Zoned Out

Every once in a while I get a question from my partner, 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 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 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, “How many zones should we create when we design a system?”  

As I tried to think about how one of my academic friends 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 whose products 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 layout. But, we really won’t know until somebody tries.

ABOUT THE AUTHOR

James M. Apple, Jr. is a Director in The Progress Group. Prior to co-founding The Progress Group in 1991, he was a Partner with Coopers & Lybrand's SysteCon division. During 1992-1995 he served as a Senior Systems Advisor with Vanderlande Industries, a major conveyor and systems provider in Europe.

Jim is an internationally recognized thought leader in the area of facility design and integrated distribution systems. His contributions to the improvement of distribution practices have been recognized by his receipt of the prestigious Reed-Apple Award, which is given for lifetime contributions to the advancement of the material handling profession. Jim has also received the Institute of Industrial Engineers' Facilities Planning and Design Award. He has written numerous articles and handbook chapters on warehousing and logistics operations and is a popular speaker on logistics seminar and conference programs.

Prior to SysteCon, Jim worked as an Industrial Engineer with IBM, was Supervisor of Facilities Planning for the Oldsmobile Division of General Motors and was Executive Vice President for an automotive aftermarket parts supplier. He holds B.S. and M.S. degrees in Industrial and Systems Engineering from the Georgia Institute of Technology.