Quantifying picker blocking in a bucket brigade order picking system

Bucket brigade is a linear order-picking process with one loading station and one unloading station. Here we model and quantify picker blocking in bucket brigade order picking systems (OPSs). A bucket brigade improves throughput and reduces variability in OPSs. However, each order picking trip fills different orders and creates workload variation per order. We show that bucket brigade order picking experiences picker blocking when there is a workload imbalance per pick face. We derive a closed-form solution to quantify the level of blocking for two extreme walk speed cases. Additional simulation comparisons validate the picker blocking model which includes backward walk and hand-off delays. We identify the relationship between picker blocking in bucket brigade OPSs and picker blocking in a circular-aisle abstraction of OPSs in which backward walk and hand-off delays as well as forward walk speed are considered. Our analytical model and simulations show that aggregating orders into batches smoothes the workload variation by pooling the randomness of picks in each order and that slowest-to-fastest picker sequencing modulates picker blocking between two pickers, i.e., the interaction between neighboring pickers.