A simulation study on the performance of pickup-dispatching rules for multiple-load automated guided vehicles

In this paper, the pickup-dispatching problem of multiple-load AGVs (automated guided vehicles) is studied. This problem is defined in the multiple-load control process proposed by Ho and Chien. Their control process identifies four problems faced by a multiple-load AGV. These problems are task-determination, delivery-dispatching, pickup-dispatching and load-selection. This paper focuses on the third problem. For this problem, nine pickup-dispatching rules are proposed and studied. The first, second and fourth problems are not the main focus of this study, thus only one task-determination rule, one delivery-dispatching rule and two load-selection rules are adopted for them. The objective of this study is twofold. First, to understand the performance of the proposed rules in different performance measures, e.g., the system's throughput, the mean flow time of parts and the mean tardiness of parts. Second, the effects that the proposed rules have on each other's performance are investigated. Computer simulations are used to achieve these objectives. The experimental results reveal a rule that dispatches vehicles to the machine with the greatest output queue length is the best in all performance measures. Also, distance-based or due-time-based rules do not perform as well as queue-based rules. It is also found that the performance of pickup-dispatching rules is affected by different load-selections rules.