Tasks sequencing in mixed-model assembly systems considering workers' competencies

Mixed-model assembly implies several models or versions of the same product assembled on one assembly line. The models differ on some characteristics despite sharing some basic parts and operations. This implies an additional layer of complexity for production scheduling. Due to the high variability of products, mixed-model assembly lines are often manual, with operators performing most of the assembly tasks. Operators require a certain level of competency to perform each task in its standard time. Operators who are less skilled than required cannot complete tasks within standard time, while overly skilled operators perform tasks faster and vice-versa. Optimising operator assignments based on their skills can enhance overall line performance.

Most research on Mixed-Model Sequencing focuses on product sequencing and overlooks operator assignment and competence variations. Competence is typically evaluated separately for each task without considering task similarities. Task competence can be viewed as a combination of elementary competencies representing individual skills and knowledge. An operator's task competence depends on proficiency in these elementary competencies and their relevance to the task.

Our research focuses on extending the classical Mixed-Model Sequencing problem to include worker assignation and thus, the variations in process time associated with operator competencies. We achieve this by including worker assignation, competence gap and corrected process times according to competence. In addition, we consider task competence to emerge from a set of elemental competencies present throughout the process. The resulting model has an extra degree of freedom, as workers can be matched to the workstations in which they are most capable, preventing line blockages in a way impossible in the classical formulation. Future research shall focus in developing faster solution methods and evaluating the effect of considering elemental competencies.