Research Of Automated Vehicle Storage And Retrieval System Based On Semi-Open Queueing Networks

The Automated Vehicle Storage and Retrieval System(AVS/RS)is an automated warehouse operation system of the operation mode based on lifts and vehicles.Accurate evaluation of this system can help enterprises'decision makers adjust and control the system's efficiency and costs.Prior research for this system mainly centered on total costs models and physical motion models.Recently,the performance evaluation models(picking efficiency and travel time)for this system has drawn huge attention from researchers.However,the majority of them take advantage of the existing simple queue network models,such as Open Queueing Network(OQN)and Closed Queueing Network(CQN),to evaluate this system.Though the above simple queue network models can be a fast solution method to measure this system's performance they can generate large errors in queue time and queue length,which are not eligible for the analysis and design of AVS/RS.This paper builds an evaluation model for AVS/RS based on Semi-Open Queueing Network(SOQN)by combining CQN and OQN,and a solution procedure is proposed in the light of Mean Value Analysis(MVA)and Matrix Geometric Method(MGM).This model,firstly,integrates the synchronization station into the CQN composed of lifts and vehicles,makes the service rate of the synchronization station be equal to the arrival rate of the external commands and then replaces the CQN with a load-dependent single server station.The resulted queue network with a load-dependent station and picking station is solved as a Quasi Birth Death process.The accuracy of the proposed solution is verified by simulation experiments using Simulink,comparing the errors of this solution method and the existing methods and analyzing the system's performance under various parameter values.Experiments results showed that this solution procedure can accurately estimate the SOQN model established in this paper.The error range is around 10%of all parameters,which is far less than the 20%of the existing methods.Our method can be an effective estimation and analysis tool in the design phase for system.From numerical examples,we find out that the picking station exerts less influence on the lift and vehicle stations,the lift could be the bottleneck of this system when its external commands rise.Therefore,the enhancement of the service rate of the lift station can effectively raise the throughput of this system.The vehicle station,however,possesses a lot of flexibility,and the throughput of the AVS/RS can be regulated through changing the number of vehicles.