Figure 2 Classification of order-picking systems (based on De Koster 2004)
In this paper we concentrate on low-level, picker-to-parts order-picking systems employing humans (and with
multiple picks per route). These systems form the very large majority of picking systems in warehouses
worldwide (based on the authors’ experience: over 80% of all order-picking systems in Western Europe).
Surprisingly, academic order-picking literature focuses more on high-level picking and AS/RS systems.
Although not the main topic of this paper, we will briefly mention some of the latter type of literature as well.
The design of real order-picking systems is often complicated, due to a wide spectrum of external and internal
factors which impact design choices. According to Goetschalckx and Ashayeri (1989) external factors that
influence the order-picking choices include marketing channels, customer demand pattern, supplier
replenishment pattern and inventory levels, the overall demand for a product, and the state of the economy.
Internal factors include system characteristics, organisation, and operational policies of order-picking systems.
System characteristics consist of mechanisation level, information availability and warehouse dimensionality
(see Figure 3). Decision problems related to these factors are often concerned at the design stage. The
organisation and operational policies include mainly five factors: routing, storage, batching, zoning and order
release mode. Figure 3 also shows the level of complexity of order-picking systems, measured by the distance
of the representation of this problem in the axis system to the origin. In other words, the farther a system is
located from the origin, the harder the system is to design and control.ms).