| Presently, X-By-Wire technology, with its superior performance, is increasinglysubjected to the world’s big automobile manufacturers’ favor. Its characteristic oftechnology is that the driver’s steering signals can be obtained through the sensors, electricalsignals transmitted to the controller by wires, the controller can identify the current driver’sintention according to the driver’s steering signals, and predict the next period of drivingbehavior, then control the vehicle in real-time according to the driver’s intention. It can helpthe driver to liberate from various complex operations, assist the driver to deal withunexpected events, and improve the vehicle handling stability and safety.The X in X-By-Wire represents the steering, braking, transmission, suspension control,and so on. The combination of X-By-Wire technology and the vehicle’s brake system formthe BBW system that gradually replaces the traditional braking system as the new trend ofbraking technology. Now, according to the difference of actuators, the vehicle’sBrake-By-Wire system can be divided into electro-hydraulic brake system (ElectronicHydraulic Brake, EHB), electronic and mechanical brake system (Electronic MechanicalBrake, EMB). Due to the vehicle’s12V power can not meet EMB of42V power demands,but it can fully meet the requirements that EHB uses the original vehicle of12V power.Therefore, realizing brake-by-wire BBW, the first step is to realize EHB.Firstly, on the basis of the EHB system’s basic institutions and operating principle, thispaper establishes the model of hydraulic system, and analyzes two position-two wayelectromagnetic valve’s characteristic of implementation of structure, and tests PWMmodulation frequency of the electromagnetic valve and the effective control of linearinterval to obtain the reasonable modulation frequency and linear range. Since EHB systemhas nonlinear characteristics, this paper adopts the fuzzy self-tuning PI controller to controlPWM, adjust the liquid of the inlet and outlet frequency through the switch ofelectromagnetic valve, achieves the tracking ability and makes EHB system have goodbraking effect at the low pressure stage, medium pressure stage, and high pressure stage.Secondly, according to the driver’s operation, such as turning the steering wheel,stepping on the brake pedal and the accelerator pedal and so on, using SVM to recognizeand predict the driver’s intention. In different conditions, using SVM to identify the roadadhesion coefficient, fix the reference value of ideal yaw angular velocity and the ideal goalvalue of wheel brake cylinder force, and complete the distribution of the wheel brakecylinder target force.Finally, this paper bases on the vehicle driving dynamics, establishes the linear twofreedom vehicle model, defines the mathematical model of desired yaw rate and its upper limit model to solve the problem of calculation of the additional yaw moment and brakingressure. For oversteering and understeering trend appearing in the process of steering, wecarry on the experiment of simulating comparison and analyzing, and draw the followingconclusions: drivers’ intent recognition module can identify which wheel cylinder should bebraken through a series of operation of current driver, but if the road adhesion coefficient isnot sure, the desired yaw rate which is determined is not reasonable, at this time the brakingforce distribution will make the vehicle unstable acceleratedly. Therefore, this paper usesSVM’s powerful study function, solving the problem of tire-road friction coefficientestimation, formulating the reasonable expectation of yaw rate. It appears the trend ofoversteer or understeer when the vehicle steers, the discrepancy of the actual yaw rate whichthe yaw acceleration sensor tests and the desired yaw rate can be a control volume. This willeffectively solve the problem of instability in the process of the vehicle steering, achievingthe purpose of the stability of the vehicle, and further improving the safety performance ofthe vehicle. |
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