Backup strategy for failures in robotic U-shaped assembly line systems

Alexander Gebel, University of Rhode Island


The application of robotic U-shaped line layouts is becoming more important for manufacturing companies. Compared to straight assembly line layouts, U-shaped assembly lines result in cost savings, easier material handling and higher production rates. The reason for this is that U-shaped lines improve visibility and skill sharing between operators, increase production quality, reduce work in process inventory and facilitate problem-solving of appearing production failures which is shown in several researches. Key companies such as Toyota and Boeing are using U-shaped assembly lines to benefit from the advantages of U-shaped line layouts. However, few breakdown strategies are designed especially for U-shaped lines even though machine breakdowns are common. Breakdowns reduce the throughput rate and product quality and therefore strategies are needed which can ensure the targeted throughput and product quality of companies during breakdowns. In this thesis a breakdown strategy IS designed for a robotic U-shaped line which uses versatile backup robots on backup stations to cover the failures of workstation robots. Versatile backup robots are only considered in one prior study for a straight line layout and, in that study, the backup robots demonstrated a better performance than other breakdown strategies used for straight lines. The concept of backup stations with versatile robots is adapted to the robotic U-shaped line layout to identify whether backup robots can be an efficient breakdown strategy for robotic U-shaped lines. This adaptation is the placement of the backup stations between the arms of the U-shaped line layout. An automotive body shop assembly line configuration is selected for the U-shaped line layout. Ten workstations are used in the line configuration. Four positions exist for the placement of backup stations. Each combination of workstations and placement positions have been analyzed to find the most efficient backup strategy for line configuration designed. The analysis starts with the one backup station, then considers two backup stations and finally three backup stations on the four possible placement options. The best option of the one, two and three backup stations are compared with four backup stations and the current breakdown strategies which are the usage of manual repair stations only and the workload reallocation of broken robots by working robots downstream the line. The criteria for the performance comparison are the cycle time and product quality which are generated for a 5%, 10% and 15%-line breakdown. For the generation of the criteria, a genetic algorithm is used which is modified from a straight line layout to the robotic U-shaped line backup strategy and current breakdown strategies. The analyses of the best placement options for the one, two, three and four backup stations options identify that the three and four backup stations options have the best cycle time and product quality for breakdowns, because they cover each workstation without the use of manual repair stations. It is shown that the three backup stations option is the best choice for the designed automotive body shop assembly line configuration. The three backup stations option has the same cycle time and product quality as the four backup stations option, but it uses one less backup station. Furthermore, the robotic U-shaped line backup strategy using three backup stations has a much better performance than the current breakdown strategies. Its cycle time for breakdowns is half as much as the cycle time of the current breakdown strategies and the robotic U-shaped line backup strategy does not use manual repair stations that generate a high product quality consciously. Due to these facts, the robotic U-shaped line backup strategy is an efficient breakdown strategy for the robotic U-shaped line, because it ensures production with a smooth line flow, a continuously high product quality and the avoidance of work in process inventories for breakdowns. Nevertheless, the robotic U-shaped line backup strategy has three major disadvantages. The first disadvantage is that the backup robots have to be maintained after each operating period to ensure that they do not break down. The next disadvantage is the requirements of an intelligent conveyor system so that the backup station can be accessed without disrupting the material flow when a breakdown occurs. The last disadvantage is that the backup robots have to been equipped with several possibly costly tools, to cover the workstation robots. The final decision on which backup strategy to use is therefore conditional on the cost of equipment, but this study can easily be extended to include these factors when the data is available. ^

Subject Area

Industrial engineering

Recommended Citation

Alexander Gebel, "Backup strategy for failures in robotic U-shaped assembly line systems" (2016). Dissertations and Master's Theses (Campus Access). Paper AAI10146193.