Synchronization in robotic forward-reserve warehouses for e-commerce logistics

Abstract

The burden from booming e-commerce business falls on order fulfillment inside warehouses. To cope with this challenge, the forward-reserve (FR) strategy is commonly applied in warehousing. In recent years, the application of automated and robotized systems has brought great opportunities for facilitating order processing. However, implementing such systems also leads to new problems, especially when combining with the original handling systems. This thesis considers the robotic forward-reserve warehouses, where pick locations are brought to workers by mobile robots in the forward area. The related synchronization issues are investigated to improve the efficiency and performance of order fulfillment across time and space. Three scenarios are studied in this thesis. The first one concentrates on the synchronization mechanism in the robotic forward area. A picking-replenishment synchronization mechanism (PRSM) is thereby proposed, which attains a balance between replenishment efforts and picking efficiency at the operational level. The resulting problem is formulated as a mixed-integer programming and solved by a tailored variable neighborhood search procedure integrated with a divide-and-conquer paradigm. Numerical results reveal remarkable benefits of PRSM over the conventional random schemes under different scenarios. The second scenario studies the efficiency imbalance issue between the manual and robotic systems. The interface between replenishment picking and order sorting is considered, where a data-driven synchronization approach (DDSA) is derived for connecting the two operations. Particularly, a two-phase solution framework is developed to handle the unknown demands in real-time. The effectiveness of the proposed approach is validated through a real case study. Compared with demand-based approaches, DDSA can reduce stock-outs and delivery costs significantly. In addition, three sources of parameters are examined and analyzed to figure out the dominating ones. The third scenario integrates decision-making in both manual reserve area and robotic forward area to synchronize intralogistics operations considering delivery requirements such that the overall performance can be optimized. This challenge is formulated as a delivery-driven intralogistics synchronization (DDIS) problem, which is solved by a tailored variable neighborhood search method. A series of comprehensive numerical experiments show the benefits of the synchronization strategy as benchmarked with sequential approaches. A substantial reduction can be achieved in both makespan and forward area size, indicating significantly improved intralogistics operational efficiency and space utilization. This thesis reveals the significance of operational synchronization in robotic FR warehousing by presenting its superiority over other solution schemes under various settings. In general, synchronized approaches show great stability and robustness faced with fluctuating demands and markets. Furthermore, utilization of resources (e.g., operators, space, workstations and trolleys) can be improved through synchronization, and thus fewer of them are required. Besides the synchronized solutions, this thesis also provides practical guidelines for warehouses, with which corresponding factors can be adjusted for better performance.