Motion Planning of Two Stacker Cranes in a Large-Scale Automated Storage/Retrieval System

Abstract

The authors propose a method for reducing the computational time of motion planning for stacker cranes. Most automated storage/retrieval systems (AS/RSs) are only equipped with one stacker crane. However, this is logistically challenging, and greater work efficiency in warehouses, such as those using two stacker cranes, is required. In this paper, a warehouse with two stacker cranes working simultaneously is proposed. Unlike warehouses with only one crane, trajectory planning in those with two cranes is very difficult. Since there are two cranes working together, a proper trajectory must be considered to avoid collision. However, verifying collisions is complicated and requires a considerable amount of computational time. As transport work in AS/RSs occurs randomly, motion planning cannot be conducted in advance. Planning an appropriate trajectory within a restricted duration would be a difficult task. The authors thereby address the current problem of motion planning requiring extensive calculation time. As a solution, they propose a “free-step” to simplify the procedure of collision verification and reduce the computational time. On the other hand, the authors proposed a method to reschedule the order of collision verification in order to find an appropriate trajectory in less time. By the proposed method, they reduce the calculation time to less than 1/300 of that achieved in former research.