Research On The Control Strategy Of EPS System Of Bus Based On Road Adhesion Coefficient Estimation

The Electric power steering system(EPS)has good steering handling performance and has gradually replaced the traditional hydraulic power steering system as the mainstream technology of modern automotive steering system.The existing EPS control strategy generally only considers steering wheel torque and vehicle speed,ignoring the influence of road surface adhesion coefficient.In order to improve the control strategy on low adhesion coefficient roads,this paper estimates the road adhesion coefficient based on neural network and conducts a study on EPS control strategy for passenger cars by exploring the influence of road adhesion coefficient on steering resistance torque.Mechanical analysis is performed by combining the three-degree-of-freedom model of the whole vehicle with the tire model to derive the influencing factors related to the road adhesion coefficient.The data related to the influence factors of the road adhesion coefficient were collected by Trucksim simulation test and driving simulator real driver in-loop test,and the original data were filtered by a combination of limit filtering and recursive weighted average filtering.The input quantities of the neural network are determined as follows: longitudinal acceleration,lateral acceleration,longitudinal vehicle speed,cross-swing angular velocity and wheel angular velocity,and the model training is optimized by K-fold cross-validation and Dropout regularization method with a loss value of 0.0404.The model was validated by simulation with the root mean square error value less than 0.5,which proved that the model has good prediction performance and generalization ability.Based on the estimation model of the road surface adhesion coefficient,the EPS control strategy of the bus is designed,including the steering mode judgment,the steering power control strategy and the active return control strategy.In the steering power control strategy,by analyzing the influence of the road surface adhesion coefficient on the steering resistance torque,a power-assist characteristic curve considering different vehicle speeds,different road surface adhesion coefficients and driver’s preferred torque is designed to improve the road feel quality on low adhesion road surfaces.The adaptive PD return control strategy based on the road surface adhesion coefficient is designed to address the problems of increased return time and return overshoot on low adhesion coefficient road surfaces.To ensure the control effect of the booster motor current closed-loop controller,the motor current vector control algorithm based on BPNNPI is designed.The simulation results show that the EPS control strategy can achieve the expected effect on low adhesion road surface.To verify the effectiveness of the control strategy in this paper,the hardware-in-the-loop test platform of the driving simulator is built based on d SPACE,including Trucksim model configuration,CAN communication model establishment,DBC file design and upper and lower controller software design,and the EPS control strategy of the bus is tested on HIL.In the low adhesion road test,the control strategy provided a larger torque gradient,and the steering lightness and active correction evaluation scores were 100 and 98.85 respectively.The test results show that the control strategy can make the driver experience light steering and clear road feeling,and has good correction performance,which meets the basic requirements of bus EPS system.