As demonstrated above, assembled products are omnipresent in both our personal as much as professional lives. However, the use of assembly lines requires setting up and efficiently organizing assembly systems. While the term assembly system frequently describes the assembly line or the feeding system (see, e.g., Arai et al. (2000); Battini et al. (2010b); Hu et al. (2011)), the term is used in a much broader sense in this thesis. As mentioned above, an assembly system consists of physical and organizational aspects. In terms of physical aspects, an assembly system is, among others, defined by geographical location, factory size, building architecture, equipment usage and placement, production and logistics areas within the building, gates, and driveways. Some of the assembly system’s organizational aspects are the assignment of products to assembly lines, the distribution of tasks among different workers, production planning, scheduling activities, material flow, and process definitions. The combination of all these and other characteristics determines an assembly system. For planners, this raises the task of making decisions on all these aspects, knowing that many decisions are interlinked. When planning the manufacturing and logistical processes of a production facility, e.g., physical characteristics may result from this planning, or they may be an input to this planning. Another option is the joint planning of physical characteristics and processes. The design of such an assembly system includes various decisions, classified as strategic, tactical, and operational decisions in this thesis.
• Strategic level: The strategic level is concerned with long-term decisions such as make-or-buy, supplier selection, single vs. multi-sourcing, and facility location (G¨opfert et al., 2016). These decisions are of the highest strategic relevance and form the basis for decisions
on the tactical and operational levels. However, lower-level decisions may be taken into account when making these decisions, or strategic decisions may require adaptation after lower-level decisions are determined.
• Tactical level: Once a facility’s location and the outsourcing of production and logistics activities are determined, tactical decisions can be taken. One tactical decision is the as-sembly line’s design, which is concerned with deciding the number of asas-sembly stations and assigning tasks to stations (Boysen et al., 2007). In literature, this is described as the ALBP. Another tactical decision is the logistical system’s determination, mainly con-cerned with feeding parts to the line. Part feeding decisions, which are made in the ALFP, determine presorting activities of parts and the delivery quantity to the line. Furthermore, those decisions have an impact on the facility’s requirements in terms of space and equip-ment. The facility’s design and the acquisition of logistical and assembly equipment such as forklifts, racks, and tools may be considered another tactical decision. All decisions on the tactical level are linked to each other.
• Operational level: On the operational level, short-term decisions such as loading and routing of in-house vehicles or the scheduling of logistical picking operations are to be taken. Another operational decision is the sequencing of products on the line: When multiple products are produced on a single line, the assembly times at any station may vary for these products, and drift may occur, i.e., some products may require less time (negative drift) whereas other products may require more time (positive drift) than the time available at a station (Hu et al., 2011). Therefore, it may be necessary to sequence products such that any positive drift is canceled out over time (Emde and Polten, 2019).
In this thesis, the focus will be on tactical decision-making, specifically the assembly line balanc-ing and the assembly line feedbalanc-ing problem. In literature, these two decision-makbalanc-ing problems are usually treated separately (see, e.g., Batta¨ıa and Dolgui (2013) and Schmid and Lim`ere (2019)).
Similarly, in practice, both problems are associated with two different departments: the in-house logistics department works on assembly line feeding decisions the production department works on assembly line balancing decisions. This thesis considers both problems individually before both problems are combined and integrated with facility design to evaluate the merits of such an integration. Before discussing the outline and contribution of this thesis, an informal explanation of both problems is given.
1.2.1 Assembly line balancing problems
Assembly line balancing problems are concerned with assigning a set of assembly tasks to work stations. The number of tasks and their characteristics needs to be known in advance. Essential characteristics are assembly times and the precedence relations of all tasks. The assembly time describes how long an operator needs to assemble a part onto the product. These times are typically estimations based on time studies such as Methods-Of-Time-Measurements (MTM), REFA, or Very Easy Work-factor (VWF). Precedence relations describe the order in which tasks
can be conducted and are primarily given in the form of a precedence graph, more specifically an acyclic digraph. An example of such a graph can be found in Figure 1.3, with nodes representing tasks and arcs (directed edges) indicate precedence relations. E.g., task 8 can only be started after task 7 is finished.
The assignment of tasks has to be done such that the sum of task assembly times at any station is smaller or equal to the cycle time. This cycle time is either given as a parameter (following a demand-driven calculation) or minimized. Similarly, the number of stations may be given as a parameter or minimized. A combination of these options results in four possible combinations, which are considered a class of simple assembly line balancing problems (SALBPs) (Baybars, 1986) SALBP-F: a feasibility problem, verifying if there is a solution with a given number of stations and a given cycle time. SALBP-1: Cycle time given, Minimizing the number of stations.
SALBP-2: number of stations given, minimizing cycle time. SALBP-E: Efficiency problem, minimizing both the number of stations and cycle time.
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Figure 1.3Precedence graph of assembly tasks (adjusted from Bowman (1960))
1.2.2 Assembly line feeding problems
The assembly line feeding problem is typically solved after the assembly line balancing problem.
Thus, the number of stations, the cycle time, and the tasks assigned to each station are known.
Next, every task is linked to all the parts needed for execution. The term part family is used to create subsets of parts. Each part family is a set of parts, also known as Stock-Keeping-Units (SKUs) or components, that serve the same or a very similar purpose. However, they are not identical as they might differ concerning color, quality, or haptics. All parts in a part family complement each other, which means that only one part from any family can be used for a specific product.
When solving the assembly line feeding problem, the goal is to assign each part to a line feeding policy while minimizing the associated logistical efforts or costs. This cost minimization problem can be complemented by various constraints and decisions that will be discussed at a later stage (Chapter 2). Line feeding policies define three aspects of part feeding:
• The execution of the associated logistical processes within the assembly facility.
• The combination of parts and part families in a single load carrier.
• The selection of a load carrier holding the material at the work station.
In this work, we distinguish five line feeding policies: (i) line stocking, i.e., the provision of a large load carrier including only one type of parts; (ii) boxed-supply, in which a smaller quantity of one part type is provided in a small load carrier or box; (iii) sequencing, for which all parts of a part family are sorted into a single load carrier according to the production sequence; (iv) stationary kits, which contain multiple sequenced part families used at a single station; and (v) traveling kits, that contain multiple sequenced part families used at multiple stations.