中文摘要：

低附着路面上,车辆自回正力矩显著降低,传统电动助力转向控制策略不能很好地克服这一不利影响,导致车辆回正性能降低,路感丧失甚至带来驾驶员对车辆的误操纵,或导致车辆不能及时回正;在建立基于整车动力学的电动助力转向系统模型的基础上,经电动机电流和转矩传感器测得转矩值,拟合得到当前路面条件下的自回正力矩,同时通过转向盘转角信息计算理想路面条件下的名义自回正力矩,结合路面估计算法,将被识别的路面附着系数等级分为高、中、低,设计基于路面附着系数的助力电流控制策略和基于时变滑模变结构控制的回正控制策略,仿真结果表明可有效改善车辆在低附着路面上的操纵路感和回正性能。建立基于LabVIEW PXI的电动助力转向（Electric power steering,EPS）硬件在环试验平台,并对控制策略进行试验验证,试验结果和仿真结果基本一致。

英文摘要：

On the slippery road surface,the alignment torque decreases evidently and the conventional control strategy of electric power steering（EPS） can not respond to this particular situation equally in time.Moreover,this status leads to poor steering-wheel returnability and false road feeling,and even results in incorrect handling.The EPS model based on a whole vehicle dynamics is established to obtain the self alignment torque on the current road based on the electric motor current and torque value measured by the torque sensor.Then the ratio of self alignment torque and nominal alignment torque are used to estimate the road friction,which is divided into three levels： high,middle and low,and a new assistant torque control strategy based on the road friction estimation and a time-varying sliding mode return control strategy are proposed.The simulation results show that the new control strategy can improve road feeling and steering-wheel returnability on the slippery road.The Labview PXI hardware-in-the-loop test platform for EPS is established,the assistant torque control strategy based on road friction and the time-varying sliding mode return control strategy are validated on the platform.The test result is consistent with the simulation result.