Browsing by Author "Toy, Ayhan Ozgur"

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A Comparative Study of Artificial Intelligence Based Methods for Abnormal Pattern Identification in SPC
(SPRINGER INTERNATIONAL PUBLISHING AG, 2022) Umut Avci; Onder Bulut; Ayhan Ozgur Toy; Toy, Ayhan Ozgur; Bulut, Onder; Avci, Umut; C Kahraman; AC Tolga; SC Onar; S Cebi; B Oztaysi; IU Sari
Statistical process control techniques have been used to detect any assignable cause that may result in a lower quality. Among these techniques is the identification of any abnormal patterns that may indicate the presence of an assignable cause. These abnormal patterns may be in the form of steady movement in one direction i.e. trends, an instantaneous change in the process mean i.e. sudden shift, a series of high observations followed by a series of low observations i.e. cycles. As long as we can classify the observed data the decision maker can decide on actions to be performed to ensure quality standards and planning for interventions. In identification of these abnormal patterns rather than relying on human element intelligent tools have been proposed in the literature. We attempt to provide a comparative study of various classification algorithms used for pattern identification in statistical process control. We specifically consider six different types of patterns to classify. These different types are: (1) Normal (2) Upward trend (3) Downward trend (4) Upward shift (5) Downward shift (6) Cyclic. A recent trend in classification is to use deep neural networks (DNNs). However due to the design complexity of DNNs alternative classification methods should also be considered. Our focus on this study is to compare traditional classification methods to a recent DNN solution in the literature in terms of their efficiencies. Our numerical study indicates that basic classification algorithms perform relatively well in addition to their structural advantages.
Citation - WoS: 2
Citation - Scopus: 3
A serial inventory system with lead-time-dependent backordering: A reduced-state approximation
(Taylor and Francis Ltd., 2023) Emre Berk; Ayhan Özgür Toy; Toy, Ayhan Ozgur; Berk, Emre
We study a serial inventory system where the external customers may have a maximum time that they would be willing to wait for delivery in cases of stock-out and the demand would be lost if the remaining delivery lead time of the next available item is longer. This lead-time-dependent backordering behavior subsumes the models of partial backordering regardless of the wait that a customer would experience. In the inventory literature this behavior has only been analyzed in single-location settings. We study this behavior in a multi-stage setting. We consider continuous review (Formula presented.) policies at all stages facing external Poisson demands. Using the method of supplementary variables we define the stochastic process representing the inventory system and obtain the expressions for the operating characteristics of the inventory system. Based on the solution structures for the special cases we propose an approximate solution which rests on replacing the state-dependent purchasing decision of the customer with an averaged-out purchase probability computed using only the age of the oldest item. An extensive numerical study indicates that the proposed approximation performs very well. Our numerical study provides additional insights about the sensitivity and allocation of stock levels across stages. © 2022 Elsevier B.V. All rights reserved.
Citation - WoS: 1
An Overview of Warehouse Operations for Cold Chain
(SPRINGER INTERNATIONAL PUBLISHING AG, 2022) Cansu Yurtseven; Banu Yetkin Ekren; Ayhan Ozgur Toy; Toy, Ayhan Ozgur; Yurtseven, Cansu; Ekren, Banu Yetkin; F Calisir
Warehouses play a significant role in cold chains as they do for regular supply chains. Although their goals are the same for both cold chains and regular supply chains the operations of cold warehouses are more sophisticated since the cost of operation is considerably higher due to energy consumption and obsolesce of products in substandard conditions. Recently there has been an enormous interest in the cold food supply chain to reduce food waste occurring along the chain. Hence efficient management of cold warehouses becomes an important issue in this direction. Design and operation requirements in a cold warehouse may be different from a traditional non-cold warehouse. In this paper we aim to provide an overview of cold chain operations mostly by focusing on cold warehouse operations. We provide some statistics from a cold chain design and technology requirements for cold warehouses as well as warehouse operations shaped according to that warehouse features. It is observed that there are quite different design parameters in cold storage.
Citation - WoS: 2
Citation - Scopus: 3
Markovian Decision Process Modeling Approach for Intervention Planning of Partially Observable Systems Prone to Failures
(Springer Science and Business Media Deutschland GmbH, 2022) Oktay Karabağ; Önder Bulut; Ayhan Özgür Toy; Toy, Ayhan Ozgur; Karabag, Oktay; Bulut, Onder; C. Kahraman , S. Cevik Onar , B. Oztaysi , I.U. Sari , A.C. Tolga , S. Cebi
In this work we consider a system which gradually deteriorates over time. The system is fully functional in the beginning. Over time the system eventually becomes malfunctional. Once malfunctional the system must be replaced with a (new) fully functional system. There is a cost associated with this system replacement. However there is an option of repair/correction of partially deteriorated system at a lower cost. Once replaced or repaired/corrected the system is as good as new. The information about the deterioration level of the system is monitored through signals which provide only partial information. These signals are based on classification of intelligent sensors for deterioration monitoring. Signals are received as green yellow or red. The green signal indicates a system in a condition from fully functional to a predefined level of partially deteriorated system, the yellow signal indicates a system in a condition from the predefined level of partially deteriorated system to malfunctional system, finally the red signal indicates a malfunctional system. We model this system as a discrete time Markovian decision process and solve it through Linear Programming. Our work herein comprises model development and extensive numerical studies for impact of system parameters on the maintenance decisions and costs. © 2022 Elsevier B.V. All rights reserved.