AMRs vs. AS/RS: Which Makes More Sense for Your Operational Needs?
Both autonomous mobile robots (AMRs) and automatic storage and retrieval systems (AS/RS) can be attractive options for order fulfillment operations looking to automate their warehouse or distribution center. Exactly which makes the most sense for a given operation will depend on the specifics of how your operation works, as well as the technical aspects of the two technologies.
Note: Some of you who are familiar with both technologies may be saying to yourselves, “This is like deciding between eating ice cream and pickles,” but before you click away keep reading and you will see they are more similar than you thought.
To help you determine whether AMRs or AS/RS would better help you reach your goals, below we explore the major differences between each technology.
Similarities Between AMRs and AS/RS
Before discussing the differences between AMRs and AS/RS, it would be helpful to first understand the similarities. It is the similarities, after all, which often lead to these two technologies being discussed together.
First and foremost, at their most basic level, AMRs and AS/RS (specifically goods-to-person technology) can be used to perform the same basic tasks of storing and delivering product in an order fulfillment operation. As such, both technologies can help an operation to increase labor productivity and combat a tightening labor market that is leading to higher labor costs and a gap in skilled labor.
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Additionally, both AMRs and AS/RS can be integrated with and controlled by a single warehouse execution software, allowing for a more streamlined order fulfillment process. Their software works functionally like a WMS and automatically prioritizes and coordinates the task in your fulfillment operation. For the sake of simplicity, we will compare the two technologies being used in order fulfillment operations.
Differences Between AMRs and AS/RS
Beyond the similarities outlined above, there are a number of very significant differences between AMRs and AS/RS that need to be considered before either is selected as the solution for your business.
1. Type of Storage Solution
Though both AS/RS and AMRs are used to store product and then deliver a specific SKU or set of SKUs to a picking station, the ways in which they do this—and the technologies upon which they rely—are very different.
Put simply, AS/RS uses a vertical storage solution and AMRs use a horizontal storage solution.
AS/RS uses specialized storage rack to store totes/cartons with the product packed loose inside. Only one machine can access an aisle/storage level at a time. Miniload cranes have one crane per storage aisle and only that crane can access the product stored in that aisle. Shuttles have multiple levels per aisle and one shuttle vehicle per storage level. Both drop the tote/carton onto conveyor which is used to route the tote/carton to one or many picking stations. (Miniloads are better for low to medium volume operations and shuttles are better high volume operations.)
AMRs, on the other hand, ride on the floor and must carry shelving units that hold the product to put away and to the picking station. This allows for very dense storage and any one of the system’s AMRs can access any storage unit. This seems better than AS/RS, but it does not necessarily use the vertical cube of the storage area. AMRs are required to deliver the product directly to the picking station if they are carrying shelving.
AMRs utilize onboard sensors and software in order to interpret and understand their environment; they use this information to dynamically create the most efficient route to follow within a facility. This means that they do not have any preset, fixed routes or rigid underlying infrastructure, and can operate in a rather fluid nature.
Thanks to this underlying technology, it is remarkably easy to change the route that an AMR follows, as well as the picking zone that they service. This flexibility makes AMRs especially well-suited for operations following an agile business model that regularly undergoes substantial changes in workflow.
Since an ASRS system has a fixed specialized storage structure, on the other hand, it is not practical to relocate them to a different part of your facility. That being said, if it is necessary for an AS/RS to service a different part of your facility, it may possible to utilize conveyor to connect the new area to the AS/RS system. It is also possible to expand an existing system to handle additional product as required.
3. Ease of Deployment
Because AMRs are relatively compact pieces of equipment, they can generally be dropped quite readily into an existing facility/workflow without much prep work, lead time, or infrastructure change. The ease comes from the fact that the AMRs themselves are the only hardware that must be deployed and configured; there is no infrastructure change.
Implementing an AS/RS or good-to-person system, on the other hand, would likely require an operation to clear out existing floor space, build out the facility, or move to a new facility in order to accommodate the machinery. AS/RS usually requires a 10-12 month lead time to allow for the manufacture of the cranes and rack. Although shuttles can be put into existing building structures, miniloads usually require a tall building (usually built specifically for AS/RS) 45-60 feet to take advantage of the efficiencies and productivity of the cranes storing product as high as possible.
4. Jobs Done
Both AMRs and AS/RS can be used to store and retrieve product for an operation. But each technology also has the potential to play other roles as well.
In addition to storing product, AMRs can be used to augment an existing picking system to make the system more efficient. For example, an AMR might be used to transport picking carts from zone to zone to reduce the amount of time that pickers spend traveling; or, an AMR might actually work alongside pickers in a pick-to-light or pick-to-voice operation transporting order totes from zone to zone so that pickers spend more time picking and less time traveling.
Automatic storage and retrieval systems can likewise be used to increase the efficiency of pickers. Goods-to-person picking keeps the picker at a station so that they don’t travel at all. Both the SKU and order totes are automatically fed to their station and they are directed by the system where to pick and place product. Other applications of AS/RS technology might include buffer storage for full case picking, and staging and consolidating multi-carton orders for packing and or shipping.
4. Speed and Capacity
In a side by side comparison in a goods to person picking scenario, rates for AS/RS and AMRs can be similar. The combination of the number of SKUs and order profile will determine which technology is more productive for a given operation or facility.
AMRs are better suited to a large number of SKUs (~5,000-10,000) with few fast movers with low to medium order volume. AS/RS is better suited to a smaller number of SKUs (~1,000-4,000) with medium to high order volume.
Though exact numbers will depend on the total number of SKUs to be picked from and the typical order profile of an operation, a shuttle-based AS/RS system can realistically operate about 300-500 lines per hour (per operator), while a fleet of AMRs can realistically operate 300-400 lines per hour per pick station. Due to this difference in operating speed, an AS/RS-based picking operation can handle a greater volume of orders than an AMR-based system.
But because AS/RS is typically more expensive to implement than AMRs (due to the added infrastructure costs, etc.), an operation must have a certain volume of orders per day in order to justify the cost. AS/RS typically begins to make sense starting at around 5,000-10,000 orders per day; below that volume, the gains in efficiency are not great enough to justify the cost, making AMRs a better choice in many cases at lower order volumes
Making the Right Decision for Your Needs
For operations looking to automate their facility, both AMRs and AS/RS can help you reach your goals. But determining which of the two technologies will best help you reach your goals will require some analysis.
We suggest you do some basic analysis to get a general overview of your operation in considering these technologies for your fulfillment operation. The answers to the questions below will help steer you to the technology that fits your operation the best:
- Total orders per day?
- What is the peak 3-4 hour period during your peak season (if you have one)?
- What is your projected annual growth?
- Order profile(s)?
- Average number of SKU’s per order
- Average number of units per order
- Total number of SKUs
- What is their volume distribution by SKU?
- Current annual labor costs by task
In addition to completing the analysis above yourself, a skilled systems integrator can help steer you in the right direction to choose the automation technology that makes the most sense for your operation, whether that is AMRs, AS/RS, or another technology altogether.