Inventory Management and Warehouse Operations.


Goods to Person Order Picking.

  By Dave Piasecki  

First, let's go over the difference between Goods-to-person picking and person-to-goods picking. Person to goods is just the classic picking process whereby an order picker goes to a location in the warehouse that contains the item they need, and picks it there. Goods-to-person picking implies the item that needs to be picked is delivered to a stationary picker so they can pick it.

Goods-to-person picking, also known as part-to-picker picking, isn't a new thing, and I'm not even sure it qualifies as a picking methodology. It just describes picking practices that are based on inventory being moved to a fixed position where an order picker can pick it. This is more the result of specialized equipment than anything else. In the past, this was accomplished by automated equipment like horizontal and vertical carousels, vertical lift modules, and Automated Storage and Retrieval systems (AS/RS cranes). What has changed though, is that in recent years, warehouse robots have provided many more options for Goods-to-Person picking. This really started with the Kiva robots that Amazon started using. Then grew with numerous manufacturers basically copying the Kiva robot system, and others creating completely different robotic systems that used various approaches to accomplish goods-to-person picking.

So what are the advantages of good-to-person order picking?

Primarily, good-to-person order picking eliminates the travel time an order picker would otherwise consume. This is especially true when other processes need to be accomplished as part of the order-picking process, such as scale counting items during picking. Normally, a scale count would require the picker to travel to a station that is set up with a counting scale, count the inventory required for the pick, then travel back to the storage location to return the remaining inventory.

Other examples would include custom packaging or light manufacturing processes that are difficult to accomplish on a picking cart or lift truck.

That said, it's important to note that there are other ways to reduce travel time. These would include slotting logic and picking methodologies such as multi-order picking. In addition, order characteristics can have a significant effect on travel time. For example, if you typically have large orders with many line items, your travel time per line may not be significant since your next pick is not that far from your current pick. Therefore, it's important to understand just how much of your order picking is travel time before looking to spend money on reducing/eliminating it.

Goods-to-Person Picking with Horizontal Carousels.

This is the classic goods-to-person picking equipment. It's been used for many decades in warehouses. Horizontal carousels in the warehouse evolved from equipment originally used in dry-cleaning businesses where garments hanging from a simple overhead conveyor were transported to and from "somewhere in the back". It eliminated the need for a person to have to go in the back to find and retrieve your dry cleaning. It not only eliminated the need to travel, but also kept track of where everything was.

Horizontal carousels in the warehouse are essentially the same thing, except, rather than a garment on a hanger, there are small shelving units hanging from the overhead conveyor. And, just as with the garment carousels, they eliminate the need to travel to get an item, but also keep track of where everything is.

A typical horizontal carousel setup in the warehouse would include multiple carousels (called a Pod) set up with a workstation. The reason you use multiple carousels is to eliminate the need to wait for a single carousel to rotate to bring the next item to pick. This would typically be two to four carousels in a Pod for a single workstation. The number of carousels required for a single workstation varies based on the number of SKUs and the number of picks per SKU in the carousel, and the time it takes a worker to complete the pick. Pick-to-light is typically used to assist the picker with a light tree or laser identifying the bin, and a digital display of the quantity to pick.

In addition, you need to have multiple picks available for the carousel to deliver. This would typically be facilitated by doing multi-order picking with a put-to-light station. For example, you may have a station that has 8 totes representing 8 orders. This will provide at least 8 picks for the carousels, but it may be substantially more than that. The picks for the orders are sequenced to make the best use of the carousels—typically picking from each carousel sequentially to make sure each carousel has the pick delivered by the time the worker is ready to pick it.

So the process goes: the carousel rotates until the needed bin is presented, then a light designates the bin to pick, and a display tells the operator the quantity to pick. When the picker picks the item, a light will designate the quantity and the tote to place the pick into; if multiple orders require the same item, each tote will display the appropriate quantity. When the quantity is placed in the tote, the operator presses a button associated with the tote to tell the system that the pick is complete. Then the next pick is presented to the picker.

You can see there is quite a bit of computer logic required to keep everything running smoothly. This is typically accomplished with software provided by the manufacturer of the carousels. Even though you may have a WMS or other inventory system that you use to pick other items not in the carousels, your WMS will generally hand off the carousel picks to the carousel software for processing through the carousels.

Zone Picking.

It's highly unlikely you will be able to do all your picking from one pod of carousels. This would be due to more picks than one person at a carousel can handle, or items to pick that don't make sense for a carousel. This means you may have to do some type of zone-picking. Pick-and-Pass has gone out of favor over the years, so you would be more likely to do some type of pick-and-consolidate zone picking using either a conveyor sortation system, put walls, or a robotic sortation system.

Other means of goods-to-person picking.

In my experience, horizontal carousels have been by far the most common means of goods-to-person picking. Horizontal carousels are very popular because they get the job done and are relatively affordable. I covered the process for horizontal carousels in detail because many of the same issues apply to other goods-to-person options. That said, there are other systems that can be used.

Vertical Carousels and Vertical Lift Modules are other options. These do a good job are utilizing overhead space—which is usually underutilized in piece-picking operations. But it is tricky picking from several at the same time.

Min-load AS/RS. Again, these are great at utilizing the full ceiling height, but are a bit pricey and are tricky picking from multiple units at the same time.

Goods-to-Person Picking with Robots.

Kiva-type Robots.

And then came Amazon and Kiva Robots. Kiva Robots first started getting attention in the 2000s with their squat little orange robots that were essentially a more sophisticated AGV. They used small shelving units that could be picked up by the robots and delivered to a picking station. I can't say that I know this for certain, but I think this was the first time this specific approach was used. So Kiva was selling systems and getting some attention in the trade magazines, but nothing substantial. Then Amazon started using them, and everybody was talking about them. Then Amazon purchased Kiva, and the industry went crazy. And when Amazon decided to stop selling Kiva Robots to other companies, it opened up a market for other warehouse automation companies to step in with their Kiva clones.

So now we have a bunch of "Kiva-type" robots to choose from. The advantages of these systems are as follows:

  • They are relatively easy to implement. Historically warehouse automation has been difficult to implement, but these robots were almost simple. That said, there is still quite a bit of work to implement these. It still takes months. I suspect that time will come down as companies get more experience implementing these.
  • They are relatively easy to expand and grow. As long as you have some room available, you can add racks and more robots at any time.
  • It's a little easier to slot larger items in the racks than it is in carousel carriers. I do want to qualify this a little, because you can put pretty big stuff in carousels, but you probably won't.
  • They almost make slotting a non-issue. If you are putting everything in the Kiva-type racks, anything can go anywhere. Now, there are some advantages to slotting items together that would likely ship together, and you need to make sure you don't overload the racks/robots. But, this is still rather simple slotting.
  • They do allow you to pick orders with fewer people. With labor shortages these days, this is the big selling point.
  • Theoretically, you can easily move the whole system to your next warehouse. This is also a big plus since businesses are constantly outgrowing their warehouses. I say "theoretically" because there still is work to accomplish this, but it is easier than most other types of warehouse automation.
  • The process of picking with Kiva-type robots is almost identical to carousel picking. You are still probably using multi-order picking to keep a buffer of picks available to the robots. You will still have several fixed picking stations, and the robots will deliver picks to these. You will still likely use some type of pick-to-light and put-to-light to assist with the picking process.
  • Similar to carousel systems, your WMS would likely hand off the picks for the Kiva-type system to software specifically designed for this system to execute the picks within the system.
  • What is different here is that, unlike carousels, specific racks are not captive to a specific pick station. So as long as everything is being picked from the Kiva-type system, you don't need to zone pick. Don't underestimate how much this helps.
  • And if a specific robot breaks down, it's no big deal because another robot can take over its work while it's being repaired. This is another huge advantage over typical warehouse automation, which can bring your operation to a stop if something breaks. This assumes it's just an isolated hardware breakdown, not a systems failure.

Disadvantages of Kiva-type robots.

Downsides include having to secure inventory in the racks. I remember working late nights in a piece-picking warehouse where I would hear things falling out of bins that were in static shelving systems long after everyone had left. These Kiva-type racks are being picked up and transported frequently, so you have to take additional steps to keep the stuff in the racks. And these restraints may interfere with picking.

But the big downside is cost. These systems are very expensive, and while you can clearly see that they will reduce your labor costs, will it be enough to pay for these systems? All I can say here is to be careful and do the ROI calculations yourself. I've seen enough overly optimistic ROIs from equipment suppliers to never trust them.

Other Goods-to-Person Picking Robots.

The Kiva-type robots were just the beginning. I already mentioned the Kiva clones. But there are numerous other robot-based systems that use the goods-to-person model, but do it differently.

The next system that seems to be getting a lot of attention is the Autostore System. This system requires you to build a 3-dimensional framework that holds totes. The totes are stacked on top of each other and are accessed by carriers that run on top of the framework, then retrieve the totes below. Obviously, this framework means that it will not be as portable as the Kiva-type systems.

Next are robots that retrieve totes from more standard shelving. An example would be the Stacker-Bot from Conveyco. They move up and down aisles, then elevate to pick totes out of the shelving. This is similar to how a mini-load AS/RS works, except rather than having an AS/RS crane dedicated to a specific aisle, the robots can move between aisles and actually transport to a pick station somewhere else if necessary.

And for palletized loads, there are numerous pallet shuttle systems that move palletized loads around in high-density pallet racks. These systems can be extended with other robots to take the palletized load to a picking station elsewhere in the warehouse if needed.

This only scratches the surface of Goods-to-person picking robots. There is extraordinary innovation going on now with warehouse robots, and it seems like every few months, I see something completely new.

Check out my article on Warehouse Robots.


More Articles by Dave Piasecki.

Dave Piasecki, is owner/operator of Inventory Operations Consulting LLC, a consulting firm providing services related to inventory management, material handling, and warehouse operations. He has over 25 years experience in operations management and can be reached through his website (, where he maintains additional relevant information. 

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