Source | Automotive lightweighting online
Abstract:As an important interior part of the automobile, the lightweight seat provides great help for the weight reduction of the whole vehicle, and the lightweight design of the seat can be carried out on the basis of ensuring safety and maintaining the beautiful appearance. Taking the electric seat of a vehicle model as an example, CATIA software is used to model, the established model is imported into HyperMesh for meshing, the finite element model is formed and the modal analysis is carried out, and finally the Optistruct solver is used to optimize the relevant model, and finally the optimized model is obtained to realize the reasonable design of the seat lightweight.
Keywords: Automotive Power Seats, HyperMesh, Lightweighting, Finite Element Simulation
With the improvement of automobile emission standards, the requirements for energy conservation and emission reduction of automobiles are becoming more and more stringent, and various countries are studying how to reduce automobile emissions as much as possible while ensuring the performance of automobiles;
Car seats not only require comfort and aesthetics, but also play a role in connecting people and the body in the car, so the lightweight design of the seat should reduce the weight as much as possible while meeting the relevant requirements and needs [1]. In recent years, the automotive industry has gradually moved closer to the direction of environmental protection and sustainable development, and how to produce products with lower cost, less materials and more advanced processes to meet the performance and requirements has become the focus of R&D and production. Foreign counterparts have proposed a seat design that measures human feedback, which can be adjusted according to different personnel to make passengers more comfortable [2]. At present, Continental is in the basic stage of automotive seat design and R&D, and has not yet made significant breakthroughs in various core technologies.
1. Establishment of seat model
1.1 Creation of 3D models
To establish a 3D model, first select the relevant physical objects, this paper selects an electric seat, after understanding the structure and function of each component, the size measurement and various data are determined, and the CATIA software is used to model the individual parts separately, and finally assemble to form a 3D model of the entire seat, as shown in Figure 1. In the modeling process, the seat was simplified and some parts that were not involved in the subsequent finite element analysis were omitted, such as the motor, one of the seat adjustment devices between the slide rail and the bidet, which was not considered in the modeling process because it had no influence on the subsequent analysis, and the curved structure of the bidet transverse tube was replaced by a straight tube.
Fig.1 3D model of the seat frame assembly
1.2 Establishment of finite element model
The finite element model is built using HyperMesh, a finite element preprocessor for meshing and finite element modeling and subsequent simulation analysis, and HyperMesh provides advanced geometry creation and meshing capabilities for high-fidelity modeling [3]. Once the 3D model of the seat was built and simplified, its finite element model was built in HyperMesh. First, the part format was converted to IGS or STP format and imported into HyperMesh for mid-surface extraction, element type selection, and meshing [4]. After meshing each individual model, it is imported into the software to determine the connection simulation method for assembly. It should be noted that some surfaces may be missing, misaligned, overlapped or deformed during the import process, and the imported geometric model needs to be checked and cleaned up, and after the repair is completed, the process holes and process parts that affect the quality of meshing are processed by relevant commands, and finally the finite element model is established, as shown in Figure 2.
Fig.2. Finite element model of seat assembly
2. Static strength analysis of seats
In the static strength analysis of the seat, the material is first assigned to the mesh model, the skeleton model is constrained and loaded according to the standard requirements, the load collector is established to place the load and constraint members, and finally the load step is established, and the static strength analysis of the model is carried out by the Optistruct solver. By consulting the data, it is obtained that in the static strength test, the main component of the force is the backrest, and the backrest skeleton transmits the load to all parts of the seat when loading, and it is approximately considered that the load is applied to the seat back. The rigid unit REB3 is used to take the seat mass unit as the main node, and the mass unit is evenly connected with each node on the backrest, so that the force is transmitted to each node of the seat backrest, as shown in Figure 3.
Fig.3 The REB3 unit simulates the stress state of the backrest
In HyperView, you can view the displacement contour and stress contour to see the number of nodes and elements of the model. From Fig. 4 and Fig. 5, the displacement changes of each part of the seat after the load is applied, as well as the stress of each part, can be obtained according to GB/T 15083-2006 "Strength Requirements and Test Methods for Automobile Seat System" [5].
Fig.4. Seat assembly displacement contour
Fig.5. Stress contour diagram of seat assembly
The analysis shows that after the seat assembly bears the load of 20 times the seat mass, the maximum displacement occurs at the position where the backrest beam meets the right pillar plate of the backrest, and the maximum displacement is 1.684 mm, and the maximum stress occurs in the middle part of the connecting plate of the recliner on the left and right sides. The material used for the seat is Q235 carbon steel, and its yield limit is 235 MPa, under the load, the maximum stress does not exceed the yield limit of the material, and is within the elastic range of the material, so the skeleton model meets the static strength requirements.
3. Optimization of the seat frame
Through the static strength analysis of the above-mentioned finite element model of the seat frame, it is concluded that the seat model meets the requirements of the national standard for the static strength of the seat. The mass of the seat frame is measured to be 20.95 kg, and the relative mass is large, which will increase the mass of the whole vehicle, affect the rolling resistance and air resistance of the whole vehicle, and increase the fuel consumption of the whole vehicle [6], which is not in line with the concept of economy and environmental protection. By consulting the data [7], it is concluded that among the various lightweight methods of the seat frame, the material lightweight has the best effect. In the selection of materials, it is necessary to combine factors such as cost, structural strength, stiffness, and weight reduction effect of materials. Magnesium alloy, high-strength steel, and aluminum alloy were compared and analyzed, and it was found that magnesium alloy had high cost and high density of high-strength steel [8], while aluminum alloy and cold-rolled steel plate had similar structural strength and stiffness, and had a good weight reduction effect. In the same way, the analysis of the seat after the replacement of materials is carried out to verify whether the characteristics of the model after the optimized material meet the requirements on the one hand, and to compare the modal parameters of the structure with the model before optimization on the other hand, as shown in Figure 6 and Figure 7.
Fig.6. Static strength displacement contour of the seat after material replacement
Fig.7. Static strength stress contour of the seat after material replacement
After the material was replaced, the weight of the seat assembly was reduced from 20.958 kg to 14.107 kg, and the weight was reduced by 32.69%, and its static strength analysis met the national standard. At the same time, the free mode analysis of the optimized seat model obtains the first 10 free mode shapes, and compares them with the modes of the steel seat frame, and it is found that the frequencies of the first orders of the two are similar, and the dynamic performance of the two is similar, and in terms of riding comfort, the first order frequency of the optimized seat exceeds the human resonance frequency range (0~30 Hz), which meets the requirements. Through static strength analysis and free modal analysis, it is concluded that the optimized seat model meets the requirements of national standards for strength, stiffness, and comfort [9].
4. Conclusion
An electric seat is extracted to establish a 3D model, a finite element model is established through HyperMesh, and the established seat model is tested for static strength in accordance with the national standard to meet the requirements of the standard. After that, the material lightweight design was carried out, and the weight of the seat was reduced by 32.96%. After research, the following conclusions were drawn:
(1) CATIA is used to establish a simplified 3D model of the seat, omitting the components that have little influence on the static strength of the seat, such as the motor, seat adjustment mechanism, and elastic elements at the recliner, and accurately modeling the recliner assembly, backrest assembly, bidet and slide rail. The model was built to meet the requirements of the standard for the static strength of the seat.
(2) Through the static strength analysis of the structure, it is found that the main load-bearing parts of the seat are the connection parts of the backrest pillar side plate, the recliner assembly, the backrest side plate and the sitting basin. For the main body of the bidet and the lower part of the slide rail, the application material can be replaced with 6061 aluminum alloy. In terms of cost, aluminum alloy is lower than magnesium alloy and acceptable, in addition, aluminum alloy has the advantages of high impact absorption capacity and good recycling performance. 6061 aluminum alloy is used to realize the lightweight of the seat frame, and on the basis of meeting the static strength requirements of the national standard, the total weight of the seat is reduced by nearly 1/3.
(3) The size of the seat back assembly is optimized and turned into a hollow structure, which not only improves the utilization rate of materials, reduces costs, but also reduces weight under the premise of meeting the static strength requirements.
Source: Periodical-Beijing Motor, Author: Chen Hongfei1, Chen Yisong2
（1. Shaanxi Automobile Holding Group Co., Ltd.;2. School of Automotive, Chang'an University)
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