Study On The Laboratory Vehicle Platform Of Electric Vehicles With In-wheel Motors And Driving Control Strategies

Energy conservation,environmental protection and safety are the development trend of vehicles and constant themes.Electric vehicle with in-wheel motors,with its unique structure and driving forms,attracts the interests of more and more researchers.Compared with traditional vehicles,electric vehicles with in-wheel motors has obvious advantages in the aspects of kinetics control,which is manifested in the following two aspects: the driving forces of each wheel is independent and can be controlled in real time;the response of the driving moment of motors is quick and the control precision is high.How to make use of these advantages,how to improve the driving stability of vehicles through controlling the torques of driving wheels of electric vehicle with in-wheel motors as well have become research focus at present.Based on this,the thesis studies the laboratory vehicle platform of electric vehicles with in-wheel motors and the stability control of their driving.First,a laboratory vehicle platform of electric vehicles with in-wheel motors based on a simple traditional frame is developed according to the need of the research.In the development of the platform vehicle the electronic control system framework and the network communication framework are defined on the level of general framework,the electric drive system,the power battery,the suspension steering system,and the braking system designed,and the electric vehicles with in-wheel motors constructed on the laboratory vehicle platform through the process of matching,checking,assembling and processing.Secondly,according to the physical structure and parameters of the laboratory vehicle platform,independently driving electric vehicles model with in-wheel motors is built based on CarSim,the traditional vehicle model,and the interface between the vehicle and the motor driving system set as well.On the basis of motor parameters and the analysis of their mathematical model,a motor kinetics model used with look-up table and in which the torque is controlled with PI is constructed based on Simulink.According to the defined interface,a universal electric vehicles with in-wheel motors simulation testing platform is constructed with the combination of CarSim/Simulink,the straight travel ability as well as the steering ability of the vehicle is verified,and the dynamic properties of the vehicle under the normal driving circumstance,including linear acceleration dynamic performance,and steering operation stability,are simulated.All these verify the accuracy of the platform.Third,the vehicle driving control system is designed based on layered structure control.Modular design is used on the upper layer of the system with clear division,vehicle reference model,instability judgment module,the total driving force deciding module,yawing moment deciding module and slip rate control module included.The vehicle reference model is responsible for providing reference signal for other control modules;meanwhile in order to make it match the vehicle model of CarSim,GA is chosen to identify its parameters.Instability judgment module is responsible for judging the stability of the driving vehicle in real time;in order to prevent the frequent unnecessary start of the stability control system,the vehicle Instability judgment method is designed based on phase plane method with the combination of yaw velocity deviation threshold value method.The total driving force deciding module is responsible for obtaining the total driving force required by the vehicle driving,and stably adaptive tracking the target vehicle speed setting.Yawing moment deciding module mainly decides the yawing moment needed to keep the vehicle stable in the case of instability of the vehicle and considers robustness.Based on sliding mode theory,second-order sliding mode controllers were designed when the body yaw rate and the sideslip angle were respectively used as controlled variable and the error rate of the two variables were also considered.The slip rate control module is responsible for ensuring that the wheel will not slip when the vehicle is driving.Once the wheel slips,the slip rate controller based on fuzzy algorithm begins to work,thus improving the driving stability of vehicles.Based on torque rule distribution,a simulation experiment of angular step and double lane change working condition is conducted on the designed driving control system.The result shows that compared with the case when no control,the designed driving control system can improve effectively the operation stability and driving safety of the vehicle,which verifies the driving control system’s effectiveness and adaptation to the working conditions.Fourth,a study on the effective distribution of on the lower layer of the three moments obtained by the upper layer is made.A method of distribution based on dynamic load factor and torque optimal distribution based on stability is proposed.With the minimum sum of the wheels utilization ratio as the target,this method takes the load transfer,the road adhesion condition constraint and motor torque output constraint into consideration,resolves the three moments obtained by the upper layer with the active set algorithm in real time and distributes the moments to each wheel optimally.Compared with the torque rule distribution method,the simulation experiment results of the three algorithms in the Fishhook and snake working conditions show that the three torque distribution algorithms can provide the driving stability for the vehicle,but the control effect of the torque optimal distribution is best.At last,the thesis summarizes the study on the laboratory vehicle platform of electric vehicles with in-wheel motors and their driving stability control,proposed the shortcomings of the thesis and the recommendation for further study as well.