Routing order pickers in a warehouse with a middle aisle

TLDR: An algorithm is presented that can find shortest order picking tours in this type of warehouses and it appears that in many cases the average order picking time can be decreased significantly by adding a middle aisle to the layout.

About: This article is published in European Journal of Operational Research.The article was published on 2001-08-16 and is currently open access. It has received 309 citations till now. The article focuses on the topics: Aisle & Order picking.

Figures

Figure 5: Average travel time as a function of the picklist size for a warehouse with 450 meters of aisles, with a middle aisle (dashed line) and without a middle aisle (solid line).

Figure 6: Average travel time as a function of the picklist size for a warehouse with 70 meters of aisles, with a middle aisle (dashed line) and without a middle aisle (solid line).

Figure 4: Average travel time for an order of 30 items as a function of the number of aisles for a warehouse with 450 meters of aisles, with a middle aisle (dashed line) and without a middle aisle (solid line).

Figure 3: 14 possible arc configurations to change from aisle j to aisle j+1.

Figure 1: Part (a) gives an example layout of a warehouse consisting of two blocks. Each solid square corresponds to a location, where items have to be picked. Part (b) gives a graph representation of this warehouse with the pick locations.

Figure 2: Six ways to traverse the edges in aisle j between bj and cj. In transition (5) only the longest double edge is not traversed. Transitions (3) an (4) are only possible if there is at least one item in this part of the aisle. Transition (5) is only possible if there are two or more items in this part of the aisle and transition (6) is only allowed if this part of the aisle is empty.

Frequently asked questions

Q1. What have the authors contributed in "Routing order pickers in a warehouse with a middle aisle" ?

This paper considers a parallel aisle warehouse, where order pickers can change aisles at the ends of every aisle and also at a cross aisle halfway along the aisles. An algorithm is presented that can find shortest order picking tours in this type of warehouses. 

Q2. How is the problem of finding shortest order picking tours in a warehouse solved?

In practice, the problem of finding order picking tours in a warehouse is mainly solved by the so-called S-shape heuristic in which order pickers move in a S-shape curve along the pick locations skipping the aisles where nothing has to be picked. 

Q3. Why is the small peak for 3 aisles in the curve for the warehouse without cross aisles?

The small peak for 3 aisles in the curve for the warehouse without cross aisles, is due to the fact that at least one of the aisles has to be entered and left from the same side to ensure that the order picker ends his tour at the front of the warehouse. 

Q4. What is the length of a subgraph of the warehouse graph?

Let Yj be the subgraph of the warehouse graph consisting of vertices bj and cj together with all edges and vertices between bj and cj and define L+yj = L − j ∪ Yj . 

Q5. What are the other activities that have to be performed?

Other order picking activities, like positioning the truck or crane at the pick location, picking items from the pick location and putting them onto a product carrier, have to be performed anyway. 

Q6. How can the algorithm be used in warehouses with only one or two possibilities for aisle changing?

the algorithm can be used in warehouses with only one or two possibilities for aisle changing, by setting the appropriate distances between the aisles to infinity.