Driving and Braking Torque Distribution Methods for Front- and Rear-Wheel-Independent Drive-Type Electric Vehicles on Roads With Low Friction Coefficient

This paper focuses on the development of a front- and rear-wheel-independent drive-type electric vehicle (EV) (FRID EV) as a next-generation EV. The ideal characteristics of a FRID EV promote good performance and safety and are the result of structural features that independently control the driving and braking torques of the front and rear wheels. The first characteristic is the failsafe function. This function enables vehicles to continue running without any unexpected or sudden stops, even if one of the propulsion systems fails. The second characteristic is a function that performs efficient acceleration and deceleration on all road surfaces. This function works by distributing the driving or braking torques to the front and rear wheels, taking into consideration load movement. The third characteristic ensures that the vehicle runs safely on roads with a low friction coefficient (μ), such as icy roads. In this paper, we propose a driving torque distribution method when cornering and a braking torque distribution method; these methods are related to the third characteristic, and they are particularly effective when driving on roads with ultralow μ. We verify the effectiveness of the proposed torque control methods through simulations and experiments on the ultralow-μ road surface with a μ of 0.1.

I. Introduction

Electric vehicles (EVs) are becoming important, not only as an environmental measure against global warming but also as an industrial policy [1]. In order for them to be used widely, the next-generation EVs must be safe and perform well. The propulsion force generation mechanism (i.e., motor-drive structure), which strongly influences the safety and running performance of the vehicle, is crucial to meeting such requirements. However, completed systematic research into the propulsion force generating mechanism of EVs has been limited [2]–[5], and studies on failsafe control in the event of failure of the electronic parts that constitute the drive systems of EVs [6]–[10] are nearly completed. Since the unexpected behavior of the vehicle that occurs at the time of failure may induce a serious traffic accident, the failsafe structure of the drive systems that generates the propulsion force should also be examined, taking vehicle stability at the time of failure into consideration. In addition, the drive structure should be constituted so that the running performance is sufficient in vehicles that must continue running in a stable way on various road surfaces. Fig. 1.

Propulsion force generation mechanisms in conventional EVs and the FRID EV. (a) Front- or rear-wheel drive-type EV. (b) Two or four in-wheel drive-type EV. (c) FRID EV.

Fig. 2.

Issues on the safety occurring during running on low- road surfaces. (a) Steering operations. (b) ABS operations.