Nozaki, H., Mizuno, K., and Yoshino, T., "Effect of Rear-Wheel Active Steering and Variable Steering Wheel Gear Ratio on Proportional Derivative Steering Assistance," SAE Technical Paper 2012-01-1936, 2012, doi:10.4271/2012-01-1936.
In a new technology called “in wheel motor,” in which the motor is installed in the wheel, the electric vehicle can become more compact, which leads to a new type of mobility. Moreover, the front wheel steering is controlled by an electrical unit instead of the traditional mechanical unit of a steering wheel inside the car. In such a “steer-by-wire” method, the motor uses an electric signal. Because the degrees-of-freedom of this steer control are increased and a variety of steer controls based on the electric signal are possible, further improvement of the control stability is needed. In other words, the steer control technique can pose a problem for drivers, and so further research in this area is needed. That is, proportional derivative steering assistance can improve emergency evasion performance and the steering delay upon counter steering. Moreover, rear-wheel active steering can improve vehicle response during emergency evasion maneuvers. In the present study, we clarify the difference in effectiveness of proportional derivative steering assistance and rear-wheel active steering. In addition, the effect of combining these methods is investigated. The results of the present study reveal that, although the effect of rear-wheel active steering is significant in the grip area, since the cornering force increase ratio according to the slip angle increase in the critical cornering area decreases, the effect of the critical cornering area is small. However, counter steering by means of proportional derivative steering assistance is clarified to be effective from the grip area to the drift area. As such, the combination of rear-wheel active steering and proportional derivative steering assistance is more effective. Next, in order to achieve early counter steering, when the body slip angle exceeds 10 degrees and the vehicle enters the drift state, the effect of reducing the steering wheel gear ratio was confirmed to be moderately effective. As a result, control during critical cornering was confirmed to be greatly improved. When this is added to proportional derivative steering assistance and rear-wheel active steering is confirmed to be more effective.