A selection procedure for the design of mixed-model assembly systems considering walking workers and fixed workers

Mixed-model assembly systems are widely used to guarantee higher flexibility, variety and throughput. In such assembly systems, fixed worker (FW) and walking worker (WW) workforce strategies are mainly adopted, influencing tactical and operational decisions with different and sometimes contrasting strengths and weaknesses. Thus, several aspects require a joint investigation to evaluate in which context a WW workforce strategy is preferable to the FW one. In this paper, the effect of assembly and station time variability on WW systems performance is investigated and compared to FW systems through a simulative study. Further, the number of workers and workstations and the effect of the travel time are also analysed. Results show that time variability has an impact on the throughput of both systems, in a different way. Starting from the target throughput and the given time variability among tasks, results derived from data elaboration allow us to propose a decision tree and a selection procedure for choosing between FW and WW systems, highlighting their most proper application conditions. According to the results, the time variability among tasks and the ratio between the number of workers and workstations are the two main drivers influencing the choice between WW and FW strategies.