Pick To Light (PTL) vs. Pick To Voice (PTV)

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ph_mloganAs I work with clients on engineering studies, we often get into a discussion on technologies that deliver picking instructions to picking operators. Inevitably, when the topic is raised, we are asked “So which is better, Pick To Light or Pick To Voice?” And, being a typical consultant, I answer “It depends!” But, in this case, it really does depend. It depends on the obvious things like number of facings, number of pickers, amount of travel between picks, etc. But there are also some other considerations, such as the amount of re-slotting done, the client’s tolerance for errors, desire for common technologies, just to name a few. However, there are some general guidelines that help to point us in one direction or another when evaluating whether PTL or PTV (or something else) is the best alternative for a picking operation.

The Obvious Ones

Generally, voice picking technology wins over PTL when there is a very large SKU environment over a large amount of real estate. Because PTL modules are placed physically at the pick location, the cost to place many of these over a large area is usually cost prohibitive. Also, if there are few order selectors (less than 10-15), Pick To Voice is also usually a better choice. And if the pick locations are constantly being re-configured, voice technology will be a higher consideration.

On the other hand, PTL is chosen for high speed picking for a lower SKU count or if the picking environment is very dense. It is often a better choice in a zoned picking operation. As the number of selectors increases, the cost per light module decreases, compared to the cost of every voice unit that must be worn by every picker.

When Accuracy Matters

I don’t really like the title of this section. I mean, when doesn’t accuracy matter? But accuracy is a relative term. Although a 99.5% pick accuracy rate may be acceptable for a DC supplying its own stores (since the inventory never leaves the internal “supply chain”), that would be dismal for a DC distributing high value or critical items to end users. And, higher levels of accuracy always come at higher costs.

According to testing done by source independent of the technologies, pickers using PTV technology make fewer errors over those using PTL. Studies show that a voice picking operation can expect an error rate of 0.2 to 2 errors per 1000 picks (99.8 – 99.98% accuracy). On the other hand, PTL operations make about 3 to 5 errors per 1000 picks (99.5 – 99.7% accuracy). At first, this information did not make sense to me. My initial reaction was that it could not be true. I would think that if a picker could see a light and the quantity to pick in a PTL operation, they would make fewer errors than an operator who is simply told where to go and how many to pick. But a few factors come into play here. First, in order to save capital costs, some PTL installations use simply a light at the pick location, but display the quantity to pick on a bay display that is not directly at the pick location. Since the lights that do not have the integrated displays are less expensive, the DCs that have installed these have made a trade-off between initial capital cost and long term accuracy. A second factor that may cause PTL to be less accurate is focus. Because the picker must change their focus from the pick to the light (to extinguish it), it has been argued that this may introduce an error factor that is not present in a voice directed picking environment. Also, in a voice directed picking environment the picker can ask the system to repeat the last instruction. In a Pick to Light environment, once the light is extinguished, the picking information is usually wiped from the display (although there are techniques for leaving this information on until the next pick). These factors may point to why voice picking is generally more accurate than PTL.

But What About Productivity?

Ah, the heart of the matter. Well, the same studies that show that Pick to Light is less accurate show that it is also more productive. Again, depending on the application, PTL operations can have rates of 350 lines per man-hour or more, while voice picking in the same applications run 300 lines per man-hour or less. Why is that so? Well in a PTL environment, the picker can “look ahead” and pre-plan their pick movements. Although this may be a subtle advantage, the benefits of these planned movements can add up over time.

But a word of caution here – there are several issues that can affect the productivity of either system. These include the balancing of work across a pick zone, the number of cartons worked concurrently by the operator, the ergonomics and slotting of the pick module, and the picking density of the operation. The key to getting the most out of a Pick To Light system (or any picking equipment) is a proper understanding of the technology and the operation.

Maturing Technologies

Although voice picking is a newer technology, both PTV and PTL are considered mature technologies. There are more choices of hardware and software for both types of systems than were available just a few years ago. Also, because of this, integrators and consultants are finding more uses for the technologies and thus increasing the value to the operations who adopt either. So neither technology is considered state-of-the-art anymore. But before plunging into a decision to implement either, a serious evaluation by those that understand the benefits of each is highly recommended.

Marvin Logan is Director of Consulting Services at Bastian Solutions. He and his team of consultants provide engineering studies, system simulations, slotting analysis, and system layout and design.

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