Joint order assignment and picking station scheduling in KIVA warehouses with multiple stations

The rapid development of e-commerce has brought new challenges to warehouse operations. Order picking plays a crucial role among all these operations, which directly affects the overall order fulfillment efficiency (Lamballais et al., 2017; Shen et al., 2020). The Robotic Mobile Fulfillment System (RMFS) is invented to improve order picking efficiency and reduce labour costs by exploiting rack-moving mobile robots (Boysen et al., 2017). The cooperation between the robots and movable racks eliminates pickers' unproductive movement in the picker-to-parts system (Battini et al., 2017). Compared with traditional manual warehouses, the picking performance of RMFS is far superior, which is reported to achieve over 600 order-lines per hour per workstation (Wulfraat, 2012; Banker, 2016). Nevertheless, order picking in RMFS needs further efficiency improvement due to the growing demand and increasingly tight delivery schedules brought by the prosperity of e-commerce (Batt & Gallino, 2019; Azadeh et al., 2017; Zhuang et al., 2021).