In today's competitive scenario of increasingly faster deliveries and smaller order sizes, material-handling providers are progressively developing new solutions. A recent, automated material-handling technology for unit load storage and retrieval consists of an autonomous vehicle storage and retrieval system (AVS/RS). The present paper presents an analytical model to estimate the performances (the transaction cycle time and waiting times) of AVS/RS for product tote movement. The model is based on an open queuing network approach. The model effectiveness in performance estimation is validated through simulation.

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Analytical model to estimate performances of autonomous vehicle storage and retrieval systems for product totes

This paper presents analytical travel time model for the computation of travel (cycle) time for shuttle-based storage and retrieval systems (in continuation SBS/RS). The proposed model considers the operating characteristics of the elevators lifting table and the shuttle carrier, such as acceleration and deceleration and the maximum velocity. Assuming uniform distributed storage rack locations and using the probability theory, the expressions of the cumulative distribution functions with which the mean travel time is calculated, have been determined. The proposed model enables the calculation of the mean travel (cycle) time for the single and dual command cycles, from which the performance of SBS/RS can be evaluated. The approximation model and a simulation model of SBS/RS have been used to compare the performances of the proposed analytical travel time model. The analysis shows that regarding all examined types of SBS/RS, the results of proposed analytical travel time model for SBS/RS correlate with the results of simulation models of SBS/RS.

Travel time model for shuttle-based storage and retrieval systems

In today’s competitive environment with increasingly faster deliveries and smaller order sizes, material handling providers are progressively developing new solutions. A more recent development in automated material-handling technology for unit load storage and retrieval is the autonomous vehicle storage and retrieval system (AVS/RS). The paper investigates the main design trade-offs for this new solution using simulation, and proposes a comprehensive design framework. Using data from a recently implemented AVS/RS, the application of the proposed framework is presented and the key design differences between the two types of AVS/RS configuration (i.e. tier-captive versus tier-to-tier) are identified.

/pdf/development-of-a-framework-for-the-design-of-autonomous-4b4meg481k.pdf

Development of a framework for the design of autonomous vehicle storage and retrieval systems

Distribution centers have recently adopted Autonomous Vehicle-based Storage and Retrieval Systems (AVS/RSs) as a potential alternative to traditional automated storage and retrieval systems for processing unit-load operations. The autonomy of the vehicles in an AVS/RS provides a level of hardware sophistication, which can lead to the improvements in operation efficiency and flexibility that will be necessary in distribution centers of the future. However, in order to exploit the potential benefits of the technology, an AVS/RS must be designed using a detailed understanding of the underlying dynamics and performance trade-offs. Design decisions such as the configuration of aisles and columns, allocation of resources to zones, and vehicle assignment rules can have a significant impact on the performance of AVS/RSs. In this research, the performance impact of these design decisions is investigated using an analytical model. The system is modeled as a multi-class semi-open queuing network with class switching...

Performance analysis and design trade-offs in warehouses with autonomous vehicle technology

Simulation based experimental design to identify factors affecting performance of AVS/RS

Approximation strategies for transaction waiting times are proposed for use in Autonomous Vehicle Storage and Retrieval Systems (AVS/RSs) with non-Poisson arrival rates and non-exponential service times. The proposed modelling tools dynamically adjust approximations based on the variance of the transaction inter-arrival times observed in a system. Simulation-based validation studies indicate that this strategy can reduce the magnitude of errors in estimated transaction waiting times by approximately 80% relative to static approximations. The design conceptualisation applications of the proposed technique are demonstrated over a series of realistically sized demonstration problems. (Received 8 April 2008; Revised 20 July 2008; Accepted 2 August 2008)

Variance-based approximations of transaction waiting times in autonomous vehicle storage and retrieval systems

This paper proposes extensions of system conceptualisation models for autonomous vehicle storage and retrieval systems (AVS/RSs) to deal with non-exponential lift service times when the number of lifts and vehicles are both design variables. These extensions are based on an adaptation of a computational algorithm for estimating the probability distribution of queue lengths in capacitated, non-Poisson queuing systems. Simulation-based validation studies suggest that the proposed model extensions yield estimation errors comparable to those observed in earlier studies restricted to cases where the number of vehicle and lifts is treated as a single design variable.

Li Zhang

Papers

Variance-based approximations of transaction waiting times in autonomous vehicle storage and retrieval systems

Performance modelling of autonomous vehicle storage and retrieval systems with generally distributed service times

Optimal design and seismic performance of base-isolated storage tanks using friction pendulum inerter systems