A labor allocation model for zone order picking systems

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] The rise in e-commerce retail is contributing to complications in distribution center order picking processes such as SKU proliferation, an increased volume of small orders, and product customization. Order picking is widely considered the most important activity in warehouse and distribution operations as it directly affects customer service and generally accounts for more than 55% of operating costs. Order picking systems can be divided into zones to which pickers are assigned and they pick items located only in their assigned zone. Generally speaking, zoned order picking systems are far less studied than traditional designs and research on operational control policies of zoned OPS's is even more sparse. This research focuses on allocating order pickers to zones while considering demand fluctuation and variation in picker service rates to help alleviate workload imbalance and increase the flexibility of the labor staff. An integer program is developed that assigns pickers to zones considering the variation stated above. Five heuristic picker-to-zone assignment methods are created and simulation is used to test the integer program solution to the other assignment methods based on performance criteria including total throughput time, order cycle time, picker idle time, and labor cost. The results show that the optimal assignment improves overall system performance in terms of productivity, efficiency, utilization and cost over all heuristic assignment methods.