Research On Classification And Storage Strategy Of Gravity-Based Multi-Deep AS/RS

The gravity-based multi-depth automated storage and retrieval system(AS/RS)has the advantages of high storage density and small footprint.Such a type of AS/RS is proposed to face the increasing logistics pressure and increasing land cost.Normally,a gravity-based multi-depth AS/RS generally applies flow-rack,directional flow-rack,single-machine flow-rack or 3D-compact rack as the main facility for storing unit-loads.Storage/retrieval(S/R)machines are deployed to the working face of gravity-based multi-deep rack to store and retrieve unit-loads.Each bin of the gravity-based multi-deep rack can store multiple unit-loads simultaneously in the depth direction.The unit-loads slide from the storage end to the retrieval end by the gravity in each bin to let stored unit-loads follow the first-in-first-out(FIFO)rule in each bin.As a result,requested unit-load has probability to be stored by other unit-loads stored in front of it in the bin.Obviously,blocking unit-loads must be removed before retrieving requested unit-loads,which requires extra travel time,increases the total working time of S/R machine and reduces the responding speed of S/R machines.In this case,the storage assignment method for gravity-based multi-deep AS/RS not only considers to allocate as many unit-loads as possible to bins near the pick-up/drop-off(P/D)station but also stores FIFO unit-loads to the same bin based on the turnover information to avoid blocking unit-loads.In class-based storage policy,unit-loads can be separated into several classes based on their turnover rates.Correspondingly,bins are divided into several zones according to their distance to the P/D station.To store unit-loads in class with high turnover rate to bins in the zone closed to the P/D station is an effective way to reduce the total travel time of storage and retrieval operations.As well,the selection of storage locations,the position of P/D station,the removing and restoring mechanism of blocking unit-loads and the zoning policy of bins are the main factors to the performance of storage and retrieval in gravity-based multi-deep AS/RSs.In this paper,class-based storage policy in flow-rack and single-machine flow-rack AS/RS are analyzed.Firstly,some improvements based on the in-deep storage are proposed.Then,zoning strategy is analyzed for the single-machine flow-rack AS/RS.Finally,zoning strategy and in-deep storage are integrated for designing the storage assignment method in a single-machine flow-rack with class-based storage policy.This research has important theoretical and practical significance.The main contributions are as follows:(1)The retrieval rules of flow-rack AS/RS with in-deep storage is modified.For reducing blocking unit-loads,Modi-InDeep is designed,in which the unit-load with higher turnover rate stored in front of unit-load with lower turnover rate in the same bin must be preferentially retrieved to avoid such a unit-load becomes blocking unit-load in future.(2)In-Deep storage is modified to single-machine flow-rack AS/RS.The modified In-Deep method divides unit-loads into two different types:fast-turnover and slow-turnover.The modified In-Deep method tries to store fast-turnover and slow-turnover unit-loads in to their special bins.As well,fast-turnover unit-loads must be stored in front of slow-turnover unit-loads in the same bin.Blocking unit-loads are removed and restored to the storage end of the same bin to minimize the cost of restoring blocking unit-load.Certainly,fast-turnover unit-loads must be stored to bins near the P/D station and slow-turnover unit-loads are stored to bins far to the P/D station to reduce the average travel time of storage and retrieval operations.(3)Zoning strategy is employed in the single-machine flow-rack AS/RS.Based on two-class division method,the bins in a single-machine flow-rack AS/RS are divided into two zones.Fast-turnover unit-loads are stored to bins in the zone near the P/D station and slow-turnover unit-loads are allocated to bins in the zone far to the P/D station.The impacts of division frontier to the performance of storage and retrieval are analyze by simulation experiments.