Malte Fliedner

A classification of assembly line balancing problems

Assembly lines are special flow-line production systems which are of great importance in the indus-trial production of high quantity standardized commodities. Recently, assembly lines even gained importance in low volume production of customized products (mass-customization). Due to high capital requirements when installing or redesigning a line, its configuration planning is of great rele-vance for practitioners. Accordingly, this attracted attention of plenty researchers, who tried to sup-port real-world configuration planning by suited optimization models (assembly line balancing prob-lems). In spite of the enormous academic effort in assembly line balancing, there remains a consider-able gap between requirements of real configuration problems and the status of research. To ease communication between researchers and practitioners, we provide a classification scheme of assem-bly line balancing. This is a valuable step in identifying remaining research challenges which might contribute to closing the gap.

Sequencing mixed-model assembly lines: Survey, classification and model critique

Manufacturers in a wide range of industries nowadays face the challenge of providing a rich product variety at a very low cost. This typically requires the implementation of cost efficient, flexible production systems. Often, so called mixed-model assembly lines are employed, where setup operations are reduced to such an extent that various models of a common base product can be manufactured in intermixed sequences. However, the observed diversity of mixed-model lines makes a thorough sequence planning essential for exploiting the benefits of assembly line production. This paper reviews and discusses the three major planning approaches presented in the literature, mixed-model sequencing, car sequencing and level scheduling, and provides a hierarchical classification scheme to systematically record the academic efforts in each field and to deduce future research issues.

Scheduling inbound and outbound trucks at cross docking terminals

At cross docking terminals, shipments from inbound trucks are unloaded, sorted and moved to dispatch points where they are directly loaded onto outbound trucks for an immediate delivery elsewhere in the distribution system. This warehouse management concept aims at realizing economies in transportation cost by consolidating divergent shipments to full truckloads without requiring excessive inventory at the cross dock. The efficient operation of such a system requires an appropriate coordination of inbound and outbound trucks, e.g. by computerized scheduling procedures.This work introduces a base model for scheduling trucks at cross docking terminals, which relies on a set of simplifying assumptions in order to derive fundamental insights into the underlying problem’s structure, i.e. its complexity, and to develop a building block solution procedure, which might be employed to solve more complex real-world truck scheduling problems.

Absalom: Balancing assembly lines with assignment restrictions

Assembly line balancing problems (ALBPs) arise whenever an assembly line is configured, redesigned or adjusted. An ALBP consists of distributing the total workload for manufacturing products among the work stations along the line. On the one hand, research has focussed on developing effective and fast solution methods for exactly solving the simple assembly line balancing problem (SALBP). On the other hand, a number of real-world extensions of SALBP have been introduced but solved with straight-forward and simple heuristics in many cases. Therefore, there is a lack of procedures for exactly solving such generalized ALBP. In this paper, we show how to extend the well-known solution procedure Salome [Scholl, A., Klein, R., 1997. Salome: A bidirectional branch-and-bound procedure for assembly line balancing. Informs J. Comput. 9 319-334], which is able to solve even large SALBP instances in a very effective manner, to a problem extension with different types of assignment restrictions (called ARALBP). The extended procedure, referred to as Absalom, employs a favorable branching scheme, an arsenal of bounding rules and a variety of logical tests using ideas from constraint programming. Computational experiments show that Absalom is a very promising exact solution approach although the additional assignment restrictions complicate the problem considerably and necessitate a relaxation of some components of Salome.

Production planning of mixed-model assembly lines: overview and extensions

Mixed-model assembly lines are of great practical relevance and are widely used in a range of industries, such as the final assembly of the automotive and electronics industries. Prior research mainly selected and discussed isolated problems rather than considering the whole planning process. In this article, mixed-model production planning is decomposed into five steps: initial configuration of the line, master scheduling, reconfiguration planning, sequencing, and re-sequencing. This article reviews and discusses all relevant planning steps and proposes general planning instruments as well as formalised decision models for those steps, which have not been thoroughly investigated in the literature thus far.

The sequence-dependent assembly line balancing problem

Assembly line balancing problems (ALBP) arise whenever an assembly line is configured, redesigned or adjusted. An ALBP consists of distributing the total workload for manufacturing any unit of the products to be assembled among the work stations along the line. The sequence-dependent assembly line balancing problem (SDALBP) is an extension of the standard simple assembly line balancing problem (SALBP) which has significant relevance in real-world assembly line settings. SDALBP extends the basic problem by considering sequence-dependent task times. In this paper, we define this new problem, formulate several versions of a mixed-integer program, adapt solution approaches for SALBP to SDALBP, generate test data and perform some preliminary computational experiments. As a main result, we find that applying SALBP-based search procedures is very effective, whereas modelling and solving the problem with MIP standard software is not recommendable.

A versatile algorithm for assembly line balancing

This paper discusses a two stage graph-algorithm, which was designed to solve line balancing problems including practice relevant constraints (GALBP), such as parallel work stations and tasks, cost synergies, processing alternatives, zoning restrictions, stochastic processing times or U-shaped assembly lines. Unlike former procedures, the presented approach can be easily modified to incorporate all of the named extensions. It is not only possible to select and solve single classes of constraints, but rather any combination of them with just slight modifications.

A Survey on Container Processing in Railway Yards

In spite of extraordinary support programs initiated by the European Union and other national authorities, the percentage of overall freight traffic moved by train is in steady decline. This development is driven by the fact that macro-economic benefits of rail traffic, such as relief of overloaded road networks and reduced environmental impacts, are countervailed by severe disadvantages from the perspective of the shipper, e.g., low average delivery speed and general lack of reliability. Attracting a higher share of freight traffic on rail requires a more efficient freight handling in railway yards, which includes technical innovations as well as the development of suited decision support systems. This paper reviews container processing in railway yards from an operational research perspective and analyzes basic decision problems for the two most important yard types, namely conventional rail-road and modern rail-rail transshipment yards. Existing literature is reviewed and open research challenges are identified.

Shunting yard operations: Theoretical aspects and applications ☆

Almost ever since freight has been transported via rail, shunting yards (also called classification or marshaling yards) are operated in order to separate freight trains and reassemble new trains. The efficient use of shunting yards has a deep impact on the efficiency and reliability of rail freight services. Thus, much research on shunting yards has been published, starting from the 1950s. Lately, several publications mostly focusing on the sorting procedures have livened up research on shunting yards. This paper reviews the literature on the operational processes at shunting yards over the last 40years and discusses the operational challenges of freight transshipment. The approaches are classified according to different sorting strategies which allows an easy access to the models for both, researchers and practitioners. The paper concludes with an overview on future research challenges.

Determining crane areas in intermodal transshipment yards: The yard partition problem

At rail-road transshipment yards, gantry cranes move containers from freight trains to trucks and vice versa. They constitute important entities in todays intermodal transportation systems. Real-world yards are often partitioned into several disjunct crane areas, so that crane interferences during container transshipment are avoided. In practice, the lengths of such crane areas are typically determined by simple rules of thumb, i.e., each crane receives an equally sized area, which might result in an unleveled division of labor among cranes and, thus, prolong train processing times. This paper provides an exact solution procedure which determines disjunct yard areas of varying size for multiple gantry cranes in polynomial runtime, so that the workload for a given pulse of trains is equally distributed among cranes. Furthermore, we investigate the potential acceleration of train processing as compared to equally sized areas in a yard simulation.