Browsing by Author "Visagie, Stephan E."
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- ItemAssignment of stock keeping units to parallel unidirectional picking(SAIIE, 2015-05) Matthews, Jason; Visagie, Stephan E.An order picking system consisting of a number of parallel unidirectional picking lines is investigated. Stock keeping units (SKUs) that are grouped by product type into distributions (DBNs) are assigned daily to available picking lines. A mathematical programming formulation and its relaxations is presented. A greedy insertion and a greedy phased insertion are further introduced to obtain feasible results within usable computation times for all test cases. The walking distance of the pickers was shown to decrease by about 22 per cent compared with the current assignment approach. However, product handling and operational risk increases.
- ItemFairness of seat allocation methods in proportional representation(Operations Research Society of South Africa, 2005) Van Eck, L.; Visagie, Stephan E.; De Kock, H. C.ENGLISH SUMMARY : In this paper the fairness of some methods of allocating seats in a proportional representation (PR) voting system is investigated. Different PR systems are in use throughout the democratic world, but the primary focus here is the method used in South Africa, namely the largest remainder method with a Droop quota. It is shown that as the number of parties increases, the number of lost votes (votes not used to allocate seats) increases when using this method. Other existing allocation methods are discussed and compared with each other as well as with three optimisation methods (based on mathematical programming) introduced in this paper. Applying these mathematical programming methods results in allocations that are more fair than the existing methods of seat allocation, if South African voting data are used. These mathematical models attempt to minimise a number of different measures of the deviation between the actual percentage of votes received and the percentage of seats allocated to a certain party. Ideally this deviation should be zero, but due to the discrete nature of seats this is virtually impossible to achieve.
- ItemGuidelines for the economic choice of transport infrastructure projects.(Southern African Association of Departments and Schools of Public Administration and Development (ASSADPAM)) c/o Prof. H.F Wissink, P.E. Technikon, Private Bag X 6011, Port Elizabeth, 6000, South Africa, 2000) Pienaar WJ; Visagie, Stephan E.
- ItemIntroducing Stellenbosch University Open Access Journals(Stellenbosch : Stellenbosch University, 2011-10) Simon, Carol; Thom, Johan; Botma, Gabriël; Botha, Willem; Brand, Gerrit; Visagie, Stephan E.; Van der Walt, Christa; Vrey, Francois; Khondowe, Oswell
- ItemOn the efficient solvability of a simple class of nonlinear knapsack problems(Operations Research Society of South Africa, 2008) Visagie, Stephan E.ENGLISH SUMMARY : In this paper the e cient solvability of a class of nonlinear knapsack problems are investigated by means of the problem's necessary and su cient conditions. It is shown that, from the general theory, it is impossible to determine su cient conditions for a solution to be globally optimal. Furthermore, it is shown that even for the smallest possible instance of this problem it is, in general, possible to have an arbitrary large number of solutions for which the necessary conditions hold. These results are then generalised to larger instances of the problem. A possible solution approach is applied to sets of randomly generated problems that utilises the necessary conditions together with the branch-and-bound technique in an attempt to limit the search space. This approach solves mixed 0/1 knapsack problems in order to nd all possible solutions satisfying the necessary conditions. Due to the large number of solutions satisfying the necessary conditions the proposed solution approach takes substantially longer than existing branch-and-bound algorithms together with linear enveloping when applied to the same set of problems. This result renders the proposed approach not very e cient.
- ItemOptimale toewysing en skedulering van werkers en take op 'n monteerbaan(AOSIS OpenJournals, 2009-03) Venter, L.; Visagie, Stephan E.Die optimale aanwending van kruisopgeleide werkslui in ’n produksielyn word ondersoek. Kruisopgeleide werkslui het die vermo¨e om haas enige taak in die produksieproses te verrig. ’n Wiskundige model om die optimale toewysing van werkers aan take te bepaal, word geformuleer. Die invloed van vaardige of onvaardige tydelike werkslui op die produksielyn word ook ondersoek. Die finale model neem verskeie doelwitte in ag. In die eerste fase word ’n nutsfunksie van produksie gemaksimeer terwyl werkers aan take toegewys word. Daarmee saam word die verbetering van die werkers se vaardigheidsvlakke gemaksimeer terwyl die effek van die leer en vergeet van vaardighede in ag geneem word. Die arbeidskoste van produksie word ook geminimeer terwyl die tipe werkers en die manier waarop hulle aangestel en afgelˆe moet word, bepaal word. Laastens word die totale produksietyd geminimeer terwyl die optimale volgorde van die take in die produksieproses bepaal word. Die praktiese bruikbaarheid van die model word ook met behulp van ’n gevallestudie gedemonstreer.
- ItemOptimising an integrated crop-livestock farm using risk programming(Operations Research Society of South Africa, 2004) Visagie, Stephan E.; De Kock, H. C.; Ghebretsadik, A. H.ENGLISH SUMMARY : Numerous studies have analysed farm planning decisions focusing on producer risk preferences. Few studies have focussed on the farm planning decisions in an integrated croplivestock farm context. Income variability and means of managing risk continues to receive much attention in farm planning research. Different risk programming models have attempted to focus on minimising the income variability of farm activities. This study attempts to identify the optimal mix of crops and the number of animals the farm needs to keep in the presence of crop production risk for a range of risk levels. A mixed integer linear programming model was developed to model the decision environment faced by an integrated crop-livestock farmer. The deviation of income from the expected value was used as a measure of risk. A case study is presented with representative data from a farm in the Swartland area. An investigation of the results of the model under different constraints shows that, in general, strategies that depend on crop rotation principles are preferred to strategies that follow mono-crop production practices.
- ItemPetrochemical blending problem instances(2013-06-27) Venter, Lieschen; Visagie, Stephan E.This file supplies the input data for four petrochemical blending problems.
- ItemPicking line data set A(2014-02-13) Visagie, Stephan E.; Matthews, JasonThis is a data set of picking lines that can by used for the comparison of the performance of different algorithms
- ItemPrysverlagings op voorraad met ’n dalende vraag(Operations Research Society of South Africa, 2004) Visagie, Stephan E.AFRIKAANSE OPSOMMING : Hierdie artikel stel ’n heuristiese benadering voor om die verliese op voorraad met ’n dalende vraag en onbekende tydshorison te minimeer. Die twee metodes wat voorgestel word, gebruik ’n benaderde vraagkromme om die verwagte verliese te voorspel. Albei heuristieke gebruik hierdie vooruitskatting om te bepaal wanneer die prys van items in voorraad afgemerk moet word ten einde verkope te versnel om verliese te vermy wat as gevolg van lang omsettye ontstaan. ’n Gevallestudie wat die werklike data van ’n CD-winkel simuleer, word gebruik om die twee heuristieke se prestasie relatief tot die huidige afmerkstrategie van die bestuur te meet.
- ItemSelection and scheduling of jobs with time-dependent duration(Operations Research Society of South Africa, 2007) Seegmuller, D. M.; Visagie, Stephan E.; De Kock, H. C.; Pienaar, W. J.ENGLISH SUMMARY : In this paper two mathematical programming models, both with multiple objective functions, are proposed to solve four related categories of job scheduling problems. All four of these categories have the property that the duration of the jobs is dependent on the time of implementation and in some cases the preceding job. Furthermore, some jobs (restricted to subsets of the total pool of jobs) can, to different extents, run in parallel. In addition, not all the jobs need necessarily be implemented during the given time period.
- ItemWisselboustelsels in kleingraangebiede(Stellenbosch : Stellenbosch University, 1997) Visagie, Stephan E.; Stellenbosch University. Faculty of . Dept. of .