Ergonomic and economic optimization of layout and item assignment of a U-shaped order picking zone

Abstract

Order picking is considered one of the most labor- and cost-intensive warehouse operating processes. Regularly, order pickers are exposed to severe physical demands, which increase the likelihood for developing muscular-skeletal disorders, such as lower back pain. Muscular-skeletal disorders cause significant compensation, recovery and deficiency costs and are expected to gain further importance in the future due to an aging workforce. Both companies and workers may therefore benefit from decision support models that explicitly take ergonomics aspects into account. The work at hand investigates a warehouses where the storage area is divided into zones with shelves in each zone arranged in the shape of a U. For this warehouse, we determine an optimal configuration of the U-zone’s layout as well as an optimal assignment of products to storage locations. We depart from prior research by considering both the minimization of the total travel distance as well as the minimization of the total ergonomic strain workers are exposed to. Both optimization problems are formalized as mixed-integer programs. An exact polynomial-runtime solution procedure, suitable for both objectives, is developed. Using this solution procedure, we illustrate how the relevant ergonomic strains can be quantified to apply them to our optimization model. Computational studies illustrate the efficacy of our proposed solution procedure. Optimal layouts and storage assignments significantly reduce the walking distance and ergonomic strain during order picking. Additionally, both objectives are only marginally conflicting, such that, mutually, an optimal solution for one objective is also a close-to-optimal solution for the other. We finally derive insights on the optimal layout and storage assignment for future research and practical application.