Armin Scholl

A survey on problems and methods in generalized assembly line balancing

Abstract Assembly lines are traditional and still attractive means of mass and large-scale series production. Since the early times of Henry Ford several developments took place which changed assembly lines from strictly paced and straight single-model lines to more flexible systems including, among others, lines with parallel work stations or tasks, customer-oriented mixed-model and multi-model lines, U-shaped lines as well as unpaced lines with intermediate buffers. In any case, an important decision problem, called assembly line balancing problem, arises and has to be solved when (re-) configuring an assembly line. It consists of distributing the total workload for manufacturing any unit of the product to be assembled among the work stations along the line. Assembly line balancing research has traditionally focused on the simple assembly line balancing problem (SALBP) which has some restricting assumptions. Recently, a lot of research work has been done in order to describe and solve more realistic generalized problems (GALBP). In this paper, we survey the developments in GALBP research.

Balancing and sequencing of assembly lines

This work deals with two main decision problems which arise when flow-line production systems are installed and operated. The assembly line balancing problem consists of partitioning the work necessary to assemble the product(s) among different stations of an assembly line. If several models of a product are jointly processed on a line, this medium-term problem is connected wih the short-term problem of determining an operating sequence of the models. In part one, balancing and sequencing problems are discussed, classified and arranged within a hierarcdhical planning system. Part two is concerned with exact and heuristic procedures for solving those decision problems.

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.

BISON: a fast hybrid procedure for exactly solving the one-dimensional bin packing problem

Abstract In this paper, we consider the well-known one-dimensional bin packing problem (BPP-1), which is to pack a given set of items having different sizes into a minimum number of equal-sized bins. For solving BPP-1, an exact hybrid solution procedure, called BISON, is proposed. It favourably combines the well-known meta-strategy tabu search and a branch and bound procedure based on known and new bound arguments and a new branching scheme. Computational results indicate that BISON is very effective and outperforms existing approaches.

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.

Balancing assembly lines effectively – A computational comparison

Abstract We report on results of a numerical experiment including the most effective branch and bound procedures for solving type 1 of the simple assembly line balancing problem (SALBP-1), namely Johnsons (1988) FABLE, Nourie and Ventas (1991) OptPack, Hoffmanns (1992) Eureka and Scholl and Kleins (1997) SALOME. It appears that SALOME clearly outperforms the other procedures with respect to hard instances of a new challenging data set. Beyond that, it is shown that the performance of SALOME can greatly be improved by adding some further features including dynamic renumbering and dominance rules.

Simple assembly line balancing—Heuristic approaches

In this paper heuristics for Type 1 and Type 2 of the Simple Assembly Line Balancing Problem (SALBP) are described. Type 1 of SALBP (SALBP-1) consists of assigning tasks to work stations such that the number of stations is minimized for a given production rate whereas Type 2 (SALBP-2) is to maximize the production rate, or equivalently, to minimize the sum of idle times for a given number of stations. In both problem types, precedence constraints between the tasks have to be considered.We describe bidirectional and dynamic extensions to heuristic priority rules widely used for SALBP-1. For the solution of SALBP-2 we present search methods which involve the repetitive application of procedures for SALBP-1. Furthermore, improvement procedures for SALBP-2 are developed and combined with tabu search, a recent strategy to overcome local optimality. Several optional elements of tabu search are discussed. Finally, the application of a nontraditional tabu search approach to solve SALBP-1 is investigated. Computational experiments validate the effectiveness of our new approaches.

Computing lower bounds by destructive improvement: An application to resource-constrained project scheduling

In this paper, two meta-strategies for computing lower bounds (for minimization problems) are described. Constructive (direct) methods directly calculate a bound value by relaxing a problem and solving this relaxation. Destructive improvement techniques restrict a problem by setting a maximal objective function value F and try to contradict (destruct) the feasibility of this reduced problem. In case of success, F or even F + 1 is a valid lower bound value. The fundamental properties and differences of both meta-strategies are explained by applying them to the well-known resource-constrained project scheduling problem (RCPSP). For this problem, only a few constructive bound arguments are available in the literature. We present a number of extensions and new methods as well as techniques for reducing problem data which can be exploited within the destructive improvement framework. Comprehensive numerical experiments show that the new constructive bound arguments clearly provide better bounds than the former ones and that further really significant improvements are obtained through an appropriate application of destructive improvement.

Incorporating ergonomic risks into assembly line balancing

In manufacturing, control of ergonomic risks at manual workplaces is a necessity commanded by legislation, care for health of workers and economic considerations. Methods for estimating ergonomic risks of workplaces are integrated into production routines at most firms that use the assembly-type of production. Assembly line re-balancing, i.e., re-assignment of tasks to workers, is an effective and, in case that no additional workstations are required, inexpensive method to reduce ergonomic risks. In our article, we show that even though most ergonomic risk estimation methods involve nonlinear functions, they can be integrated into assembly line balancing techniques at low additional computational cost. Our computational experiments indicate that re-balancing often leads to a substantial mitigation of ergonomic risks.