Multi-Objective Optimization Design Of Lightweight Bus Chassis Frame Based On Multi-section Design

With the vigorous development of the bus industry,the concept of low-carbon travel is increasingly gaining weight in people’s minds.The lightweight structural design of electric buses is of great significance for a series of topics such as improving the mileage of buses,saving energy,reducing emissions,protecting the environment,and protecting battery life.Therefore,the topic "Research on lightweight technology of composite structure plug-in bus body" benefits from the strong promotion of the national key research and development plan project "High cost performance commercial vehicle hybrid system development and vehicle integration".Based on the bus frame model provided by a domestic bus company,this paper conducts a lightweight structural optimization design for the bus bottom frame,striving to achieve a reduction in the structural mass of the bus bottom frame while ensuring the performance of the entire vehicle,thereby achieving a lightweight design of the entire vehicle mass.This article uses the widely used finite element software hypermesh in the industry to perform finite element mesh generation on the 3D model of the bus skeleton provided by the enterprise,and simulates the typical operating conditions that are easily encountered during the driving process of the bus by applying appropriate external loads and constraints,namely,full load horizontal bending conditions,emergency braking conditions,extreme torsion conditions,and emergency steering conditions.By solving the strength,stiffness,and free vibration modal frequencies of the bus frame under the four typical operating conditions described above.Through the calculation results,the accuracy of the bus finite element model establishment and the mechanical performance of the bus model are analyzed,which lays the foundation for the lightweight optimization design of the structure in the following text,and serves as a control group for the optimization results in the following text.Divide the design space into objective functions for the passenger car underbody skeleton,comprehensively consider four working conditions,conduct topology optimization design for the passenger car underbody skeleton,and reshape the middle structure of the passenger car underbody skeleton based on the results of the topology optimization design.Modify the layout of the beams in the middle area of the passenger car underbody skeleton,and couple the new passenger car underbody skeleton with the original passenger car surrounding skeleton,Solve the mechanical properties of the optimized finite element model of the bus skeleton under typical working conditions,and ultimately achieve weight reduction based on topology optimization to improve the stiffness and strength of the bus skeleton.On the basis of topology optimization,a two-step sensitivity analysis was conducted for the structure of the bus underbody skeleton to screen out the beam structures involved in the multi-objective optimization design of the underbody skeleton and obtain the optimized dimensional variables.Firstly,group all skeleton beam structures according to their thickness and perform relative sensitivity analysis to complete the first screening of beam structures;Secondly,the selected beam structures are grouped according to their symmetry and functionality,and a second relative sensitivity analysis is completed to determine the final optimal design variables.Orthogonal experimental design was used to optimize the cross-sectional shape of the beam structure selected through two sensitivity analyses.For each beam structure,three levels of square section beam,"cap" shaped section beam,and "C" shaped section beam were designed,with a total of 7 factors.Analyze the importance of each factor and level,and choose the most powerful combination for lightweight passenger car underbody framework.By coupling the designed bus underbody frame with the four sides of the original bus,the stiffness,strength,and free vibration modal frequencies of the new bus structure under four typical operating conditions are calculated,achieving a controllable reduction in stiffness and strength while reducing the mass of the bus frame.The stability and accuracy of the Gaussian process regression model and the traditional kriging model are compared with a specific example.Using the thickness as a variable for the seven beam groups selected above,the design goal is to have the lowest mass of the entire bus skeleton and the lowest strain value under torsional conditions.The proposed constraints are the confidence information predicted by the Gaussian process regression model,the first order torsional modal frequency,and the first order bending modal frequency.According to the above design goals and constraints,a multi-objective optimization design is conducted for the bus underbody skeleton.After the establishment of the proxy model,particle swarm optimization is used to optimize the established proxy model to obtain a pareto solution set.After rounding off the optimized thickness value,it is substituted into the bus underbody skeleton model and coupled with the original bus vehicle skeleton model to solve the stiffness and strength information of the bus under four typical operating conditions,as well as the first six order free vibration modal frequencies of the bus vehicle skeleton.The final results show that the mass of the passenger car underbody skeleton is reduced by 120 Kg,the weight reduction rate is 9.3%,and the performance losses such as stiffness and strength are within the allowable range,achieving a good overall lightweight effect.