Free balancing for a shuttle-based storage and retrieval system

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.