drivers to decelerate the vehicle are sensed by sensor. Brake pressure is generated by electric motor which is controlled by computer. BBW system provides users with many advantages such as quick response, simplified modular structure, easy improvement and adding new functions, et al. Thus, it has recently aroused a research fever ranging from industry to academic field. How to distribute brake force of four wheels when the driver pushes down the brake pedal is a pivotal problem for designing BBW system. Brake force distribution determines whether the brake ability of each wheel is used sufficiently or not and simultaneously determines the stability of vehicle during braking, and, at the same time, it influences driver's brake feeling. The main research of this thesis included:The thesis expatiated on the structure of BBW system. The procedure of designing BBW system contained the well consideration of both achievement of the realization of brake function and the improvement of the fault tolerance ability of the system. Vehicle simulation model that extremely similar to real car was established using CaiSim, a simulation software of vehicle dynamics. This model will be used for the simulation of brake force distribution method.Under the consideration of driver's brake feeling sufficiently, the relationship between brake pedal force and brake deceleration was designed. Therefore, we could determine the total brake force according to driver's brake intention. The relationship provides a basis for distributing brake force to each wheel.The brake force distribution methods when driving on straight and crooked road were investigated respectively. In order to make the brake ability of each tire being used more sufficiently, the brake force of each tire was designed to be proportional to normal load. In order to compensate the error between real brake force distribution and ideal brake force distribution caused by error of estimated normal load, change of vehicle mass, and ramp or inclination of the road, a brake pressure modulation method on the basis of the wheel speed variance between each wheel was explicitly proposed. The wheel speed of each wheel was modulated nearly equal at last, and real brake force distribution was adjusted close to ideal brake force distribution. The brake force distribution method was finally tested through co-simulation of MATLAB/Simulink and CarSim. The result revealed that the effect of brake force distribution method was favorable.When the arrester of one wheel was caught in failure, the brake force distribution method of the other three wheels was investigated. In order to prevent braking deviation caused by brake force unblance between left and right wheel after redistribute of brake force, the established speed resulting from brake force unbalance was designed to be inverse proportional to the square of vehicle speed. Accordingly, the driver would have enough time to respond and turn steering wheel in a reverse direction. At the same time the brake distance would not become too much longer than the normal braking condition. In the end, the brake force distribution method was tested through co-simulation of MATLAB/Simulink and CarSim. The result revealed that the behavior of failed vehicle performed almost the same as the normal vehicle in the situation of light braking. The braking deviation was prevented during heavy braking and the brake distance was not become longer. |
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