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

Louw, Louis (2003-04)

Thesis (PhD)--Stellenbosch University, 2005.

Thesis

ENGLISH ABSTRACT: To maximise the throughput of a production system the capacity constrained resource needs to be protected from variation and uncertainty. In the Theory of Constraints philosophy such protection is provided by means of time buffers and protective capacity. Time buffers are protective time that is allowed in the production schedule to buffer against disruptions, whereas protective capacity is defined 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 time buffer lengths. This procedure uses an open queuing network modelling approach where workstations are modelled as GIIG/m queues. A simulation experiment was performed to evaluate the time buffer estimation procedure on the operations of an actual fifteen station flow shop. The results from the study suggest that the analytical procedure is sufficiently accurate to provide an initial quick estimate of 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 constraint resource as well as at the other non-constraint resources on the mean flow time, the bottleneck probability of the primary constraint resource, as well as the output of flow production systems using simulation 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 fashion and the number of stations are not fixed. The experimental studies show that flow shop performance in the form of flow time and line output is not that much influenced by low protective capacity levels at the secondary constraint resource. Low protective capacity levels at a single station however can significantly reduce the bottleneck probability for the primary constraint resource when it is located before and relatively close or near to the primary constraint in the process flow, or after but relatively far from the primary constraint. An after-far secondary constraint location also causes slightly longer job flow times, and should therefore be avoided if possible. The research further shows that quite high protective capacity levels at the nonconstraint resources 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 ensure that the maximum designed output level as determined by the utilisation of the primary constraint resource is obtained. The results for the assembly line experiment showed that an unbalanced line configuration where less work is assigned to the non-constraint stations than to the primary constraint station (but nonconstraint stations have an equal work content) can lead to significant reductions in the mean flow time while maintaining the same line output, without resulting in too many additional stations. Low protective capacity levels in the range of 2% to 5% are sufficient to cause substantial improvements in flow time without resulting in too many additional stations in the line.

AFRIKAANSE OPSOMMING: Om die finale uitset van 'n produksiestelsel te maksimeer is dit noodsaaklik dat die bottelnek beskerm word teen fluktuasies en onderbrekings. In die "Theory of Constraints" filosofie word van twee soorte beskermingsmeganismes gebruik gemaak: tydbuffers en beskermende kapasiteit. Tydbuffers is beskermende tyd wat in die produksieskedule gevoeg word om sodoende die bottelnek teen onderbrekings te beskerm, terwyl beskermende kapasiteit ekstra produksiekapasiteit relatief tot die bottelnek se kapasiteit is wat by nie-bottelnekke gevoeg word. In hierdie navorsing IS 'n analitiese prosedure ontwikkel om meer akkurate berekenings van tydbuffergroottes te verkry in produksiestelsels wat volgens 'n "Theory of Constraints" filosofie bestuur word. Die prosedure maak gebruik van oop toustaan netwerk modellering waar werkstasies gemodelleer word as GIIG/m toue. Die analitiese prosedure is ge-evalueer met 'n simulasie eksperiment op 'n werklike vyftien stasie vloeiwinkel. Die resultate dui aan dat die analitiese prosedure akkuraat genoeg is om vinnig aanvanklike beramings vir die benodigde tydbuffergroottes tydens die ontwerpsfase van die produksiestelsel te verskaf. Verder is ook ondersoek ingestel na die effek van beskermende kapasiteitsvlakke by die sekondêre bottelnek asook die ander nie-bottelnekke op die gemiddelde deurvloeityd, die totale uitset, asook die bottelnek waarskynlikheid vir die primêre bottelnek in vloei produksiestelsels deur gebruik te maak van simulasie modelle en ANOVA. Twee verskillende tipes vloei produksiestelsels is ondersoek: (1) 'n vloeiwinkel met 'n vaste aantal stasies en 'n onbeperkte buffer spasie tussen stasies, en (2) 'n monteerlyn waar 'n totale werksinhoud op 'n bepaalde wyse onder stasies verdeel moet word en die aantal stasies nie vas is nie. Die eksperimentele studies dui aan dat die deurvloeitye en totale uitset van 'n vloeiwinkel me noemenswaardig beïnvloed word deur lae beskermende kapasiteitsvlakke by die sekondêre bottelnek nie. Hierdie maatstawwe word meer beïnvloed word deur die gemiddelde beskermende kapasiteitsvlakke by al die nie-bottelnekke. Lae beskermende kapasiteit by 'n enkele werkstasie kan egter die bottelnek waarskynlikheid vir die primêre bottelnek aansienlik verlaag indien dit voor en relatief na aan die primêre bottelnek in die prosesvloei geleë is, of na, maar relatief ver, vanaf die primêre bottelnek. 'n Sekondêre bottelnek ligging na maar relatief ver vanaf die primêre bottelnek in die prosesvloei veroorsaak ook langer deurvloeitye, en moet dus vermy word. Verder dui die navorsing aan dat redelike hoë gemiddelde beskermende kapasiteit by nie-bottelnekke benodig word om 'n meer stabiele primêre bottelnek te verseker. In vloeiwinkels met lae vlakke van variasie en onderbrekings is egter lae gemiddelde vlakke van beskermende kapasiteit by nie-bottelnekke voldoende om te verseker dat die maksimum ontwerpte uitset soos bepaal deur die benutting van die primêre bottelnek behaal word. Die resultate vir die monteerlyn eksperiment dui aan dat 'n ongebalanseerde lynkonfigurasie waar minder werk aan die nie-bottelnek stasies as aan die primêre bottelnek stasie toegeken word (maar niebottelnek stasies het 'n gelyke werksinhoud), aansienlike verlagings in deurvloeityd teweeg kan bring terwyl dieselfde lyn uitset behou word. Dit is moontlik sonder te veel addisionele stasies in die lyn. Die eksperimentele resultate dui aan dat lae beskermende kapasiteitsvlakke van tussen 2% tot 5% voldoende is om beduidende verlagings in deurvloeityd teweeg te bring sonder te veel addisionele stasies.

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