Journal of Optimization in Industrial Engineering Vol.13, Issue 1, Winter & Spring 2020, 67-80 DOI: 10.22094/JOIE.2018.537.37 67 A Mathematical Model for Multiple-load AGVs in Tandem Layout Behnam Rahimikelarijani a , Mohammad Saidi-Mehrabad a,* , Farnaz Barzinpour a a Department of Industrial Engineering, Iran University of Science and Technology, Tehran, Iran Received 11 October 2015; Revised 18 April 2018; Accepted 24 April 2018 Abstract Reducing cost of material handling has been a big challenge for companies. Flexible manufacturing system employed automated guided vehicles (AGV) to maintain efficiency and flexibility. This paper presents a new non-linear mathematical programming model to group n machines into N loops, to make an efficient configuration for AGV system in Tandem layout. The model minimizes both inter-loop, intra-loop flow and use balanced-loops strategy to balance workload in system simultaneously. This paper significantly considers multiple-load AGVs, which has capability of reducing fleet size and waiting time of works. A modified variable neighborhood search method is applied for large size problems, which has good accuracy for small and medium size problems. The results indicate that using multiple load AGV instead of single load AGV will reduce system penalty cost up to 44%. Keywords: AGV, Tandem, Multiple-load, Machine-to-loop assignment, variable neighborhood search 1. Introduction An AGV is an automated guided vehicle, using for horizontal transportation of goods and materials. AGVs traveling on a network of guide-paths. These vehicles are designed to perform repetitive, though not continual, tasks. However, their designs vary according to their usage, capacity and environment. They use in warehousing system, production line, container terminal, public transportation for material handling. The flexible manufacturing system (FMS) composed of different work cells, where different workstations are located in these cells. Categorizing workstations into different cells or loop and assign an AGV with appropriate workload capacity is one the initial decisions. To design such a system, first, the facilities should be located. The systems can vary by the flow path configuration, which is known as a method of classification in AGV system. (Choobineh, Asef-Vaziri and Huang, 2012) enumerate these configurations to be conventional, single loop, tandem and segmented. The advantages of tandem configuration, as mentioned by (R.Tavakkoli-Moghaddam et al., 2008), include control over AVG, elimination of collision and deadlocks, and simplification in production planning. All these features encouraged several researchers to apply tandem configuration in their studies. Tandem configuration distributed stations into non- overlapping loops, where each loop is served by one AGV, which is not allowed to move between loops. Hence, to perform inter-loop transportations, transfer points are considered ( Fig 1) Fig. 1. tandem configuration as proposed by (Bozer and Srinivasan, 1989) This paper addresses a new mathematical to group n machines into N loops and minimize both inter-loop and intra-loop flow simultaneously. 2. Literature Review The problems in Tandem AGV system are categorized as follow (Salehipour, Kazemipoor and Moslemi Naeini, 2011);  Determine number of loops (fleet size)  Assigning workstations to loops  Arranging stations in each loop  Specifying optimum transfer point between loops *Corresponding author Email address: mehrabad@iust.ac.ir