Automated Storage and Retrieval Systems (AS/RS)

An Automated Storage and Retrieval System (AS/RS) is a fixed-infrastructure technology that automatically places loads into and retrieves them from predefined storage locations. AS/RS systems are the oldest form of warehouse automation, dating to the 1960s, and remain one of the most capital-intensive and space-efficient options available.

System Types

Unit-Load AS/RS (Stacker Cranes)

Designed for full pallets or large containers. A stacker crane travels along a rack aisle (single- or double-deep), with the mast spanning the full rack height.

Aisle height: up to 45 m in high-bay warehouses
Throughput: 30–60 dual cycles/hour per crane (one combined store + retrieve)
Weight capacity: 500 kg to 2,000 kg per load
Key vendors: Dematic, Swisslog, SSI Schäfer, Daifuku, Mecalux
Best for: cold storage, automotive parts, beverages, and other high-cube, heavy-load environments

Mini-Load AS/RS

Scaled-down stacker cranes for totes, trays, and cartons. Operates in the same rail-bound aisle format but for smaller loads (up to ~50 kg).

Throughput: 100–500 totes/hour per aisle
Common in: spare parts, e-commerce fulfilment, apparel

Vertical Lift Modules (VLMs)

A VLM is a self-contained enclosed column with two columns of trays and a central extractor that retrieves the requested tray and presents it at an ergonomic access opening at the operator level.

Height: 2 m to 16 m; maximises vertical space in existing buildings
Access time: 20–40 seconds per tray
Storage density: 60–80% floor space reduction vs open shelving
Key vendors: Kardex, Hänel, Modula
Best for: slow-to-medium velocity parts, spare parts rooms, healthcare supplies

Vertical Carousels

A chain of shelves rotating on a vertical oval track — like a Ferris wheel for inventory. The target shelf rotates to the operator, who picks without bending or reaching.

Simpler and less expensive than VLMs
Lower density than VLMs but higher throughput
Best for: parts rooms, MRO, print consumables

Horizontal Carousels

Oval-shaped horizontal track carrying bins that rotate to bring the target bin to the operator. Usually deployed in banks of 2–4 with a single operator servicing all simultaneously.

Throughput: 250–400 picks/operator/hour with an optimised pick sequence
Key vendors: Hänel, Remstar (Kardex), BEUMER

Shuttle Systems

A rack structure with one or more shuttles (small robotic vehicles) per aisle level that slide in and out to store and retrieve totes. Compared to stacker cranes, shuttles offer:

Higher throughput via multi-level shuttles (one shuttle per level)
More redundancy — failure of one shuttle doesn't take the whole aisle down
Faster deployment (modular racking)

Sub-types:

Single-level shuttles — one shuttle per level, handles that level only
Multi-level shuttles — lift within the rack to change levels
Grid-based systems (AutoStore, Ocado) — robots on a grid roof retrieve bins stacked below

Grid / Cube Storage (AutoStore model):
Bins are stacked in a dense grid without aisles. Robots travel on top of the grid, dig down through stacks to retrieve target bins, and deliver them to workstations.

Density: highest of any system — ~4× denser than conventional shelving, ~2× denser than shuttle racks
Throughput: scales linearly with robot count
Key vendors: AutoStore, Ocado, Exotec (Skypod)
Limitations: retrieval latency increases for bins buried deep in the stack (hot SKUs must be near the top)

Comparison

System	Throughput	Density	CapEx	Flexibility
Unit-load crane	Medium	High	Very high	Very low
Mini-load crane	Medium-high	High	High	Low
VLM	Low-medium	Very high	Medium	Low
Carousel (horiz.)	Medium-high	Medium	Low	Low
Shuttle rack	High	High	High	Medium
Grid / cube	High (scalable)	Very high	High	Medium

Integration with Other Systems

All AS/RS types interface with the WMS through a Warehouse Control System (WCS), which translates high-level pick/store orders into device commands. Totes retrieved from AS/RS systems typically flow onto conveyor and sorting lines for onward routing to packing stations.