Research On Complex Mechanical System Modeling And Its Application In Multi-axis Chassis Development

Based on the fractal theory, the fractal decomposition method is proposed to decompose the complex mechanical system by the author through the exploratory way. The main idea of fractal decomposition method is to decompose the complexity of the model to the local optimization model in each level uniformly, which makes the model designed and optimized easily through reducing the complexity of each local optimization model. At the same time, the integrity of original model will not be violated, because all the local optimization models are related through the transition index parameters.The proposed method is used in the design of multi-axis chassis. The vehicle dynamic theory of the multi-axis chassis and double-wishbone oil suspension are complemented. The framework and flow of optimization model of multi-axis chassis based on the fractal decomposition method are constructed, and the numerical optimization example of an 8-axis chassis is presented.The original research of this work can be summarized as follows:Firstly, the fractal decomposition method is proposed to decompose the complex mechanical system based on the fractal theory in this thesis. From both the science and technology theory and the practical engineering, a new modeling and optimization framework is proposed, where this framework can solve some problems which are large size, low-solving efficiency, irregular, difficult to optimization, and model decomposed inconsistent with industrial manufacture system when traditional modeling methods are used in complex mechanical systems. In specific optimization, a rotational optimization method among the independent local optimization models is proposed, and the numerical example shows that this method is more efficient than traditional method.Secondly, the vehicle dynamic theory is complemented. The vehicle dynamic theory is extended to the multi-axis vehicle with any wheelbase layout, which provides the theory for multi-axis chassis modeling and simulation. In detail, the optimum distributed condition of each wheel slip angle for multi-axis vehicle is proposed, and some related formulas are introduced; the analysis expression of understeer and correlative handling criteria of multi-axis vehicle are induced; the stabilization condition of straight brake is proposed; to obtain high brake efficiency of each axle, the conditions of each suspension performance and brake force on each axle are proposed, and some related formulas are introduced.Thirdly, the suspension dynamic theory is introduced by screw theory method. The shortcoming of some concepts such as roll center, trim center and so on are indicated, and they are re-defined as lateral force inducing factor and longitude force inducing factor, which transforms the original static performance index to dynamic performance index, so the suspension dynamic theory supports the vehicle dynamic more directly and dynamically. The analysis expressions of lateral force inducing factor and longitude force inducing factor of double wishbone suspension are introduced in this thesis.Lastly, based on the proposed fractal decomposition theory, multi-axis chassis optimization model framework is constructed, and an 8-axis double wishbone hydro-pneumatic suspension chassis is optimized.