In conventional Automated Storage and Retrieval Systems (AS/RS), stacker cranes are used to deliver loads to the AS/RS racks and to retrieve loads from the racks. Stacker cranes travel simultaneously in the vertical and horizontal directions. However, the stacker cranes are inadequate for extra heavy loads. In this paper a new kind of Storage/Retrieval (S/R) mechanism is presented to handle such loads. Unlike stacker cranes, the new S/R mechanism has one vertical platform and N horizontal platforms to serve N tiers of an AS/RS rack. The vertical platform provides the vertical link among the tiers of the AS/RS rack, whereas the horizontal platforms transfer loads to the individual storage cells on a given tier. The operations of the vertical platform and the horizontal platforms may be independent and concurrent. In this paper, the design, operation and advantages of the new mechanism are presented. We present a travel-time model under the dwell point policy that the platforms remain where they are after completing a storage/retrieval operation. The model is validated by computer simulations. The results show that our model is reliable for the design and analysis of the new kind of AS/RS. We also present guidelines for the optimal design of a rectangular-in-time AS/RS rack with the new S/R mechanism.
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