Influence of task time variation in adopting Walking Worker assembly systems: a design approach

Abstract

Unlike traditional Fixed Worker (FW) assembly systems, in which an operator remains in the same workstation performing repetitive tasks, in the Walking Worker (WW) ones the operators travel along the line stations to produce the finished product. Interestingly, such a configuration enables to achieve a greater level of production volume flexibility, without changing the assembly system layout, parts disposition, and tools assignment. The basic requirement of a WW is to be cross-trained and able to fully assemble the product from beginning to end. The WW travel activity, which increases when the number of stations increases, negatively impacts on the system performance. On the other hand, the possibility to have extra-stations, i.e., a greater number of stations compared with the number of operators, makes the WW system able to dynamically balance the work overload differences between the operators.The aim of this paper is to investigate the effect of stations time variability, typical for example of mixed model assembly lines, on the system productivity, comparing the performance of a WW system and a FW one. A simulative study has been developed, considering as variables the task time coefficient of variation, the travel time, the number of stations, and the number of operators, to analyze when and how the WW system can outperform the FW one. The results show an assembly system design approach that defines the better system to adopt (WW or FW), the number of operators and the number of extra-stations to include, as a function of the considered target productivity and task time variation.