Development of a digital laser welding system for body-in-white.

Park, H.S. ; Lee, G.B.

Abstract: To increase the competitiveness of the products, manufacturers try to replace conventional manufacturing system by new technology. In automobile production field, laser welding system is the ideal solution for building car body and the planning method for designing manufacturing system. The main objective of this paper is to present an implementation of laser welding system for automobile side panels with digital manufacturing. The developed system is analyzed using IDEF0 and UML diagram to generate a strategic plan for designing and implementing laser welding system. Based on this plan, the alternative systems are considered and the optimal one is decided by using TOSIS method.

Keywords: Laser welding system, Digital Manufacturing, Layout planning, TOPSIS method.

1. INTRODUCTION

In general, configuration of manufacturing system is accomplished through three stages i.e. planning-, process- and operation level. All requirements of product are grasped in planning level. To define functions of system and relationships of system components based on these requirements, IDEF0 diagram and ULM diagram are used in process level. Using these results, system components are determined and modeled using 3D CAD tools in operation level.

[FIGURE 1 OMITTED]

The selected components are allocated to execute the functionalities of system in correct manner, and then various alternative systems are proposed. The functionalities of alternative systems are tested using OLP (Off-Line Programming) in digital environment modeled with DELMIA. TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) [4] method is used to select optimal manufacturing system (Fig.1).

2. GROUPING LASER WELDING STITCHES THROUGH PRODUCT ANALYSIS

For the application of laser welding, the side panels which require welding and sealing process have to be analyzed exactly for the welding conditions. There are some areas or structures where the application of laser welding not only difficult or even unacceptable , but also generate more unnecessary jigs than that of the existing spot welding. For solving these problems, spot welding is applied to the inapplicable points such as 4 layers and the inappropriate points of the reduction of the number of jigs. After identification of jig points and spot welding points, laser welding stitches are generated in consideration of jig points and spot welding points. Process parameters are chosen in order to guarantee the quality of stitches and 7 groups of process parameters were created

3. PROCESS MODELING FOR THE CONCEPT FOR THE LASER WELDING SYSTEM

[FIGURE 2 OMITTED]

For the configuration of manufacturing system, the UML (Unified Modeling Language) and the IDEF0(ICAM Definition methodology) model are chosen to describe dynamic and static relationships of the assembly system of the car side panels. In IDEF0, the highest diagram describes activity of the whole system. All boxes in the middle diagram present the functions of each station. The lowest diagram details the processes carrying out in the station by identifying the major information involved in each process, i.e. process requirements and constraints, practical welding know-how, existing system components and so on (Fig.2). The activities of components of the assembly system are expressed in the UML to execute the defined process of the stations. As the results of process modeling using IDEF0 and UML, the task and sequence of process for assembling the side panels were decided. After that, the time table for the whole assembly system was generated based on the cycle time and the time required for executing the task of each process. With this data, the fixed process sequence and the available space of working station, the required number of stations and robots was calculated.

4. GENERATION OF THE ALTERNATIVE SYSTEM FOR WELDING SIDE PANELS

In order to evaluate alternative system, MADM (Multiple Attribute Decision Making) [4] method, AHP (Analytic Hierarchy Process) and eigenvector method [4] were used, base on criteria divided into four attribute groups such as cost, time, system and product. The evaluation goal is stated first, and then the relative weight of the criteria is determined in consideration of the comparison between the criteria was applied. After finding out the weight of the criteria, Hwang and Yoon introduced TOPSIS method, based on the concept that the chosen alternative should have the shortest distance to the ideal point and the furthest distance from the negative ideal point. The closeness (C) was decided by the following equation

C = [S.sup.-]/[S.sup.-] - [S.sup.+] (1)

[S.sup.+]: the distance of the alternative system from the ideal solution [S.sup.-]: the distance of the alternative system from the negative ideal solution

The results of the distance [S.sup.+] and [S.sup.-] as well as the closeness were presented a table. From the table, the alternative was selected as optimal due to the high score in load balancing, the simplification of system and the improvement of product quality

5. SYSTEM IMPLEMENTATION

[FIGURE 3 OMITTED]

In order to find out the problems to be occurred when applying, assembly system was implemented in the digital environment with the commercial tool, DELMIA (figure 3).

To implement the digital laser welding system, the optimal components which were selected were modeled by using 3D CAD tool, with data collected from the participated automobile companies.

These 3D models of the components are stored in library and used for new configuration of cells by calling back through a search [7][8]. In order to test the satisfaction of cycle time, insurance of welding quality and minimization of transfer time for material flow, the OLP (Off-Line Programming) technique is applied

6. CONCLUSION

With the purpose of reducing manufacturing cost and to improve the product quality, automotive enterprises are on the effort to change the conventional spot welding to the laser welding system. For the application of this technique, the process varies depending on the shape and material of product. As the result, the stitches needed for laser welding were grouped according to the welding process parameters. The most appropriate equipments were selected through the comparison between the requirements and their capability for welding each group. With the selected equipments, the assembly system for automobile side panels was implemented by using digital manufacturing technology. Under the consideration of the technical and functional requirements, the alternative assembly systems were generated. The optimal assembly system was selected by TOPSIS method for evaluation.

7. ACKNOWLEDGMENTS

This research was supported by the MIC (Ministry of Information and Communication), Korea, under the ITRC (Information Technology Research Center) support program supervised by the IITA (Institute of Information Technology Assessment).

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