New Automated Storage And Retrieval Modes And Optimizations For Logistics

The floor space utilization in warehouse storage systems is closely related to the storage density of automated storage and retrieval systems (AS/RS). Flow-rack is a multi-deep rack with the first-in-first-out AS/RS rule. Driven by gravity, stored unit-loads slide from the storage face to the retrieval face. The blocking unit-loads have to be removed by restoring them to the storage face using restoring conveyors before retrieving the requested outgoing unit-loads. The performance of flow-rack systems heavily depends on the sequence of storage and retrieval (S/R) operations, the deployment of S/R machines, and the restoring mechanism of blocking unit-loads. It is necessary to design new flow-rack S/R modes and optimize S/R operations to improve the storage density, decrease the investment costs, and reduce the operational costs. The main contributions of this dissertation are as follows.(1) Sequencing S/R operations in a flow-rack S/R mode with the duration-of-stay (DOS) storage policy. A group-matching heuristic is proposed for the DOS flow-rack S/R mode with the given arrivals and departures of unit-loads within a planning horizon. As many as possible unit-loads are assigned to the bins close to the pickup/drop-off stations to minimize the number of blocking unit-loads and the total travel time of operations in the whole planning horizon.(2) Dual-retrieval operations in the flow-rack S/R mode. Dual-shuttle machines, which con-duct S/R operations on two unit-loads independently, are deployed to the retrieval face of the flow-rack. The dual-shuttle machine can remove two blocking unit-loads or to retrieve two outgoing unit-loads simultaneously. The dual-retrieval operation generation problem is formulated as an assignment problem. Based on the Hungarian method, a heuristic is presented for the considered problem.(3) Designing and optimizing parallel flow-rack S/R mode. The number of S/R machines in the flow-rack system is the key to the S/R performance. A parallel flow-rack S/R mode is designed in this dissertation, in which multiple flow-racks are parallel deployed. Unit-loads can be stored to or retrieved from different flow-racks by the S/R machines between flow-racks. The S/R machines work in the dual-command (DC) mode. The travel time model for the parallel flow-rack S/R mode is established. A three-phase heuristic is introduced to generate the DC operations.(4) Designing and optimizing bi-directional flow-rack S/R mode. Blocking unit-loads in flow-rack systems cost too much. A bi-directional flow-rack S/R mode is designed, in which the bins in adjacent columns of the flow-rack slope in opposite directions. The S/R machines work in the DC mode, which store/retrieve unit-loads to/from the same face. Blocking unit-loads are restored to the available bins on the same face, which considerably reduces the restoring time. The travel time model is constructed for the bi-directional flow-rack S/R mode. A batching-greedy heuristic is proposed to generate DC operations.