Zoned out: Deciding on how many zones to put in your picking system

Deciding how many zones to put in your picking system is not as easy as you might think.

By Jim Apple -- Modern Materials Handling, 8/1/2007

Every once in a while I get a question from my partner at the Progress Group Steve Mulaik that sets me back a little. I think: "How could I have not thought about that before?"

This time it was, "How do we decide how many zones we should have in a serial zone picking system?"

It would have been nice to be able to say that we simply calculate the number of pickers required to meet the demand and then create a zone for each one of them. But, that answer ignores too many of the factors that make a system work well.

First of all, the number of pickers varies greatly during the year, month, week or even the day. When zones are established with conveyor support they are not easy to change. And, the number of pickers surely will be smaller when the system is new, than when volumes reach the planning goal.

In a simple pick-and-pass system with paper pick lists, it is easy to vary zone sizes. But with light-directed picking, zones are more likely to be fixed in size and number. With RF terminals or voice-directed systems, we can change the zone sizes a bit more dynamically.

I think that the question can best be addressed with a list of things to consider, and then some guidelines for reaching the best conclusion.

Some important ones are:

How do we direct the pickers?
Can we support more than one picker in a zone? On the same order or on different ones?
Can we change the zone boundaries easily? Every once in a while, by wave or by order?
Are we picking one order at a time or a batch of several orders?
Can zones overlap?
How does the problem differ for simple pick-and-pass, serial zones with by-pass capability and parallel zone picking?

So, Steve, I don't have a simple answer to your question.

As I tried to think about how one of my friends in the academic world might approach the question, my mind was freed of all the traditional physical constraints. I imagined the picking area as a circle with the pick-up and drop-off point at the center.

Fast movers would be slotted closest to the center and slower mover toward the perimeter. Pickers would be assigned an order, or perhaps a small batch of orders, with products that were grouped in a segment of the circle, reducing the distance to travel.

Larger orders with picks all around the circle might be recombined as part of another batch for another segment.

ZONES??? There are no zones, at least not in the traditional sense. "Zones" are created uniquely for each order or batch. Workload would be easily balanced because pickers are free to work in any segment of the circle without having to travel to another zone.

Conventional wisdom says that such a system would be impossible to design or lay out. But, we really won't know until somebody tries.

Author Information

Jim Apple can be contacted at japple@theprogressgroup.com

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