Hydraulic Character Modeling And Vehicle Stability Control Algorithm For EHB System Of Passenger Car

As a new type of vehicle active control braking systems, EHB(Electronic Hydraulic Braking) system is the development product of the transition from the vehicle conventional hydraulic braking system to EMB(electronic mechanical braking) system.EHB abolished the mechanical connection between brake pedal and brake wheel cylinder, and the driver's braking order is transmittedto the ECU (Electronic Control Unit) in the form of electronic signal, then ECU identify the driver's braking intentention while collecting the other vehicle sensors information, making out the optimal cylinder brake pressure by controlling the solenoid valve to regulat cylinder pressureor.The characteristics structure of EHB system bring about revolutionary innovation for the design of modern vehicle.As a new braking system, EHB can significantly improve the braking performance and vehicle handling and stability, while providing a variety of support functions, sharing information with other vehicle control systems to cater the development of the vehicle chassis integrating and realize the ultimate intelligent vehicle control.Based on the study of EHB system basic structure and working principle, the paper mainly completed EHB hydraulic system modeling and the stability control algorithm research based on EHB. Control methods are used to control the switching of the solenoid valve to regulate the wheel brake cylinder pressure. Change modulation frequency and performance parameters of the electromagnetic valve, and find the factors of influencing vehicle stability control effect. Finally, hardware in-the-loop test of EHB system and the structure of the EHB hydraulic system are built to provide a platform for the following development of the hydraulicsystem hardware and the improvement of the brakeing control algorithm.(1) Hydraulic System ModelingEHB and the conventional hydraulic braking system are quite different in structure, based on structural features of the EHB system to establish the EHB components hydraulic model, which is of great significance for the development of active control algorithm research. This paper creates an effective brake hydraulic circuit system architecture model, which is verified by conventional hydraulic brake control system's wheel cylinder hydraulic pressure increase and release process. The hydraulic model can be used for the initial simulation research on EHB. This paper also establish the ESP system hydraulic structure model, and analyse the hydraulic response between EHB and ESP system in theory when come to the intervention of stability control, a conclusion is made that EHB outstanding in brake pressure applying quickly and releasing downright.(2) PWM control logic of solenoid valveA number of factors impacts the hydraulic braking system charecteretics, wheel cylinder pressure is regulated by in-solenoid and out-solenoid valve, the dynamic response of the control strategy willt impact the accuracy of wheel cylinder pressure greatly. Solenoid valves of HCU are the highest manufacturing processing hydraulic components, and their performance parameters and control strategies are of the great significance on the characteristics of the hydraulic system. In this paper the structure of high-speed on-off valve and the valve core principle are discussed, the paper also analysis the response of the valve core under different modulation frequency and duty cycle and the change of .wheel cylinder pressure. Setting a proper modulation frequency to make sure that electromagnetic valve can be worked in a greater linear range. While modeling, the response time will be incorporated into the electromagnetic valve opening or closing time, through the simulation analysis of restricting the duty cycle of electromagnetic valve or not, in the spool in a certain relative displacement standard conditions, the electromagnetic valve duty cycle may not be restricted due to the spool in a certain relative displacement to amend the solenoid valve lag time because of simplifying the process of modeling electromagnetic with neglecting lag time to ensure that the duty cycle of solenoid valve is set under the conditions of not work in their death or saturated region. The PWM control method of solenoid valve can effectively regulated wheel cylinder pressure. The simulation results show that the spool relative displacement were almost 0.5 above, and the electromagnetic valve did not enter dead or saturated zone.(3) Primer study on v ehicle stability control method based on yaw rateBased on vehicle dynamic stability control theory, a single-variable control of vehicle yaw rate stability control strategies and algorithms was established combined with structural characteristics of EHB hydraulic system. Reference to vehicle linear model with two degree of freedom, an additional vehicle yaw torque is decided by comparing vehicle actual state to normal one. Translate the torque into the corresponding additional wheel brake pressure, wheel cylinder pressure is controlled in closed-loop from getting pressure sensor information which provided by pressure sensor in wheel cylinder.(4) Simulations analysis on vehicle stability control system under typical conditionsSimulations for stability control system which is based on EHB are implemented, the results illustrate that the stability control algorithm proposed in the paper can ensure significant control effects to prevent vehicle from losing control. Furthermore, the control system can make brake application on individual wheel properly and quickly with the wheel cylinder pressure following the target pressure appropriately. The duty cycle of solenoid valve is in its linear area and the relative displacement of valve core is 0.5 or more and never goes into dead zone and saturation region.(5) The influence of EHB key hydraulic components performance and control frequency on vehicle stability control effectIt provides theoretical basis for the requirements of the parameters of EHB hydraulic system and the development of control system to estimate the influences of PWM frequency and dynamic response time of the high speed switch valve on the control effects of vehicle dynamic control system. On one hand, turning down the PWM frequency of solenoid valve may cause the delay of the wheel cylinder pressure modulation and bigger fluctuation and eventually impact the control effect. On the other hand, system pressure can compensate the delay of pressure building using the high-pressure source when the dynamic response time is big, but this can cause the solenoid valve with no action and delay of cylinder pressure modulation because under this condition the solenoid valve can not fully response and has small relative displacement in a long time, and eventually may impact the control effect. So the conclusion is only proper design of components of hydraulic system especially the performance parameters which can ensure great dynamic response of hydraulic system that consistent with the control strategy can make good use of stability control system.(6) Scheme of EHB hardware-in-the-loops test and hydraulic systemThe EHB system hardware in the loop test bench blue print and EHB hydraulic system structure scheme are proposed based on the lab condition after extensive survey and argumentation. This constructs a platform for the validation of the research results in this paper, study of the characteristics of hydraulic system and improvement of active braking system. The test bench can fulfill basic braking function of EHB system as well as active control logic of ABS, ASR and ESP, and produce failure model for EHB while maintaining basic brake function of vehicle. The plan has advantages of low-cost, low-risk, shorter development cycle and perform well.