NVH analysis of steering system and instrument pan

2022-08-24
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NVH analysis of automobile steering system and instrument panel assembly

Abstract: using MSC Nastran finite element analysis method, the modal, energy analysis and structural optimization of the steering system and instrument panel assembly structure of a certain vehicle model are carried out. Improve the natural frequency of the steering system, avoid the resonance area between idle speed and BIW, improve the subjective feeling of NVH, and make the performance meet the design requirements

Keywords: NASTRAN, NVH, steering system, modal, finite element

1 Overview

with the development of the automotive industry, people are more and more strict in the control of automotive NVH. Improving the NVH performance of products to meet the users' hearing, comfort and operational stability is the research and development direction in the automotive NVH field. In the process of vehicle development and design, it is essential to investigate, study and optimize the NVH of IP (instrument panel) system and steering system. Steering system and instrument panel system are important components of automobile body. The vibration and noise of this system can give people the most direct feeling of this kind of technology beyond the traditional method of using prepreg carbon fiber or resin to convert injection molding - safety, comfort and stability. In this paper, it is considered as a system, and the finite element method is used to analyze and optimize the vibration frequency of the steering system and the instrument panel assembly, so as to avoid the idle excitation frequency of the engine and meet the design requirements

2. Steering system vibration and its modal analysis

2.1 analysis of steering system excitation source

engine idle speed and road roughness excitation are the main reasons for the vibration of the steering system and instrument panel system. The idle speed vibration of the steering system and instrument panel system is mainly generated by the reciprocating inertia force excitation under the idling condition of the engine, and its frequency is related to the number of engine cylinders and idling speed of the vehicle [2]

the calculation formula of excitation frequency of four stroke engine is:

where n is the engine speed (r/min); M is the number of cylinders of the engine

the idle speed of a four cylinder engine is 800rpm, and the excitation frequency is 26.7hz; When the air conditioner is turned on, the idle speed is 850rpm and the excitation frequency is 28.3hz. According to the modal planning, the bend twist mode of the trimmed body should be within the Hz range. In order to avoid engine idle speed frequency and interior body frequency, the yaw or vertical bending frequency of the steering wheel shall be at least 36Hz

2.2 steering system and instrument panel model processing

2.2.1 steering system model

the frequency changes of steering column and steering wheel are the focus of this case, and the model must be detailed. The universal joints are rigidly connected and release the rotational freedom between each other. The steering column and the steering wheel are rigidly connected to release the rotational freedom of the steering wheel. See Figure 1 for the finite element model of the steering system

Figure 1 steering system model of a certain vehicle model

2.2.2 instrument panel crossbeam (CCB) model

the steering system is installed on the CCB through a bracket a. There are vehicle body connection brackets at both ends of CCB, middle channel connection bracket B, electrical box bracket, safety air bracket E and IP connection bracket C, D, etc. See Figure 2 for CCB finite element model

Figure 2 a model of instrument panel crossbeam

2.2.3 instrument panel system model

instrument panel (IP) is the main interior trim of the car. Airbags, various electronic instruments, entertainment systems, glove boxes and metal brackets are all installed with IP. These non structural parts are replaced by centralized mass, and their centroid positions are connected through each installation point. See Figure 3 for the finite element model of the instrument panel system

Figure 3 model of instrument panel assembly of a vehicle model

2.2.4 steering system and instrument panel system combination model

steering system is connected with CCB through bracket. The instrument panel is connected with the CCB through the brackets at both ends of the CCB, constraining the 6 degrees of freedom at the connection point with the vehicle body. See Figure 4 for the finite element model of the assembled steering system and instrument panel system

Figure 4 steering system + instrument panel assembly model of a vehicle type

2.3 mode, frequency and energy analysis of steering system and instrument panel assembly

analyze the mode and frequency of steering system and instrument panel assembly system, and get the steering column vertical (Z-direction) frequency of 34.57hz, which does not meet the design goal

Figure 5 the vertical frequency of the steering wheel is 34.57hz

the reason why it has become the main equipment for material testing is that it has the advantages of easy operation and high precision. The energy analysis of the steering system and instrument panel assembly system is carried out to find out the energy concentration position in the structure. From the modal strain energy nephogram in Figure 6 and Figure 7, it can be seen that the energy of the structure is mainly concentrated in CCB mounting bracket a, bracket B, bracket C, bracket D and bracket e, in which bracket a and bracket e contribute the most to resonance, and their energy concentration location is the weak link of the structure, so the optimization analysis of the weak parts of the structure is carried out

Figure 6 cloud diagram of modal strain energy of the original design scheme a

Figure 7 cloud diagram of modal strain energy of the original design scheme b

3 Optimization analysis of steering system structure

aiming at the weak position of the structure, the following three optimization schemes are proposed

scheme A: change the support B connected between the left side of CCB and the channel in the car body into a closed structure to strengthen the connection stiffness between it and the front floor of the car body, as shown in Figure 8

figure 8. Modification schemes a and B of steering column bracket

scheme B: for the insufficient connection stiffness between instrument panel and CCB, increase the thickness of instrument panel and CCB connection bracket C from 1mm to 2mm, and change L-shaped bracket d to U-shaped structure, as shown in Figure 8

scheme C: in view of the insufficient stiffness of the connection between the steering column and CCB, the thickness of the steering column mounting bracket a is changed from 2.5mm to 3.5mm, and the passenger safety airbag (PAB) bracket e is changed to a new structure to improve its stiffness, as shown in Figure 9

Figure 9 steering column support modification scheme c

combining the above three optimization schemes, the vertical vibration frequency of the steering column can be increased to 38.21hz, meeting the target requirements. All employees of the company are determined to achieve win-win with customers with excellent products and services

Figure 10 modal strain energy comparison results

4 Conclusion

steering system and instrument panel system are the most sensitive areas of vehicle vibration, and CCB is the main bearing part of steering system and instrument panel system. Therefore, in the early stage of design, we should focus on the natural frequency of its structure, and at the same time, we should also pay attention to the natural frequency of other connecting brackets of CCB, so as to avoid the CCB structure difficult to change after freezing in the later stage of design

in this paper, the instrument panel and steering system structure are considered as a system, and the modal strain energy method is used to optimize the CCB connection bracket structure, which improves the vertical frequency of the steering wheel, achieves the design goal, and improves the vehicle NVH performance

references

[1] Fenji fujikawa Analysis of steering column Vibration[J]. Motion & Control No..

[2] Tian Guannan et al Analysis and design process for NVH performance of automotive steering system September, 2006 Computer Aided Engineering Volume 15 (end)

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