Maximizing Utilization with Pick and Pass Zone Picking

Pick and Pass Zone Picking

A common zone picking approach in large distribution centers with manual operations is Pick and Pass.

In a typical pick and pass system, the distribution center is divided into multiple pick zones. Multiple pickers work simultaneously in each pick zone. A pick zone is divided into sub-zones, and a single picker is assigned to each sub-zone. The sub-zones are organized in pick sequences. A work task is started in the first sub-zone of the sequence. When the task is completed in the sub-zone, it is passed to the next sub-zone. This pick and pass process continues until the task completes in the last sub-zone. A work task can be a single order, a cluster of orders, a batch of order lines, or a set of batches of order lines. There is staging capacity between the sub-zones to stage orders while they wait to be started in the next sub-zone.

Pick and pass is an efficient way to organize pickers in pick zones. Multiple pickers can work simultaneously in the same pick zone without congestion issues as there is never more than one picker in each sub-zone.

In traditional pick and pass, the sub-zones are static with pre-defined boundaries. Normally, the sub-zones are re-configured from time to time to balance work effort across the sub-zones. To add pickers, remove pickers, or re-configure sub-zones all the pickers in the pick zone need to stop working. Typically, in wave-based systems, re-configuration is done between waves.

Work Starvation Issues

A key risk of pick and pass is work starvation. Pick and pass systems are serial processes that move at the pace of the slowest link in the chain. Based on my experience working with customers over the last 30 years, even the most efficient DCs waste 10% to 20% of their labor costs due to starvation in pick and pass processes. Waste upwards of 30% is common due to idle workers waiting for work. Unfortunately, many operators lack the visibility or tools to truly understand how much money they are losing to starvation issues. But they see tell-tale signs on the floor of pickers waiting for work, or moving slowly with little to do.

Traditional pick and pass systems attempt to address work starvation by rebalancing sub-zones, however, the rebalancing is normally done at the total work level, not at the task level. Different tasks require different efforts. As a result, pickers with light workloads end up waiting for upstream pickers with heavier workloads to pass work.

Order processing in traditional pick and pass operations can also be slowed down by:

- Pickers working at different paces. Even if the work effort is perfectly balanced, the time to complete tasks will be different based on picker skills. ‍

- Pick zone reassignments. Moving pickers across pick zones to keep the zones synchronized requires stopping the work in all involved pick zones.‍

- Orders idle in staging areas. Work staged between sub-zones extends the time to process work in the pick zone.

Traditional Bucket Brigades

A powerful approach to address pick and pass starvation issues is using bucket brigades. The bucket brigade process was developed at Georgia Tech^[1] for manufacturing lines. With bucket brigades, tasks flow through a sequenced line of stations. When the last member of the brigade completes a task, the worker releases their work and walks back to the previous member of the brigade. The worker then takes over the task of the previous member and continues processing it. The previous member then walks back to the next previous member and takes over the task from the next previous member. The process continues until it gets to the first brigade member. The first member does not have a previous member to walk to, so this member starts a new task (e.g., a new order).

Bucket brigades eliminate the starvation issue with traditional pick and pass systems. Using bucket brigades for zone picking makes the sub-zones dynamic. Sub-zones expand and contract based on workload and picker velocity. Pickers do not have to wait for upstream pickers to pass work. Additionally, moving pickers between pick zones to keep zones synchronized can be done without stopping work. There is also no need to buffer work between sub-zones. Without buffering work between sub-zones, lead times to process tasks is minimized.

However, one shortcoming of bucket brigades that is not well addressed by the original Georgia Tech approach is blockage. Blockage occurs when pickers move faster than the pickers in-front and catch up. Blocked pickers need to wait for the front pickers to move to continue working. Rear pickers will remain blocked while there is no separation between the pickers in-front.

The original Georgia Tech approach tries to prevent blockage by sequencing pickers in the brigade by skill level. The slowest picker is assigned as the first brigade member (starting work). The fastest picker is assigned as the last brigade member (completing work). This solution reduces the risk of blockage and works well in manufacturing environments where the time and effort to perform the same task is relatively consistent. Unfortunately, in a picking operation, the time and effort to pick individual orders can be very different. As a result, the sequencing solution does not work as well as in manufacturing.

Another approach to avoid blockage is balancing work intelligently across pick zones by releasing orders that are heavier in work in pick zones that are currently less busy. Intelligent order release can help to a point, but it’s limited by the available orders. When there are many orders to select from, it is easier to find “good” orders. However, at the end of work periods, when there are fewer orders to choose from, finding good orders can be difficult or impossible. Often, the orders left at the end of periods are the ones that were never “good” to release, creating a challenging end of day.

Fulfillment Engines Enhanced Bucket Brigades

We believe warehouse operators need a better way to avoid blockage that is not limited by the available orders or order profiles, so we created Fulfillment Engines enhanced bucket brigades. Our approach handles the shortcomings of traditional bucket brigades, without depending on good orders or static order profiles. How?

- Real-time decision making. Fulfillment Engines uses advanced applied mathematics and real-time decision making to minimize the risk and impacts of blockage proactively. Pickers catching up to pickers in front is almost unavoidable in a real operation. Our algorithms detect when there is risk of blockage and automatically begin to adapt pick zones and tasks to mitigate the risk. If a picker does become blocked, Fulfillment Engines enhanced bucket bridges minimize the duration of blockage by updating tasks to create separation between the congested pickers.

- Intelligent pick zone orchestration. Fulfillment Engines enhanced bucket bridges also use real-time intelligence to move pickers between pick zones to keep the zones synchronized. Our bucket brigade algorithms monitor synchronization across pick zones and relocate pickers automatically, without supervisor intervention or stopping work. To avoid moving pickers too frequently, the algorithms weigh the benefit of the move against the productivity cost of moving the picker, and when the picker was last moved (customers can set a minimum time in zone).

- Automatic brigade rebalancing. Fulfillment Engines monitors real-time picker performance and works to re-sequences pickers continuously from slowest to fastest, without supervisor intervention or impacting picker productivity. This feature also comes in very handy when pickers move across pick zones.

Because we know change management in a warehouse is critical, we made it easy to roll out Fulfillment Engines enhanced bucket brigades in stages.

- You can quantify the potential benefits of enhanced bucket bridges using our no-cost, utilization benchmark and simulation. Our benchmark and simulation tools give you deep visibility into your current utilization metrics and the potential productivity improvements from using Fulfillment Engines – all without making any changes in your DC.

- Implementation can start in just a few pick zones (as few as one) to test and quantify the benefits for the facility’s operation. This process can co-exist with any other picking technology in the facility.

- The implementation can include just a limited number of SKUs.

- The different features of the Fulfillment Engines enhanced bucket brigades can be turned on and off through configuration parameters, allowing you to quantify the yielded benefits, and decide what works well and not so well for your operation.

If you have pick and pass processes in your operation, and are dealing with low utilization or starvation issues, contact Fulfillment Engines to get your no-cost utilization assessment and learn how we can help.

^[1] https://www2.isye.gatech.edu/~jjb/bucket-brigades/papers/bbwh.pdf