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Tricks and Tags for Hard-to-Read Products

Based on numerous studies, the University of Arkansas' RFID Research Center says companies can overcome the challenge of tagging products that include metal or water.
By Brad Adams and Maurizio Turri
Jun 01, 2008—One of the most common questions we get from companies looking to use Gen 2 ultrahigh-frequency passive tags in the supply chain is: "How do I read my tagged product if it has high water or metal content?"

The problem, simply stated, is that metal reflects RF energy, and water absorbs RF energy. RFID tags and interrogators communicate with each other through transmission of RF energy in the form of electromagnetic waves. When RF energy is reflected by metal, the waves interfere with themselves, decreasing efficiency. When this interference occurs, the interrogator cannot understand the message in the signal. And when RF energy is absorbed by water, its power is reduced to a level insufficient for communication.

It doesn't take a lot of metal or water to cause problems with read rates. Sometimes companies are surprised to learn that the packaging for their products is the culprit. For example, corrugated boxes of dishwashing powder are notoriously difficult to read because of the metal-foil lining in the box. Even the metallic ink on packaging for DVDs, toothpaste and many other products reduces read rates.

Here at the lab, we often use certain "tricks" to achieve good readings. And when those fail, we turn to another solution: special tags designed for metal and water products.

Tricks of the Trade
When dealing with hard-to-read products, we first try to find the optimal location—or "sweet spot"—on the case or package where a conventional passive tag will work effectively. The RFID Research Center uses a sweet-spot tester that provides a visual representation of proper tag location. This device is not available commercially, but companies can test for the sweet spot using their own tags and interrogators. Here's how:

1. Place a tag on a piece of cardboard and hold it in front of a reader antenna for a few seconds. Note the number of times the tag was read in a given time period.

2. Place the same tag on the product in a test location, and note the number of reads in the same amount of time. A minimum acceptable read rate would be more than 50 percent of the reads obtained in step 1.

3. Repeat this process until a usable tag location is found.

We have used this manual method on a variety of products with good results.

Another "trick" we've discovered concerns tagged products that are difficult to read at a pallet level. Often they can be read once the pallet is broken down into individual cases or items. This became clear when we tested large plastic buckets of paint, which contain high amounts of water. By placing a tag on the lid, we were able to read the individual buckets. But when the buckets were stacked on a pallet and the tags on the lids were covered by the buckets above them, we were unable to read the individual buckets on the pallet.

This is satisfactory if it's sufficient to read just the pallet tag when the pallet passes through an RFID portal. But if it's necessary to read every case or item on the pallet, you'll probably need to use special tags designed for metal and water products.
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