Robotic Sortation Systems: A Practical Guide for Warehouse Teams

A sorter is fast once an item is on it. Getting the item there, and dealing with the items it can't take, is a different job. Someone still separates and places every item at induction, clears full chutes, and works the exception lane where no-reads and oversize items pile up. Choose the wrong sortation technology for your item mix and that manual work grows instead of shrinking.

Sortation shows up all over the supply chain. A 2019 survey of automated sortation conveyors in the European Journal of Operational Research describes applications ranging from airport baggage handling to parcels sorted by tilt-tray conveyors in postal distribution centers to batches of picking orders in ecommerce warehouses. This guide walks through the main technology categories, where each fits by item type and volume, and the work that stays manual whichever one you choose.

What every sortation system has to do

Whatever the hardware, a sortation system does four jobs:

1. Induct. Get one item at a time onto the sorter, spaced and positioned so it can be read and diverted.

2. Identify. Read a barcode or tag and look up the destination in the warehouse control system or WMS.

3. Divert. Move the item off the main flow into the right chute, lane, pocket, bag, or tote.

4. Clear. Empty full destinations, close them out, and send them to the next step.

The technology categories below differ mostly in how they divert. Induction and clearing are where people tend to stay.

Types of robotic sortation systems

Sliding shoe sorters

Slats carry items along a straight conveyor, and shoes slide across the slats to push each item into a divert lane. They suit cartons, cases, and totes with a stable base. MHI's Conveyor & Sortation Systems group notes that shipping operations with high throughput, or with a wide range of product sizes and weights, are more likely to use a sliding shoe or narrow belt sorter. Small loose items, soft bags, and round items are a poorer fit, because the shoe needs a stable surface to push against.

Cross-belt sorters

A loop of carriers, each with its own short belt running at right angles to the direction of travel. To divert, the belt runs and delivers the item into a chute on either side. Because each item rides on its own carrier and is discharged under control, cross-belts handle a broad mix: small boxes, polybags, and soft goods. They take significant floor or mezzanine space and are sized for a planned volume.

Tilt-tray sorters

A loop of trays that tilt to slide each item into its destination chute. Tilt-tray and cross-belt modules both appear on circular sorters that keep open orders moving until the remaining items arrive, then divert the complete order to consolidation. Tilt-trays are simple and handle mixed items, but the item leaves the tray by gravity, so very light, round, or fragile items need testing.

Bomb-bay and pocket sorters

Bomb-bay sorters drop items through doors in the carrier into destinations underneath, which saves width. Pocket, or pouch, sorters hang each item in its own pouch on an overhead track. Pouches can be buffered and released in sequence, which suits ecommerce orders, apparel, and returns, where items wait for the rest of their order.

AMR-based sortation

Fleets of small mobile robots each carry an item from an induction station to its destination, usually a chute or bag on a raised platform or a floor grid. Capacity grows by adding robots rather than extending a machine. A 2021 study in Transportation Science starts from the observation that many distribution centers use expensive conveyor-based sorting systems that need large buildings, and finds that a robotic sorting system typically has lower overall annual cost than a cross-belt sorter when the required throughput is not too large. Sortation robots share floor space with people, so plan traffic rules and a safety risk assessment for the fleet.

Put walls

The lowest-capital option. In the common setup, the wall is a reach-through rack whose shelves are each temporarily assigned to one customer order: a worker takes items from arriving bins and puts each into its order's shelf, guided by put-to-light, while packers empty completed shelves from the other side. Put walls take almost any item a person can hold. Their limit is labor, since every item is handled by hand. Robot arms can do the put step at some walls, which turns a put wall into a robotic sortation cell.