The Eternal Dilemma While Designing a Pick/ Put to Light System

Sorting is one of the most essential activity in any distribution or fulfillment center. With meteoric rise of eCommerce and new age startups going with hyper local business models spoiling the end users like us, the level of throughputs expected out of Distribution or Fulfillment centers (DC or FC) has increased to a whole new level. To process hundreds of thousands of orders daily after picking from a pool of thousands of unique SKUs there needs to be very reliable sorting mechanism. When it comes to sorting, it would be hard to believe if any solution designer said that old school Pick/ Put to Light (PTL) didn’t came to his/ her mind as first thought. Despite being decades old technology, it is one of the most popular, cost effective, easy to maintain technology used for sorting. Now PTL has found its way to survive in modern Goods to Person (GTP) but in country like India due to cheap labor full stack PTL based sorting is still a go to sorting technology for most of the companies.

While PTL system are easy to understand while designing but there are two tricky questions that every solution designer face: Which I call an eternal dilemma while designing a PTL system.

1. Should I decide number of lights required based on locations required for PTL system (SKU pick faces in pick to light and Orders open at a time in Put to Light)?

2. What should be the optimum zone length of a zone?

If you freeze a zone length you would get productivity per person which will define number of zones required. But those number of zones may be insufficient to accommodate SKUs or Open orders?

If you freeze the number of zones (Or total length of racks) required there is possibility with given throughput you may employ more people in sorting than required under utilizing manpower.

Now, this is an iterative process to change the length of zone and observe the productivity achieved and then balancing it with total number of zones required. There is no fix equation to get an optimum zone length which would balance the manpower required and sort locations. But there is certainly some indicative relation between the zone length and productivity for which two very important factors are;

1. Distance travelled by sorting employee to complete one sorting task which is governed by the zone length

2. Number of lines and task obtained from an SKU tote in Pick to Light system and Order tote in a Put to Light system

On an overall basis, while zone length is simpler part to understand but for increasing the tasks per tote in zone solution designer can do very little on his/ her end and this is more of an must have WMS functionality. WMS should be capable enough to slot SKUs such a way that maximum tasks should be fulfilled by travelling minimum number of zones. WCS should be smart enough to divert and balance the load between different zones.

• Every meter increase in the length of a picking zone there is “7% to 2%” drop in the sorting productivity based on lines per tote

• Every line added per tote in a zone there is gain of “4% to 43%” productivity, but maximum effect lies when increasing lines per tote from 1 to 3, after this gain in productivity is absorbed by actual sorting time due to increased number of physical tasks.

These productivities are based on MOST and may vary by use of different method to estimate productivity and by the nature of product being handled while sorting. However, with above findings, if you estimate productivity for zone length and particular number of tasks per tote per zone above findings will help to quickly get new productivity numbers. This will help to continue the iterative process in a bit easier manner to find optimum length of a zone balancing the location requirement to overcome the eternal dilemma of designing an PTL system.