A COMPREHENSIVE ANALYSIS OF GROUP SCHEDULING HEURISTICS IN A JOB-SHOPCELL

This paper describes a broad-based simulation study of the performance of two-stage group scheduling heuristics in a job shop cell. The obje ctive of this study was to examine the direct and interactive effects of a variety of shop factors on the performance of the best, previousl y reported, group scheduling heuristics. A set of traditional single-s tage scheduling heuristics were examined as well. Shop factors conside red include: setup to runtime ratio, cell load level and variability o f inter-arrival times. An assumption common to group scheduling resear ch which provides for an equal division of the part family into subfam ilies is also examined. This is accomplished through the creation of a n alternative scenario where the majority of the parts are assigned to one subfamily, i.e. one subfamily dominates the part family populatio n. The effects of set up to runtime ratio and cell load have been exam ined in previous group scheduling research, but not in conjunction wit h the inter-arrival time variability factor. Further, no study has exa mined the impact of subfamily dominance on group scheduling heuristics in a full-scale simulation study. The results indicate that performan ce comparable to that of the two-stage heuristics can be obtained with the easily implementable single-stage heuristics when factors which l essen the impact of setup times are in place. In particular, the tardi ness performance of two-stage scheduling heuristics deteriorates when subfamily dominance is in effect while the single-stage heuristics exh ibit dramatic improvements in tardiness performance. Low setup to runt ime ratio, shop load, and less variable inter-arrivals all induce dram atic performance gains across all measures among the single-stage heur istics, while yielding only marginal improvement in the performance of the two-stage heuristics. As a result, in many instances when combina tions of these factors are in effect, the single-stage heuristics yiel d similar performance to the two-stage heuristics.