ISTS: A new procedure for the integrated scheduling of multi-stage systems

This paper addresses the problem of scheduling a two-stage manufacturing system. Material requirement planning (MRP) systems have been developed to synchronize production in multi-stage systems, assigning due dates to production at each stage. However, MRP systems allow just a broad scheduling and it is therefore up to the short term scheduling system to meet such due dates. Previous research work, and industrial practice, have developed independent scheduling systems for each stage, relying on MRP to synchronize them. Resulting performances are not satisfactory because, quite often, many components are late, and assembly schedules are disrupted. Recent research works on assembly shops have shown that improvements in performances can be achieved if assembly constraints and dependencies between jobs are also considered at the short term scheduling level. Based on the idea that these findings can be valid for multi-stage manufacturing systems, this research work presents a new short term scheduling procedure which, on the one hand adopts two different scheduling systems for the machining stage and the assembly stage, and on the other, highly integrates short term scheduling of different stages. The former exploits the different characteristics of the two sub-systems. In particular, unlike all previous research work, assembly constraints and dependencies between jobs are taken into consideration at the machining stage, both at the dispatching level, and at the Order Review and Release level.