  Robotic-based compact storage and retrieval systems provide high-density
storage in distribution center and warehouse applications. In the system, items
are stored in bins, and the bins are organized inside a three-dimensional grid.
Robots move on top of the grid to retrieve and deliver bins. To retrieve a bin,
a robot removes all bins above one by one with its gripper, called bin digging.
The closer the target bin is to the top of the grid, the less digging is
required to retrieve the bin. In this paper, we propose a policy to optimally
arrange the bins in the grid while processing bin requests so that the most
frequently accessed bins remain near the top of the grid. This improves the
performance of the system and makes it responsive to changes in bin demand. Our
solution approach identifies the optimal bin arrangement in the storage
facility, initiates a transition to this optimal set-up, and subsequently
ensures the ongoing maintenance of this arrangement for optimal performance. We
perform extensive simulations on a custom-built discrete event model of the
system. Our simulation results show that under the proposed policy more than
half of the bins requested are located on top of the grid, reducing bin digging
compared to existing policies. Compared to existing approaches, the proposed
policy reduces the retrieval time of the requested bins by over 30% and the
number of bin requests that exceed certain time thresholds by nearly 50%.
