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Managing environmental issues related to RFID

RFID's main benefit lies in the ability to recognize individual items from a-far. The business benefits related to the technology especially for apparel retailers are lucrative and offer what no technology has been able to do before.

Authors: Jessica Säilä, Toni Heijari and Sini Syrjälä

THE SCHOOL OF RFID INVESTIGATES:
WHITE PAPER: How to address environmental issues related to RFID

How many technologies can you name that can offer 4,5 months ROI as American Apparel (a US based vertical retailer) states? For the first time in approximately 30 years, the Auto ID industry is living a technology revolution as RFID changes the way businesses can run their stock-related processes. Many consider the transition to RFID technology as radical as the introduction of barcode technology throughout retail.

RFID technology spreads fast with a growing number of companies piloting also in the retail environment worldwide. As the technology spreads, one should bear in mind that it is based on radio and hence laws of physics apply. Thanks to the nature of radio one cannot always be 100 % sure about the area, which RFID readers are able to cover. From the radio point of view, wooden structures do not attenuate RF-signals, hence in practice walls made of wood and sometimes also from concrete, are "invisible" to RFID readers. Hence the signal will pass through walls. Let's use an example of a retail store in which all items are equipped with RFID tags: In retail stores the stock room is often located right behind the shop floor area. Let's imagine that an employee wants to count all items in the inventory and use RFID reader for it: as the shop employee walks close to the stock room walls, it is possible that the RFID reader also recognizes tags from the stock room and adds them to the stock count as well even if the person performing the count thinks (s)he is only counting on the shop floor. Additionally, when the store personnel is scanning in order to search for an item, they might see the item from the stock room to the shop floor and vice versa. This is not a problem, it is just an example of the environment and the purpose of this white paper is to discuss how one works with the technology in order to fix such situations.

In this white paper we introduce two different challenging scenarios for the RFID technology:

Scenario 1: Cross-reading from shop floor to stock room and vice versa
Scenario 2: RFID in fitting rooms / blocking the RF signal

SCENARIO 1: Cross-reading from shop floor to stock room and vice versa

Cross-reading is caused by the fact that wooden and concrete structures do not attenuate RF signals and hence the RFID interrogator sees no "walls". As a guideline one can state that all conductive materials like metals and liquids will block / attenuate RF signals. So for example mirrors and human body will block the propagation path of the reader's signal and may prevent a successful read. Unfortunately there is no exact way of predicting what an RFID reader can and cannot see when it comes to shop floor conditions, hence why the following tips will help when encountering cross reading.

Tips related to software

"LOCATION"

In case all of the stores have stock room, it is recommendable to add the information about the individual item's location to the business application. This will not only help with the cross-reading issues, but also make it easier for the staff to locate items at a later point. Many RFID pilot projects in the apparel industry have shown positive results when using such a feature. In practice this means that during goods in, all items are registered to a specific location. Usually these locations refer either to stock room or shop floor. When items have a specific location, the inventory (stock take) application can be written to only detect items registered to a specific location.

"EXPECTED LISTS / EXPECTED TAG CONTENT"

The more companies adopt RFID, the more likely is also the chance for cross-reading between stores, especially in shopping centers. One potential fix for such a case is to start the inventory count round by downloading an "expected list" to the mobile unit. In practice it means that the application on the mobile unit still queries all RFID tags in its field, but only the tags originally in the database will be calculated to the stock count. Once the inventory count is finalized, a list of unknown tags can be produced as well as a list of non-found tags. As many companies are interested of the "not-found" items, many RFID applications already create such listings. Using expected tag content is another method of realizing the same; this means that during inventory round, only tags with specific content will be included in the inventory count.

"RSSI VALUES"

RSSI gives an indication of the "closeness" of an individual tag. However, RSSI signals alone are somewhat unreliable as the RSSI signal might appear higher than the item's actual location would suggest (this due to reflections from metal for example), or lower than it should be in reality (due to tag being somehow hidden or signal being blocked for some reason for example). We do not recommend the use of RSSI values for preventing cross-reading, although it might be better than not using any feature at all. 

Changing the physical environment

The most reliable method of fixing cross-reading between the stock room and shop floor, or between different retail stores (in shopping malls for example) is changing the physical environment to be more RFID friendly. In practice this means blocking the RF signal from the stock room to shop floor (and vice versa). This can be done by covering the walls between the areas that need isolation from each other. This kind of shielding only needs to be applied on one side of the wall, meaning for instance that it only needs to be administered to the side of the stock room in a retail store, hence changes on the area the consumer sees are limited. The shielding can for instance be aluminum tapestry, aluminum foil or carbon paint (shielding paint). The influence of different shielding material chosen is further discussed while describing scenario 2.

SCENARIO 2: RFID IN FITTING ROOMS / BLOCKING THE RF SIGNAL

Field test by the School of RFID

During different RFID pilots and roll-outs with different apparel retailers Nordic ID has had a similar discussion several times: It would be interesting to collect data from fitting rooms and perhaps other special areas that consumers visit in the stores. In order to understand the store environment and the collection of such data, Nordic ID School of RFID decided to set up as realistic settings as possible in a laboratory, for a shop. In this case the shop refers to a fitting room as well as a cabinet for items in reserve. The fitting room was built from three pieces of chipboard; each sized 130x200 cm. Additionally the cabinet for reserve items was brought to the same, empty lab room. Regular t-shirts were tagged with three different tag types to understand a little bit the influence of the tag itself. The chosen tags are all recommended for apparel use, and the chosen tags were: UPM WEP (Impinj Monza 3 /4), UPM Belt (Impinj Monza 5) and QID smartPIN. Tags were attached both vertically and horizontally.

UNDERSTANDING RFID AND FITTING ROOMS

The reader antenna position and the RFID tag's orientation are important when deciding how to equip a fitting room with a UHF RFID system. Before anything, one should decide the main objective for RFID; at least the two following pop up in conversations all the time: 1) to be able to read tagged articles, which at some point have been taken to a fitting room (and register the time they were kept there) and 2) the ability to identify in which of the fitting rooms a specific item still is or has been. The first scenario is much easier to achieve as in that case all the items in any which fitting room can be registered. The School of RFID set out to test the second scenario to answer the most challenging issue.

WEB-Fitting -room -test

THE TEST IN THE FITTING ROOM

On top of everything already said about RFID physics, one more important point for discussion is the orientation of the tag relative to the reader antenna. From the read success point of view this in fact is quite sensitive. The picture below illustrates the tag's position in relation to the antenna. As a guideline, the antenna always recognizes a tag better, once it sees the "long side" of the tag. So for example on the left hand side in the picture, the vertically placed tag is easy to read.
 

Tag -position -pic


In order to fully understand different possibilities in fitting rooms, three different antenna positions were selected for the test. The aim was to find the best position as well as to consider the possibility of hiding the antenna from the consumer.

ANTENNA POSITION 1

The reader antenna was attached to top center of the test fitting room (see also picture). 

No metal materials were used at this point. The antenna was attached to Nordic ID Sampo S1 reader using a 3m RF cable (3dB attenuation).

Reader -and -tag -position -Top -center

In this position the reader antenna is easy to install, and it is also invisible to the consumer. As the reading field points to the floor instead of out of the fitting room, the cross-reading is in fact fairly minor. According to the test the vertically attached tags (short side towards reader antenna) were quite difficult to read and the reliability of the read increased a once when the tags were horizontally attached to the items. If you only want to monitor which articles are being taken to any of the fitting rooms, then the read accuracy of an antenna placed like this will be enough even if the tags were attached vertically. This is due to the fact that the item in reality is moving around in the fitting room as the customer tries it on or looks at it otherwise. If the aim is to monitor also items that are left to fitting rooms (for example the item is left back and hangs from a hook in the fitting room), a vertically placed tag would be difficult to read once antenna is positioned as in this example. The least sensitive tag to orientation in this test was the UPM WEP tag.

Antenna position 2

The reader antenna was attached to top corner of the fitting room and tilted about 45 degrees. If antenna is facing towards the entry, the reading field yields quite far outside of the fitting room. So it might be appropriate to aim the field towards the back wall of the fitting room.  


Again the RFID reader used was the Nordic ID Sampo S1.

The test crew felt that this antenna position was a compromise. The reader could read vertically oriented tags slightly better than on position 1, but at the same time the reading accuracy with horizontal tags decreased a little bit. This antenna configuration also had the same challenge as position 1: reading stationary tags did not present a reliable read. Compared to position1, the performance increased thanks to the tilted antenna configuration used.

Reader -and -tag -position -Top -corner -and -tilted

Position 3

The reader antenna was attached to the side of the fitting room. Both the side wall and the back wall were tested and the result showed little difference. To our understanding usually the back wall would be impossible to use for the antenna installation due to mirrors usually being placed there. Reading through mirrors would not be possible as it tends to block the signal due to materials used in it.


Reader -and -tag -position -Side

The team would suggest for example integrating the reader antenna inside the wooden side wall of the fitting room so that it is not visible to the customer. At this point one should bear in mind that any conductive material between the reader antenna and read zone will block at least some of the tag reading.

The read reliability and accuracy were the best when using this antenna position. One reason to explain this is the fact that the antenna itself is located closer to the tags. This configuration also reads stationary tags with high reliability. Hence we would recommend this antenna position for anyone wishing to read stationary (left in fitting room) items.

The downside of using this installation is that it would be harder to fit to existing fitting rooms, yet could be done. This one also has higher cross-reading rate than the earlier two cases. Should you choose to go with this installation, it is highly recommended to shield the RF signal. 

BLOCKING THE RF SIGNAL

Cross-reading proposes to cause you problems especially when you wish to monitor items in individual fitting rooms, making inventory on shop floor in stores where stock room is close and sometimes when monitoring stationary items left to fitting rooms. This chapter looks at ways to deal with a situation in which you wish to isolate areas. 

One way to address such a situation would be dropping the output power of the antenna. However, the read reliability drops as a cause of it as well. For example someone working with info kiosk would solve this by indicating to the consumer to bring the item closer to the reader and this would not really be an issue. However, this would not be suitable if you are trying to read a fitting room full of items. 

In order to solve cross-reading some kind of attenuation should be added. Any conductive  material could be used. UHF RFID doesn't need much attenuation to block misreads. A 10dB attenuation will almost completely block the cross-reading. 

The School of RFID team tested few materials that could be used: 

• Aluminum foil -  Attenuation @ UHF Frequency: ~20dB
• Aluminum paper (attached like tapestry) - Attenuation @ UHF Frequency: ~20dB
• Carbon paint (YSHIELD HSF54) - Attenuation @ UHF Frequency: ~17dB

The test was conducted by first testing with no blocking material, then repeating the test with all the different materials. The results are listed next to the material name.


As a conclusion, any of the materials is suitable for the blocking of the RF signal: it is rather a question of which one is the easiest to realize. Additionally one should consider that it may not be wise to shield the whole fitting room (due to blocking away for example mobile phone signal), but rather it would be wise to choose just two walls, or parts of the walls. In most cases this would be enough to get the read reliability discussed in this paper.

 

 

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