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How Can I Read 1,000 Tagged Apparel Items Within a Small Area?

Posted By RFID Journal, 10.21.2011
The goods in question are located inside a 4-foot-square box, and I've only been able to successfully read about 50 percent of them or so.

—Name withheld

———
There are many factors that could influence your ability to read tags quickly and consistently. One is tag sensitivity. Older Electronic Product Code (EPC) tags are not nearly as sensitive as newer models. Since the newer tags require less power to respond to a reader, it is easier to power up all tags within a read field. You might also need to create a tunnel, with reader antennas on the top, bottom, left and right of the box, in order to be able to interrogate the tags.

In addition, there are several features of the EPC Gen 2 protocol that might help you read every tag. For an explanation of these features, I reached out to Victor Vega, NXP Semiconductors' director of marketing for RFID solutions. Here is his advice:

"For high-density tag reads, it really becomes an issue of tweaking the Gen 2 reader settings. The protocol offers several 'knobs' that you can adjust. One is called the inventory 'session,' where you can choose between session 0, 1, 2 or 3.

"The session defines the duration of time and conditions that an inventoried tag remains in either of two 'states,' A or B. Think of A as being the state in which a tag has not been inventoried, and B as the state in which the tag has been inventoried. Of course, you could mask tags and place all tags not of interest in state B as well, but in general, let's assume the tag's default state is A. So as you go through the inventory process, reading one tag at a time, you would instruct the reader to switch the tag's state to B once the read has been confirmed. Since tags in the B state have presumably already been accounted for, you would then only focus on those in the A state.

"Which session you choose to use plays an important role in reading many tags over a short period of time, because the session defines the duration and condition for which that tag remains in the B state. For large tag populations, you don't want to waste time counting tags more than once, so you want to retain the B state for as long as possible, and focus only on tags in the A state.

"In session 0, the tag persists in the B state only as long as it is within a continuous RF field. So when the reader power is removed, the tags in the B state revert back to the A state and are candidates to be read again. This state is good for reading a small number of tags quickly and repeatedly, but it is not good for dense tag populations. Keep in mind that a tag loses power when antennas are multiplexed (that is, when you shut down one antenna and power up another), or when the reader cycles through its frequencies (e.g., every 400 microseconds).

"In session 1, the tag persists in the B state for a limited duration (persistence), even after being removed from the RF field. The persistence in the B state is defined between a minimum of 500 microseconds and a maximum of 5 seconds. Persistence may vary from IC manufacturer to manufacturer, IC model to model, and IC to IC, but should fall within these boundaries. When the persistence time elapses, the tag reverts to the A state.

"In sessions 2 and 3, the tag persists in the B state for least 2 seconds after being removed from the RF field. The sessions are similar in duration, but operate independently. So, if I were to set up a system for large tag populations and wanted to minimize duplicate reads (they only need to be read once), I would select session 2 or 3.

"I would additionally multiplex the system to alleviate nulls, firing in a push-pull manner (e.g., one antenna on one side, and another on the other side). I would probably position the antenna at the mid-point of the box, and back it up enough to ensure coverage from top to bottom (assuming a 60-degree beam width)—or, one at 1 foot and another at 3 feet, and on the other side, one at 2 feet and one above, so as to cover all tag orientations. Rotating the box—on a stretch wrapper, for example—would also be helpful in ensuring that all tags are read, because the tags can be exposed to the reader in a variety of orientations. These are just a few things that come to mind which might help."

I hope the above suggestions will help you read all 1,000 tags. Please let me know how it works out.

—Mark Roberti, Founder and Editor,
RFID Journal

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