Traction control is an important element in modern vehicles to enhance drive efficiency, safety, and stability. Traction is produced by friction between tire and road, which is a nonlinear function of wheel slip. In this paper, a sliding-mode control approach is used to design a robust traction controller. The control objective is to operate vehicles such that a desired wheel slip ratio is achieved. A nonlinearity observer is employed to estimate tire tractive forces, which are used in the control law. Simulation and experimental results have illustrated the success of the proposed observer-based controller. 1. Introduction Electric vehicles (EVs) have become very attractive in replacing conventional internal combustion engine vehicles because of environmental and energy issues. They have received a great attention from the research community. Control methodologies have been actively developed and applied to EVs to improve the EVs performances [1–8]. Traction control plays an important role in vehicle motion control because it can directly enhance drive efficiency, safety, and stability [9, 10]. Traction is the vehicular propulsive force produced by friction between tire and road. The characteristics of the friction are nonlinear and uncertain, which make traction control difficult. The friction depends on many factors such as tire type, road surface, road condition, and wheel slip. Accordingly, an objective of the traction control is to operate vehicles such that a desired wheel slip ratio is obtained. The slip ratio yielding the maximum friction coefficient is usually desired because it yields the maximum torque from the propulsion system to drive the vehicle forward. Traction control of electric vehicles has drawn extensive attention since electric motors can produce very quick and precise torques compared to conventional internal combustion engines. In [1], traction control based on a maximum transmission torque estimation (MTTE) approach was proposed. The estimation was carried out by an open-loop disturbance observer, which required only the input torque and the wheel speed. The estimated maximum transmission torque was used in the control law as a constraint to prevent the slip. Experimental results illustrated the effectiveness and practicality of the proposed control design. The MTTE approach was extended by replacing the open-loop observer with a closed-loop observer in [2]. By doing this, the robustness of the control system was markedly enhanced. In [3], traction control of electric vehicles using a sliding-mode observer to estimate the
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