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 RFID Primer: Where the WLAN Hits the RFID Fan By Alex Goldman and Ken Crawford of Tech Center
December 23, 2003
Tag complexitiesAll RFID tags are not the same! Tag capabilities and performance vary widely amongst the different brands and makers. Intermec currently makes dozens of different RFID tag designs based on the same IC chip, and that number should increase as new applications emerge. That's because tags are designed and tuned for particular target items, environments, and mounting materials, with each presenting different challenges to the tag designer. Each vendor's IC chip functions also vary greatly. Temperature ratings, radiation tolerance, arbitration (handling data collisions), simultaneous multiple tag reads and read rates, reading groups of tags (group select), reading only particular tags (item search), are vendor specific features and conditions. Since RFID is RF technology, many of the same principle issues apply. Any experienced RF user will know that each frequency requires a slightly different antenna shape. Tags mounted on metal require an "air gap" between the tag and the item to which the tag is attached. If the size of the air gap changes (for example, if the metal bends), the tag will be detuned. And, of course, there are interference issues, as with any RF application. Each tag antenna has a magnetic field. If its field overlaps with that of another tag, readings can be subjected to interference, and whole tags can be hidden. For example, a jeweler who wanted to tag every ring on a jewelry counter would find that the tags were too close together. RFID readers and interrogatorsTags transmit data to readers or interrogators. They can be fixed mounts, handheld, or PCMCIA cards (at this time, only Intermec makes a PCMCIA card reader). Each reader can have several antennas. For example, a fixed reader on a factory's conveyor belt might need only one antenna but a fixed reader on the same factory's massive doors might need several. Linear polarization is usually appropriate for applications such as conveyor belts where both antenna and tag will be aligned in a fixed manner. PCMCIA card readers can be used to enable RFID data collection in places where the other two options would not work. For example, Tech Center has used the card to enable data collection on computers mounted on forklifts (more on this later). For handheld readers, circular polarization is usually appropriate. People are accustomed to lining up infrared readers with barcode tags, but they can see the infrared whereas they cannot see RF. Since RF is invisible, its alignment cannot be eyeballed. Handheld interrogators typically have only one antenna, and can use linear or circular polarization. A handheld typically performs at 60 percent to 70 percent shorter ranges than a fixed mount, and writing to a tag can also reduce range by 70 percent. For example, the maximum read distance for a fixed antenna operating in the 915 MHz band is 20 feet. Its write distance is about 14 feet. A handheld using the same frequency band could read at about 15 feet and write at about 9 feet. Tech Center usually customizes the reader and antenna design for a particular application. This includes design consideration for connecting the RFID subsystem to the client's network and backend systems. Switches, hubs, network traffic, port redirection software, client and network security, etc. are all important considerations in a successful RFID subsystem design and implementation. Intermec has designed a handheld interrogator, the IP3, incorporating the company's Model 750 mobile computer based on the PocketPC OS, and a custom plastic handle containing a circular polarized antenna. It complements the Intermec. Sabre. 1555 linear polarized antenna that handles both RFID and infrared (for bar codes). These units provide Tech Center and other VARs a diverse set of tools for tethered and portable RFID system designs. No stranger to customization, Tech Center designed a tether less 2D Bar Code imager for a major automotive client by adding a radio/camera battery package to an Intermec 1470B imager. "The Blob" allows Datamatrix codes to be read on tires and aids efficiency by allowing worker mobility. For a warehouse application, Tech Center customized a forklift-mounted computer, turning it into what the company calls its "TC Smart-Lift." For OSHA safety considerations, the reader had to be inoperable while the forklift was in motion, so Tech Center designed patent-pending technology that blanks the screen whenever the forklift's transmission is not in Neutral. Smart-Lift antenna array designs for pallets and large cartons are also ready for supply chain readiness programs.
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