Balancing assembly lines with variable parallel workplaces: Problem definition and effective solution procedure

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 subject to a strict or average cycle time. Traditionally, stations are considered to be manned by one operator, respectively, or duplicated in form of identical parallel stations, each also manned by a single operator. In practice, this assumption is usually too restrictive. This is particularly true for large products like cars, trucks, busses and machines, which can be handled by several operators performing different tasks at the same time. Only restricted research has been done on such parallel workplaces within the same station though they have significant relevance in real-world assembly line settings. In this paper, we consider an extension of the basic ALBP to the case of flexible parallel workplaces (VWALBP) as they typically occur in the automobile and other industries assembling large products. The problem is defined and modelled as an integer linear program. As a solution approach a branch-and-bound procedure is proposed which also can be applied as a heuristic. Finally, computational experiments documenting the solution capabilities of the procedure are reported.