1,721,262 research outputs found
Stochastic Reorder Point-Lot Size (r,Q) Inventory Model under Maximum Entropy Principle
Full text linkThis paper takes into account the continuous-review reorder point-lot size (r,Q) inventory model under stochastic demand, with the backorders-lost sales mixture. Moreover, to reflect the practical circumstance in which full information about the demand distribution lacks, we assume that only an estimate of the mean and of the variance is available. Contrarily to the typical approach in which the lead-time demand is supposed Gaussian or is obtained according to the so-called minimax procedure, we take a different perspective. That is, we adopt the maximum entropy principle to model the lead-time demand distribution. In particular, we consider the density that maximizes the entropy over all distributions with given mean and variance. With the aim of minimizing the expected total cost per time unit, we then propose an exact algorithm and a heuristic procedure. The heuristic method exploits an approximated expression of the total cost function achieved by means of an ad hoc first-order Taylor polynomial. We finally carry out numerical experiments with a twofold objective. On the one hand we examine the efficiency of the approximated solution procedure. On the other hand we investigate the performance of the maximum entropy principle in approximating the true lead-time demand distribution
Economic production quantity for a decaying item with stochastic demand and positive lead time
No full textDiffusion theory applied to tool-life stochastic modeling under a progressive wear process
No full textIn this paper a novel approach to the derivation of the tool-life distribution, when the tool useful life ends after a progressive wear process, is presented. It is based on the diffusion theory and exploits the Fokker-Planck equation. The Fokker-Planck coefficients are derived on the basis of the injury theory assumptions. That is, tool-wear occurs by detachment of small particles from the tool working surface, which are assumed to be identical and time-independent. In addition, they are supposed to be small enough to consider the detachment process as continuous. The tool useful life ends when a specified total volume of material is thus removed. Tool-life distributions are derived in two situations: (i) both Fokker-Planck coefficients are time-dependent only, and (ii) the diffusion coefficient is neglected and the drift is wear-dependent. Theoretical results are finally compared to experimental data concerning flank wear land in continuous turning of a C40 carbon steel bar adopting a P10 type sintered carbide insert. The adherence to the experimental data of the tool-life distributions derived exploiting the Fokker-Planck equation is satisfactory. Moreover, the tool-life distribution obtained when the diffusion coefficient is neglected and the drift is wear-dependent is able to well-represent the wear behavior at intermediate and later times
Improving Tool-Life Stochastic Control Through a Tool-Life Model Based on Diffusion Theory
No full textIt is known that estimating the wear level at a future time instant and obtaining an updated evaluation of the tool-life density is essential to keeping machined parts at the desired quality level, reducing material waste, increasing machine availability, and guaranteeing the safety requirements. In this regard, the present paper aims at showing that the tool-life model that Braglia and Castellano [23] developed can be successfully adopted to probabilistically predict the future tool wear and to update the tool-life density. Thanks to the peculiarities of a stochastic diffusion process, the approach presented allows deriving the density of the wear level at a future time instant, considering the information on the present tool wear. This makes it therefore possible updating the tool-life density given the information on the current state. The method proposed is then experimentally validated, where its capability to achieve a better exploitation of the tool useful life is also shown. The approach presented is based on a direct wear measurement. However, final considerations give cues for its application under an indirect wear estimate
The average-cost formulation of lot sizing models and inventory carrying charges: A technical note
Full text linkEconomic production quantity with inventory rationing for a decaying item
No full textIn this paper, we develop an economic production quantity (EPQ) model with inventory rationing for a decaying item under deterministic conditions. Two demand classes are considered, and inventory rationing is implemented by introducing a critical level: when stock is below the critical level, low-priority demand is rejected. The cost model is developed considering two situations, depending on whether the critical level is in the inventory build-up phase or in the inventory depletion phase. We then formulate the optimization problem and propose a solution procedure. Numerical experiments are finally carried out to demonstrate the benefits of implementing inventory rationing. Copyright (C) 2022 IFAC. Copyright (C) 2022 The Authors
Just-In-Time parts feeding policies for paced assembly lines: possible solutions for highly constrained layouts
No full textThis paper deals with the dimensioning of a just-in-time (JIT) parts feeding system, for a paced assembly line. Specifically, we consider the quite frequent case of an old-fashioned, push-oriented warehouse arrangement, characterized by materials and subassemblies that are stocked in different and peripheral areas of the manufacturing plant. In these circumstance, layout changes are almost impossible and warehouses cannot be substituted by supermarkets. Nevertheless, this does not preclude the possibility to rationalize material flow and to reduce work in process (WIP) using a JIT parts feeding system. To this aim, we propose two alternative approaches, differentiating in terms of the order with which warehouses and workstations are visited. In order to dimension both systems and to estimate their expected performances, a mathematical model is also presented. At first, the model is presented in a basic formulation, which is very effective and easy to use. A more sophisticated formulation, based on Bayesian probability, is introduced next; this is not so much for practical purposes, but rather to demonstrate the potential of the proposed JIT system in very constrained operating settings
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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