Study On The Control System Of Vehicle Dynamic Stability Via Simulation

Vehicle handling and stability plays an important role in the field of vehicle safety. Itmainly refers to the abilities that vehicles can follow driver’s intentions and keep stability nomatter external interventions such as lateral wind. With the sharp development of our nationaleconomy, the volume of private car ownership is gradually increasing each year. At the sametime, vehicle running speed is much higher than before along with the infrastructureconstruction of highway. Besides these, the diversity of drivers’ abilities is another importantfactor that leads the majority of road traffic accidents in China. Therefore, more and morepeople are paying their attentions on vehicle safety. Many electronic devices such asElectronic Stability Program (ESP) to improve vehicle safety are equipped in vehicles. ESPcan ameliorate vehicle lateral stability effectively and minimize traffic accidents.Usually, ESP control strategy includes two parts, the upper and the lower control sections.Based on the principle of Direct Yaw moment Control (DYC), the output of upper controlsection is control torque needed to stabilize vehicles. Then, a tire longitudinal force can becalculated with a tire model in the lower section. The lower section uses low frequency pulsewidth modulation (LPWM) to control a switch valve and achieve the goal torque. The outputfrequency of LPWM is smaller than the response frequency of high speed switch valve (HSV).It results in many shortcomings such as the noise made by the collision between valve coreand its seat when working, the high pressure fluctuation and inaccuracy, the uncomfortabilityinduced by the shaking of brake-pedal. In addition, hydraulic control unit is also an importantpart in evaluating control strategy for vehicle stability. In a lot of literatures, thecharacteristics are not paid attention to in the study of vehicle stability control strategybecause it is difficult to build the model of hydraulic system. These leave a challenge tocurrent research on vehicle handling and stability.In the paper, two different control strategies for vehicle stability have been purposed. Thefirst one is the slip ratio control strategy which is based on fuzzy theory. The target slip rate isgiven by fuzzy controller in the upper control section. A tire braking torque isapplied at corresponding tire in order to achieve the actual slip rate close to the target slip ratein the lower control section. The advantage of this control strategy is that it doesn’t need tocalculate braking torque and computational efficiency is improved. At the same time, tire slipratio regulating capability is the basic function of vehicle braking system. So, the realization ofslip ratio control is feasible. Another one is the braking pressure control strategy which isbased on high frequency pulse width modulation (HPWM). According to the researches on theuse of HPWM to adjust pressure in hydraulic systems, hydraulic pressure controlled byhigh-speed switch electromagnetic valve (HSV) could be changed continuously such as the ability of servo valve or proportional valve. The extra yaw moment is given by fuzzy controllerin the upper control. In order to make the braking pressure of hydraulic control unit (HCU)mostly close to demanded pressure and realize the accurate control of braking force, theduty cycle of HPWM acts on HSV is adjusted by PID in the lower control.Numerical simulations are developed and carried out to verify the effectiveness of thementioned two control strategies. Firstly, a complete HCU is established which includes themaster barking cylinder, the wheel braking cylinder, HSV, plunger pump, throttling valve,accumulator and so on. At the same time, a vehicle model with14Degrees of Freedoms (14DOFs) is constructed. It includes three translational motions (longitudinal, horizontal andvertical motions) and three rotating motions (pitch, roll and yaw). The other eight DOFs arerotation and vertical motions of four wheels, respectively. Then, HCU,14DOFs vehicle modeland control strategy are combined to analyze the performance of the presented control strategy.Three typical cases are used to evaluate the proposed control strategies for improving vehiclestability by the numerical model. The typical cases are composed of step input of steeringwheel, Fishhook and emergency double lane change. Furthermore, the influence of dataupdate frequency such as the refreshing rate of vehicle dynamic parameters (25Hz) and thedynamic response frequency of HCU (not exceeding10Hz) of actual ESP hardware isconsidered in the study.The result shows that the presented two control strategies are capable to improve vehiclelateral stability and increase vehicle safety in critical conditions. The characteristics of HCUcan be investigated in detail with the proposed numerical model. Because the normal force ofinner rear wheel is reduced due to load transferring, the further development of ESP for vehiclestability will depend on integrated vehicle chassis control system such as the utilization ofactive suspension system and DYC.