We study a single location supply system for repairable spare parts. The system consists of a multi-server repairshop and an inventory with ready-to-use spare parts. When a failed part is received, a new (or as-good-as-new) replacement (if available in the inventory) is sent back and the failed part is sent to the repair shop. In case of unavailability, failed requests are backordered and fulfilled when a ready-for-use part of same type is received from the repairshop. The repair shop has several multi-skilled parallel servers (technicians) that are capable to handle certain types of spares. In this paper, we propose a Particle Swarm Optimization heuristic combined with Discrete-Event Simulation for optimizing the cross training policy (skill assignment scheme) while minimizing the total system cost (consisting of inventory costs, backorder penalty cost, server cost and skill cost).

Design SolutionsWith reviewed papers that have generated several design solutions using the TRIZ solution principles, almost all of them employed AHP as their method for selecting their best design. The remaining number of papers reviewed had not generated a list of design solutions, and were satisfied with one. Vinodh 2014 used the multi-criteria decision making (MCDM) approach, in this case the AHP approach, to select the best design out of the ones generated as a solution to the problem. A hierarchy was constructed and the criteria for the best design were identified. The opinions of experts were collected in order to carry out the pairwise comparison of the criteria, and the weights were calculated accordingly. In order to confirm that the judgement was acceptable, the consistency ratio CR was calculated. Rosli 2013 also used AHP in the selection process of their best design, utilizing the Expert Choice software to aid in this process. They had begun by conducting a survey in order to determine the main and sub-criteria weights that are based on customer's preferences when buying the product they were improving. The pairwise comparisons were done using the author's experiences and their knowledge, and following that they used the Expert  Choice Software to rank the design concepts generated. After the design solutions were prioritized, Rosli 2013 went further, by selecting the highest two ranking design solutions that are compatible in design and combining them into one final design of which they were satisfied with. Chakroun 2014 has also applied AHP in order to select their best design, however did not mention how they carried out the pairwise comparison of their criteria. Chen 2012 had likewise used AHP in order to select the best design out of three cases, and after ensuring that the judgement was consistent, they selected one case that ranked the highest based on their identified main and sub-criteria for the best design. One the other hand, in order to determine the optimal solution out of the three which have been generated, Yeh 2010 had three different solutions targeting one problem: to extend the battery service life of a Notebook computer. However, after listing out three solutions derived from the TRIZ solution principles, they combined them into one final design that incorporates all three and thus had ended up one design. They then made an ideal comparative assessment of their final results, and in that way had determined whether it was an optimal solution or not. EvaluationWhen it comes to evaluation, there was only one paper that had evaluated their design. Chen 2012 had verified their design by conducting a questionnaire survey among 10 consumers and experts to check whether there is any statistical significant difference between the surveyed groups' criteria of the three design solutions, or as they have referred to them, the three cases proposed. At the same time, they had also determined whether the selected case conforms to the settings of each criteria. The statistical test conducted was ANOVA, and according to it, there was indeed a significant difference (P<0.05). Therefore consistency was verified and along along with it, their selected design.

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