Relative positioning of a load extractor for a storage carousel

The use of carousels in manufacturing for the storage of work-in-process items is very common. A major reason for the popular use of carousels is to store work-in-process items closer to the workstations and therefore minimize handling and the time taken to satisfy an item retrieval request from storage. Although carousel load retrieval times are considerably less than those of normal warehouses, the average response time can be further improved through proper prepositioning of the load extractor machine in anticipation of storage/retrieval requests. Two models are presented in this paper to strategically preposition the extractor machine when idle to minimize the system response time. In the first model the emphasis is on preposition of the extractor machine to minimize the maximum system response time when the extractor machine becomes idle. In contrast, the emphasis on the second model is to preposition the machine to minimize the expected system response time when the machine becomes idle. Two modes of carousel operation, (a) unidirectional rotation capability and (b) bidirectional rotation capability, are considered. Solution procedures based on mathematical models are developed to obtain optimal solutions. The developed procedures can be embedded in a carousel controller and used for on-line control and prepositioning of the extractor machine to improve the carousel system's operational efficiency. Computer simulation is performed under various traffic rates and storage methods to analyze the system performance under these two prepositioning strategies and one other traditional carousel prepositioning strategy commonly in use today. An on-line control architecture to implement the proposed prepositioning strategies is also presented.