The impact of restricting the flow of inventory in serial production systems

This research uses simulation technique to compare the output performance of pull production systems with different inter-station storage capacities (ISSC) under various levels of total inventory (INV) in the system, station coefficient of variation (CV), and station down-time (DT). There are two primary findings from this study. First, it shows pull systems with larger ISSC yield a significantly higher output at all levels of INV in the system, station CV, and station DT. Second, it shows that lines using a strict kanban-based pull system can increase output while decreasing WIP inventory simply by allowing the inventory to flow freely between stations. These are major findings considering that the modifications necessary in the system to achieve these results are fairly uncomplicated and should require minimal investment.     

The simplification of material flow systems

There is a widely accepted philosophy of production organization which has provided the foundation for the design of nearly all the world's factories. It is based on a set of beliefs which until recently were never questioned. This philosophy provides restraints which make it difficult to increase efficiency. It inevitably produces very complex, material flow systems, makes it difficult to delegate responsibility for the completion of products or components and leads to highly bureaucratic systems with centralized decision making and a low level of job satisfaction. Since the Industrial Revolution we have learned to plan the production of components with some precision. Until recently we did not plan material flow systems; they only evolved as the results of thousands of independent component planning decisions. We now know how to preplan simple and efficient flow systems and to achieve by this means major gains in both productivity and workers’ job satisfaction.     

The impact of lot sizing and sequencing on manufacturing performance in a two-stage hybrid flow shop

This paper deals with the problem of production scheduling in a two-stage hybrid flow shop, which consists of one machine in the first stage, and multiple process lines in the second stage. In the first stage, materials are batch-processed, which are then fed into one of several process lines in the second stage, depending on the final product specifications. First, we formulated the total flow time or makespan, the capacity utilization and the maximum work-in-process level as measures of manufacturing performance. We analysed, through a series of numerical experiments, how these performance measures are affected by lot sizing, sequencing rules and the scheduling scenario. The following points were clarified. (1) The ratio rule yields the best performance with regard to the makespan, but the LPT rule yields the best performance with regard to the maximum work-in-process level, (2) The optimum lot size is also affected by such operation conditions as the unit processing time, the setup time, and the ratio of unit processing times in the two stages. (3) The starting point for making a schedule should be set in the bottleneck stage with regard to the makespan.     

Practical considerations in the optimization of flow production systems

In this paper we consider the problems faced by an industrial planner who is responsible for the design of real-life asynchronous production lines under stochastic conditions that may be due to breakdowns or random processing times. Based on real-life system data, it is shown that a number of available algorithms for the performance evaluation of a given system configuration as well as an algorithm for determining the optimum buffer configuration can be successfully applied in industrial practice.     

The segmented bidirectional single-loop topology for material flow systems

This paper introduces the segmented bidirectional single-loop (SBSL) flow topology for carrier-based material handling systems. This configuration is based on a single-loop flow-path structure that is divided into non-overlapping segments, each containing a single carrier operating in a bidirectional mode. The design procedure comprises a 0 - 1 mixed-integer formulation to determine the single-loop including the pickup and delivery station location. The second stage is a segmentation procedure to determine the non-overlapping segments in the loop. Finally the performance of the SBSL is evaluated by means of simulation.     

Managing two-stage serial inventory systems under demand and supply uncertainty and customer service level requirements

We consider a two-echelon serial inventory system with demand and supply uncertainty, non-zero lead times for component procurement and end-product assembly, and a minimum customer service level requirement. We present two supply models which incorporate both quantity and timing uncertainty; these models correspond to current and proposed supply environments. Assuming that installation base-stock ordering policies are followed and that the demand distribution is quasi-concave, we show that the chance-constrained problem of determining optimal base-stock levels which minimize the total inventory investment (cost-weighted stock levels) subject to a service constraint is a convex programming problem. We characterize the relation between the optimal base-stock levels of the component and the end-product. We also illustrate how an optimal internal (component) service level can be computed, which permits decomposition of the two-stage serial system into two coordinated single-echelon systems. Computational experiments illustrate insights on the effects of supply uncertainty and other problem parameters on stock-positioning in a two-echelon serial system. In particular, we evaluate the benefits of switching from one supply environment to another.     

Decentralized inventory control in a two-stage capacitated supply chain

This paper investigates a two-stage supply chain consisting of a capacitated supplier and a retailer that faces a stationary random demand. Both the supplier and the retailer employ base stock policies for inventory replenishment. All unsatisfied demand is backlogged and the customer backorder cost is shared between the supplier and the retailer. We investigate the determination of decentralized inventory decisions when the two parties optimize their individual inventory-related costs in a noncooperative manner. We explicitly characterize the Nash equilibrium inventory strategies and identify the causes of inefficiency in the decentralized operation. We then study a set of simple linear contracts to see whether these inefficiencies can be overcome. Finally, we investigate Stackelberg games where one of the parties is assumed to be dominating.     

Thermoeconomic considerations in the allocation of heat transfer inventory for irreversible refrigeration and heat pump systems

In this paper, thermoeconomic considerations are given to heat exchanger inventory allocation in irreversible refrigeration cycles and heat pumps with finite thermal capacitances. Investigation is made with respect to specified rate of cooling and heating for the refrigeration cycle and heat pump, respectively. Exact expressions are obtained without the use of an internal irreversibility parameter. The optimum hot-to cold-end unit cost ratio resulted in unequal division of heat exchanger conductances for the heat pump in contrast to the endoreversible case where they were the same. At a constant evaporator to condenser fluid temperature ratio, the COP of both refrigeration and heat pump systems was found to vary in an almost asymptotic manner in contrast to the endoreversible case where it was constant. In conclusion, it is best to measure the effect of internal dissipation by deriving exact mathematical expressions as the effect on some performance parameters could be lost due to the averaging effect of the internal irreversibility parameter.     

Real-time disruption management in a two-stage production and inventory system

This paper presents a general disruption management approach for a two-stage production and inventory control system. A penalty cost for deviations of the new plan from the original plan is incorporated and the concept of a disruption recovery time window is introduced. We define two classes of problems: one with fixed setup epochs and another with flexible setup epochs. With linear or quadratic penalty functions for production/ordering quantity change and fixed setup epochs, the best recovery plan is obtained by solving a quadratic mathematical programming problem. With convex penalty functions for quantity changes and flexible setup epochs, it is shown that the second stage orders have identical order quantities within each production cycle. Therefore, in a lot-for-lot system, the ordering and production quantities for both stages are the same. As a special case, we consider disruption recovery problems with short time windows spanning one or two production cycles. We also discuss solution procedures for both major and minor disruption problems and give an extension for the case of multiple retailers. Throughout the paper managerial insights are presented that indicate how a company should respond to various types of disruptions during its operations.     

The role of inventory banks in flow-line production systems

Many production managers of flow-line systems, such as in the steel and metal processing industries, observe the high levels of in-process inventories. They feel that it should be possible to reduce these inventories; however, before doing so it is necessary for them to understand their function. Although the general rule that inventories exist because of imbalance in supply and demand at a point within the system is true, it is necessary to show more precisely how such an imbalance might arise in single-product systems due to variability in processing times at the stations, or interruptions in production due to breakdown and subsequent repair of stations. Quantitative results have been obtained which indicate how such factors as the number, location and capacity of inventory bank, affect the system production rate. These results enable system designers and operators to check whether their systems could be improved.     

The impact of the number of parallel warehouses on total inventory

In the strategic design of a distribution system, the right number of stock points for the various products is an important question. In the past decade, a strong trend in the consumer goods industry led to centralizing the inventory in a single echelon consisting of a few parallel warehouses or even a single distribution center for a Europe-wide distribution system. Centralizing inventory is justified by the reduction in total stock which mostly overcompensates the increasing transportation cost. The effect of centralization is usually described by the “Square Root Law”, stating that the total stock increases with the square root of the number of stock points. However, in the usual case where the warehouses are replenished in full truck loads and where a given fill rate has to be satisfied, the Square Root Law is not valid. This paper explores that case. It establishes functional relationships between the demand to be served by a warehouse and the necessary safety and cycle stock for various demand settings and control policies, using an approximation of the normal loss function and its inverse. As a consequence, the impact of the number of parallel warehouses on the total stock can be derived. The results can be used as tools in network design models.     

An introduction to the segmented flow approach for discrete material flow systems

Due to increases in versatility and the complexity of demands in modern manufacturing systems a growing need has evolved t0 simplify the control of these systems while maintaining a high degree of efficiency and flexibility in operation. The segmented flow approach (SFT) suggested in this paper is a possible solution to these needs. This structure simplifies the control task by reducing the need for dispatching and by eliminating the need for routeing material handling devices and scheduling traffic at intersections. Each task involves only one material handling device and one possible route. This structure maintains a high degree of efficiency by operating material handling devices in a bidirectional mode and eliminates congestion and blocking by operating them on mutually exclusive tracks. This paper introduces a design procedure for an SFT implemented system using a minimum cost objective, and compares the throughput and time in system performance of this system to a conventional system and a tandem type system by means of simulation.     

The interaction of location and inventory in designing distribution systems

Many companies face the strategic decision of deciding on the number of Distribution Centers (DCs), their location, and which customers they serve. One objective for a company facing this decision is to maintain acceptable service while minimizing the fixed costs of operating the DCs, inventory holding costs at the DCs, and transportation costs between plants and DCs, and DCs and customers. For insight into this problem, we develop an analytical model for a stylized version of it. However, since the general version of the problem is NP-Hard, we also develop heuristic procedures. We solve a variety of example problems to test the performance of these heuristics relative to optimal solutions and a lower bound based on a relaxation of the original problem. Managerial insight based on our computational studies is provided. We also present a small case-study example motivated by our interaction with Frito-Lay, Inc     

两段式结构赝三元材料热电模块的研究

在实验研究单段式结构赝三元材料的热电模块基础上,利用现有半导体致冷器生产工艺,依据热电材料的温度特性和冷热端工作温度,采用不同电阻率的赝三元材料,设计制作了两段式结构的热电模块．研究表明,当冷、热端工作温差为80K时,其输出功率和发电效率均比单段式结构的热电模块提高80％左右．     

Minimising makespan in the two-stage assembly hybrid flow shop scheduling problem using artificial immune systems

Two-stage hybrid flow shop (HFS) scheduling problem followed by single assembly machine is addressed in this paper. To produce the final product, parts need to be processed on the HFS stages and thereafter, several parts are joined under the assembly operations based on the predefined Bill of Materials of the product. The aim of this research is to find the schedule which minimises completion time of the last product, i.e. makespan. For the considered problem, lower bound, heuristic algorithms and two metaheuristic techniques based on artificial immune system are developed. Computational results demonstrate that the proposed lower bound and heuristic algorithms outperform the existent lower bounds and heuristic algorithms.     

On the structure and logic of typical material flow systems†

A queueing network technique (Solberg's CAN-Q) and a flow control simulator (SINDECS) are applied to investigate general, fundamental performance properties of material flow systems (MFS). The effects of a system's structure and flow logic on performance are compared for three typical flow systems: transfer line, recirculating conveyor loop, and job shop. The concepts of Most Economical MFS for a given performance level, Iso-Performance MTS combinations, and Overall MFS Configuration Measures are introduced.     

A study of material flow systems (input/output) in high-bay warehouses

A method of planning and designing product sorting systems in warehousing is described. The theoretical background of the analytical models and simulation modelling is given. A calculation of the capacity of elements for joining and dividing for different priorities of material flow is described. New parameters are introduced in the formula for calculating the average number of unit loads in a system for joining, in the direction of the slave flow and for a simple analytical model for joining material flows with two different priorities with an exponential distribution of inter-arrival time. Simulation results are obtained using the GPSS/FON simulation language. Some characteristic results, used in the process of planning and design of two new distribution centres in Belgrade, are shown.     

Performance evaluation of two-stage multi-product kanban systems

We propose an approximate evaluation procedure for a kanban-controlled production system with two stages and multiple products. In the first stage, single-product manufacturing facilities produce items that are the input material for a single multi-product manufacturing facility in the second stage. Each manufacturing facility is controlled by a distinct kanban loop with a fixed number of kanbans. Processing and setup times are exponentially distributed, demand arrivals at the output store of the second stage are Poisson and independent for each product. If a customer's demand cannot be met from stock, the customer either waits or leaves the system, depending on the admissible number of backorders and the current number of waiting customers (partial backordering). We describe a new decomposition-based approximation method for the evaluation of such systems in steady state. We focus on the performance measures average fill rate, average fraction of served demand, and average inventory level. We report the results of several numerical tests. The results indicate that the approximation is sufficiently accurate for a large variety of systems. We also illustrate the effects of increasing the maximum number of backorders on the performance of the system.