Simulation Evaluation of Single Shuttle and Twin Shuttle AS/RS

An Automated Storage and Retrieval System (AS/RS) operates a shuttle with computer process control to execute the storage and retrieval of warehouse stock. It can be equipped with the traditional single shuttle or the newly developed twin shuttle. There has been a significant literature on the comparison of single shuttle systems versus twin shuttle systems. Researchers agree that twin shuttle systems have better operational performance. However, a twin shuttle system costs about 40% more than a comparable single shuttle system. Therefore, given the same operating environment, we believe that the capacity or effectiveness of a twin shuttle system should outperform a single shuttle system by at least 40% to justify the cost. This study develops Arena 8.0 simulation programs to evaluate the performance of single shuttle and twin shuttle systems. The comparison is based on average storage/retrieval waiting time, the effective storage/retrieval capacity, and the ratio of dual command. We assume different rack shapes, time periods of storage and retrieval operations may or may not overlap, input and output stations may not be in the same location, and there may be batch demands for storage and retrieval operations. Simulation results suggest that twin shuttle systems would be the better investment if the input/output stations are in separate locations, time periods of storage/retrieval operations do not overlap, or there are significant batch demands. The result of our study can provide valuable information to practitioners who intend to establish an AS/RS system in the future.