Browsing by Subject "Production control."

Now showing 1 - 18 of 18

Open Access
Analyses of serial production lines and assembly systems for throughput and inyerdeparture time variability
(1998) Kök, Abdullah Gürhan
In this thesis, we study three different but closely related production system design problems. First, we investigate the effects of various design factors such as number of stations, buffer capacity, allocation of bulfers and location of a bottleneck on the interdeparture time variability of serial production lines. In the second part, we study the effects of number of component stations, processing time distributions, buffers and buffer allocation schemes on throughput and interdeparture time variability of assembly systems. As an alternative to work transfer, we introduce variability transfer and assess its effectiveness. We analyze the anomaly displayed by optimal throughput for some processing time distributions and uncover the underlying details of this behavior. In the third part, we analyze serial production lines and assembly systems under constant workload condition. In addition to investigating the problem of determining the optimal system size, we examine the effects of other design factors such as buffers and material handling time on throughput, interdeparture time variability and cost related measures. Each part reveals several important findings. We also discuss the managerial implications of these findings to present guidelines for the practitioners.
Open Access
Analytical loading models and control strategies in flexible manufacturing systems: a comparative study
(1990) Kırkavak, Nureddin
There are three problem areas in designing and implementing a manufacturing line : the part family selection and grouping, system configuration and toolings and the operational control of manufacturing. The manufacturing process has to be stream-lined by considering resources and products to achieve flow lines operating around product families with acceptable levels of utilization. The stream-lined processes have to be assigned to tandem machines in the manufacturing lines. Then, interactions between production and inventory levels should be controlled at the operational level. Based on this framework, first a system configuration and tooling problem is modeled. The model turns out to be a large mLxed integer linear program, so that some alternative optimal seeking or heuristic techniques are used to solye the model for constructing a flow line structured Flexible Manufacturing System. Push systems of the Material Requirements Planning type or pull systems like the base-stock or Kanban schemes are often seen as alternatives for controlling manufacturing systems. The differentiating features of push, pull and a hybrid strategy are studied by discrete event simulation under different system and environmental characteristics for Flexible Manufacturing Systems. The impact of assignment of operations to machines on the performance of the system is also discussed.
Open Access
Continuous time control of make-to-stock production systems
(2010) Bulut, Önder
We consider the problem of production control and stock rationing in a make-tostock production system with multiple servers –parallel production channels--, and several customer classes that generate independent Poisson demands. At decision epochs, in conjunction with the stock allocation decision, the control specifies whether to increase the number of operational servers or not. Previously placed production orders cannot be cancelled. We both study the cases of exponential and Erlangian processing times and model the respective systems as M /M /s and M /Ek /s make-to-stock queues. We characterize properties of the optimal cost function, and of the optimal production and rationing policies. We show that the optimal production policy is a state-dependent base-stock policy, and the optimal rationing policy is of state-dependent threshold type. For the M /M /s model, we also prove that the optimal ordering policy transforms into a bang-bang type policy when we relax the model by allowing order cancellations. Another model with partial ordercancellation flexibility is provided to fill the gap between the no-flexibility and the full-flexibility models. Furthermore, we propose a dynamic rationing policy for the systems with uncapacitated replenishment channels, i.e., exogenous supply systems. Such systems can be modeled by letting s --the number of replenishment channels-- go to infinity. The proposed policy utilizes the information on the status of the outstanding replenishment orders. This work constitutes a significant extension of the literature in the area of control of make-to-stock queues, which considers only a single server. We consider an arbitrary number of servers that makes it possible to cover the spectrum of the cases from the single server to the infinite servers. Hence, our work achieves to analyze both the exogenous and endogenous supply leadtimes.
Open Access
Design and implementation of an MRP-II system in electronics industry
(1991) Cankat, A Burç
Manufacturing Resource Planning (MRP II) system is a modern production planning and control method that is relatively new in our country. In today's competitive and continuously improving industrial scene MRP II systems play an important role in increasing productivity. Electronics industry is a field that is highly suitable for MRP applications and can easily set a good example for further improvements. This work aims at applying a professional MRP II softv>^are to a relatively large electronic equipment manufacturer. In this aspect the semi-formal operating system of the company is totally reconstructed and formalized to meet the requirements of the new system. To achieve these a phased implementation plan is developed and executed and thus a successful implementation is performed. Finally, the initial results of the new system are discussed.
Open Access
Design and scheduling of periodic review kanban systems
(1997) Erhun, Feryal
In the last years, the term just in-time (JIT) has become a common term in repretitive manufacturing systems. It can be defined as the ideal of having the necessary amount of material available where it is needed and when it is needed. One of the major elements of JIT philosophy and pull mechanism is the Kanban system. This system is the information processing and hence shop floor control system of JIT philosophy. In this study, we propose an algorithm to determine the withdrawal cycle length, kanban size and number of kanbans simultancously in a periodic review Kanban system under multi-item, multi-stage, multi period modified flowline production setting. The proposed algorithm considers the impact of operating characteristics such a.s scheduling and actual lead times on design parameters.
Open Access
Efficiency of CONWIP production lines: a simulation study
(1994) Bölükbaş, A. Ercüment
Today in industiy production lines are stochastic systems. These systems are designed to optimize some performanee measures; the perfoimance measures are generally the throughput and WIP of the systems. Managers want to maximize throughput and minimize WIP. To achieve these goals, several designs are tested. Aim of this study is to develop some guidelines for the design of CONWIP type of systems.
Open Access
Generating a robust model for production and inventory control
(1993) Sencer, Aslı
In tills stud}', we generate a production and inventory control model which gives h'obust‘ solutions against demand estimation errors. This model is applied to a real production and inventory system; howe\’er, it is a general model where the demand rate is stochastic with a known probability distribution and other parameters of the system are constant. The proposed model is a bi-objective chicision making model, with two decision variables. .A ‘compromised* solution is found for the problem using the trade-off curve generated by a constrained sequential optimization technique, applied on a nonlinear programming model parametrically. Robustness against parameter estimation errors is tested by sensitivity analysis. Here a new dimension is added to sensitivity analysis methodology by including a sensitivity measure as a ‘cost of error* of parameter estimation. By so doing, the proposed model is tested against the classical EOQ model and it is shown that the proposed model ])erforms far better.
Open Access
Inventory control system: a case study
(1992) Kocaoğlu, Neriman
There are two objectives of inventory control system. The first one is to maximize the level of the costumer service by having the right goods in sufficient quantities, in the right place, and at the right time. The other is to minimise the cost of providing the right level of customer service. The inventory problem is to determine an ordering policy, determinig when to order and how much to order. In this study,an ordering policy is attempted to be determined for a small merchandise company. The A B C approach is one method to classify inventory items according to their importance. Different ordering policies could be used according to the result of A-B-C classification. The required data are collected and processed in order to determine the ordering polic for each item.
Open Access
An inventory model for randomly perishing goods
(2000) Yüksel, Banu
In this study, we consider an (s, S) ordering policy with backordering for a continuous review inventory system, where the items have a random shelflife. Assuming zero lead time and no decay until the shelflife, we derive the exact expressions for both unit and random batch demand cases with renewal demand arrivals. We present some analytical results on the cost rate function for unit demand case. A detailed numerical analysis is also provided to investigate the performance of the model which incorporates the random shelflife and comparisons with flxed shelflife are given.
Open Access
Joint economic lot-sizing approach to the just-in-time purchasing problem
(1993) Durusoy, İhsan
One of the important concepts of JIT philosophy is the high frequency with small lots in the delivery process. However, this issue is settled between the purchaser and vendor depending on the existing balance of power. The result of such decisions could end with ordering policies, not suitable for JIT logic and place some disadvantages to one of the parties or both. Additionally, these policies have not considered the effect of transportation cost on the optimal ordering and shipment size quantities; despite the fact that purchased materials must bear transportation charges. This paper develops joint economic lot-size model under deterministic conditions, focusing on the shipment size and its effect to the joint total cost, which also includes transportation cost. For that purpose, the joint model is arranged according to the shipment size. Then a computational analysis is made between each parties shipment size policy with the joint model. Consequently, a full factorial design is generated with four factors at three levels. By using the analysis of variance, the effects of the factors on the joint total cost are investigated.
Open Access
A joint pricing and replenishment policy for perishable products with fixed shelf life and positive lead times
(2009) Bayramoğlu, Könül
Most of the existing inventory models in the literature are based on the assumption that the items have infinite shelf life and do not deteriorate no matter how long they stay on the shelf. However this assumption may not be applicable in many situations since there are also many types of products with limited shelf lives. In the inventory literature stored items with fixed finite lifetimes are usually referred to as perishable items. Examples of perishable products include fresh foods, medical products, whole-blood units, packaged chemical products and photographic films. In this study, we consider the joint pricing and ordering policy, (Q, r, P1, P2), for an inventory model with perishable items, with constant shelf lives and positive lead times. The demand process is assumed to be Poisson. If there is a single batch on hand, the items in a batch are sold at price P1. If there are two batches in stock, the items in the older batch are sold at price P2, where P1 > P2. The younger batch is not sold until the older one is totally depleted. Although the shelf lives are constant, the sequence of remaining shelf lives of the items at the instances where stock level hits Q, is a random sequence. The limiting distribution of this sequence is obtained and the analytical derivations of the operating characteristics of the model is based on this limiting distribution. Numerical results are also presented.
Open Access
Medium range production planning of "Tacer" company: a case study
(1994) Süer, Burak
In order to foresee the potential problems related with production, a production plan is undertaken for the year 1994. Product classification is determined according to the annual sales volume of the company "Tacer". A production schedule is prepared in relation with the forecast data of 1994. The schedule is examined by capacity requirements and financial analysis. Some capacity problems that are highlighted were investigated in the analysis section. Short and long term solutions were made to overcome the insufficient capacity
Open Access
Minimazing the total completion time in a two stage flow shop with a single setup server
(2012) Kolay, Muhammet
In this thesis, we study a two stage flow shop problem with a single server. All jobs are available for processing at time zero. Processing of a job is preceded by a sequence independent setup operation on both machines. The setup and processing times of all jobs on the two machines are given. All setups are performed by the same server who can perform one setup at a time. Setups cannot be performed simultaneously with job processing on the same machine. Once the setup is completed for a job, processing can automatically progress without any further need for the server. Setup for a job may start on the second machine before that job finishes its processing on the first machine. Preemption of setup or processing operations is not allowed. A job is completed when it finishes processing on the second machine. The objective is to schedule the setup and processing operations on the two machines in such a way that the total completion time is minimized. This problem is known to be strongly NP-hard [3]. We propose a new mixed integer programming formulation for small-sized instances and a Variable Neighborhood Search (VNS) mechanism for larger problems. We also develop several lower bounds to help assess the quality of heuristic solutions on large instances for which optimum solutions are not available. Experimental results indicate that the proposed heuristic provides reasonably effective solutions in a variety of instances and it is very efficient in terms of computational requirements.
Open Access
Modelling and analysis of pull production systems
(1995) Kırkavak, Nureddin
A variety of production systems appearing in the literature are reviewed in order to develop a classification scheme for production systems. A number of pull production systems appearing in the classification are found to be equivalent to a tandem queue so that accurate tandem queue decomposition methods can be used to find the performance of such systems. The primary concern of this dissertation is to model and analyze non-tandem queue equivalent periodic pull production systems. In this research, an exact performance evaluation model is developed for a singleitem periodic pull production system. The processing and demand interarrival times are assumed to be Markovian. For large systems, which are difficult to evaluate exactly because of large state spaces involved, an approximate decomposition method is proposed. A typical approximate decomposition procedure takes individual stages or pairs of stages in isolation to analyze the system and then it aggregates the results to obtain an approximate performance for the whole system. An experiment is designed in order to investigate the general behavior of the decomposition. The results are worth attention. A second aspect of this study is to investigate an allocation methodology to achieve the maximum throughput rate with providing two sets of allocation parameters regarding the number of kanbans and the workload at each stage of the system. Together with some structural properties, the experimental results provide some insight into the behavior of pull production systems and also provide a basis for the proposed allocation methodology. Finally, we conclude our findings together with some directions for future research.
Open Access
On the distribution of throughput of transfer lines
(1998) Deler, Bahar
A transfer line corresponds to a manufacturing system consisting of a number of work stations in series integrated into one system by a common transfer mechanism and a control system. There is a vast literature on the transfer lines. However, little has been done on the transient analysis of these systems by making use of the higher order moments of their performance measures due to the difficulty in determining the evolution of the stochastic processes under consideration. This thesis examines the transient behavior of relatively short transfer lines and derives the distribution of the performance measures of interest. The proposed method based on the analytical derivation of the distribution of throughput is also applied to the systems with two-part types. An experiment is designed in order to compare the results of this study with the state-space representations and the simulation. They are also interpreted from the point of view of the line behavior and design issue. Furthermore, extensions are briefly discussed and directions for future research are suggested.
Open Access
Serial production lines under pull environment
(1996) Eskigün, Erdem
Uie aim of this thesis is to evaliuite the performance of transfer lines under pull environment. In this study, we have modelled a two-stage trcuisfer line separated by finite buffers by using Markov-Chcun representation. Experiments with different parameters were carried out to evaluate the different performance measures of the system. A two-stage simulation model was also constructed by using the package PromodelPC 2.0. Since two-stage models are limited to analyze most prcictical systems in manufacturing, we have extended it to an N-stage model. From the experiments performed for both two-stage and Nstage models, we hcive observed that we can decreci.se the in-process inventory substantially by just exchanging the positions of the nicichines in the system. Assembly-disassembly systems under pull environment have also been modelled by using simulation.
Open Access
A simulation study to determine raw material safety stocks and production batch sizes
(1995) Yılmaz, Gökşın
MRP II controlled production systems are large in scope and generally complex in operation. Consequently the total time required to fill a customer order through procurement and production activities is very long. This study proposes a method to increase the throughput of a real MRP II controlled production system by using the data available in the system. Critical raw materials and subassemblies of a product are determined by using project scheduling techniques. A simulation model is developed by using SIMAN. Simulation experiments are performed for determining raw material safety stock quantities and production batch sizes with the objective of decreasing the time required to fill a customer order.
Open Access
Two-machine flowshop scheduling with flexible operations and controllable processing times
(2011) Uruk, Zeynep
In this study, we consider a two-machine flowshop scheduling problem with identical jobs. Each of these jobs has three operations, where the first operation must be performed on the first machine, the second operation must be performed on the second machine, and the third operation (named as flexible operation) can be performed on either machine but cannot be preempted. Highly flexible CNC machines are capable of performing different operations as long as the required cutting tools are loaded on these machines. The processing times on these machines can be changed easily in albeit of higher manufacturing cost by adjusting the machining parameters like the speed of the machine, feed rate, and/or the depth of cut. The overall problem is to determine the assignment of the flexible operations to the machines and processing times for each job simultaneously, with the bicriteria objective of minimizing the manufacturing cost and minimizing makespan. For such a bicriteria problem, there is no unique optimum but a set of nondominated solutions. Using ǫ constraint approach, the problem could be transformed to be minimizing total manufacturing cost objective for a given upper limit on the makespan objective. The resulting single criteria problem is a nonlinear mixed integer formulation. For the cases where the exact algorithm may not be efficient in terms of computation time, we propose an efficient approximation algorithm.