DSpace Community:
https://dspace.ewha.ac.kr/handle/2015.oak/171699
2024-03-29T07:07:48Z
-
Preventive maintenance for the constrained multi-attempt minimal repair
https://dspace.ewha.ac.kr/handle/2015.oak/267924
Title: Preventive maintenance for the constrained multi-attempt minimal repair
Ewha Authors: 차지환
Abstract: In practice, at some instances, the repair of a failed item can be unsuccessful and should be repeated due to various reasons. This can happen in practice with a minimal repair when a failed part of a system is replaced, e.g., by the faulty spare. The corresponding stochastic model that takes into account this possibility can be described probabilistically in terms of the Polya-Aeppli process, which is a generalization of the Poisson process to the case of multiple occurrences of events. Some useful properties for considering optimal preventive maintenance problems are derived. The optimal replacement policy for a system with the multi-attempt minimal repairs is defined and analysed. The characteristic feature of this policy is that the system is replaced not only on reaching the predetermined age but also after a fixed number of unsuccessful minimal repairs. The detailed numerical examples illustrate our findings. © 2023 Elsevier Ltd
2024-01-01T00:00:00Z
-
nlmeVPC: Visual Model Diagnosis for the Nonlinear Mixed Effect Model
https://dspace.ewha.ac.kr/handle/2015.oak/267699
Title: nlmeVPC: Visual Model Diagnosis for the Nonlinear Mixed Effect Model
Ewha Authors: 이은경
Abstract: A nonlinear mixed effects model is useful when the data are repeatedly measured within the same unit or correlated between units. Such models are widely used in medicine, disease mechanics, pharmacology, ecology, social science, psychology, etc. After fitting the nonlinear mixed effect model, model diagnostics are essential for verifying that the results are reliable. The visual predictive check (VPC) has recently been highlighted as a visual diagnostic tool for pharmacometric models. This method can also be applied to general nonlinear mixed effects models. However, functions for VPCs in existing R packages are specialized for pharmacometric model diagnosis, and are not suitable for general nonlinear mixed effect models. In this paper, we propose nlmeVPC, an R package for the visual diagnosis of various nonlinear mixed effect models. The nlmeVPC package allows for more diverse model diagnostics, including visual diagnostic tools that extend the concept of VPCs along with the capabilities of existing R packages.
2023-01-01T00:00:00Z
-
Termination versus operation extension for degrading systems
https://dspace.ewha.ac.kr/handle/2015.oak/266999
Title: Termination versus operation extension for degrading systems
Ewha Authors: 차지환
Abstract: Optimal cost-wise termination of operation for degrading engineering systems is considered that maximizes the expected profit for the infinite horizon. Termination reduces probability of the future failure that usually results in substantial loss. The cases of non-repairable and minimally repairable systems are discussed. The black-box scenario without additional information on stochastic process of degradation and that with additional information are considered. The novel sequential optimal inspection policy is described. It is proved that the termination time can be postponed in an optimal way if the degradation observed at inspection is smaller than the defined level. The developed approach allows for optimal usage of system’s “resource.” The detailed numerical examples illustrate the findings. © IMechE 2022.
2023-01-01T00:00:00Z
-
Acceptance reliability sampling plan for items from heterogeneous populations
https://dspace.ewha.ac.kr/handle/2015.oak/266998
Title: Acceptance reliability sampling plan for items from heterogeneous populations
Ewha Authors: 차지환
Abstract: Existing reliability sampling plans reported in the literature assume that the items belong to homogeneous populations. However, at many practical instances, a population of the manufactured items is a mixed one. In this paper, we consider the case when a population consists of items that belong to two subpopulations, that is, the subpopulation with better reliability characteristics and that with worse reliability characteristics. Thus, a variables acceptance reliability sampling plan is proposed and its properties are investigated under this assumption. A sequential numerical algorithm for finding the parameters of the proposed sampling plan is developed and applied. Furthermore, the lifetime of the population before the acceptance test and that of the population which have passed the sampling test are stochastically compared. © IMechE 2022.
2023-01-01T00:00:00Z