Performance analysis of carousel systems

Abstract

Carousel storage systems are automated storage/retrieval systems for small to medium sized unit loads. They have been widely used in warehousing applications to support order picking, assembly and kitting operations. A typical carousel storage system has a driving mechanism, which rotates a set of multi-shelf storage carriers, allowing a S/R machine or picker to select the desired item while remaining in one position. Recent developments in this system include the automation of S/R operations where both the rack and the automated S/R machine are operated under computer control. In the first part of this thesis, we consider the problem of optimal design of automated double carousel system which is a variation of the standard carousel system. The standard carousel system is composed of single carousel body and a S/R machine. On the other hand, the carousel body of double carousel system consists of two sub-carousels, a lower and an upper one. Each sub-carousel has its own driving mechanism to bring the desired rack location to the S/R machine column. It is expected that the throughput rate of a double carousel system can be substantially increased by adopting the two class storage assignment policy. In this policy, the items and storage region are partitioned into two classes, i.e., the items with high turnover rates are stored at the lower carousel and those with low turnover rates at the upper carousel. Within any given class, items are assigned to storage locations randomly. We derive the expected cycle time models to estimate the throughput rate of the two class-based double carousel system. Also, a proper choice of class boundary is shown to further enhance the system efficiency. The second part attempts to measure analytically the effects of double shuttle in the storage/retrieval machine on the throughput of standard and double carousel systems. For the double carousel system, a retrieval sequencing heuristic is proposed which utilizes the characteristic...