Sequencing dynamic storage systems with multiple lifts and shuttles

New types of Automated Storage and Retrieval Systems (AS/RS) able to achieve high throughput are continuously being developed and require new control polices to take full advantage of the developed system. In this paper, a dynamic storage system has been studied as developed by Vanderlande Industries, consisting of a conveyor, two non-passing lifts that share a mast, multiple transfer shuttles, and a storage rack. This study is concerned with the scheduling problem of these two lifts, i.e. which lift is going to handle which (storage or retrieval) request, and in which order. An integrated look-ahead strategy heuristic to simultaneously assign a set of pre-defined requests to the lifts and the order in which they will be handled taking into account delays caused by interference between the lifts. A practical methodology to characterize the system to identify and resolve situations where the lifts would interfere with each other is presented. Experimental results compare several priority rules to handle interference between lifts, and provide empirical evidence that the dynamic system with multiple lifts controlled by the proposed look-ahead strategy achieved average improvements of 44.34% and 35.82% in terms of total handling times (82.49% and 60.08% in terms of throughput) compared to a single lift system and to a simple rule of thumb, respectively.