2018-01-0557 Published 0 3 Apr 2018 © 2018 SAE International; General Motors LLC.A Nonlinear Slip Ratio Observer Based on ISS Method for Electric Vehicles Bingtao Ren and Weiwen Deng Beihang University Hong Chen Jilin UniversityJinsong Wang General Motors LLC Citation: Ren, B., Deng, W., Chen, H., and Wang, J. “A Nonlinear Slip Ratio Observer Based on ISS Method for Electric Vehicles,” SAE Technical Paper 2018-01-0557, 2018, doi:10.4271/2018-01-0557. Abstract Knowledge of the tire slip ratio can greatly improve vehicle longitudinal stability and its dynamic perfor - mance. Most conventional slip ratio observers were mainly designed based on input of non-driven wheel speed and estimated vehicle speed. However, they are not applicable for electric vehicles (EVs) with four in-wheel motors. Also conventional methods on speed estimation via integration of accelerometer signals can often lead to large offset by long-time integral calculation. Further, model uncertainties, including steady state error and unmodeled dynamics, are considered as additive disturbances, and may affect the stability of the system with estimated state error. This paper proposes a novel slip ratio observer based on input-to-state stability (ISS) method for electric vehicles with four-wheel independent driving motors. Instead of estimating vehicle speed, the proposed method employs the estimated error of motor torque as the correction output by taking the advantage of electric vehicles that the torque of the driving motors can directly reflect the tire force. Also vehicle acceleration is directly used as a time-varying parameter of the system to reflect the longitudinal dynamic characteristics of the vehicle. The error dynamics is input-to-state stable subject to the disturbances, such that the nonlinear longitudinal character - istics of each tire can be effectively dealt with. Some extensive simulation has been conducted to verify the proposed slip ratio observer with an AMESim-based electric vehicle model. The results show that the designed nonlinear slip ratio observer has the better performance compared with the conventional EKF method. Introduction Vehicle stability control system is a very important research topic in the automotive industry. Even for intelligent and autonomous driving vehicles, vehicle chassis control remains to be the core in controlling tire-road friction forces. The antilock braking system (ABS) and traction control system (TCS) represent both classic and effec - tive approaches to longitudinal vehicle dynamics control. Among them, knowledge of the tire slip ratio plays a crucial role in achieving precise tire force control to improve the longitudinal stability and dynamic performance of vehicle itself. One of the most critical problems related to the develop - ment of the tire force controllers is the estimation of tire slip ratio, particularly for electric vehicle (EV) with independent in-wheel motors as in this case. Compared with traditional internal combustion engine driven vehicles (ICVs), this kind of EVs can perform better with more precise and faster tire force control. The advantages are that the torque response is very quick, the generated torque can be detected accurately, and motors can be directly controlled by either speed or torque. Therefore, these are favorable for accurate estimation of tire slip ratio information.Most of the conventional slip ratio observers depend on the non-driven wheel speed and vehicle speed [ 1, 2]. However, this kind of method is not applicable when the EVs decelerate or accelerate by the independent in-wheel motors, and also the speed detection through accelerometer cannot avoid the problem of speed offset by the long-time integral calculation. Many studies have focused on the TCS without detecting the vehicle velocity and acceleration, for example, the model-following control (MFC) method [ 3, 4, 5], which only requires torque and wheel rotation state as input variables. Referenc