Automated Storage/Retrieval Systems in Production and Operation Management

An automated storage/retrieval system (AS/RS) is defined by the Materials Handling Institute as, “A combination of equipment and controls which handles, stores and retrieves materials with precision, accuracy and speed under a defined degree of automation”.

AS/R systems are custom-planned for each individual application, and they range in complexity from relatively small mechanized systems that are controlled manually to very large computer-controlled systems that are fully integrated with factory and warehouse operations.

The AS/RS consists of a series of storage aisles that are serviced by one or more storage/ retrieval (S/R) machines, usually one S/R machine per aisle. The aisles have storage racks for holding the materials to be stored. The S/R machines are used to deliver materials to the storage racks and to retrieve materials from the racks. The AS/RS has one or more input stations where materials are delivered for entry into storage and where materials are picked up from the system. The input/output stations are often referred to as pickup and deposit (P&D) stations in the terminology of AS/RS systems. The P&D stations can be manually operated or interfaced to some form of automated handling system, such as a conveyor system or AGVS.

1. Types of AS/RS

Several important categories of automated storage/retrieval systems can be distinguished. These include:

1. Unit load AS/RS: This is typically a large automated system designed to handle unit loads stored on pallets or other standard containers. The system is computer-controlled and the S/R machines are automated and designed to handle the unit load containers. The unit load system is the generic AS/RS.
2. Miniload AS/RS: This storage system is used to handle small loads (individual parts or supplies) that are contained in bins or drawers within the storage system. The S/R machine is designed to retrieve the bin and deliver it to a P&D station at the end of the aisle so that the individual items can be withdrawn from the bins. The bin or drawer is then returned to its location in the system. The miniload AS/RS system is generally smaller than the unit load AS/RS and is often enclosed for security of the items stored.
3. Man-on-board AS/RS: The man-on-board AS/RS system represents an alternative approach to the problem of storing and retrieving individual items in the system. Whereas the miniload system delivers the entire bin to the end-of aisle pick station, the man-on-board system permits the individual items to be picked directly at their storage locations. This offers an opportunity to reduce the transaction time of the system.
4. Automated item retrieval system: These systems are also designed for retrieval of individual items or small unit loads such as cases of product in a distribution warehouse. In this system, the items are stored in single-file lanes rather than in bins or drawer. When an item is to be retrieved, it is released from its lane onto a conveyor for delivery to the pickup station. The supply of items in each lane is generally replenished from the rear of the retrieval system, so that there is flow-through of the items, thus permitting first in first out (FIFO) inventory control.
5. Deep-lane AS/RS: The deep-lane AS/RS is a high density unit load storage system that is appropriate when large quantities are to be stored but the number of separate types of material is relatively small. Instead of storing each unit load so that it can be accessed directly from the aisle, the deep-lane system stores up to 10 or so loads in a single rack, one load behind the next. Each rack is designed for ‘flow-through’ with input on one side and output on the other side. Loads are picked from one side of the rack system by a special S/R type machine designed for retrieval and another special machine is used on the entry side of the rack system for input of loads.

2. Basic Components of an AS/RS

All automated storage/retrieval systems consist of certain basic building blocks. These components are:

• Storage structure
• Storage/retrieval (S/R) machine
• Storage modules (e.g, pallets for unit loads)
• Pickup and deposit stations.

1. The storage structure is the fabricated steel framework that supports the loads contained in the AS/RS. The structure must possess sufficient strength and rigidity that it does not deflect significantly due to the loads in storage or other forces on the framework. The individual storage components in the structure must be designed so to accept and hold the storage modules used to contain the stored materials.
2. The S/R machine (sometimes called a crane) is used to accomplish a storage transaction, delivering loads from the input station into storage, or retrieving loads from storage and delivering them to the output station. To perform these transactions, the storage/retrieval machine must be capable of horizontal and vertical travel to align its carriage with the storage compartment in the storage structure, and it must also pull the load from or push the load into the storage compartment.
3. The storage modules are the containers of the stored material. Examples of storage modules include pallets, steel wire baskets and containers, tote pans, storage bins, and special drawers (used in miniload AS/RS systems). These modules are generally made to a standard base size that can be handled automatically by the carriage shuttle of the S/R machine.
4. The pickup and deposit stations are used to transfer loads to and from the AS/RS. They are generally located at the end of the aisles for access by the S/R machine and the external handling system that brings loads to the AS/RS and takes loads away. The pickup stations and deposit stations may be located at opposite ends of the storage aisle or combined at the same location. This depends on the origination point of the incoming loads and the destination of the output loads. The P&D stations must be designed so that they are compatible with the S/R machine shuttle and the external handling system.

Source: KumarAnil, Suresh N. (2009), Production and operations management, New Age International Pvt Ltd; 2nd Ed. edition.