Journal of Control Theory and Applications 3 (2004) 229 - 238 Pick sequencing optimization problem in the rotary rack S/R system Pan ZHANG, Lei JIA, Guohui TIAN (School of Control Scienceand En~neering,ShandongUniversity,Jinan Shandong 250061, China) Abstract: The problem of"pick sequencing in the rotary rack S/R system (PPS-ICIKS) is investigated with the objective of minhnizing the execution time. The rotary rack S/R system consists of one S/R na~ctfine and nmltiple levels of carousels that can rotate independently in bi-directions. The routing polio T, namely the decision on the storage or retrieval sequence, dominates the efficiency and the throughput for such S/R systems,due to the complicated relationship between all levels of carousels and the S/R machine. For the purpose of optimizing the PPS-P,IKS,a computational model is developed in tenns of execution time for picking multiple items in one trip. Characteristics of" tbe PPS-1KRS are analyzed and a local search heuristic based on a newly proposed neighborhood is presented. Integrated with the proposed local search procedure a new hybrid genetic algorithm is developed. Experimental results deruomtrate the structure characteristics of good sequence and the efficiency and effectiveness of the proposed sequencing algorithm. Keywords: Rotary rack S/R system; Hybrid genetic algorithm; Sequencing problem; Level-blank neighborhood; Local search 1 Introduction Carousel storage and retrieval (S/R) systems are conmlonly referred to as a Stock-to-Operator systena, where the operator stays in one or a series of pick stations and the carousels bring the candidate bins or items to the operator. As an effective warehousing facility, it can be used to store and pick small, light and highly demanded items, mad also provides relatively high throughput rate, efficient space utilization, and high reliability [ l ]. Standard carousel system consists of a carousel body and a picker. The body comprises a series of bins which are linked together and mounted on an oval track. In every bin, there are multiple shelves, where various stocked items are stored. The carousel body is operated by the computer which rotates the linked bins and brings the requested item to the picking location. The Stock-to-Operator operation removes the walking for a picker, and hence gain the throughput perfomlance as two or three times as the performance achieved for conventional shelving systems [2,3]. To expand the merits and further reduce the waiting time for a picker, multiple carousels are often integrated into one S/I< system so that while a picker picks from one Carousel, others can convey items simultaneously. The rotary rack S/1K system is a special variation of such a system, which is typically composed of a rotary- rack body and one storage/retrieval (S/R) machine. Within the rotary stack body, there are multiple levels of horizontal carousels, and each level owns a drive unit and consequently rotates independently. The S/1K machine can shuttle anlong a series of pick stations beside the corresponding carousels to pick the reached items on the associated level. This feature makes it possible for all the levels of carousel to cooperate to transfer the items needed and hence reduce the S/IK machine waiting time. Though this integrated S/R system brings us the benefits of potential high throughput rate, the thorough realization of high efficiency remains challenge. The factors are multi-fold, including the scheduling logics for levels of carousels, routing policies, and storage policies et al ~4]. Due to the complicated logic relationship among all the levels of carousels and the S/R machine, the routing optimization policy is the dominative factor. This policy determines the route of the S/R machine and carousels in a picking/storing tour, specifically the sequence in which candidate items are to be picked /stored. The routing process is identical for both storage and retrieval, thus without loss of generality we here only considered the pick sequencing process and define the routing op6mization process as the combinatorial opfmization problem of pick Received 26 April 2004; Revised 10 August2004. This work was supported by the National Natural Science Foundation of China (60104009);the Natural Science Foundation of Shandong Province,China (Z2000G01).