A solution approach for scheduling assembly line feeding operations

The problem addressed in this thesis is motivated by a real problem in the assembly lines of a plant that produces washing machines. The problem is the scheduling of the operations for feeding the materials to the assembly lines. There are several assembly lines in the system with, potentially, stations in both sides of the line. Multiple models are produced in the lines in a single shift. The component and parts used in the assembly are transfered from a central depot location to the lines in two different size containers using the material handling equipment called transpallets. Each transporter carries material to the stations that are assigned to his responsibility. The scheduling of material feeding to the lines is handled as two separate problems. First problem is about balancing the workload of transporters to carry the required materials to the lines during a shift based on the production plan. The second main problem is the scheduling of the tours to carry the required materials for each transporter separately. For the first problem, we developed a mathematical model that minimizes the absolute differences of the assigned containers to different transporters in a shift. We also suggested an effective heuristic approach for the load balancing problem and compared its results with optimal solutions. The second main problem has two objectives; First objective is minimizing the number of tours that the transporter makes in a shift, while the second objective is minimizing the inventory accumulations at the assembly stations. For the second problem we introduced a mathematical model based on dividing the shift into equal time intervals This model takes the tour minimization as the objective while assuming a limit for the inventory at the stations as a constraint. The problem is shown to be NP-hard. We also suggested a heuristic solution approach for the problem in order to solve the bigger problems of real life in short amount of time and we showed the effectiveness of the solution approach on a set of problems based on a real data set obtained from the washing machine producing plant. Keywords: Assembly line balancing, Material feed scheduling