Research On Functional Safety Of Vehicle Brake By Wire System For Automatic Driving

In recent years,with the increasing popularity of electrification,networking,intellectualization and sharing of vehicles,autonomous vehicle technology is also booming.Compared with traditional vehicles,autonomous vehicles without driver control need to have stricter safety levels.The increase of electronic and electrical components makes the integration of autonomous vehicles higher and higher,and the electrical architecture is more complex.When autonomous vehicles have design flaws,they can lead to serious safety accidents and waste a lot of human,material and financial resources,undermining public confidence in autonomous driving technology.As one of the core of the chassis integrated control system,the brake by wire system is the basic foundation to realize the braking operation of autonomous driving vehicle.It eliminates some complicated hydraulic pipelines in the traditional braking system,and is replaced by some electronic components and electric actuators,with simple layout and low development cost.The automatic driving vehicle receives external information through sensors,and the automatic driving computing unit calculates the braking strength required by the braking system,and the braking controller directly sends instructions to the actuator to realize the braking operation.When electrical and electronic faults occur in the brake by wire system,the whole system may fail,which brings unpredictable risks to the vehicle.Therefore,how to identify the potential danger of the brake by wire system in the design stage,and design a reliable safety strategy to avoid the occurrence of harm has become the focus of current research.ISO 26262 functional safety standard is one of the most widely used vehicle design criteria.It provides guidance for the development of functional safety of vehicle electrical and electronic systems.The safety related vehicle electrical systems should follow this design standard to reduce the risk of failure to a reasonable range.This topic refers to the ISO 26262 functional safety standard,and carries out research on the brake by wire system of autonomous vehicles,in order to improve the safety of the brake by wire system.The specific research content is as follows:（1）Functional safety conceptual design of brake by wire system.According to the third part of the functional safety standard concept stage content,first determine the functional items and system application scope of the system,and then use hazard analysis means,hazard analysis and risk assessment of the system.Hazard analysis and risk assessment mainly adopt HAZOP and STPA analysis methods,firstly determine the driving mode and operation scenario of the vehicle,and then identify the possible system failure through the safety key guidance words in HAZOP.On this basis,the control structure of the brake by wire system was established based on STPA method,and the unsafe control behaviors among internal components were identified,and the causes of the unsafe control behaviors were analyzed.Finally,according to the hazard events caused by unsafe component interaction,according to S,E and C three evaluation indexes,the ASIL grade classification is carried out to establish the safety objectives and functional safety requirements of the brake by wire system.（2）Functional safety design of brake by wire system controller.In order to meet the corresponding functional safety requirements,it is necessary to design the braking controller safety mechanism and develop the monitoring strategy.The main functional safety monitoring strategy includes: MCU redundancy check functional safety strategy,which improves the fault tolerance ability through the cooperative control of master and auxiliary MCU;The monitoring strategy of CAN communication fault is explored for node short circuit,short circuit fault,node transceiver fault and data link layer fault.Monitoring strategy of signal conditioning module,relevant detection mechanism and troubleshooting measures are designed for pressure sensor failure and ADC module failure;Solenoid valve drive module fault,according to the solenoid valve drive chip drive feedback state,design the fault detection algorithm of the drive module,complete the functional safety design.（3）Fault diagnosis of high-speed switching solenoid valve based on wavelet packet decomposition and BP neural network.Solenoid valve is the core electrical component of the actuator of the brake by wire system.The design idea of functional safety is to establish a fault diagnosis scheme with high real-time performance and strong diagnostic ability.First of all,starting from the working principle of the solenoid valve,analyze the failure mechanism and failure mode of the solenoid valve,based on the circuit model of the solenoid valve working process,magnetic circuit model and motion model,build Matlab/Simulink simulation platform.Combined with the simulation model of solenoid valve working process,the common faults of solenoid valve are simulated by changing the performance parameters of solenoid valve.Based on the analysis of the transient response characteristics of the solenoid valve,the current change rate of the driving end is extracted as the original data for fault diagnosis.The energy feature vector of reconstructed signal is normalized and used as the input of fault diagnosis by three-layer wavelet packet decomposition.The research on fault diagnosis algorithm based on BP neural network,the selected number of input and output neurons and excitation function,establish the final three layers neural network topology structure,complete the data sample training and testing,diagnosis,according to the results of this fault diagnosis algorithm can achieve higher diagnostic accuracy,proved that the functional safety mechanism of high-speed switching solenoid valve is feasible.