Meet the fast 3D storage solution that boosts logistics

So-called cube-storage systems have been in the automated material handling market for over ten years now. They are very popular and for good reasons. Cube-storage solutions are very compact, reasonably affordable and offer an efficiency increase in order fulfillment.
 
A cube-storage system typically has products stored in bins and those bins are stacked on top of each other in a metal structure. Robots drive on top of the grid (or, in some solutions, below the grid) to retrieve the bins. In most cases, if a bin somewhere in the middle of a stack is required, all bins on top of it are placed to the side by robots that“dig out” the required bin. This technology makes efficient logistics and order fulfillment possible in distribution centers, taking warehouse operations to another level.

A natural limit – storage-to-throughput ratio

All this works very well, up to a limit. That limit is defined by the “storage-to-throughput ratio”. The throughput of these systems is limited by the number of robots that fit on (or below) the storage and retrieval system. Use too many robots, and the operation will get congested.  
 
In the last ten years, this limit has increased. In the beginning, the maximum throughput that could be achieved in a system storing 10.000 bins was around 600 to 700 bins per hour. For a system with 20.000 bins, that would be double, etcetera. 
 
Smarter Warehouse management systems (WMS), or Warehouse Execution systems (WES), faster robots and better layouts have increased this limit significantly, but there is still a maximum that can be achieved. The only thing that could be done to reach beyond that is to make the cube system lower. That will increase the surface area to create more space for the robots to navigate the system with more efficiency.

In real-world and real-time terms, what do these numbers mean? Let’s assume an average bin holds 30 pieces. And that for each bin presented, you pick out two pieces. With an average of 8 hours of picking per day, this would equal 15 to 30 days of inventory in the system. Or a stock rotation of 10 to 20 times per year.
 
For many warehouse operations, this is more than enough. A lot of them will never be close to this limit.

Sometimes it needs to be faster, e.g. for e-commerce and groceries

In some cases, however, things just need to be faster. A lot faster. For example, in fast-moving consumer goods and (grocery-)retail. Also, in e-grocery there is a need for high-stock rotations. Two or three days of forward inventory are not exceptionally low in these warehouse operations.

Many of the so-called micro-fulfillment centers are operated with low inventory. Because these MFCs are close to the consumers, warehouse space is often at a premium. And with spreading inventory across multiple smaller locations, there is a risk that the total inventory goes up. To minimize these effects, micro-fulfillment centers are operated with a minimum of stock. 

The alternative is to use a shuttle system. These are “classic” aisle-based solutions. Typically there is a shuttle on each level, in each aisle. With these automated storage and retrieval systems, higher stock rotations can be achieved. The downside of these ASRSs is that they are less flexible and less compact.

Storage and throughput are highly connected. If more throughput is required, another aisle will need to be built. That will also give more storage capacity, even if it is not needed. Furthermore, these shuttle systems need conveyor systems to connect storage with picking workstations. In high-throughput systems, these conveyors will become complicated and expensive.