Sequencing and scheduling in flowshops with task redistribution
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Sequencing and scheduling in flowshops with task redistribution RL Burdett and E Kozan* Queensland University of Technology, Brisbane, Australia In some manufacturing situations, station tasks or operations can be shifted or redistributed to adjacent stations. This can be done when these stations have the appropriate equipment, and the workers on that station can perform the shifted work to a reasonable level of competency. This paper addresses such an environment and provides a general framework for applying the shifting or redistribution of tasks methodology to the intermediate storage, no-intermediate storage and nowait flowshop problems. The outcome of this research is a way in which to utilise more efficiently the general-purpose facilities of this type of production environment. It includes mathematical models, recurrence equations and solution techniques for sequencing and scheduling. From an extensive numerical investigation, the benefits achieved by the application of this methodology are detailed. Keywords: sequencing; scheduling; flowshop; mixed model assembly; heuristics
Introduction This paper provides a foundation for the more efficient sequencing and scheduling of final assembly operations in a mixed-model assembly process. The basis of this is the reduction of idle time inefficiencies that arise as a result of upstream and downstream blocking of stations, and the utilisation of the available multi-skilled workforce. Figure 1 below displays the difference between the two types of idle time inefficiencies. The term mixed-model, above, refers to the production of more than two model variants, which are intermixed and processed simultaneously on the assembly line. The particular type of mixed-model assembly line (MMAL) that is of interest here is a single stationary line that has moveable products, open stations, and follows a variable rate launching (VRL) discipline (see Dar-El1). Operators work under unpaced conditions, and no intermediate storage generally exists between work stations so that the blocking of stations may occur; however, assembly lines with intermediate storage and no-wait conditions are also addressed.
Figure 1 Upstream versus downstream blocking.
*Correspondence: E Kozan, School of Mathematical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane 4001 Australia. E-mail: [email protected]
In practice, idle time inefficiencies can have a large effect on the schedule makespan even when a good sequence is determined. However, since workers are multi-skilled and perform pure assembly operations, it is possible to reduce the makespan and to improve plant efficiency by shifting or redistributing tasks so that idle time inefficiencies are reduced or eliminated. The classification of three particular and general taskshifting scenarios result from this idea. In the first and second scenario, the shifting of station tasks to upstream or downstream locations respe
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