We study a next generation of storage systems: live-cube compact storage systems. These systems are becoming increasingly popular, due to their small physical and environmental footprint paired with a large storage space. At each level of a live-cube system, multiple shuttles take care of themovement of unit loads in the x and y directions. When multiple empty locations are available, the shuttles can cooperate to create a virtual aisle for the retrieval of a desired unit load. A lift takes care of the movement across different levels in the z-direction. Two-class-based storage, in which high turnover unit loads are stored at storage locations closer to the Input/Output point, can result in a short response time. We study two-class-based storage for a live-cube system and derive closed-form formulas for the expected retrieval time. Although the system needs to be decomposed into several cases and sub-cases, we eventually obtain simple-to-use closed-form formulas to evaluate the performance of systems with any configuration and first zone boundary. Continuous-space closed-form formulas are shown to be very close to the results obtained for discretespace live-cube systems. The numerical results show that two-class-based storage can reduce the average response time of a live-cube system by up to 55% compared with random storage for the instances tested.

Additional Metadata
Keywords Live-cube compact storage system, Logistics, Material handling, Response time analysis, Two-class-based storage policy
Persistent URL dx.doi.org/10.1080/24725854.2016.1273563, hdl.handle.net/1765/101783
Journal IISE Transactions
Zaerpour, N, Yu, Y, & de Koster, M.B.M. (2017). Response time analysis of a live-cube compact storage system with two storage classes. IISE Transactions, 49(5), 461–480. doi:10.1080/24725854.2016.1273563