Invited ReviewDesign and control of warehouse order picking: A literature review
Introduction
As more companies look to cut costs and improve productivity within their warehouses and distribution centres, picking has come under increased scrutiny. Order picking – the process of retrieving products from storage (or buffer areas) in response to a specific customer request – is the most labour-intensive operation in warehouses with manual systems, and a very capital-intensive operation in warehouses with automated systems (Goetschalckx and Ashayeri, 1989, Drury, 1988, Tompkins et al., 2003). For these reasons, warehousing professionals consider order picking as the highest-priority area for productivity improvements.
Several recent trends both in manufacturing and distribution have made the order-picking design and management become more important and complex. In manufacturing, there is a move to smaller lot-sizes, point-of-use delivery, order and product customisation, and cycle time reductions. In distribution logistics, in order to serve customers, companies tend to accept late orders while providing rapid and timely delivery within tight time windows (thus the time available for order picking becomes shorter). Many smaller warehouses are being replaced by fewer large warehouses to realise economies of scale. In these large warehouses, the daily pick volume is large and the available time window is short. In order to be more responsive to customers, many companies have adopted a postponement strategy (Van Hoek, 2001) leading to various value-adding activities (like kitting, labelling, product or order assembly, customised packaging or palletisation) that take place in the distribution centre and which have to be scheduled and integrated in the order-picking process. Warehouses are also involved in recovering products, materials, and product carriers from customers in order to redistribute them to other customers, recyclers, and original-equipment manufacturers (De Koster et al., 2002).
The organisation of order-picking operations immediately impacts the distribution centre’s and thereby the supply chain’s performance. Between the time an order is released to the warehouse and the time it takes to reach its destination, there is ample opportunity for errors in both accuracy and completeness, not to mention time lost. There is also room for improvement. Industry has come up with innovative solutions, making it possible to attain productivity up to 1000 picks per person hour. Science is also progressing rapidly. Over the last decades, many papers have appeared studying order picking processes. New problems have been studied and new models have been developed. Still, there is a gap between practice and academic research, since not all new picking methods have been studied and the optimal combinations of layout, storage assignment, order clustering, order release method, picker routing and order accumulation have been addressed to a minor extent only. This paper presents a systematic overview of these recent developments in academic literature. We structure typical decision problems in design and control of order-picking processes by focusing on optimal (internal) layout design, storage assignment methods, routing methods, order batching, and zoning. Several areas appear to have received only little attention from researchers. Innovations from practice also lead to new research challenges.
The remainder of the paper is organised as follows. In the next section, we briefly highlight warehouse missions and functions and give an overview of order-picking systems. In Sections 3 Layout design, 4 Storage assignment, 5 Zoning, 6 Batching, 7 Routing methods, 8 Order accumulation and sorting, we review recent literature on design and control of order-picking processes, focussing on layout design, storage assignment, batching, picker routing, and order accumulation. We conclude and discuss potential research directions in Section 8.
Section snippets
Warehouses and order picking
According to ELA/AT Kearney (2004), warehousing contributed to about 20% of the surveyed companies’ logistics costs in 2003 (other activities distinguished are value added services, administration, inventory costs, transportation and transport packaging). Warehouses apparently form an important part of a firm’s logistics system. They are commonly used for storing or buffering products (raw materials, goods-in-process, finished products) at and between points of origin and points of consumption.
Layout design
In the context of order picking, the layout design concerns two sub-problems: the layout of the facility containing the order-picking system and the layout within the order-picking system. The first problem is usually called the facility layout problem; it concerns the decision of where to locate various departments (receiving, picking, storage, sorting, and shipping, etc.). It is often carried out by taking into account the activity relationship between the departments. The common objective is
Storage assignment
Products need to be put into storage locations before they can be picked to fulfil customer orders. A storage assignment method is a set of rules which can be used to assign products to storage locations. Before such an assignment can be made, however, a decision must be made which pick activities will take place in which storage system.
Zoning
As an alternative to single order picking, the order picking area can be divided into zones. Each order picker is assigned to pick the part of the order that is in his assigned zone. Compared to other planning issues, the zoning problem has received little attention despite its important impact on the performance of order-picking systems. Possible advantages of zoning include the fact that each order picker only needs to traverse a smaller area, reduced traffic congestion, and furthermore the
Batching
When orders are fairly large, each order can be picked individually (i.e. one order per picking tour). This way of picking is often referred as the single order picking policy (or discrete picking or pick-by-order). However, when orders are small, there is a potential for reducing travel times by picking a set of orders in a single picking tour. Order batching is the method of grouping a set of orders into a number of sub-sets, each of which can then be retrieved by a single picking tour.
Routing methods
The objective of routing policies is to sequence the items on the pick list to ensure a good route through the warehouse. The problem of routing order pickers in a warehouse is actually a special case of the Travelling Salesman Problem, see also Lawler et al. (1995). The travelling salesman problem owes its name to the problem described by the following situation. A salesman, starting in his home city, has to visit a number of cities exactly once and return home. He knows the distance between
Order accumulation and sorting
When batching and/or zoning is applied, usually some additional effort is needed to split the batch and to consolidate the items per customer order or per destinations to which orders will be shipped. These processes are often called accumulation/sorting (A/S).
Fig. 9 shows an example of a typical A/S system (mentioned in Meller, 1997, Johnson, 1998). Items of a group of orders (a pick-wave) that are to be loaded onto a certain number of trucks are picked from the picking area. In general, items
Conclusions
We can draw the following conclusions from the literature. First, in spite of their dominance in practice, pickers-to-parts order-picking systems have received less research attention compared to parts-to-picker order-picking systems. Less than 30 percent of the about 140 papers we considered concerns pickers-to-part order-picking systems. The reasons for this may have something to do with the complexity and diversity of picker-to-parts order-picking systems. Furthermore, parts-to-picker
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