Protective capacity and time buffer design in theory of constraints controlled discrete flow production systems

[摘要] ENGLISH ABSTRACT: To maximise the throughput of a production system the capacity constrained resource needs to beprotected from variation and uncertainty. In the Theory of Constraints philosophy such protection isprovided by means of time buffers and protective capacity. Time buffers are protective time that isallowed in the production schedule to buffer against disruptions, whereas protective capacity isdefined as a given amount of extra capacity at non-constraints above the system constraint's capacity.In this research an analytical procedure was developed to more accurately determine the required timebuffer lengths. This procedure uses an open queuing network modelling approach where workstationsare modelled as GIIG/m queues. A simulation experiment was performed to evaluate the time bufferestimation procedure on the operations of an actual fifteen station flow shop. The results from thestudy suggest that the analytical procedure is sufficiently accurate to provide an initial quick estimateof the needed time buffer lengths at the design stage of the line.This dissertation also investigated the effect of protective capacity levels at a secondary constraintresource as well as at the other non-constraint resources on the mean flow time, the bottleneckprobability of the primary constraint resource, as well as the output of flow production systems usingsimulation models and ANOV A. Two different types of flow production systems were investigated:(1) a flow shop with a fixed number of stations and unlimited queue or buffer space between stations,and (2) an assembly line where a total work content is distributed among stations in a certain fashionand the number of stations are not fixed.The experimental studies show that flow shop performance in the form of flow time and line output isnot that much influenced by low protective capacity levels at the secondary constraint resource. Lowprotective capacity levels at a single station however can significantly reduce the bottleneckprobability for the primary constraint resource when it is located before and relatively close or near tothe primary constraint in the process flow, or after but relatively far from the primary constraint. Anafter-far secondary constraint location also causes slightly longer job flow times, and should thereforebe avoided if possible. The research further shows that quite high protective capacity levels at the nonconstraintresources are needed to ensure a more stable and therefore manageable primary constraint.However low average levels of protective capacity at non-constraint resources are sufficient to ensurethat the maximum designed output level as determined by the utilisation of the primary constraintresource is obtained. The results for the assembly line experiment showed that an unbalanced line configuration where lesswork is assigned to the non-constraint stations than to the primary constraint station (but nonconstraintstations have an equal work content) can lead to significant reductions in the mean flowtime while maintaining the same line output, without resulting in too many additional stations. Lowprotective capacity levels in the range of 2% to 5% are sufficient to cause substantial improvements inflow time without resulting in too many additional stations in the line.

[发布机构] Stellenbosch University