Comparative Simulation Study of Production Scheduling in the Hybrid and the Parallel Flow

What is it about?

Scheduling is one of the most important decisions in production control. An approach is proposed for supporting users to solve scheduling problems, by choosing the combination of physical manufacturing system configuration and the material handling system settings. The approach considers two alternative manufacturing scheduling configurations in a two stage product oriented manufacturing system, exploring the hybrid flow shop (HFS) and the parallel flow shop (PFS) environments. For illustrating the application of the proposed approach an industrial case from the automotive components industry is studied. The main aim of this research to compare results of study of production scheduling in the hybrid and the parallel flow, taking into account the makespan minimization criterion. Thus the HFS and the PFS performance is compared and analyzed, mainly in terms of the makespan, as the transportation times vary. The study shows that the performance HFS is clearly better when the work stations’ processing times are unbalanced, either in nature or as a consequence of the addition of transport times just to one of the work station processing time but loses advantage, becoming worse than the performance of the PFS configuration when the work stations’ processing times are balanced, either in nature or as a consequence of the addition of transport times added on the work stations’ processing times. This means that physical layout configurations along with the way transport time are including the work stations’ processing times should be carefully taken into consideration due to its influence on the performance reached by both HFS and PFS configurations.

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Why is it important?

Scheduling is an important and necessary issue to deal with in every production system. Good scheduling ensures good use of resources and timely delivery of orders to customers. Due to the large number of criteria to be considered in scheduling problem, it is recommended to used methods supporting decision-making, which effectiveness is proven in numerous publications. Based on an industrial case in this paper we study and compare the makespan performance of two alternative manufacturing systems configurations, namely the Parallel Flow Shop (PFS) and the Hybrid Flow Shop (HFS: The best performing evaluated efficient scheduling algorithm for both was used, i.e. the General Shifting Bottleneck Routine (SBR), available in the scheduling system LEKIN. The study was based on a physical setting that is unchanged, as it is the routing of the jobs’ operation to the second stage of manufacturing, provided with a set of identical machines that configures the system to be operated as either a PFS or a HFS configuration. One important practical conclusion of this study is that system’s configuration along with the specification of transportation means/time does influence the overall manufacturing system performance, having an influence on what operating scenario to chose to run the manufacturing system. It means that this factors should be carefully taken into consideration due to its influence on the performance reached by both HFS and PFS configurations. In general, and also based on a previous study we are able to state, based on an analysis of 25 problem instances of problems of the same extend of 12 jobs, that the PFS performs better than the HFS when the processing times among stages are well balanced and transport times are homogenously distributed over the working stages’ processing times but loses advantage, and becomes clearly worse than the performance of the PFS configuration as the working stages’ processing times, including or not transport times become less homogeneous and unbalanced. Therefore, we may state that practitioners must be aware of the importance of transport times and the way these times are considered or added over the working stages’ processing times, when operating production systems with direct flow either as Parallel Flow Shops or Hybrid Flow Shops, as this can determine which of this two configurations will be best suited for each production scenario. The authors intend to apply the approach proposed for studying the performance of different operating systems configurations dependent on job routings and transport times, in a fixed layout of a manufacturing system, to more complex systems, namely for more than two processing stages.