Stok kontrol simülasyon sistemi

Şirincan, Serap
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Fen Bilimleri Enstitüsü
İşletmelerde maliyetleri en aza İndirme» siparişleri zamanında yetiştirebilme gibi zorunluluklardan dolayı etkin bir stok kontrol politikasının uygulanması zorunludur. Bu politikayı oluşturmak için ise mevcut stok sisteminin elemanlarının yapısının iyi bilinmesi ve buna uygun bir model kurularak optimum sonuçların bulunması gereklidir. Bazen problemlerin analizi için tanımlanan bir sistemin modeli çok karmaşık olabilir. Sistemlerin analitik yöntemlerle çözümünün bulunmadığı bu gibi durumlarda simülasyon tekniklerinden yararlanılmaktadır. Bu çalışmada Stok Kontrol Sistemleri ele alınarak, bu sistemleri çeşitli koşullarda inceleyen bir simülasyon programı geliştirilmiştir.
Inventory Control Simulation In Today's competitive marketing conditions, inventory planning and control is one of the most important factors which effects the profitability of industrial organizations. It can either increase or decrease cash flow, can improve or destroy customer service and can make or break company profits. So, it is necessary for the nature of interrelated elements in the inventory system, to establish the relationships among them, and to develop inventory planning models based on those relationships. Classes of inventory found in the typical manufacturing company are of many types, and the reason for every type and variation is the final use. Inventories are classified and segregated according to purpose to which they will finally be applied. Some of the more common types of inventories are as follows s - Raw materials, - Finished products, - Work-in-process, Raw materials, items acquried by the company in a form that needs further processing to make them a part of or to convert them into an end product. Examples are materials in a natural state, such as iron ore, crude oil, and wood fiber, or processed materials for general use, like steel rods. Finished products, completed parts or components used in the manufacture of an end product. Vii Work-in-process, all product materials on which the company has performed some manufacturing, processing, or converting operations, but which are not yet in finished form, and are not ready for sale or for storage as component parts. The costs incurred in operating an invetory system play a major role in determining, what the operating doctrine should be. The most widely adopted approach is to consider three elements of cost: 1. Administrative costs, i.e. the cost of obtaining quantity, 2. Holding costs, i.e. the cost of maintaining a stock. 3. Shortage costs, i.e. loss of profit, goodwill, opportunity etc., Whatever type of stock system is used, two questions need answering: 1. When should stock orders be placed ? 2. How much should be ordered ? When the demand rate is constant the answer to the questions lead to the same policy. However, in practice, due to variations in demand, emphasis is placed on one or the other. This leads to two basic types of system. When the quantity ordered is fixed, control is obtained by varying the time at which an order is placed. Orders are placed when the stock level reaches a re-order point. This is called as the Fixed Order Quantity System. viii Q System can be described as follows: Place an order for Q units whenever a withdrawal brings the inventory position to the re-order point R, that is, the minumum level allowed. The Q System has just two parameters: Q and R. Each new order is of the same size, or some other quantity selected by management. V An alternative approach is to fix the time at which an order is to be placed, say once a month and then order a varying amount depending upon the stock level at the review. This is called as the Periodic Review System or P System. Demand is a random variable, so the total demand between reviews varies, and the lot size Q changes from one order to the next. Note the differences this system and the Q system, in which Q is fixed and the time between orders varies. P System works as follows: Review the item's inventory position every P time periods. Place an order equal to (target inventory- inventory position). Whichever type of system is followed, the calculations leading to order quantity or review period would be based upon the mean demand whenever they arrise, some form of safety stock must be carried. Neither the Q ve P system is the best in all situations. The advantages of one system are implicitly disadvantages of the other one. IX Advantages of Q system ı - Can individualize the replenishment intervals. More suited for quantity discounts or capacity limitations, - Less safety stock. Advantages of P systems - Fixed replenishment intervals, - Can combine orders to some supplier, - Perpetual inventory system not mandatory. Various hybrid inventory control systems are also used. They include some but not all the features of the Q and P systems. One of them is base stock system. A base stock system is an inventory system in which a replenishment order is issued each time a withdrawal is made. The order quantity Q is equal to the amount of the withdrawal. This one-for-one replacement policy maintains the inventory position at a base stock level to the expected demand during the lead time plus safety stock. However, order quantities now vary to keep the inventory position at R at all times. Because this is the lowest inventory position possible to maintain a specified service level, the base stock system can be seen as a way to minimize cycle inventory. The information that are told us up to now summarized the knowledge which are given in the first and second parts. The third part consists of the explanation x of a computer program. The aim in building the program is to determine values for the desition variables which are optimal. The simulation program is written in the Pascal. It consists of about 800 lines and six subroutines. When the package is run, first comming part is the main menu for the system. There are nine choices in this menu. The choices in first-second and third ones are about Q system, in the fourth-fifth and sixth ones are about P system. The seventh and eighth choises are about Base Stock System and change. The last choise of the main menu is for exit. Running the program that is explained above, some outputs is obtained. As all systems in a company, inventory control system is a part of MIS (Management Information System). It can be considered as an independent system from other systems. Basically, every company, even if in the same sector, is different from each other. For this reason, each of them has to choose the best system which is suitable for its activities. As the results of this system change according to parameters used and the conditions, it is a neccesitty and the company to be aware of the conditions. It has and to decide which system is the best one for its activity.
Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 1992
Anahtar kelimeler
Benzetim, Stok denetimi, Simulation, Stock control