Abstract: 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.

Key words: Control strategy, Electric power steering system, Returnability, Road feeling, Slippery road