This paper presents a novel Fault Tolerant Control (FTC) method for Electric Vehicle (EV) with wheel hub motors and by-wire steering system, in order to improve the vehicle safety and directional stability in case of steering failure. The driving force of this Four Wheel Independent Drive (FWID) EV was generated from four in-wheel motors using by-wire technology. The main objective is to track the desired motion to turn around even the steering breaks down. A dynamic vehicle model with the function of four-wheel driving and front-wheel steering is established. The transient dynamic behaviors of steering failure cases are conducted by comparing with the normal cases. Based on functional redundancy of over-actuated chassis, the potential of fault tolerant control by independent braking was analyzed. Subsequently, a sliding mode based motion controller is designed for generalizing the desired yaw moments. According to the concept of tire usages, an optimal control allocation strategy based on Non-linear quadratic programming is implemented to deal with the steering failure by driving/braking torque allocation. Numerical simulations have been conducted to verify the proposed FTC algorithm. The result suggests that steering failure could be effectively covered by driving/braking torque allocation for the functional redundancy of over-actuated chassis system.
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