Batching and scheduling in FMS hubs: Flow time considerations

Many flexible manufacturing systems (FMS) involve complex queueing networks in the processing of lots (jobs) in a production facility. In particular, the feedback of lots to a common, multipurpose workstation, or hub, processing different tasks for the same lot at various points in the production process, can create serious scheduling problems. This paper models the hub structure and presents some results for dual measures of flow time in the facility, namely, total flow time required for a fixed number of lots (or makespan), and average flow time for all lots (or average completion time). The flow time performance is a function of different batch sizes and priority scheduling rules for lots arriving at the hub. The results can be used to establish strategies for batching and scheduling lots according to adjustments in the importance of the cycle time versus the work-in-process tradeoff. Extensions of the results to more general production facilities are also discussed.