The specific warehousing situation assumed is shown in
Figure 1 where an order picking vehicle travels between a
storage area and input/output (I/O) point. It is assumed
that the order picking vehicle interleaves a storage and a
retrieval in each order picking transaction. Specifically, dual
command transactions are assumed where the order picking
vehicle obtains a unit load from the storage queue, travels
to the appropriate address within the storage area, deposits
the unit load and then travels directly to the retrieval location.
At the retrieval location, the unit load is picked fro,n
storage and transported back to the I/O point where it is
deposited onto a conveyor. The conveyor routes the load
to workstations in a separate part of the staging area where
orders are accunmlated. After the necessary items are
picked from the unit load, it is routed by the conveyor to
a storage queue for placement back in storage on a subsequent
transaction. Meanwhile, the order picking vehicle
obtains another unit load from the storage queue and
repeats the dual address cycle for another storage/retrieval
pair. In all cases, it is assumed that replenishlnent of stock
within the storage area occurs separately from order picking.
Storage transactions merely represent the placement of
unit loads back in storage after order picking occurs.
To formulate the inventory and order picking cost
model, the following notation will be used: