Sport utility vehicles typically feature high ground clearances that allow them to be used in off-road conditions. Their use is not limited to off-road conditions and they are often used as day-to-day family vehicles. On the road, where high friction surfaces are prevalent, their high centres of gravity can make them susceptible to un-tripped rollovers during severe dynamic manoeuvers such as an emergency obstacle avoidance. The detection of a high risk of rollover and the avoidance thereof has great potential to improve vehicle safety, as the consequences of rollover incidents are generally quite severe.
Rollover mitigation systems are triggered when a rollover threshold index is exceeded, indicating a high risk of rollover. The metric implemented in this study is known as the zero-moment point method, which allows for vehicle parameters and terrain to be taken into account. Previous research has indicated that mitigation systems that trigger braking intervention are some of the most successful methods in reducing rollover risk, as it not only stabilises the vehicle, but also reduces the speed.
Brake based rollover prevention systems typically implement electronic stability program methods that use yaw rate reduction as the primary tool for reducing rollover risk, which often comes at the expense of the vehicle's path following ability. This means that the stability control system may lead to the vehicle leaving the road and causing an even more severe accident. The control algorithm implemented in this study gives preference to reducing the forward speed of the vehicle which in turn reduces lateral acceleration, a major contributor to rollover propensity. Braking is however apportioned to all four wheels and distributed so as to achieve vehicle yaw rate targets. Emphasis is placed on maintaining good path following capability to prevent the vehicle from leaving the road.
The detection and mitigation system was tested on a Land Rover Defender 110 for a variety of manoeuvers in simulation as well as experimental testing. The results indicate that the rollover mitigation system managed to successfully reduce the rollover threshold index of the vehicle during the manoeuver whilst simultaneously maintaining the path following ability of the vehicle and improved the yaw rate tracking.
Dissertation (MEng)--University of Pretoria, 2017.