In this paper, a four-wheel driving force distribution method based on driving stiffness and slip ratio estimation is proposed. In previously proposed distribution method, vehicle velocity is measured by an expensive optical sensor and moreover the response speed of distribution is limited by that of Driving Force Control (DFC), the traction control proposed by the authors' research group. Therefore, driving stiffness and slip ratio estimation is applied to improve the distribution method. Due to the slip ratio estimation, vehicle velocity sensor is not needed and the distribution speed depends on that of driving stiffness estimation, which is faster than DFC. If the length of a slippery surface is shorter than the vehicle's wheel base, the total driving force is retained by distributing the shortage of driving force to the wheels that still have traction. On the other hand, when either the left or right side run on a slippery surface, yaw-moment is suppressed. The effectiveness of the improved method is verified by experiments.
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