A note on picker blocking models in a parallel-aisle order picking system

Abstract

This note develops analytical picker blocking models to simply and accurately assess picker blocking in parallel-aisle order picking systems when multiple picks occur at a pick point. The Markov chain--based models characterize the two bounding walking speeds for modeling picker movement: unit walk time and instantaneous walk time. The unit walk time model has a state-space transition matrix that is reduced by a factor of 16 for both narrow-aisle and wide-aisle systems. Additionally, the model improves upon the existing literature by providing a closed-form expression for the narrow-aisle system with instantaneous walk time. Experimental results are provided to demonstrate how picker blocking is influenced by pick density in a variety of scenarios under varying assumptions regarding the maximum number of picks at a pick point. These results broaden those previously presented in the literature, as well as demonstrate the improved efficiency of the proposed model.¹

Keywords:

• Warehousing
• picker blocking
• discrete-time Markov chains

Acknowledgments

A shorter and less technical version of this article was published previously as “Analysis of Picker Blocking in Narrow-Aisle Batch Picking” in Progress in Material Handling Research: Proceedings of 2010 International Material Handling Research Colloquium.