Research On Driving Force Distribution Of4WID-EV Based On Composite Sliding Ratio Control

As the electric vehicle of four-wheel independent drive (4WID-EV) possesses theadvantages of zero emission, low noise and excellent speed characteristics,4WID-EV hasbecome a research focus in automotive technology and will be the most promisingtransportation tool in the future. In electric vehicle field, it is very important andsignificant to improve coordinated operation of four wheels and the running performancefor4WID-EV through studying its driving system.Two electric vehicles of four-wheel independent drive and four-wheel steering(4WID-4WS-EV) have been developed for testing. The first one was driven by fourindependent rim motors, that is, axle driven; while the second one was driven by fourindependent in-wheel motors, that is, wheel driven. The sample car was equipped withfour motors for four wheel separately. Besides power system, load carrying system,steering system, braking system, monitoring and control system are involved in the samplecar. In this paper, the second sample car was studied by means of theoretical analysis,simulation and test verification.Firstly, a vehicle dynamics model was established based on the simulation platformADAMS/View to analyze dynamics performance of the vehicle. System identification wasconducted for the vehicle with the corresponding theory and method, and a mathematicalmodel was then established for the driving system of each wheel motor. Moreover, themotor model was established using electric drive theory in Matlab/Simulink. The vehiclewas tested via system identification step respond as well as the mathematical model of themotor drive controller was founded through combining co-simulation with vehicle test.Finally, a vehicle co-simulation model was built.The distribution strategy of driving force with the principle of equal-torque andequal-power was proposed and the four-wheel drive system was designed with optimalcontrol theory. Then the reference input was determined. After completing the distributionstrategy of driving force and co-simulation platform, the vehicle model and the optimalcontroller model were simulated under different conditions. In order to prove the models mentioned above, the results were analyzed and showed that the driving force can beallocated by the driving system according to control strategy and the designed controllercan operate fast, stably and accurately.The distribution strategy of four-wheel driving force was first proposed according tothe composite sliding ratio control and a mathematical model of driving force opticalallocation was established. The basic idea is that weighted sum of square of four-wheelsliding ratio and the change of sliding ratio was minimal through distributing the motor’scurrents. The driving currents were distributed on the basis of each drive wheel’s status,which results in accordant sliding ratio for four wheels and reduce power friction due tono mutual promotion or drag between each other. Through establishing the driving forcedistribution controller in Matlab/Simulink, co-simulation models were achieved which canbe used to optimize and control the vehicle performance.Two degree freedom linear model for single-track two-wheeled vehicle wasestablished to ensure the lateral stability of electric vehicles during turning. Thefeedforward controller was designed by using ideal vehicle yaw rate and centroid rollangle in order to optimize the four-wheel steering angles and the yaw moment. Feedbackcontroller was designed by using optimal control theory. Both the four-wheel angles andthe yaw moment gained from feedforward controller and feedback controller wereinputted into the vehicle model to control the lateral movement and improve lateralhandling and stability of the vehicle.The vehicle co-simulation model was used to simulate the conditions of uniformstraight driving, uniform acceleration and turning in order to verify the driving forcedistribution controller, feedforward and feedback controllers.On the basis of Visual C++, the optimal controller and the control strategies ofequal-torque, equal-power and composite sliding ratio are incorporated into the program,in which the control strategy of composite sliding ratio was solved by the method of thePoint Penalty function. All the three control strategies were applied to distribute thecurrent of four-wheel drive. Finally, road tests were carried out under the conditions ofuniform driving, uniform acceleration and turning. The test results show that all thecontrol strategies are feasible and the simulations are credible.