Soutenance mémoire
Vibration model
In general, there are two kinds of approaches in dynamic analysis of a vehicle. The first approach relies on experimental analysis and field test; the second one utilizes computer simulation to conduct a numerical analysis. The application of the first approach is not as expanded as the second approach; the first approach is known by its high cost and the required equipments are so expensive. In contrast, the second approach has been popular, because of its low cost and flexibility in testing different scenarios of a model.
However, it must be mentioned that to have a computer analysis of a vehicle, the former of a vehicle must be available which can be expensive . Hence, in order to use the second approach, a vehicle needs to be simplified to develop a vehicle model for simulating the real operation conditions. Based on the vehicle model, the dynamic response at any position in the vehicle can be approximated numerically.
Pseudo excitation method
In the design of Long-span bridges, the spatial effects of earthquakes, including the wave passage effect, the incoherence effect, and the local site effect, must be taken into account. The random vibration method can fully account for the statistical nature as well as the spatial effects of earthquakes; so it has been widely regarded as a very promising method. Unfortunately, the very low computational efficiency has become the bottle-neck of its practical use .
In the last 30 years, a great number of civil engineering projects, dams and long-spam bridges, have been carried out in China; many of these projects are located in earthquake regions. Over the last 20 years, a very efficient method, known as the pseudo-excitation method (PEM) to cope with the above computational difficulty, has been developed. This method can easily compute the 3D random seismic responses of long-span bridges using finite element models. It can be used with up to thousands of degrees of freedom on a small personal computer, in which the seismic spatial effect is accounted for accurately. This method is now being applied and developed in China by a great number of scholars .
Vibration analysis
The mechanical and mathematical model of suspension systems is usually simplified as a multiple-mass and complicated vibration system. Due to road roughness, suspension system may come into complicated vibration, which is disadvantageous to its components such as lower arm. Therefore, it is important and necessary to control the suspension system’s vibration within a limited grade in order to ensure proper operation of lower arm, safety steering and physical health of drivers and passengers, as well as the operating stability of man-vehicle-road system .
Regarding to vehicles movement, the random and changeable of road surface are the main factors to induce vehicle vibration. Therefore, investigation of stochastic vibration induced to suspension system by road roughness has been a significant problem of suspension system design and its performance simulation. In order to satisfy this problem, the Fourier transform analysis must be used to investigate the dynamic characteristics of vibrating problems of suspension system based on stationary random vibration theory.
After completion of a vibration model for lower arm of McPherson suspension system, it is important to derive the frequency characteristic of lower arm vibration responses. It is also necessary to establish power spectrum density function of road excitation and lower arm vibration responses.
Dynamic analysis
Vector analysis mostly has been used to express dynamic behavior of mechanical systems, as well as, suspension system. This method can develop the understanding of suspension operation and its effects on total vehicle performance. The calculations for dynamic analysis will include a series of analyses including:
Velocity analysis
Acceleration analysis
Dynamic force analysis
Most practical vehicles have some form of suspension, particularly when there are four or more wheels. The suspension system in general must reduce the vertical wheel load variations which are imposed to the wheel by bumps of the road. However, the introduction of a suspension system introduces some tasks of its own; each additional interface and component brings some specific load condition for suspension system during its operation.
These three categories are considered as an important load condition of suspension system that has been encountered: wheel load variation, handling load and component loading environment .
Finite element analysis
Finite element analysis is a powerful numerical procedure used to get information about designed components that would be difficult, if not impossible to be determined analytically. In order to perform finite element analysis for every part, important information must be provided as the geometry of the part to be analyzed and the material properties of the part to be analyzed. Properties of part for finite element analysis can be listed as: elastic modulus, shear modulus, Poisson’s ratio and the type for the material of the part for instance being homogenous.
Modeling for FEA requires a thorough understanding and accurate representation of the part to be analyzed. Accurate modeling is not easily to be done, particularly where loading and boundary conditions are concerned. The geometry of the part is divided into thousands of little pieces called « elements »; the vertex of every element is called a node.
Inside the software, there are equations called shape functions that tell the software how to vary the values of x across the element.
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Table des matières
CHAPTER 1 :INTRODUCTION
1.1 Introduction
1.2 Problem definition
1.3 Objectives
1.4 Methodology
CHAPTER 2 :LITERATURE REVIEW
2.1 Introduction
2.2 suspension system
2.3 Road profile
2.4 Vibration model
2.5 Frequency response
2.5.1 Pseudo excitation method
2.6 Optimization
2.6.1 Genetic algorithm
2.6.2 Outline of genetic algorithm
2.7 Vibration analysis
2.8 Dynamic analysis
2.9 Finite element analysis
2.10 Conclusion
CHAPTER 3 :MATHEMATIC OBJECTIVE FUNCTION AND OPTIMIZATION
3.1 Introduction
3.2 Road profile
3.3 Vibration model
3.4 Objective function
3.5 Optimization by using genetic algorithm
3.6 Conclusion
CHAPTER 4 :DYNAMIC ANALYSIS
4.1 Introduction
4.2 Velocity analysis
4.3 Acceleration analysis
4.4 Dynamic analysis
4.4.1 Linear momentum of a rigid body
4.4.2 Angular momentum of a rigid body
4.4.3 Equation of motion
4.5 Case study of McPherson suspension system
4.5.1 Velocity analysis
4.5.2 Acceleration analysis
4.5.3 Dynamic force analysis
4.6 Conclusion
CHAPTER 5 :VIBRATION AND STRESS ANALYSIS
5.1 Introduction
5.2 Vibration analysis
5.1.1 Natural frequency of vibration model
5.1.2 Natural frequency of Lower arm
5.1.3 Unsprung mass vibration
5.3 Stress analysis
5.1.4 Material properties
5.1.5 Force and boundary condition
5.1.6 Mesh properties
5.1.7 Stress condition
5.4 Conclusion
CHAPTER 6 :CONCLUSIONS AND RECOMMENDATIONS
6.1 Introduction
6.2 Contribution and conclusion
6.3 Recommendation for future work
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