Implementations of shuttle based automated storage and retrieval systems (AS/RS) for goods-to-person (GtP) order fulfillment — particularly in direct-to-consumer e-commerce applications — are on the rise. These systems enable operations to hold, organize, track, and access a large volume of vastly different stock keeping units (SKUs) profiles in a highly dense footprint.
Certain AS/RS feature powered shuttles that travel independently throughout a grid of multiple, closely spaced levels to store and retrieve totes, trays, or cases of product. Each shuttle is equipped with an inserter/extractor device that automatically stores or retrieves items as directed by the AS/RS’ control software and the facility’s overarching warehouse management system (WMS) or warehouse execution system (WES).
Indeed, shuttle-based systems have become the most deployed AS/RS technology. According to ARC Advisory Group, they are ideal for high-throughput operations filling highly variable e-commerce orders. With picks presented automatically to operators at workstations, time is spent filling orders instead of searching for SKUs.
That said, there are a variety of shuttle based AS/RS types, designs, and configurations available in the marketplace. For that reason, matching the right shuttle technology to the optimal application is critical for deployment success. There are four types, with the main difference among them being their directional movement capabilities: forward and backward, side to side, and up and down.
One-Dimensional Shuttles were the first designs available. They travel forward and backward within one storage aisle and are restricted to a single storage level. These limitations make the system less adaptable to changing order profiles or volumes, and often result in low shuttle utilization. They are best suited to applications storing a limited number of SKUs and with consistent order profiles, such as manufacturing and kitting.
Two-Dimensional Shuttles offer improved utilization rates, as they are outfitted with two sets of wheels for movement forward and backward and side to side. They also incorporate one or more lifts on the side of the storage structure for on-demand transport of one shuttle at a time to other levels with higher pick volumes. Lifts enable placement of shuttles on every other level to increase utilization while reducing overall system cost. These systems are ideal for GtP picking applications, operations with lower throughput requirements, or handling of medium- to low-velocity SKUs.
Three-Dimensional Shuttles move independently in all three planes — forward and backward, side to side, and up and down — delivering payloads directly to a picking workstation, no lifts required. Although this increases shuttle availability, it can also inhibit overall system capacity and performance if shuttles get held up waiting their turn to travel to the pick stations or to return items to storage. The potential for shuttle traffic jams makes these systems better suited to lower throughput operations, including GtP picking and handling of medium- to low-velocity SKUs.
Roaming Shuttles address the limitations of two- and three-dimensional shuttles with a combination of two-dimensional shuttles for SKU placement and retrieval, integrated product lifts, and buffering conveyors. When an item is needed, the shuttle closest to its storage location travels to it, removes the tote, tray, or case, and carries it to a lift platform designated for picks. It deposits its payload for movement to a picking station via conveyor. Separate lifts pick up totes sequenced for return to storage.
Sequenced and buffered tote delivery to inbound and outbound lifts ensures that all totes with items required for an order are routed together to a specific workstation. This enables roaming shuttle AS/RS to deliver up to 600-plus totes to a single workstation per hour. That makes them ideal for high volume picking operations like e-commerce order fulfillment. They also deliver:
- Improved order accuracy
- Increased flexibility in day-to-day operations
- Resolved labor challenges
- Shortened order lead/cut-off times
- Reduced downtime and higher availability