IFAC PapersOnLine 50-1 (2017) 1211–1216 ScienceDirect ScienceDirect Available online at www.sciencedirect.com 2405-8963 © 2017, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. Peer review under responsibility of International Federation of Automatic Control. 10.1016/j.ifacol.2017.08.344 © 2017, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. Keywords: Assembly line balancing problem; Multi-manned workstations; Multi-skilled workers; Mathematical programming. 1. INTRODUCTION Assembly lines are a class of serial production systems that includes a series of workstations arranged along a material handling system (Tonelli et al., 2013). This kind of production lines are usually utilized to manufacture products composed of some subitems that have been systematically joined together to build the main products. One of the problems that managers of such systems usually encounter is to balance the assembly line; that is, to assign the assembly task of the products to the workstations in order to optimize a specific objective, such as minimizing the number of workstations for a given production cycle time. Salveson (1955) was the first researcher that presented a mathematical formulation for the assembly line balancing problem (ALBP). The problem is characterized by some basic assumptions such as: mass-production of one homogeneous product, paced line with fixed cycle time, serial line layout within single- manned workstations, etc. Since then, several researches have dedicated their efforts to develop solution approaches for the simple ALBP (SALBP) (Baybars, 1986). Many studies and works have been done for SALBP; a comprehensive survey of the SALBP research can be found in Becker and Scholl (2006) and Batta¨ ıa and Dolgui (2013). However, the assumptions of SALBP are very restrictive and the model might not represent the industrial reality. In recent years, most of researchers tried to model more realistic and generalized problems of the assembly line bal- ancing through the formalization of the General Assembly Line Balancing Problem (GALBP). Basically, GALBP is the assembly line balancing problem when other consider- ations are added to those characterizing the SALBP fam- ily. The Multi-manned Assembly Line Balancing Problem (MALBP) is a new type of GALBP in which there is the possibility of assigning more than one operator to each workstation, according to the product features. Such a class of balancing problems typically occurs in industries producing large size and high volume of products, such as the automotive industry in which the size of the product is reasonably large to utilize the multi-manned assembly line configuration (Dimitriadis, 2006). A Multi-manned Assembly Line (MAL) has substantial advantages over the simple assembly line, such as reducing the length of the assembly line, the amount of throughput time, the cost of tools and fixtures, the material handling, and setup time. These advantages are sufficient to justify the use of the MAL for the assembling of large-size products. An example of the structure of a MAL is reported in Fig 1. Even if MALs are frequently used in various industries with high volume of products such as the automotive industry, the literature on this subject is not abundant. * Department of Informatics, Bioengineering, Robotics and Systems Engineering (DIBRIS), University of Genova, Via Opera Pia 13, 16145 Genova, Italy (e-mails: massimo.paolucci@unige.it, abdolreza.roshani@edu.unige.it) ** Department of Mechanical, Energy, Management, and Transportation Engineering (DIME), University of Genova, Via Opera Pia 15, 16145 Genova, Italy (e-mails: davide.giglio@unige.it, flavio.tonelli@unige.it) Abstract: Multi-manned assembly lines are usually found in plants producing large-sized high- volume products such as automotive sector. In this paper, the balancing problem of this kind of assembly lines with skilled workers is addressed. A new mixed integer programming formulation is presented to solve this problem optimally with the objective of minimizing the total operating cost of the assembly line. The main advantage of the proposed model is to allow the workers in each multi-manned workstation to perform the different assembly tasks of same product simultaneously. The proposed formulation has been used to solve some experimental problems found in the literature. The comparison between the results obtained with the proposed model and those obtained with the model proposed by Moon, Logendran, and Lee (2009) shows that the proposed model can improve the operating cost of the system by reducing the number of workers and workstations. Davide Giglio ** Massimo Paolucci * Abdolreza Roshani ** Flavio Tonelli ** Multi-manned Assembly Line Balancing Problem with Skilled Workers: A New Mathematical Formulation