Media Coverage
June 2010
Ants and Zones in Warehouses
In warehouses with a large number of SKU’s it is common to divide the warehouse into zones and then to allocate pickers to a zone. As orders come in, they are automatically split into zones and each sub-list sent to a picker in each zone. The sub-picks are then combined back into the full order.
This zone-based approach seems to make sense but there is one big flaw. Even, if the zones are well constructed so that on average the amount of work allocated to each zone is equal, on any given day, or any subset of orders there can be a very disproportionate amount of work between the zones. Also, it is well know that there is a large variation in worker efficiency, with the fastest workers being up to four times quicker than the slowest. This means that at times some pickers will be standing around waiting for work while others are scurrying around trying to keep up. You may feel that this problem is insoluble or somehow inherent in the warehouse setup. How can you prevent this kind of imbalance in workload?
It turns out that by watching ants and other swarming insects, Eric Bonabeau and Christopher Meyer, in a very famous article that appeared in the Harvard Business Review in 2001 entitled “Swarm Intelligence: A whole new way to think about business”, have come up with a novel and intriguing way of dealing with this problem. They observed that when ants bring food back to the nest they don’t each carry a load back all the way, rather, they use a bucket brigade approach where the ants pass food along a chain. Each ant runs along the chain and passes the food over to the next ant he meets, he then runs back up the chain to get the next bit of food. The transfer points are not fixed and slide up and down depending on the speed of each ant. The only fixed locations are the source of the food and nest.
Their idea was that as an order is received, the picker in the first zone starts picking until he runs into a returning picker. This may be in his original zone or even deep into the next zone. He then hands over the order and runs back for the next order. In the mean time, the next picker continues picking the order until he meets a returning picker and hands it over. Then, at the end of the process, the order is complete and does not need to be assembled from several zones.
You may still think that pickers will have to wait if there are no orders coming down the chain, but if we sequence the workers from slowest to fastest then everyone will always be working and there will be no need to stand around waiting for work while pickers from other zones complete their tasks.
This swarm based approach has been implemented in many large distribution centres with improvements of up to 31% reported over the zone method.
One drawback of this method is that pickers may have to work in, and be familiar with, several zones, so there will be an additional training overhead. This should be quite small in most cases.
Another possible drawback is that orders may be quite large and heavy and bigger than can be easily handled by one person, so that it is not desirable for the entire order to pass along a chain. In such a case it may make sense to split the warehouse into larger zones and use the bucket brigade approach within each zone.
If you have any comments or questions, please e-mail me at david.lubinsky@opsi.co.za.
