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

Abstract

We 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. We thereby address the current problem of motion planning requiring extensive calculation time. As a solution, we propose a "free-step" to simplify the procedure of collision verification and reduce the computational time. On the other hand, we proposed a method to reschedule the order of collision verification in order to find an appropriate trajectory in less time. By the proposed method, we reduce the calculation time to less than 1/300 of that achieved in former research.

Using Two Stackers on One Rail in a Production System

“…This solution for a Storage/Retrieval system (Fig. 3) was published in [3] with methods for how to coordinate these two stackers to prevent collisions. Stackers must pass by at different heights, cannot operate with forks on the same length positions and nearby.…”

Integrated Optimization of Input/Output Point Assignment and Twin Stackers Scheduling in Multi-Input/Output Points Automated Storage and Retrieval System by Ant Colony Algorithm

“…e solutions of collisions in the above literature are based on the premise of determining the operation sequence of one stacker first, and then optimize the operation sequence of another stacker combining with noncrossing constraint, so as to minimize the makespan on the premise of avoiding collisions. When Kung et al [39,40] solved the stacker operational optimization problem with noncrossing constraint, they divided the problem into two stages: grouping and sorting. In the first stage, the clustering method was applied to group S/R requests.…”

Research on scheduling strategy for automated storage and retrieval system

“…However, some researchers have different opinions. Kung et al [19] considered that trajectory planning in a warehouse equipped with two cranes is very difficult. As a solution, they proposed a 'free-step' and a way to choose a trajectory that is likely to be free from collision.…”