| Double-deep multi-aisle automated storage/retrieval systems are increasingly applied for storing and retrieving unit loads, with advantages of increased space utilization, reduced number of aisles, and improved efficiency of S/R machines. Besides, using a double-shuttle S/R to implement four command cycles also improved efficiency of the systems. In such systems, the retrieval process may consist of the rearrangement of blocking loads based on the assumptions of uniformly distributed storage locations and random storage policy. Different rearrangement rules make different effect on the expected travel time of the S/R machine. When implementing the storage/retrieval orders, the S/R machine can also choose different dwell point policies.This paper formulates analytical travel-time models of four command cycles under three rearrangement rules. This paper validates the analytical travel-time models by simulation and conduct numerical experiments to analyze the effect of the number of aisles an S/R machine serves, the fill-grade factor and the rearrangement rules on the expected travel time of the S/R machine. Besides, this paper formulates analytical travel-time models of dual command cycles under two dwell point policies. And this paper validate the analytical travel-time models by simulation and conduct numerical experiments to analyze the effect of the number of aisles an S/R machine serves, the fill-grade factor, the number of aisles an S/R machine serves and dwell point policies on the expected travel time of the S/R machine. The results show that the expected travel time of the S/R machine is increasing with the increase of the number of aisles an S/R machine serves and the increase of the fill-grade factor, the RTN rule(The S/R machine will rearrange the blocking TUL to the nearest empty storage location) is the best choice rearrangement rules, the POSC is the best dwell point policies and using dual-shuttle S/R machine is better than using single-shuttle S/R machine. |
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