Abstract
Keywords: brake force, µ slope, fixed trace method, brake control, tire hysteresis
This paper proposes a method of estimating the road surface friction condition from the brake torque and tire slip ratio, and based on that result, a braking control method for correcting the target slip ratio so as to obtain the shortest possible braking distance for various road condition.
The road surface friction condition is divided into a stable region, where brake force increases accompanying an increase in the slip ratio, and an unstable region. The ratio between the rate of change in the slip ratio and the rate of change in the brake friction coefficient is used to judge which region the present condition is in. (shortly as 'µ slope'.) A robust fixed trace method has been proposed for dealing with such issues as the deterioration of calculation accuracy when the rate of change in the slip ratio is small and noise accompanying a differential calculation. However, this method has remained at the research level because the resultant estimation accuracy is still insufficient.
Against this backdrop, the authors have found that tire hysteresis is one of the reasons why estimation accuracy deteriorates. To deal with this problem, we have devised an algorithm for updating the estimated µ slope value only when the slip ratio increases. The effectiveness of this method has been validated on the basis of vehicle test results. A braking control method has also been developed for correcting the target slip ratio on the basis of the road surface friction condition obtained with the algorithm. Vehicle test results have also confirmed the effectiveness of this control method.