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dc.contributor.advisorProf. Chang Soo Han-
dc.description.abstractVehicle rollover is one of the most dangerous accident that can happen to a vehicle and can results into driver fatality. According to National Highway Traffic Safety Administration (NHTSA) data 80% of the rollover accidents take place without interaction of any other vehicle and it only involves the vehicle and the driver, for this reason it is also called single car accident. Vehicle rollover is one of the major cause of road fatalities. Only 3% of the total road accidents are rollover but it account for about 33% of all driver fatalities in USA (NHTSA). 40% of rollover cases are related to high speed driving as per NHTSA database. Vehicle at high speed rollovers as the lateral acceleration goes beyond the threshold limit of lateral acceleration that the vehicle can withstand while turning. Causes of the accidents involve driver inattentiveness, alcohol, misjudging of sharp turn, load variation (cause a change in center of gravity). NHTSA data also shows the majority of such accidents are involved in daily routine driving (going straight or negotiating a turn). This shows that the driver behavior in very significant in such accidents. Tripped rollover are externally triggered by soft soil or road guardrail and so cannot be avoided by the controller action. Secondary safety systems such as air bags, belts and safe structure can help reduce the damage in such accidents. Unlike, untripped rollover are internally triggered and controller proper action can save the vehicle from rollover using primary safety systems. Active safety for rollover prevention involves two steps on rollover detection and rollover mitigation. Several metrics have been devised for rollover detection based on vehicle states such as lateral acceleration, roll angle, lateral load transfer ratios, time-to-rollover and roll energy methods. Simple schemes alerts the driver of impending rollover by generating warning to the driver about the rollover danger. The driver may take action or may ignore these warning if he feels them unnecessary. More advance rollover prevention schemes involve the role of one or combination of actuators installed on vehicle for rollover mitigation such as active steering, differential braking, anti-roll bars, active suspension or braking systems. In rollover avoidance control strategy the controller possesses information about the vehicle dynamics from sensors that measure lateral acceleration, roll-rate, and yaw rate to establish proper control action for rollover avoidance. Most of the rollover prevention schemes are based on the threshold values of lateral acceleration and roll angle for detecting the rollover propensity and followed by controller triggering action for rollover prevention. These systems are designed to trigger only in excessive lateral acceleration, yaw rate and roll angle. The action in such system have a time delay as they are not designed to run routinely. Also the triggering action cause driver discomfort by sudden actuation. Most of the rollover prevention schemes reduce the yaw rate to reduce the lateral load transfer and bring the roll angle down. This cause the reduction in lateral acceleration. This change in yaw rate however changes the driver’s intended path and vehicle moves with comparatively a large radius. This change in trajectory is undesirable as it may cause another accident if there are other vehicle moving on a laned road alongside. Also in autonomous mode of driving the planed trajectory is more important to follow for safety reasons. The approach adopted in this study for rollover safety is decelerating the vehicle rather than changing the yaw rate for lowering the lateral acceleration. For rollover stability we can impose certain constraints on vehicle states such as lateral acceleration, and provided these constraints are met, the rollover stability will be guaranteed. In this study we propose a control method for rollover stability, by coupling the lateral and longitudinal acceleration of vehicle with help of a diagram called g-g diagram - whose abscissa represents the lateral acceleration of vehicle a_y and ordinate represents the longitudinal acceleration a_x. The motion of vehicle at any time can be represented by a point on this diagram. The threshold value of lateral acceleration can be represented as a line on g-g diagram to constraint the vehicle motion. The lateral and longitudinal acceleration are coupled using a virtual potential field that will cause the vehicle to reduce the speed as the vehicle lateral acceleration enters the field band of lateral acceleration. Lateral potential field function are devised to produce a repulsive force between this constraints and point moving in this diagram which represents the vehicle. This repulsive potential field will be realized by the decelerating the vehicle with braking force. The vehicle state will have a natural tendency towards the origin as it is pushed back away from the threshold value of lateral acceleration. This control concept is evaluated on two models; a rear wheel drive EV COMS and eight wheel drive mobile platform. Both of these models are developed in TruckSim software. The simulation results show that this proposed method is effective for rollover stability of ground vehicles.-
dc.titleCoordinated Control of Longitudinal and Lateral Acceleration by using Potential Field Function for Rollover Stability of Ground Vehicles-
dc.contributor.googleauthorMian Ashfaq Ali-
dc.contributor.affiliationVehicle Dynamics-
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