WHAT IS IT?
RFID, or radio-frequency identification, is a short-range wireless technology. It goes beyond bar coding to mark, identify, and track everything from products to people.
RFID uses electronic tags or smart labels to electronically store a unique identifying number instead of a printed bar code. Each tag consists of a single chip with an EEPROM containing the ID number and a radio transceiver or transponder. A popular tag format is a chip mounted on a plastic sheet that contains an antenna in the form of a small loop (Fig. 1).
Unlike bar codes, readers don't need a line-of-sight (LOS) scan. Furthermore, reading can be automated because orientation of the labeled device isn't an issue. Reading or scan time thus becomes much faster. In addition, automatic reading cuts costs and human reading errors, especially in supply-chain applications. It's also possible to write to an RFID tag and change its code or content. Another huge time-saving benefit is the ability to scan or read multiple items concurrently.
The other major piece of the RFID system is the reader. Also called an interrogator, this higher-power transceiver (usually 1 W or so) uses a larger antenna to interrogate the tag. Passive tags receive the RF signal from the reader. Then they rectify and filter the signal into a dc that powers up the transponder circuits, which in turn transmit the tag's ID code back to the reader. Working with a computer, the reader recognizes and/or records the tag info and takes the appropriate action, depending upon the application. Most readers are fixed in place, but handheld readers are available, too.
To read a passive tag, the distance between the reader and the tag must be relatively close (Fig. 2). A typical range is several inches. But this varies with the frequency of operation as well as the antenna size, and it can range from a few centimeters to about 20 ft. This short range is within what wireless engineers call the near field. The near-field range extends up to about one wavelength (λ) from the transmitting antenna, where λ = 300/fMHz in meters. The far field is beyond one wavelength.
Most wireless systems use the far field, in which the radio wave breaks away from the antenna and becomes a self-supporting electromagnetic wave. The electric and magnetic fields regenerate one another along the way according to Maxwell's equations. The near field is essentially the magnetic field produced by the reader's antenna.
RFID uses transformer theory in the near field. The interrogator antenna is the primary winding of the transformer, while the tag antenna is the secondary. Signal strength at the tag is proportional to the cube of the distance (r) from the reader, or 1/r3. The larger the tag antenna, the larger the signal and the greater the range. Typical tag antenna sizes range from less than an inch to 4 or 5 in.
RFID uses the Federal Communication Commission's unlicensed industrial-scientific-medical (ISM) frequency assignments. The most popular frequencies are 125 kHz, 13.56 MHz, 915 MHz, and 2.4 GHz. The original tags used the lower frequencies, but the most popular today is 13.56 MHz. The UHF frequency of 915 MHz is growing in use because it provides a longer reading range and is the key operating frequency of the new Gen 2 standard.
Backscatter modulation, the most common modulation scheme, is a form of amplitude shift keying (ASK). When the reader is turned on, the tag powers up and begins to transmit its ID data. The binary pulses modify the impedance of the tag's antenna, which in turn causes an amplitude shift in the reader signal. The process loads and unloads the secondary winding to reflect an impedance back into the primary. The result is an AM wave with a very low percentage of modulation. The actual amplitude shift on a 100-V p-p carrier may only be several hundred millivolts peak-to-peak or less, depending on the range. This signal is peak-detected and reshaped into a serial data signal. Manchester coding typically is used to make clock recovery reliable.
High-speed data rates aren't a major requirement in most applications. Speeds from a few kilobits per second to several hundred bits per second are typical. The higher the operating frequency, the higher the potential data rate. When reading physically close multiple tags, higher speed is necessary.
RFID tags use EEPROM memory. The manufacturer usually programs it, but users can buy programmers to program the data themselves. The smallest memories are only 64 or 96 bits, though memories up to several kilobits are available. Greater memory sizes consume more power. They also provide less for transmission back to the reader and have shorter reading distances. Readers can reprogram some tags as well. Write distance is usually about one half the read distance or less.
Newer tags feature anticollision resolution, as multiple tags powered up simultaneously interfere with one another. Many available schemes prevent such collisions. One uses a time-division multiplexed arrangement, assigning each tag a time slot in which to transmit. Others use an access scheme, where a tag waits a random time before transmitting. Anticollision is a must in applications where multiple items will appear within the reader's antenna field. Otherwise, you'll wind up with read errors or no usable data.
While passive tags are the most widely used, active tags also are available. Active tags have a battery to boost the transmit power back to the reader, so they have a longer read range. Such tags use thin button batteries, which make the tag bulkier and much more expensive. An active 433-MHz tag features a read range to 300 ft.
Finally, tag cost is a huge issue. Passive tags have dropped in cost over the years from a few dollars each to as low as 25 cents in large quantities. Lower prices are the key to wider usage.
Cost-wise, the holy grail is a 5-cent tag. At this stage, RFID tags can begin to replace the lowly printed bar code on a wider scale. Many in the industry wonder if that will ever happen.
Does RFID spell the demise of bar codes? No way, say most people in the industry. Bar-code labels always will be cheaper than electronic tags, and the need for them will never subside. Bar-code reading systems have been in place for decades, and virtually every product uses one. Even as RFID tags evolve to increase reading distance and lower price, there still will be items whose price better fits a printed code. Expect many years of coexistence between RFID and bar codes.