Monthly Archives: April 2015

Successful Defenses



Congrats to Dr. Faraz Ramtin, who successfully defended his Ph.D. dissertation thesis entitled, “Modeling and Analysis of Automated Storage and Retrieval Systems with Multiple in-the-aisle Pick Positions,” and to Patrick Reilly, who successfully defended his M.S. thesis entitle, “Propagation of Unit Location Uncertainty in Dense Storage Environments.”

I am super proud of both students, who are excellent researchers and human beings.



Faraz’s dissertation consists of three contributions all focusing on a special type of case-level order fulfillment technology – an “Automated Storage and Retrieval System with Multiple in-the-aisle pick positions.” These semi-automated systems are common in temperature-controlled warehouses.  Our first contribution includes the first study to analyze AS/RS with multiple in-the-aisle outputs. We develop expected travel time models for random storage policies and provide design insights into these systems.  In our second contribution, we considered the use of MIAPP-AS/RS to fulfill orders for non-identical items’ demand, which relaxed some of the assumptions we made in the first contribution. Specifically, we focused on an important practical design decision, the optimal SKU assignment problem. We studied the impact of different pick position assignments on system throughput, as well as system design trade-offs that occur when the system is running under different operating policies and different demand profiles. We developed optimization models to find the optimal assignment that minimizes the expected travel time.  Finally, we developed optimization models for the SKU-to-pick position assignment problem for dedicated and class-based storage policy for MIAPP-AS/RS.  By exploiting the structure of these optimization models, we decomposes the problem using Benders decomposition.


The first two contributions of Faraz’s dissertation work has been accepted for publication:

  • Ramtin F., Pazour J. A. “Analytical Models for an Automated Storage and Retrieval System with Multiple in-the-Aisle Pick Positions”. IIE Transactions, 46(9), 968-986.
  • Ramtin F., Pazour J. A. “Product Allocation Problem for an AS/RS with Multiple in-the-Aisle Pick Positions”. IIE Transactions, Accepted Manuscript.

He is working on the manuscript of his third contribution, which explores a dedicated storage policy in these systems.

Patrick’s work focuses on dense storage environments and adds an additional dimension to the warehousing literature in that area, specifically item location uncertainty.  Effective space utilization is an important consideration in logistics systems and is especially important in dense storage environments. Dense storage systems provide high-space utilization; however, because not all items are immediately accessible, storage and retrieval operations often require shifting of other stored items in order to access the desired item, which results in item location uncertainty when asset tracking is insufficient. Given an initial certainty in item location, we use Markovian principles to quantify the growth of uncertainty as a function of retrieval requests and discover that the steady state probability distribution for any communicating class of storage locations approaches uniform. Using this result, an expected search time model is developed and applied to the systems analyzed. We also develop metrics that quantify and characterize uncertainty in item location to aid in understanding the nature of that uncertainty. By incorporating uncertainty into our logistics model and conducting numerical experiments, we gain valuable insights into the uncertainty problem such as the benefit of multiple item copies in reducing expected search time and the varied response to different retrieval policies in otherwise identical systems.IMG_1745