Minimizing operators' walking times into a linear system layout

This paper deals with the assignment of stations to operators into flexible manufacturing systems by minimizing the operators’ walking times over a given planning horizon. The manufacturing system is supposed to present a linear layout where the stations are aligned along a longitudinal axis. Stations are flexible (so that different jobs can be scheduled on the same station along the planning horizon) and may differ from each other in terms of degree of automation and, consequently, in terms of amount of human labor involved. Thus, two or more stations might share the same operator within the working shift and, consequently, the assignment of the optimal subset of stations to each operator is anything but trivial. Hence, a mixed-integer linear programming model is proposed which takes into account (and minimizes) the travel walking distances of the operators according to the assigned subset of stations. Other realistic constraints, such as the actual availability of the operators and the requirement stating that each scheduled job has to be assigned to a single operator for its entire duration, are included in the model. The validity of the model is proved by discussing a real-world case study from the plastic industry.