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

2025-05-28

AI Summary

In high-density grid-based storage systems (e.g., automated parking facilities, container terminals), dynamic inbound/outbound operations frequently trigger costly relocations under full-load conditions. Method: This paper investigates the feasibility and construction of relocation-free storage layouts under full-load operation. We establish, for the first time, that fully relocation-free access is achievable for any sequence of insertions and deletions when the grid’s aisle width is at least three—breaking the classical stack-like constraints and enabling free lateral translation of loads by mobile robots. Leveraging combinatorial optimization and grid reachability modeling, we derive necessary and sufficient conditions for relocation-free layouts and propose an efficient constructive algorithm. Contribution/Results: We theoretically prove the guaranteed existence of relocation-free solutions across multiple realistic scenarios. Experiments demonstrate significant improvements in throughput and energy efficiency compared to conventional approaches.

Technology Category

• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems
• Intelligent Robots: Localization, Mapping, and Navigation
• Search and Optimization: Combinatorial Optimization

Application Category

• Graph Algorithms and Modeling for the Web: Efficient manipulation of static and dynamic Web-related graphs
• Systems and Infrastructure for Web, Mobile and WoT: Experiences and lessons learnt from Web-based algorithms and system deployments
• Responsible Web: Human-perceived consequences of algorithmic deployment on the web

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.

Problem

• Maximizing space utilization in grid-based storage systems
• Minimizing rearrangement efforts during load retrieval
• Ensuring rearrangement-free solutions for dense storage scenarios

Innovation

• Grid-based storage with free load movement
• Rearrangement-free solution for dense storage
• Access requires grid opening width ≥3 cells