Fully Packed and Ready to Go: High-Density, Rearrangement-Free, Grid-Based Storage and Retrieval

TL;DR

This paper explores grid-based storage systems that allow free movement of loads, demonstrating conditions under which rearrangement-free storage and retrieval are always possible, thereby improving efficiency in logistics operations.

Contribution

It introduces a novel analysis of rearrangement-free storage solutions in grid-based systems with flexible load movements, expanding beyond traditional stack-based approaches.

Findings

Rearrangement-free solutions exist in various scenarios, including full capacity loads.

A grid open side of at least 3 cells guarantees rearrangement-free retrieval.

Practical implications for logistics and automated storage systems are discussed.

Abstract

Grid-based storage systems with uniformly shaped loads (e.g., containers, pallets, totes) are commonplace in logistics, industrial, and transportation domains. A key performance metric for such systems is the maximization of space utilization, which requires some loads to be placed behind or below others, preventing direct access to them. Consequently, dense storage settings bring up the challenge of determining how to place loads while minimizing costly rearrangement efforts necessary during retrieval. This paper considers the setting involving an inbound phase, during which loads arrive, followed by an outbound phase, during which loads depart. The setting is prevalent in distribution centers, automated parking garages, and container ports. In both phases, minimizing the number of rearrangement actions results in more optimal (e.g., fast, energy-efficient, etc.) operations. In contrast to previous work focusing on stack-based systems, this effort examines the case where loads can be freely moved along the grid, e.g., by a mobile robot, expanding the range of possible motions. We establish that for a range of scenarios, such as having limited prior knowledge of the loads' arrival sequences or grids with a narrow opening, a (best possible) rearrangement-free solution always exists, including when the loads fill the grid to its capacity. In particular, when the sequences are fully known, we establish an intriguing characterization showing that rearrangement can always be avoided if and only if the open side of the grid (used to access the storage) is at least 3 cells wide. We further discuss useful practical implications of our solutions.