Order release and sequence-dependent setup times

Sequence-dependent setup times lead to a potential for savings in setups if an appropriate sequencing rule is used at the detailed scheduling level. The benefits of savings in setups have to be taken into account by the higher planning levels of a production planning and control system. If we assume a complex production planning situation (complex products, multi-stage production, job-shop production), a workload control concept is often a good way to keep flow times and capacity utilization under control. In such a planning concept, order release has the task to keep work-in-process (WIP) at an appropriate level. In the case of sequence-dependent setup times, the total amount of setup time (for given lot sizes) depends on the level of WIP in the shop, and this relationship must be known to the order release function to determine the appropriate level of WIP in the shop. In this paper, the functional relationship between WIP and total setup time is examined. An approximate queueing model is developed to show the function for a certain setup-saving sequencing rule and the results of the model are compared with simulation results.