AS/RS Picking Systems Case Study

A study of an automated storage and retrieval system using Flexsim Simulation Software


Skarnes Inc., a material handling company located in Plymouth, MN, wanted to increase the throughput of one of its client’s warehousing processes. Skarnes’ client used an Automated Storage and Retrieval System (AS/RS) that interfaced with a conveyor loop that transported pallets to a picking station and then back to the AS/RS.

The AS/RS consisted of four aisles, each with a Storage and Retrieval Vehicle (S/RV). Each aisle also had an input and output conveyor that connected to the main conveyor loop. All of the conveyors in the system are on the same level. The system is currently capable of picking 70 pallets per hour.

Issues to Solve

After observing this operation, Skarnes thought if conveyor congestion could be decreased in front of the AS/RS then the number of pallets picked per hour would increase. Skarnes came up with the idea of adding another mainline conveyor for outbound pallets. This new conveyor would be located above the existing mainline conveyor. In order to prove the validity of this option to their customer, Skarnes contacted Flexsim about developing a simulation model.

Simulation Approach

In the simulation model, an S/RV retrieves a pallet from the storage rack and moves it to the first section of the lower level output conveyor. The pallet is then conveyed forward to the second section of the lower level output conveyor. Once a pallet is completely in the second section of the lower level output conveyor, a lift raises the pallet to the upper level mainline conveyor. The pallet is then transferred to the upper level mainline conveyor which conveys the pallet to a lift that lowers the pallet to the lower level mainline conveyor. The pallet is then conveyed to a picking station where an operator removes cases from the pallet. After the required number of cases have been removed from the pallet it is routed to the empty pallet conveyor if it is empty or routed back to the AS/RS to be restored if it is a partial pallet.


By adding this upper level mainline conveyor, the simulation model demonstrated that the number of pallets picked went from 70 to 100 pallets per hour, an increase of 40%. Additionally, the 3D animation of the system visually demonstrated that conveyor congestion was greatly reduced.


Model and case study completed by Bill Strong. Bill is a simulation engineer with Flexsim Software Products and can be reached at [email protected] or 801-224-6914.

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