Cost-effective Sheet Metal Assembly by Automatic Path Planning and Line Balancing, Integrated with Dimensional Variation Analysis

Doctoral thesis, 2011

Automobiles are an integral part of modern society and its way of life. In addition, automotive manufacturers are important institutions in society, summing the work of a vast array of sub suppliers, employing many. The margins are however moderate and competition is fierce. The automotive industry furthermore faces shifts of paradigms regarding propulsion as well as styling. Fundamental technical innovations, primarily to meet environmental requirements, that are moreover not a trivial issue to foresee, will affect all parts of the operation. It is thus important that product realization can respond quickly and effectively to market impulses using no more resources than necessary. Virtual models of products and processes are now developed long before any physical prototypes are built. However, whereas product and process now are represented in a virtual environment, the design parameters are still separately determined based on experience rather than mathematical analysis. Moreover, cost/capacity analysis is mainly separated from quality analysis, leading to either suboptimal cost/capacity or suboptimal quality. This work targets the car body, i.e. the Body-in-White, which is perhaps the most defining part of any car, with a significant influence on safety, aesthetics, handling, fuel economy and top speed. In the automotive industry, sheet metal assembly design has moved in the past 20 years from a physical to a virtual engineering environment. Typically, an automotive Body-in-White consists of about 300 steel sheet metal parts, joined by about 4000 spot welds. The workload is distributed to several hundred industrial joining and handling robots in about 80 stations, mainly organized in production lines. Sheet metal assembly is indeed an investment intense type of production. Thus it is important that the equipment is efficiently utilized. The balancing of weld work load between the executing stations and robots has a significant influence on achievable production rate and equipment utilization. Therefore the overall research question of this work is: How can Automatic Path Planning & Line Balancing, integrated with Dimensional Variation Analysis, make sheet metal assembly more cost-effective? This work presents a world first automatic simulation based sheet metal assembly line balancing method, which significantly increases assembly equipment utilization. Applied on industrial stud welding lines, the method produces line cycle times significantly better than those of the corresponding manually optimized running production programs. Using the proposed method on a stud welding line of 3 stations, 10 robots and about 200 stud welds, the workload can be executed with one station (of 2 robots) less and still with lower cycle time than that of the current running production programs. Furthermore, the time required to balance a line is reduced from several months to about one day. Due to the significant reduction in line balancing times, the method also has a significant impact on detailed process design iteration times, supporting concurrent product and process design. The method is moreover implemented in a widely used CAE-tool and is thus ready for broad based industrial application. This work furthermore increases knowledge regarding the trade-off between assembly equipment utilization and geometrical assembly variation, and how the two criteria can be jointly treated, in particular with respect to welding sequences. Furthermore, chronological and functional couplings between the design parameters that influence the two criteria are identified.