Invited ReviewA survey of literature on automated storage and retrieval systems
Introduction
Automated storage and retrieval systems have been widely used in distribution and production environments since their introduction in the 1950s. An automated storage and retrieval system (AS/RS) usually consists of racks served by cranes running through aisles between the racks. An AS/RS is capable of handling pallets without the interference of an operator, thus the system is fully automated. Both in production and distribution environments AS/RSs are used for putting products (e.g., raw materials or (semi-) finished products) in storage and for retrieving those products from storage to fulfill an order. Between 1994 and 2004, there has been a significant increase in the number of AS/RSs used in distribution environments in the United States (Automated Storage Retrieval Systems Production Section of the Material Handling Industry of America, 2005). The usage of AS/RSs has several advantages over non-automated systems. Examples are savings in labour costs and floor space, increased reliability and reduced error rates. Apparent disadvantages are high investments costs (approximately $634,000 for a single aisle AS/RS, Zollinger, 1999), less flexibility and higher investments in control systems (about $103,000, Zollinger, 1999).
In designing an AS/RS, many physical design and control issues have to be addressed in the right way to fully take advantage of all its pros. This paper intends to present a critical overview of all important issues concerning AS/RS design and control in both production and distribution environments while studying recent and past literature. Previously, several overview papers have been published that discuss part of the AS/RS literature. Almost all of these overview papers, however, have a focus different from AS/RSs, for example, general warehouse design. Because of this, only a limited number of aspects of AS/RSs and a limited number of references with respect to AS/RSs are presented in those papers. Matson and White (1982) review a number of material handling research areas one of which is concerned with AS/RSs. Kusiak (1985) describes design and operational decision problems for flexible manufacturing systems with a focus on automated guided vehicles and AS/RSs. The author discusses design, storage and batching (i.e., consolidation of orders) policies for AS/RSs. Johnson and Brandeau (1996) discuss stochastic models for the design and control of automated guided vehicles and AS/RSs. Manda and Palekar (1997) discuss some papers on travel time estimation for AS/RSs and storage assignment rules.
General overviews of warehouse design and control include Cormier and Gunn, 1992, Van den Berg, 1999, Rouwenhorst et al., 2000, De Koster et al., 2007, Gu et al., 2007. Due to their broad scope, these five papers only discuss a fraction of the AS/RS issues and literature. To our knowledge, Sarker and Babu (1995) is the only paper discussing exclusively AS/RSs, however, this paper only reviews some design aspects of AS/RSs while focussing on travel time models. We conclude that our paper seems to be the first overview paper in over 10 years devoted exclusively to AS/RSs, and the first ever to give a broad overview of all design and control issues in AS/RSs.
The main structure of the paper can be described as follows. First, we present a general description of AS/RSs and a classification of related design and control issues. The main body of the paper consists of an overview of literature discussing solution methods for these design and control problems. At the end, we indicate relevant open research questions. In more detail, Section 2 defines various types of AS/RSs and describes some important technical characteristics. Section 3 presents a classification of both physical design and control issues. This broad introduction will be followed in Section 4 by a more detailed description of methods that support each of the discussed physical design issues. Papers addressing individual control policies for storage assignment (Section 5), batching (Section 6), parking of idle AS/RSs (Section 7) and sequencing (Section 8) will be discussed in subsequent sections. Travel time estimates and other performance measures will be treated in Section 9. Section 10 presents conclusions and issues for further research.
Section snippets
AS/RS types
An AS/RS system is defined as a storage system that uses fixed-path storage and retrieval machines running on one or more rails between fixed arrays of storage racks (Automated Storage Retrieval Systems Production Section of the Material Handling Industry of America (2005)). AS/RSs are used to store and retrieve loads in various settings. The main components of an AS/RS are racks, cranes, aisles, I/O-points, and pick positions. Racks are typically metal structures with locations that can
Overview of design decisions
It is crucial to design an AS/RS in such a way that it can efficiently handle current and future demand requirements while avoiding bottlenecks and overcapacity. Due to the inflexibility of the physical layout and the equipment, it is essential to design it right at once. In Fig. 2 a schematic view of design issues and their interdependence is presented.
It is important to realise that the AS/RS is usually just one of many systems to be found in a warehouse. The true performance of the AS/RS is
Physical design
Only a few papers address the design of AS/RSs in combination with the design of other material handling systems in the facility. Most of these papers consider manufacturing environments. Chincholkar and Krishnaiah Chetty, 1996 use approaches as Petri Nets and the Taguchi method to simultaneously address the scheduling of jobs to an AS/RS system and the scheduling of jobs to machines in a flexible manufacturing system. The AS/RS is both responsible for storing and retrieving loads and for
Storage assignment
Several methods exist for assigning products to storage locations in the racks. Five often used storage assignment policies for AS/RSs are described here in more detail (see e.g., Hausman et al., 1976 or Graves et al., 1977). These rules are:
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dedicated storage assignment
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random storage assignment
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closest open location storage assignment
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full-turnover-based storage assignment
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class-based storage assignment
For the dedicated storage method each product type is assigned to a fixed location.
Batching
Suppose we have a number of orders that need to be retrieved from storage in a person-on-board item-picking AS/RS. We could retrieve the orders one at a time or we could try to combine several orders in a single tour of the crane. The latter approach is called batching. Batching problems for person-on-board AS/RSs are quite similar to batching problems for order pickers in warehouses. For a detailed review of this type of research, refer to De Koster et al. (2007). In this section, we will only
Dwell-point location
Several methods have been proposed to deal with the decision where to position an idle crane, i.e., how to determine the crane’s dwell-point. Bozer and White (1984) introduced four simple static dwell-point strategies. Table 5 summarises these rules and indicates the resulting one-dimensional parking location. Park (1991) showed that the input station rule returns an optimal dwell-point if the probability, that the first request after an idle period is a storage request, is at least 1/2.
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Sequencing of storage and retrieval requests
Storage requests in distribution or production environments are usually not time-critical. The exact point in time at which loads are stored is not of much importance for the performance of the system. Therefore, storage requests are usually stored according to the first-come-first-served principle. In sequencing retrievals usually due times of retrievals should be met, which makes it necessary to look beyond simple first-come-first-served. Furthermore, by sequencing the retrievals in a smart
Performance measurement
In evaluating the design and control rules of an AS/RS several performance measures can be used. Based on the literature overview presented in this paper, we can at least consider the following performance measures:
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travel time per request,
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number of requests handled per time period (e.g., Azadivar, 1986, Foley et al., 2002),
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total time required to handle a certain number of requests,
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waiting times of cranes of the AS/RS,
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waiting times of products to be stored/retrieved,
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number of requests waiting to
Conclusions and further research issues
In both manufacturing and distribution environments, AS/RSs are used to store products and to retrieve products from storage in response to production orders or customers’ orders. In designing an AS/RS, various physical design problems and control problems need to be solved (see Section 3). Literature related to the various interrelated physical design and control problems, such as storage assignment, batching, dwell-point location and request scheduling was treated in Sections 4 Physical design
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