Travel time analysis of a new automated storage and retrieval system

Abstract

In conventional automated storage and retrieval systems (AS/RS), stacker cranes are used to access (store or retrieve loads into/from) the storage cells. The stacker cranes can travel simultaneously in the vertical and horizontal directions. However, because the combined motions generally require heavy machineries, the stacker cranes are inadequate for extra heavy loads such as sea container cargo. For such applications, we present a new kind of storage/retrieval (S/R) mechanism, designed with input from AS/RS manufacturers. 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 different tiers of the AS/RS rack, whereas the horizontal platforms access the storage cells on a given tier. The vertical platform and the horizontal platforms may move independently and concurrently; and the separation of the mechanisms for vertical/horizontal movements also makes the platforms lighter and hence they can operate at a higher speed than the conventional design. We then present a travel-time model under the stay dwell point policy, i.e. the platforms remain where they are after completing a storage/retrieval operation. The model is validated by computer simulations. The results show that our analytical 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.