Optimal dwell point policies for new automated Storage/Retrieval systems

In conventional automated storage and retrieval systems (AS/RS), stacker cranes are used to access the storage cells. The stacker cranes can travel simultaneously in vertical and horizontal directions. However, since the combined motions generally require heavy machineries, stacker cranes are inadequate for extra heavy loads. For such applications, a new kind of storage/retrieval (S/R) mechanism has been designed. 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 given tier. The vertical platform and the horizontal platforms may move independently and concurrently, and the separation of the mechanism for vertical/horizontal movements also makes the platforms lighter and they can hence operate at a higher speed than that of a conventional design. In this paper, authors present a travel-time model aimed to the investigation of the effect of different dwell point policies. The model is validated by computer simulations. The results show that this analytical model can help in effective way the design and the analysis of this new kind of AS/RS. Guidelines for the optimal design of an AS/RS rack with the new S/R mechanism is also presented