Research On Four-wheel Independent Braking System Based On Functional Safety

The rapid development of the automobile industry has greatly facilitated people’s life.The applications of new technology in automotive electronic control system have been constantly emerging,and four-wheel independent brake by wire system is one of various forms helping drivers driving.Compared with the traditional mechanical brake system,it has the advantages of simple structure,stable operation and being able to realize advanced functions such as ABS.However,how to ensure the normal work of the system has become an important topic.This dissertation studies the hydraulic four-wheel independent braking system with high reliability from the point of functional safety.Firstly,four-wheel independent brake system structures were analyzed.To build functional safety goals,five failure modes of braking failure,spontaneous braking,braking error,braking deviation and brake hysteresis were evaluated in four different environmental,then the overall design goal was set.The fault trees were set up to analyze the five failure modes.Above all,the corresponding functional safety requirements were established.In the next place,based on the analysis of vehicle braking stability,the method of ideal force distribution was confirmed.The vertical load of each wheels were calculated by analyzing load transfer,which was the base of four wheels’ barking forces.What’s more,the four wheels’ barking forces were adjusted as the final according to the wheel speed.The normal four-wheel single brake system control strategy was established as result.Next,according to the functional safety requirements,the functional designs of the normal control strategy were carried out.There were hardware redundancy of the pedal sensor and the power supply with control algorithm,failure determination method with wheel speed,wheel speed change rate and the limit value of the wheel speed sensor,selected Flex Ray communication network and redundant design of the ECU and the brake controller.In addition,taking into account the external and other factors,a single-wheel failure braking system was designed.The control strategy of four-wheel independent braking system in accordance with ASIL D was established.Finally,the method of joint modeling of Car Sim and MATLAB/Simulink software was adopted.A four-wheel independent braking vehicle model was established to meet the functional safety requirements.Established fault injection on the basis of ISO 26262-8,and put it into the system.After that,the simulation tests were carried out under the conditions of linear braking and curve braking in comparison with the normal four-wheel braking system.It showed that the results after fault injections were in line with the normal one.A single-wheel failure braking control system model was established,and the feasibility of the single wheel failure control strategy was verified by comparing the normal four-wheel independent braking system.In results,the whole system had reached the ASIL D from the analyses.