Coordinated Control For Straight Driving Stability Of Four Wheel Independent Drive Electric Vehicle

In recent years, the electric vehicle has become a development boom due to its characteristics of non-polluting and energy recycling. As the wheel is driven directly by the motor, the emerging electric wheel drive vehicle(EWV) has the advantages of high transmission efficiency, the electric wheel being convenient to control independently and responding quickly etc, which is more conducive to vehicle dynamics control and the development of miniaturization, intelligentialize and environmental protection to electric vehicle in the future. The theory of straight driving stability control and driving coordination control among electric wheels for four wheel independent drive electric vehicle(IEV) is studied in this paper.Firstly, synthesizing the research status of EWV, the reasons and processes for deviation of IEV in straight driving are analyzed, and the factors and conditions leading to deviation are classified and summarized. Taking four wheel speed being the same, output torque of hub motors on both sides being equal and yaw rate being zero as the control target, yaw rate has been selected as the controlled variable to design a coordination control system for ensuring straight driving stability of IEV after analyzing their corresponding control effect.Secondly, based on the vehicle dynamics and other related theories, the dynamic model of IEV is established, mainly including sub models of car body dynamic model,tire model and motor model etc. According to the testing data of hub motor, the motor model is built by methods of curve fitting, interpolation and look-up table.Then, with the purpose of the vehicle straight driving stability, coordinated control system of IEV is designed by researching the aspects of driving force coordinated allocation and anti-slip for four wheels. In order to ensure the robustness of the designed system, the additional yaw moment is calculated to correct the deviation of IEV utilizing sliding mode variable structure control. The additional yaw moment is distributed in way of four wheels equal and wheels on both sides opposite,and it is converted to the voltage control signal adjustment of four hub motors applyingPID control method to adjust driving forces between left and right wheels dynamically.With the target to equal slip rates of front and rear wheels on same side, the PID control method is employed to redistributed the voltage control signal of hub motors for driving forces real-time distribution of front and rear wheels. In the process of control and allocation, the wheel will be into anti-slip PID control when it has a serious slip, while the control and distribution of normal driving wheels remain unchanged.Finally, integrating the vehicle dynamic model and the coordination control system above, simulation platform of IEV is built in Matlab/Simulink. And the simulation analysis is carried out in different types of conditions, such as unequal torque output by hub motors on both sides, the imbalance of the wheel driving force on both sides caused by external disturbance and the disturbance of vehicle lateral force.The results show that the coordinated control system can control deviation of IEV driving straight 1km in 5m, slip ratios between the front and rear wheels being equal and a single slip rate not exceeding 0.25, which ensures that driving among four wheels is coordinating and IEV is provided with good driving stability, and has verified the effectiveness of the proposed system.