INTRODUCTION The vehicle steering system has gone through manual steering, hydraulic steering, electronic-hydraulic steering, electric power steering (EPS) and the latest steering-by-wire system (SBS) [ 1]. However, in order to realize EPS or SBS, one or more extra motors are required, which makes the steering system structure more complicated and expensive. The distributed drive electric vehicle (DDEV), which is driven by four in-wheel motors, has advantages not only in environmental protection but also in dynamics control: Driving or braking torque on each wheel can be controlled independently, more quickly and more precisely. With those properties, unequal driving torque on each front wheel can be generated. As the existence of the kingpin lateral offset, the unequal moments to the kingpins will be produced. So there will be a surplus torque, which is called the differential assisted torque, can help the driver to steer without requirements for extra motors or mechanical structures [ 2,3]. In this paper, a closed-loop control method of differential drive assist steering (DDAS) was developed for a distributed drive electric vehicle to improve the vehicle steering properties. Simulations and experiments were conducted to verify the validity of the control strategy.PRINCIPLE AND FEASIBILITY ANALYSIS Principle of DDAS For a distributed drive electric vehicle, the longitudinal force of each wheel can be controlled according to control requirements within a motor's capacity. Because of the existence of the kingpin lateral offset, the two unequal longitudinal forces produce unequal steering moments to each kingpin separately if the longitudinal forces on the left and right front wheels are different. The superposition of the unequal moments is a surplus steering moment which can be transmitted to the steering wheel at last and help the driver to make an anticipated steering angle ( Figure 1). Figure 1. Schematic diagram of DDAS.Differential Drive Assisted Steering Control for an In-wheel Motor Electric Vehicle Bo Leng, Lu Xiong, Chi Jin, Jun Liu, and Zhuoping Yu Tongji University ABSTRACT For an electric vehicle driven by four in-wheel motors, the torque of each wheel can be controlled precisely and independently. A closed-loop control method of differential drive assisted steering (DDAS) has been proposed to improve vehicle steering properties based on those advantages. With consideration of acceleration requirement, a three dimensional characteristic curve that indicates the relation between torque and angle of the steering wheel at different vehicle speeds was designed as a basis of the control system. In order to deal with the saturation of motor's output torque under certain conditions, an anti-windup PI control algorithm was designed. Simulations and vehicle tests, including pivot steering test, lemniscate test and central steering test were carried out to verify the performance of the DDAS in steering portability and road feeling. Results showed that with the proposed DDAS, the reference steering wheel torque can be tracked satisfactorily, drivers' handling burdens were reduced efficiently and the vehicle steering performance was improved. CITATION: Leng, B., Xiong, L., Jin, C., Liu, J. et al., "Differential Drive Assisted Steering Control for an In-wheel Motor Electric Vehicle," SAE Int. J. Passeng. Cars – Electron. Electr. Syst. 8(2):2015, doi:10.4271/2015-01-1599.2015-01-1599 Published 04/14/2015 Copyright © 2015 SAE International doi:10.4271/2015-01-1599 saepcelec.saejournals.org 433Downloaded from SAE International by Brought to you by the University of Kansas (Technical reports: 1998 to Present), Sunday, August 26, 2018Feasibility Analysis of Structure The effectiveness of the differential assisted steering is closely related to the layout form of the electrical motor in a distributed drive electric vehicle. In general, there are two kinds of layout forms: In-wheel motor layout ( Figure 2) and wheel-side motor