Won-Young Yun

AN ECONOMIC ORDER QUANTITY MODEL WITH A RANDOM PLANNING HORIZON

ABSTRACT The Economic Order Quantity (EOQ) model and its variations have received a lot of attention by researchers for a long time. However, an infinite planning horizon in practice is of rare occurrence because of changes in product specifications and design or its abandonment or substitution by another product due to rapid development of technology. This paper studies an EOQ Model which relaxes the assumption of the infinite planning horizon. In addition, the discounted cash flow (DCF) approach is used to properly recognize the financial implication of the cash flows in an inventory analysis. We derive the general expression of the expected present value for the inventory carrying and ordering costs, and investigate the characteristics of the function for the exponential distribution.

Fault tree analysis with fuzzy gates

Abstract Fault tree analysis is an important tool analyzing system reliability. Fault trees consist of gates and events. Gates mean relationships between events. In fault tree analysis, AND, OR gates have been used as typical gates but it is often difficult to model the system structure with the two gates because in many cases we have not exact knowledge on system failure mechanism in early design stage. In this paper, we apply the fuzzy sets theory to modeling the fuzzy system structure, propose the new procedure to calculate the system reliability and a new importance index of basic events.

The distribution free job control problem

Abstract The job control problem is to determine an optimal release time where the flow time is a random variable with a known probability distribution. There is a trade-off between the penalty cost for late delivery and the holding cost for early finish. C. Liao [ Computers & Industrial Engineering , 22 , 163–169 (1992)] formulated this problem and provided a practical decision procedure. This paper addresses the job control problem where only the mean and the variance of the flow time are known. The motivation of this paper comes from the fact that the distributional information on the flow time is usually limited for some job shops. We provide a simple line search algorithm to find a job release time which can be used in the absence of the distributional information.

A NOTE ON EVALUATING INVESTMENTS IN INVENTORY SYSTEMS: A NET PRESENT VALUE FRAMEWORK

ABSTRACT Recently, Roumi and Schnabel wrote an addendum to attempt to fill gaps found in the article by Kim, Philippatos and Chung. They claimed that the objective functions developed by Kim, Philippatos and Chung are concave. However, we present a counter example which disproves their claim. In addition, we give clarifying explanations that the unique inflection point is a global maximum even though the objective functions are not concave. Consequently, we justify the optimally of solutions derived from the first order conditions in Kim, Phillippatos and Chung.

System analysis of a multi-product, small-lot-sized production by simulation: a Korean motor factory case

In this article, a simulation model of an existing motor production line is developed to identify system parameters to improve the system performances such as facility utilization, flow time, and buffer sizes (work-in-process inventories). System parameters investigated include dispatching rules, setup time reduction, overwork, demand increase, and productivity improvement. The simulation results have been applied to the development of a new layout. The model has been developed using SIMAN simulation language which has been demonstrated to be a powerful tool to simulate complex manufacturing systems.

A time-varying lot sizes approach for the economic lot scheduling problem with returns

We consider the economic lot scheduling problem with returns by assuming that each item is returned by a constant rate of demand. The goal is to find production frequencies, production sequences, production times, as well as idle times for several items subject to returns at a single facility. We propose a heu ristic algorithm based on a time-varying (TV) lot sizes approach. The problem is decomposed into two distinct portions: in the first, we find a combinatorial part (production frequencies and sequences) and in the second, we determine a continuous part (production and idle times) in a specific production sequence. We report computational results that show that, in many cases, the proposed TV lot sizes approach with consideration of returns yields a relatively minor error.

Economic lot and supply scheduling problem: a time-varying lot sizes approach

We study the economic lot and supply scheduling problem (ELSSP) that arises in the distribution and manufacturing industries. The ELSSP involves the simultaneous scheduling of end-item production and inbound transportation of input materials over an infinite time horizon to minimise the average costs of inventory, production set-up and transportation. We present a new methodology based on a time-varying lot sizes approach for the ELSSP. We also provide computational experiments showing that the developed algorithm outperforms the existing heuristic for improved integrated scheduling.

Positioning empty containers under dependent demand

Owing to trade imbalance, shipping companies position empty containers between ports or depots periodically. The most difficult problem for positioning is that it is not easy to know the exact amounts of empty containers required in the future. The paper deals with the problem of positioning empty containers in a port area with multiple depots. Customer demands and returning containers in depots per unit time period are assumed to be serially-correlated and dependent random variables. Three options are considered to prepare the required extent of positioning: positioning from other overseas ports, inland positioning between depots, and leasing. The policies for empty-container management consist of three parts as follows: a coordinated, (S, s) inventory policy for positioning from other ports, (Ri, ri) policy for inland positioning between depots; and a simple leasing policy with zero lead-time. The objective is to minimize the expected total costs including inventory holding, overseas positioning, inland positioning and leasing costs. The optimal results are then compared with those in case that customer demands and returning containers are independent and identically distributed random variables. A genetic-based optimization procedure is developed to find the optimal parameters (S, s) and (Ri, ri). Some numerical examples are given to demonstrate the results.