In summary, previous studies have either considered only the case of a single server or have not
considered product and server location simultaneously with allocating products to multiple servers.
3. Problem description
Consider a picking line using a carton or pallet How rack system, which stores N products. Storage
locations on the line are arrangedin n stacks of shelves, spacedequally on the picking line. Each
stack contains k¿1 shelves of equal size so that there are kn = N storage locations. Each product
consumes a single storage location. The picking line is assumedto be long andlow, such that it
wouldbe reasonable for a picker to reach all storage locations within a vertical shelf (henceforth
calleda bin) in the same amount of time.
Customer orders, which consist of demands for one or more products, are 2lled by s¿1 pickers
(or servers) in sequence by using a ‘pick andpass’ strategy. Partially 2lledord ers are transported
down the line, from server to server by a material handling device such as a roller conveyor (refer
to Fig. 1 for a conceptual diagram). Each server operates from a ‘home base’, and is responsible for
picking products from a speci2edset of bins on the line, calleda ‘zone’. When an order container
arrives at the servers’ home base, (s)he begins picking items in the order from the set of products
within his or her zone.