This paper aims to realize the four-wheel differential steering function of an in-wheel motor (IWM) driven electric vehicle (EV) with a steer-by-wire (SBW) system. The dynamic models of four-wheel differential steering vehicle (4WDSV) and four-wheel-steering vehicle (4WSV) are established, and a frontwheel-steering vehicle (2WSV) with the neutral steering characteristics and a drop filter of the sideslip angle is employed as the reference model to provide the ideal yaw rate and sideslip angle.

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... ccording to the reference model, the decoupling and fractional PI  D  controllers are designed for the 4WSV to obtain the standard inputs, i.e., the front and rear wheel steering angles, which is finally drawn as the MAP. Furthermore, the sliding mode controller (SMC) and torque distributor are designed to control the 4WDSV to have the same yaw rate as the 4WSV, whose inputs are consulted from the MAP drawn in advance according to the vehicle speed. The simulation results of several vehicle models with/without the controller of two working conditions (reverse and in phase steering) are presented, which indicates that the proposed decoupling and fractional PI  D  controller for the 4WSV is effective to obtain its standard inputs, and the proposed SMC and torque distributor for the 4WDSV ensure that it has the same yaw rate as the 4WSV, as well as the good robustness. INDEX TERMS Four-wheel differential steering, in-wheel motor driven, decoupling, fractional PI  D  control, sliding model control.