Design and Operational Analysis of Automated Guided Vehicle-Based Goods-to-Person Order Picking and Sortation Systems

Abstract

Design and analysis of warehouse automation systems continues to be an active topic of interest both in academia and practice, especially in light of the significant increase in online retail sales. This work concerns the operational analysis and modeling of warehouse automation systems that rely on Automated Guided Vehicles (AGVs). Although, as a technology, AGV systems are not new, recently they have been used in “goods-to-person” order picking (OP) systems and in sortation systems.

AGV-based OP systems, also known as Robotic Mobile Fulfillment Systems (RMFS), were first introduced under the name of Kiva systems. A key component of an AGV-based OP system is the fleet of “robots” (or AGVs) that pick up the “pods” (or “racks”) and transport them to the appropriate pick station (PS), where a human picker picks the items ordered by customers.

We examine two types of well-known, goods-to-person order picking systems, namely, a miniload system and a Kiva system. Using a simulation model, we compare the performance of the two systems on the basis of expected throughput and expected container retrieval times to process the same set of customer orders. We also discuss some of the advantages and limitations of the above two systems.

The performance of AGV-based OP systems depends on the number of AGVs, which in turn depends on the time it takes the AGV to retrieve a pod from the storage area. We derive closed-form analytic expressions for the expected travel distance of the AGVs operating under two possible order assignment rules. Under the random assignment rule, an order is assigned to any PS with equal probability. Under the closest assignment rule, the order is assigned to the closest PS. We also examine the impact of the shape of the storage area on the expected AGV travel distance and the impact of alternative PS configurations. The results offer valuable insight concerning the expected AGV travel distances, which are also needed for analytic design and performance evaluation models.

AGV-based sortation systems (AGV-SSs) have recently been adopted by some of the leading online retail companies. An AGV-SS performs the same basic function as a traditional, conveyor-based sortation system except that it relies on AGVs instead of conveyors to sort the items. In virtually all automated and semi-automated sortation systems, there is at least one “induction point” (IP) that serves as an identification and entry point for the items to be sorted by the system. In an AGV-SS, each (empty) AGV is loaded at the IP with an item to be sorted. The loading process may be manual or automated. Once the item is loaded onto the AGV, the AGV travels to the appropriate destination point (DP), where the item is automatically discharged, and the empty AGV travels to the same or another IP to pick-up the next item. Given the location of the DPs, we are concerned with determining the optimum location of the IPs in order to minimize the expected AGV travel distance. We propose heuristic methods for the problem and compare their performance through a computational study. We also present results for and compare interior versus perimeter placement of the IPs. Finally, we develop and solve an analytic model for the optimal location of two IPs in a continuous plane that represents the sortation region.