Ride Comfort Optimization of Buses
Sammanfattning: As the design of buses is constantly improving, passengers are becoming more fastidious and the demand for a more comfortable travelling environment is constantly increasing. The main objective of this thesis is to contribute to the understanding of how the design of buses affects the ride comfort characteristics. An optimization toolbox for numerical ride comfort optimization of buses is therefore developed in this thesis work. The optimization toolbox consists basically of three components: a dynamic analysis of a FE model of a bus subjected to road excitations, an evaluation (according to the ISO 2631:1997 comfort standard) of the vibrations to which human bodies are exposed and a module including optimization algorithms and sensitivity calculation methods.Three optimization problems have been studied. In the first problem, the vertical stiffness and the related damping of the power unit mounts are used as design variables and the discomfort within the bus due to simulated road irregularities is minimized. In the second problem, physical dimensions of structural members in bus body structure have been used as design variables and the transient discomfort due to an obstacle on the road is minimized. In the third problem, a ride comfort design sensitivity analysis is carried out with the purpose to find the design variables in the complete bus that most effectively reduce the discomfort in the bus due to different road excitations. The optimization results from the first two problems show that it is possible to reduce the discomfort by optimizing the above mentioned design variables. The reduction of the discomfort by optimizing the power unit insulation was, however, considered to be quite small. The results from the second problem show that the comfort is generally improved when the stiffness of the structural members in the bus body is increased. The results from the sensitivity analysis show that stiffness and damping associated with the rear axle suspension, the structural properties of profiles around the rear axle and the internal damping of the bus body structure had great influence on the discomfort within the bus. The existence of several local optima was discovered in the optimization problems above and a comparison of global optimization algorithms applied to the first optimization problem was therefore carried out. A better optimum than the one that was located by a local optimization algorithm was found and a proposed modified zooming method turned out to be an efficient algorithm for that particular optimization problem. In the last part of this thesis, a roll control system for a bus is considered. A rigid body model of a bus is created and the perceived lateral force during a curve passage is studied. The results show that by using the roll control system the lateral ride discomfort during the curve passage is significantly reduced although the required roll torque from the system is large. The handling characteristics are also affected by the system and further studies to improve the system are therefore recommended.
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