Automated Storage/Retrieval Systems

Building the Model: Advanced Elements > Material Handling Systems > Automated Storage/Retrieval Systems

Automated Storage/Retrieval Systems

An automated storage/retrieval system (AS/RS) is “a combination of equipment and controls which handles, stores, and retrieves materials with precision, accuracy, and speed under a defined degree of automation” (The Material Handling Institute 1977). The goal of an AS/RS is to provide random, high density storage with quick load access, all under computer control.

Modeling AS/RSs

At a simple level, an AS/RS move time may be modeled by taking a time from a probability distribution that approximates the time to store or retrieve a load. More precise modeling incorporates the actual crane (horizontal) and lift (vertical) speeds. Each movement usually has a different speed and distance to travel which means that movement along one axis is complete before movement along the other axis begins. From a modeling standpoint, it is usually only necessary to calculate and model the longest move time.

In modeling AS/RS, the storage capacity is usually not a consideration and the actual inventory of the system is not modeled. It would require lots of overhead to model the complete inventory in a rack with 60,000 pallet locations. Since it is primarily only the activity that is of interest in the simulation, actual inventory is ignored. In fact, it is usually not even necessary to model specific stock keeping units (SKUs) being stored or retrieved, but only distinguish between load type insofar as it affects routing and subsequent operations.

Modeling an AS/RS as a Vertical Bridge Crane

One way to model an AS/RS with greater accuracy is to use the bridge crane construct. An AS/RS is really just a vertical crane with the same characteristics as a bridge crane.

AS/RS Simulation Benefits

Simulating your AS/RS will help you address design and operational issues, answering the following questions:

How many aisles do you need to handle the required activity?
What is the best sequence of stores and retrievals to maximize throughput?
What is the best stationing of empty S/R machines to minimize response time?