Design model for assembly lines including fractional tasks and parallel workstations

In recent years, the use of robots and cobots allow to increase productivity and quality of products. Due to the higher investment, the robustness and efficiency of flow lines are crucial to reduce the throughput loss. The solution of installing buffers between stations increases costs and factory space. To improve the efficiency and robustness of assembly lines, the literature proposed some variants to the simple assembly line balancing problem. The introduction of fractional tasks and parallel workstations are two promising models proposed in recent works to reduce throughput loss caused by short failures. The potential of the two approaches has been studied individually, but no work has evaluated the integration of fractional and parallel tasks can further improve the efficiency of the production lines. This paper proposes a matheurstic method to design assembly lines integrating fractional tasks and parallel workstations. The approach proposed aims to reduce the computational complexity of the design of the assembly lines and provides a series of design alternatives. The simulation model tests the robustness of the design alternatives against short failures. The numerical results highlight how the proposed model improves the performance and the robustness of the assembly line when unforeseen events such as failures occur. The integration of fractional tasks and parallel tasks can improve the robustness against short failures. This benefit is relevant for robotic assembly lines, and the increasing use of cobots that are mainly used in the automotive, electronics sector, and metal machinery industries.