Catálogo de publicaciones - libros

Im folgenden werden flexible Fließproduktionssysteme mit Montagestationen untersucht. An solchen Stationen werden Komponenten von mehreren Zulieferstationen zur Bildung eines neuen Werkstücks zusammengefügt. Das ist die sog. Synchronisationsbedingung. Der Materialfluß ist asynchron. Die Puffer sind beschränkt. Die Bearbeitungszeiten sind beliebig verteilt. Der folgende Beitrag beschreibt ein Verfahren zur Abschätzung der Produktionsrate eines solchen Systems. Hierfür wird ein Dekompositionsansatz verwendet. Die betrachteten 2-Stationen-Subsysteme werden als Warteschlangenmodelle abgebildet, für die die virtuellen Ankunfts- und Bearbeitungsraten sowie die zugehörigen Variationskoeffizienten zu ermitteln sind. Die Approximationsgüte des Verfahrens wird mit einem Simulationsexperiment untersucht.

Im Rahmen einer Zusammenarbeit mit der Firma Stahlwerk Ergste-Westig GmbH, einem Unternehmen der ZAPP AG, wurde ein Verfahren zur Lösung eines 1.5-dimensionalen Verschnittproblems entwickelt und prototypisch implementiert. Das Problem zeichnet sich durch einen begrenzten Lagerbestand mit sehr heterogenem Sortiment und zusätzlichen Nebenbedingungen zur Materialverwendung aus. Es wird durch ein lineares ganzzahliges Modell abgebildet und mittels einer Branch-and-Cut Strategie unter Einsatz der MIP-Solver MOPS und CPLEX gelöst. Dabei wird durch ein um die Berücksichtigung von Range-Bedingungen erweitertes MOPS IP-Preprocessing eine entscheidende Verbesserung der Lösungsqualität und Verkürzung der Rechenzeit erreicht. Das implementierte Entscheidungsunterstützungssystem konnte die realen Probleminstanzen effizient und im Sinne des Unternehmens lösen und erzielte in den durchgeführten Vergleichsrechnungen ein Reduzierungspotential für den Verschnitt von etwa 30% zum bisherigen Vorgehen. Unter Verwendung des Systems ist neben der Ersparnis durch die Verschnittreduzierung auch ein deutliches Einsparpotential an personellen Ressourcen durch den im Vergleich zur manuellen Disposition geringen Zeitbedarf vorhanden.

We present an algorithm for the problem of routing Automated Guided Vehicles (AGVs) in an automated logistic system. The algorithm avoids collisions, deadlocks and livelocks already at the time of route computation (conflict-free routing). After a preprocessing step the real-time computation for each request consists of the determination of a shortest path with time-windows and a following readjustment of these time-windows. Both is done in polynomial-time. Using goal-oriented search we get computation times which are appropriate for real-time routing. Additionally, in comparison to a static routing approach, used in Container Terminal Altenwerder (CTA) at Hamburg Harbour, our algorithm had an explicit advantage.

telecommunication networks allow for switching connections by which can pass several network links without any opto-electronic conversion. Upon arrival of a connection request, it must be decided online, i.e., without knowledge of future requests, if it is accepted and in that case on which lightpaths the connection is routed. This with the goal of maximizing the total profit gained by accepted requests is called (Dsca). We present existing and new algorithms for Dsca as well as their theoretical and practical evaluation.

In this paper we discuss the value of accurate inventory information for distribution planning purposes in a reverse logistics system. Three levels of information are introduced and the value is assessed in a simulation loop. For every planning period the operational vehicle routing problem is solved to optimality by route enumeration and set partitioning. Extensive sensitivity analysis is performed and discussed to analyze the factors that influence the value of accurate inventory information for reverse distribution planning. Network density appears to be one of the main determinants.

We consider a production inventory problem under periodic review with stochastic demand and stochastically proportional yield. Defective products (i.e. the yield loss) arising from the unreliable production process can be temporarily stored in an inventory and passed through a rework process. Rework is completely reliable and brings defective products up to the same quality level as new ones, so that they can be used for demand fulfilment. We focus on the application of linear heuristic policies similar to those emerging from MRP application, and develop simple expressions for the computation of policy parameters. Results of a numerical study indicate that the heuristic approach performs quite well.

Remanufacturing is the most valuable product recovery option since here the value added to the product can be obtained. The goal of this contribution is to develop an instrument which supports decision makers in their considerations about implementing a remanufacturing strategy. The framework is built on a two-stage proceeding, where in the first step an approach for modeling this strategic decision process is presented. In the second step, the contribution aims to show how to evaluate alternative decisions economically by developing a life cycle model for remanufacturing and calculating the life cycle costs.

We investigate the interest of migrating from a call center where all agents are pooled and customers are treated indifferently by any agent, towards a call center where customers are grouped into clusters with dedicated teams of agents. Each cluster will be called a portfolio. Customers of a same portfolio are always served by an agent of the corresponding team. There is no specialization involved in this organization in the sense that all customer portfolios as well as all agents teams have (statistically) identical behaviors. The purpose of this paper is to investigate how the benefits of moving to this new organization in terms of the management of the workforce can outweigh its drawback that comes from the increasing of variability.

In the production of perishable products such as dairy, meat, or bakery goods, the consideration of shelf life in production planning is of particular importance. Retail customers with relatively low inventory turns can benefit significantly from longer product shelf life as wastage and out-of-stock rates decrease. However, in today’s production planning and control systems shelf life issues with regard to specific products or customers are seldom taken into account. Therefore the objective of this paper is to pay attention to these issues. The way to do that is by means of optimization models in which shelf life aspects are integrated into operational production planning and scheduling functions. Specifically we make use of so-called Mixed Integer Linear Programming (MILP) models. Our research is based on an industrial case study of yogurt production. Relying on the principle of block planning, an MILP model for weekly production planning is presented that is based on a combination of a discrete and a continuous time representation. Batch sizing and scheduling of numerous recipes and products on several packaging lines are considered in the model. Overnight production and, hence, the necessity for identifying two different shelf life values for the same batch is also included in the model formulation. Numerical experiments show that near-optimal solutions can be obtained within a reasonable computational time. Finally, the proposed MILP model can be adapted to cover specific features arising in other fresh food industries.

We consider a semiconductor manufacturing facility with multiple products and associated routes, single servers and batch servers, job class dependant service times and external arrivals. The system is modelled as an open queueing network. The aim is to optimize routing such that cycle time constrained capacity is maximized and can be checked in reasonable computation time. We use a decomposition approach based on the connected components of a properly defined fab graph. Taking into consideration arrival rate vectors and service time matrices the routing problem for the network is formulated as a Quadratic Programming Problem (QP) involving averages and variances. The strategy for use of the manifold routing options as they typically occur in semiconductor manufacturing is to distribute load in a way such that each connected component, also called closed machine set (CMS), approaches heavy traffic resource pooling behaviour. In the presence of batch servers, mainly in the furnace area of a fab, results for the batch service queue /1 with threshold server starting policy are combined with a new result for a batch service system with infinitely many job classes and Round Robin service discipline and applied along with the QP solver.