Yaw-control enhancement for buses by active front-wheel steering

The number of bus accidents is rather small in comparison with that of cars or trucks. However, bus accidents always attract high public attention due to the severity of each accident. According to a bus-accident survey conducted at PTI (The Pennsylvania Transportation Institute), loss of yaw stability is one of the major causes leading to bus accidents. Maintaining yaw stability is difficult and sometimes impossible for a human driver under critical driving situations. In this case, it is very natural to consider the use of automatic driver-assistance systems to avoid accidents. In order to improve the safety level of buses as well as that of the overall transportation system, a yaw-stability enhancement system needs to be developed for buses. However, very few studies have been conducted on handling characteristics and yaw-stability improvement for buses.; This thesis focused on studying bus handling characteristics and developing a yaw-stability enhancement system for heavy-duty transit buses.; In the thesis work, an active front-wheel steering (AFS) system was developed for a typical 40-foot transit bus. A crucial part in the design of an AFS system is dealing with the nonlinear characteristics of the tire forces. Tires are the parts in vehicle dynamics afflicted with the highest degree of nonlinearity due to vehicle motions, tireroad friction, vertical load, and many other factors. It is obvious that an AFS system has to be robust with respect to the huge uncertainty caused by the nonlinearity in the tire forces. Following the approach recommended by Ono et al., the nonlinear tire force was characterized by uncertain cornering stiffness. Ono suggested that if a controller is able to regulate the motions for a linear vehicle model with uncertain cornering stiffnesses, the same controller can be applied to the vehicle model with nonlinear tire forces. Based on this approach, a proportional-integral (PI) controller was designed using the constrained optimization method proposed by Astrom et al.. The designed AFS controller was evaluated on a three degree-of-freedom nonlinear bus model using a series of test scenarios. The computer-simulation results demonstrated the effectiveness of the AFS system in yaw-stability enhancement for buses. In addition, a comparison between the PI controller and a Hinfinity loop-shaping controller revealed that, for the specified test cases, the robustness about road friction variation the simple PI controller achieved was similar to that of the advanced H infinity loop-shaping controller.; This thesis also investigated the handling characteristics of a typical 40-foot transit bus both experimentally and numerically. The experimental data showed that heavy-duty buses exhibit unique handling characteristics, which are different from those of cars or trucks. Compared to a car, a bus has a narrower linear operating range and a much slower yaw response. Compared to a truck, a bus has a much higher rollover-threshold. The results from the computer simulation suggested that vehicle weight has only a minor effect on bus handling during normal operation.