01 March 2005
RFID unites the supply chain
Radio frequency database tags are not yet in wide commercial use. Eventually they'll be prevalent because they solve a very difficult problem in the integration of data between companies on both sides of the supply chain.
Although they have been around for over 16 years, it has been only recently that radio frequency tags have emerged into a unique market. The first applications for RF tags were in the identification of animals where tiny tags about the size of a grain of rice resided under the skin of pigs.
The latest and most prominent use of RF tags is in highway toll collection applications where they operate as E-Z Pass, FastPass, or similar names. They allow vehicles to pass through toll collection stations without stopping to speed up the collection process on toll bridges and toll roads.
Not all RF tags are identical. They all have a common identification field, usually 64-128 bits in length, and different numerical value for each tag. They all must have a source of electrical power in order to respond when queried by a reader.
Different tag technologies work to hold the cost of the tag to the lowest possible levels for the intended applications. Finally, tags all have memory that is always readable, but may or may not be writeable.
RFID passive tags: The simplest tag is the radio frequency identification (RFID) passive tag, which supplies no more than its own identity, the 64-bit value, when read. The manufacturing process for the least expensive RFID tag etches a different number onto each tag that is unalterable.
Power for the RF transmission of the tag's ID data field comes from the reader that emits a low energy electromagnetic field (EMF) in order to power the tag. When the tag energizes by being in this EMF radiation, it repeatedly transmits its own identity field value. A recent variation on this protocol allows the transmission to occur only once after a fixed time delay unless it detects a query message generated by the reader. This allows many tags in the same field of the reader to register individually.
The value of the RFID passive tag is approximately the same as for a barcode; the tagged item has a unique identity. The data for the tagged item is usually located in a computer database, not on the tag. Simplicity of the tag keeps the cost at a minimum.
However, the fact that the RFID tag needs only to be located within the EMF field of the reader, not necessarily in a direct line-of-sight as required to read a barcode, enhances its functionality for item tracking over barcodes that need direct line-of-sight to the reader.
The RFID passive tag in the form of a credit card works now in automatic fare collection for public transportation systems. The card is similar to a magnetic stripe identity card, except it does not need to come into physical contact with the reader, only near the read station.
RFID active tags have an on-board battery enabling higher power transmissions to cover longer distances. Reading an active tag is usually a transaction in which the reader continuously polls to determine if any tags are within its reading range. Often, an electromagnetic field comes from the reader signaling that it is ready to read the active tag. When the tag receives the poll read-request, it returns its ID value. This pattern avoids wasting the active tag's battery life since the power required to receive is far less than the power required to transmit a signal. Because the reading range of an active tag is usually much larger than for a passive tag, there is a strong likelihood that more than one tag will be within reading range at the same time. A reading protocol usually exists to make sure that only one tag clocks in at a time.
The most common uses of active RFID tags are in automatic highway toll collection and in tracking of railroad boxcars. Some systems, such as EZ-Pass and FastPass, use active RFID tags that are readable at distances of up to 10 meters when located behind the windshield of an automobile.
In North America, all railroad freight cars carry an active RFID transponder as part of the Rail and Intermodal Asset Tracking System. As the rail cars pass in front of readers located at strategic rail switching yards scattered all over North America, their identity is read and reported to a common tracking system.
RFID programmable tags: "Programmable tags" is an old name but means only that the user can write the ID number on the tag. Most often, the technology used is the Electronically Erasable Programmable Read-Only Memory (EEPROM). Both passive and active programmable tags are available, although the use of these tags is diminishing and giving way to more connected databases using RFID tags or by more flexible RF Data tags.
RF data tags are readable, and one can write to them. Usually there is an ID field of the same 64-128-bit length as for both passive and programmable RFID tags, but there is extensive read/write memory located on the tag as well. In all cases, RF data tags are active with a long-life battery on-board. They generally have a read range equivalent to other active tags, up to 10 meters, depending upon the reader's antenna gain. Writing distance is less, typically to about 3 meters. The read/write memory is usually Flash RAM with capacity up to 256 Mb. Flash memory is usually organized into blocks similar to disk and is supported to maintain a file system similar to that used for disk drives. RF Data tags are similar to USB memory devices except for the wireless connection to an RF reader. Flash memory does not require battery power to retain data. Read and write speeds for RF Data tags with Flash RAM are comparable to that of USB memory, slower than disk drives.
It is also possible to produce a high performance RF Data tag using battery-powered CMOS RAM memory. Read/write times are similar to that of computer main memory; however radio speeds necessary to actually use this high speed memory are not attainable. Although it does not require much power to retain memory in CMOS RAM, any loss of battery power erases memory content. Therefore, there are no commercial RF Data tags using CMOS RAM.
Location tags or beacon tags are proprietary VHF radio devices that usually attach to pallets or containers stored in a large flat warehouse space. The tag generates an intermittent signal with the tag's ID value that broadcasts throughout the warehouse space. Usually, the tag is equipped with a motion sensor so the interval between broadcasts becomes shorter when the tagged item is in motion. Readers are logistically located at the corners of the warehouse space to receive the RF signals. When a reader receives the beacon from the tag, it also receives the strength of the signal, indicating the approximate distance between the tag and the reader. In order to map the two-dimensional location of the item in the warehouse space, the beacon signal identification and signal strength must simultaneously read into a second reader.
Nicholas Sheble edits the Networking & Communications department. This information comes from Dick Caro's forthcoming addition to his book Wireless Networks for Industrial Automation, ISA Press, 2004.
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