# Multi-AGV Scheduling under Limited Buffer Capacity and Battery Charging Using Simulation Techniques
###### Abstract  
In recent years, automated guided vehicles (AGVs) have been widely adopted to automate material handling procedures in manufacturing shopfloors and distribution centers. AGV scheduling is the procedure of allocating a transportation task to an AGV, which has large impacts on the efficiency of an AGV system with multiple AGVs. In order to optimize the operations of multi-AGV systems, AGV scheduling decisions should be made with consideration of practical issues such as buffer space limitations and battery charging. However, previous studies have often overlooked those issues. To fill this gap, this paper proposes a simulation-based multi-AGV scheduling procedure for practical shopfloors with limited buffer capacity and battery charging. Furthermore, we propose three kinds of rules: job selection rules, AGV selection rules, and charging station selection rules, for AGV scheduling in practical shopfloors. The performance of the rules is evaluated through multi-scenario simulation experiments. The FlexSim software v.2022 is used to develop a simulation model for the experiments, and the experimental findings indicate that the job selection rules have larger impacts on the average waiting time than the other kinds of rules.  
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kinds of rules: the job selection rule, the AGV selection rule, and the charging station selection rule. Conventional AGV scheduling methods prioritize the first two types of rules; however, it is crucial to acknowledge the significant impact of battery charging on the efficiency of AGV systems.  
Another practical issue considered in this paper is the limited buffer capacity, where a buffer denotes work-in-process at a specific place. Many manufacturing plants impose limitations on the buffer capacity and maximum buffer size within their production process due to congestion control and safety concerns. In such shopfloors, production is halted when the buffer size reaches the limitation, and it is resumed after the buffer size decreases. The buffer capacity limitation can also affect the performance of AGV scheduling.  
This paper applies the discrete event simulation (DES) technique to multi-AGV scheduling under limited buffer capacity and battery charging. A simulation model for shopfloors with a multi-AGV system is developed using the commercial DES software, FlexSim. Note that the structure of the simulation model is motivated by the practical shopfloor of an automotive part manufacturer in Korea. Experimental results demonstrate that optimal AGV scheduling can be obtained through multi-scenario simulation experiments.  
The remainder of this paper is organized as follows: Section 2 reviews existing studies addressing the AGV scheduling problem. Section 3 discusses the main issues related to buffer capacity limitations and battery charging in a practical shopfloor. We also propose some scheduling rules related to work and AGV selection, as well as charging station selection. Section 4 presents a simulation model for multi-AGV scheduling in a practical shopfloor and demonstrates that the optimal AGV scheduling policy can be derived through multi-scenario experiments. Finally, Section 5 summarizes the conclusions of this study, presents future research directions, and concludes.