Free balancing for a shuttle-based storage and retrieval system

https://doi.org/10.1016/j.simpat.2017.12.006Get rights and content

Highlights

  • We propose free balancing in a shuttle-based storage and retrieval system (SBS/RS).

  • SBS/RSes usually require equal numbers of shuttles and tiers.

  • Free balancing logic enables fewer shuttles while maintaining throughput.

  • Blocking and collisions, which occur in tier-to-tier SBS/RSes, are prevented.

  • The effectiveness of free balancing logic is verified through simulation analysis.

Abstract

Shuttle-based storage and retrieval systems (SBS/RSes) – a type of automated storage and retrieval system (AS/RS) – have recently been developed to increase throughput capacity. An SBS/RS can increase throughput by using multiple shuttles as storage and retrieval machines (SRMs), while traditional AS/RSes are limited because they use a single stacker crane as an SRM to handle multiple jobs. However, when setting up an SBS/RS, a significant investment is required to integrate a shuttle for each tier in the storage construction. To lower costs, a reduction in the number of shuttles is an adequate solution if the SBS/RS can maintain the designated throughput. However, this could also lead to operational problems. Namely collision prevention and workload balances could be affected because each shuttle would be able to travel to any tier for pickups and drop-offs. Thus, shuttle position control is important when the system operates with a mismatch between the number of shuttles and tiers. Ideally, free balancing should be achieved. When free balanced, each shuttle's position is monitored to prevent collisions or blockages, as well as to make sure jobs are evenly assigned and the required throughput is maintained. This paper suggests system controls to prove the effectiveness of free balancing, and it runs a simulation analysis to verify the system with the suggested controls. As a result, free balancing shows better performances in terms of throughput and utilization compared to a basic system control. Specifically, in our case, free balancing can achieve targeted throughput with less number of shuttles.

Introduction

Automated storage and retrieval systems (AS/RSes) are the most well-known automated warehouse technology. These systems offer significant benefits over non-automated systems; they save workforce costs and floor space, increase reliability and lower error rates [23]. However, transaction mechanisms have recently changed, and there is now a tendency toward smaller quantities in terms of orders and production batches. This requires technology advances.

As one of these advanced technologies, shuttle-based storage and retrieval systems (SBS/RSes) have been developed to process more loads and achieve higher speeds than mini-load crane-based automated storage and retrieval systems (CBAS/RSes). The latter systems are not suitable for loads that require frequent transactions because the storage and retrieval machine (SRM) consists of a single stacker crane. In contrast, an SBS/RS is able to satisfy the new transaction mechanisms because the SRM employs multiple shuttles that service multiple tiers. Fig. 1 describes the SBS/RS shuttle carrier in a simulation, and Fig. 2 shows the physical configuration of the SBS/RS.

There are two main SBS/RS configurations – one with tier-captive shuttles and the other with tier-to-tier shuttles. In a tier-captive SBS/RS, an individual shuttle carrier is configured for each tier. One of the significant drawbacks of this system is its low flexibility. Although efficient tier-captive SBS/RSes have been constructed with high throughput capacities and short cycle times in numerous industries, they rely on fixed configurations and cannot simply adjust to rapid changes confronting storehouse management. To overcome this drawback, tier-to-tier SBS/RSes have been employed. These systems achieve greater flexibility than their tier-captive counterparts by adding or eliminating extra shuttles.

Despite the advantages of tier-to-tier SBS/RSes, they suffer from operational issues. Because the number of shuttles changes to achieve flexibility, the system requires inter-tier movement that does not occur in tier-captive SBS/RSes (Fig. 3). In SBS/RSes using bi-directional guide paths, this inter-tier movement can cause operational difficulties as well as system failures due to shuttle collisions. As a result, tier-to-tier SBS/RSes can exhibit significant differences in system performance according to operational method. In other words, the efficient use of tier-to-tier SBS/RSes is very important in achieving cost-effective operations.

This paper defines the problems inherent in tier-to-tier SBS/RSes and presents free balancing as a solution. In free balancing, the system is dynamically operated by controlling each shuttle and load position to prevent inefficiencies such as blocking delays and collisions. To analyze the effects of free balancing, this paper compares the performance of free balancing and non-free balancing operations by exploring several examples of transaction process logic and load assignment policies in a simulation. The detailed simulation is modeled using AutoMod®.

This study is organized as follows. Section 2 provides a review of the literature on AS/RSes, SBS/RSes and automated guided vehicles (AGVs). In Section 3, the details of the tier-to-tier SBS/RS and shuttle collision prevention are explained. Section 4 describes the main assumptions of this paper and exhibits a comprehensive simulation model. The results of the experiments and analyses are presented in Section 5. Section 6 discusses the conclusions of this research.

Section snippets

Literature review

AS/RSes have been discussed by a lot of researchers. Roodbergen and Vis [23] have provided a review of AS/RSes. According to their paper, AS/RS design decisions can be divided into two categories – physical designs and control policies. The physical design determines the physical configuration, including factors such as the number of cranes per aisle. In contrast, control policies determine the actions conducted by the system. A control policy is composed of storage assignments, batching,

Tier-to-tier SBS/RS

As mentioned, tier-to-tier SBS/RSes have fewer shuttles than tiers. Thus, if a shuttle carrier does not exist in the ith tier where a storage or retrieval transaction has occurred, one of the shuttle carriers has to move to that tier to complete the transaction.

There are two ways (Fig. 7) to transfer a shuttle carrier from its current tier to another tier; the shuttle carrier can ride the elevator, or the shuttle carrier can be captured by two elevators to move vertically. Both methods have

Simulation modeling in an SBS/RS

The simulation is modeled to analyze the effects of shuttle free balancing from the view of the entire system. To implement the above two strategies in the simulation, the simulation must control not only the shuttle positions by using transaction process logic but also the load storage locations by applying a load assignment policy. In short, the entire system will be controlled via transaction logic and a load assignment policy.

The performance of the system can change depending on the

Simulation analyses

Table 4, Table 5, Table 6, Table 7 show the results, which reveal the performance measures. These measures are the average utilization of the shuttle carrier (USCAR) and the elevator (ULIFT), the average cycle time per storage transaction (Tstor) and that of the retrieval transaction (Tretr), and lastly, the average cycle time per S/R transaction (Ttotal) and lastly, the number of shuttle's inter-tier movement per hour (Ntier). In these results, utilization refers to the percentage of time

Conclusion

This paper suggests three system controls and analyzes the effects of free balancing (FB) in an SBS/RS via a modeling simulation. The system controls in the simulation consist of the load assignment policy and transaction process logic. Among the system controls, FB is composed of STPL, RTPL2 and the shuttle priority policy. With STPL and RTPL2, FB can prevent shuttle collisions and blocking delays in tier-to-tier SBS/RSes. In addition, with the shuttle priority policy, loads decide storage

Acknowledgments

This research was supported by the MSIP(Ministry of Science, ICT and Future Planning), Korea, under the ITRC(Information Technology Research Center) support program (IITP-2017-2014-0-00678) supervised by the IITP(Institute for Information & communications Technology Promotion; and the Ministry of Land, Infrastructure and Transport, Republic of Korea, under the Light Weight Material Handling Machine Development for Improving Logistics Efficiency project (13PTSI-C065358-01-000000).

References (29)

  • M. Fukunari et al.

    An efficient cycle time model for autonomous vehicle storage and retrieval systems

    Int. J. Prod. Res.

    (2008)
  • V. Giordano et al.

    Integrated supervisory and operational control of a warehouse with a matrix-based approach

    IEEE Trans. Autom. Sci. Eng.

    (2008)
  • S.S. Heragu et al.

    Analysis of autonomous vehicle storage and retrieval system by open queueing network

  • S.J. He et al.

    Deadlock control of autonomous vehicle storage and retrieval systems via coloured timed petri nets and digraph tools

    Int. J. Prod. Res.

    (2009)
  • Cited by (35)

    • Retrieval request scheduling in a shuttle-based storage and retrieval system with two lifts

      2023, Transportation Research Part E: Logistics and Transportation Review
    • A joint optimisation of multi-item order batching and retrieving problem for low-carbon shuttle-based storage and retrieval system

      2022, Cleaner Logistics and Supply Chain
      Citation Excerpt :

      An assembly line parallel job problem generating scheduling task queue model with time window is established with the consideration of the movement characteristics of SBS/RS. Ha and Chae (2018) present free balancing into SBS/RS operation, in which the system inefficiencies such as blockings and collisions are prevented by controlling the position and movement of shuttles and loads. The experiment shows the effectiveness of free balancing that the target throughput can be achieved with a smaller number of shuttles.

    • Travel time models for tier-to-tier SBS/RS with different storage assignment policies and shuttle dispatching rules

      2021, Transportation Research Part E: Logistics and Transportation Review
      Citation Excerpt :

      For tier-to-tier SBS/RS, there are no existing studies investigating shuttle dispatching rules even though shuttle relocation is the major difference between a tier-to-tier system and a tier-captive one. Ha & Chae (2018) proposed strategies to avoid shuttle collision and policies to store incoming identical SKUs to reduce transaction time in a tier-to-tier SBS/RS. The efficiency of the proposed strategies was tested through simulation.

    • Energy efficient automated warehouse design

      2020, Solving Urban Infrastructure Problems Using Smart City Technologies: Handbook on Planning, Design, Development, and Regulation
    View all citing articles on Scopus
    View full text