Robust Control For Four-Wheel Independently Actuated Electric Vehicles Based On Differential Steering

Electric vehicles (EVs) have attached world-wide research attentions because of the advantages on emissions reductions and fuel economic. As for the in-wheel motors (IWM) driven electric vehicles (EVs), the physical structure is totally different with the conventional internal combustion engine driven vehicles. Thus the design, manufacturing and control strategy should be reconsidered to improve the quality of IWM EVs. Note that the in-wheel motors are mountedEVs, then more flexibility actuation can be realized due to the electric motors' fast and precise torque responses. Consequently, the IWM EVs are revealed a promising vehicle architecture for its capability in considerably enhancing the vehicle maneuverability, stability and safety in defective driving conditions. Moreover, when the Steer-By-Wire system (SBW) is equipped on the IWM EVs,vehicle handling performance can be enhanced by the fast response of electric steering motors.However, there is no mechanical connection in the SBW system, which might degrade the reliability of the vehicle steering system, or even cause dangerous accident when the SBW system completely fails in high speed driving.This paper investigates the vehicle stability control and handling issue for IWM EVs based on the differential steering in the presence of the complete failure of the SBW system. Differential steering, generated from the differential torque between the left and right wheels in IWM EVs, is an emerging steering mechanism. In case that the regular steering system is defective, differential steering can be utilized to act as the sole steering power, and thus avoiding dangerous consequences for vehicles. The main contents and contributions of the paper are as follows.First, the vehicle model and the differential steering model are presented. The steering dynamics is analysized based on the overall model of the tire-suspension-steering system. The tire cornering stiffness,the suspension characteristics and the steering system characteristics are considered in the model. The vehicle yaw model is established based on analysizing the yaw rate,tire later force and the differential moment. The steering model is established based on the steering-suspension dynamics. Combining the yaw model with the steering model, choosing the proper state vectors, the system model is established.Second, a robust H? output-feedback controller based on differential steering is designed.Note that the front-wheel steering angle is a state variable rather than a control input in this work,since we investigate the failure situation of the active steering system in this study. The yaw control in this paper is realized through making the yaw rate and the steering angle track their respective values. The reference yaw rate can be obtained based on the driver's steering command, however,the desired steering angle is uncertain and hard to obtain. That is because the desired steering angle for generating a certain yaw rate is related with the vehicle parameters and states, tire-road friction coefficient, as well as the yaw moment control input. Thus it is difficult to calculate the reference steering angle only by the reference yaw rate. To handle the problem, the output-feedback approach is adopted in this study. Parameter uncertainties for the cornering stiffnesses and the external disturbances are considered to make vehicle robust to different driving conditions.Carsim-Simulink joint simulation is presented to validate the effectiveness of the proposed controller.Third, vehicle differential steering control and the anti-rollover control are simultaneously considered to enhance the performance of the controller. Note that the driven force of the wheel mainly depends on the vertical load of the tire. With severe steering maneuvers, the vertical load on one side of wheels may be decreased sharply. Note that the differential steering schema relays on the differential tire driven forces of the front two wheels. Without the vertical loads, the tire can not provide sufficient friction driven force, resulting in failure of the differential steering. Therefore, it is necessary to balance the vertical load on the front two wheels by rollover prevention system to ensure the efficiency of the differential steering system. To handle the problem, an observer-based feedback control strategy based on differential steering with rollover consideration is designed to achieve lateral control and rollover control, simultaneously. Carsim-Simulink joint simulation is presented to validate the effectiveness of the proposed controller.Finally, the experimental study of the differential steering control problem is investigated. An experimental vehicle is constructed. There are four in-wheel motors equipped on the vehicle. The control system is based on the Matlab/Simulink and the MicroAutobox. Both the open-loop and close-loop experiments are tested with the vehicle. In the open-loop experiment, the accerlation,steering and braking charactertics are tested. In the closed-loop experiment, the control goal is to miminize the yaw rate tracking error. A PID controller is given and the corresponding gains are tested. Once we get the proper control gains, the control performances are validate under several experiment cases. Experimental results verify the effectiveness of the proposed controller.