Product Focus: Passive Radio Frequency Identification

By: Brian Reuter

Supply chain management relies on the ability to track parts, components, finished goods, and various other things and be able to locate those items at any time. The workhorse of this task has long been the bar code. Using an optical bar code scanner, goods coming into a warehouse are scanned by the bar code reader, transferred by radio frequency to the main computer, which then automatically updates the inventory without having to resort to paper documentation of every increase and decrease of the inventory. Now a new technology has been rolling onto the scene that can be used to replace or enhance the bar code, that of radio frequency identification (RFID).1

The features of radio frequency identification that make it attractive to applications in the supply chain are found in its inherent ruggedness, omnidirectional reading, data read/write capability and the fact that RF can pass through obstacles such as packaging, tote boxes, and pallets. RFID is an automatic identification and data capture technology that is truly coming of age. Technological advances and wider acceptance are bringing prices down and standards are being developed that will aid manufacturers in producing interchangeable systems and components.2

Some recent advances in technology have opened the door for RFID to many new applications, specifically with the arrival of passive RFID tags that are disposable. These are thin, flexible tags that can be laminated into standard baggage tags or package labels, and are rugged enough to endure the heat of being printed over by thermal printing. This allows tags to be incorporated directly into packaging or to be used as a label that is adhered to a package. Although called disposable, these tags are rugged, reprogrammable and do not require batteries. RFID tags can store much more information than a typical 1-D bar code. Passive tags may hold around 600 bits of user programmable memory and some battery-powered tags can hold up to 1MB of data.3 Some of the specific advantages that RFID systems have to offer are:

1) RFID systems transfer their data via radio waves and therefore are not affected by obstacles such as dust, dirt, containers, etc. Bar code technology transfers data optically and therefore requires direct line of sight.

2) Reading of the tags is not dependent on alignment. The tags just need to be in range, which can be from a few inches to 100 feet or more. Bar codes must be aligned properly or they may read incorrectly or not read at all.

3) RFID tags can hold much more information than 1-D bar codes and on many styles of tags, the information is rewritable. To rewrite information with bar codes, new labels must be made.

4) RFID is completely a non-contact technology – information is transmitted via radio waves.

5) Initial cost of RFID systems is more than that for bar code systems, but they pay for themselves over time. Minimum lifetime of tags is usually around 300,000 write cycles with unlimited read.

6) A wide variety of tag sizes, memory configurations, low and high frequency options and interface options make RFID flexible and easily customized to each application.

7) RFID systems are fast, communicating data from tags to readers in milliseconds.

8) They are virtually impossible to copy.

Technology Description

Basic radio frequency identification systems are made of up three components – an antenna or coil, a transceiver with a decoder and a transponder or tag. The antenna gives off radio signals that activate the tag and they are also used for reading and writing data to the tag. Antennas can be placed in a variety of places, such as a doorframe or a tollbooth, depending on the application. Often times the antenna, transceiver and decoder are packaged together as a single unit referred to as the reader or interrogator. The reader can emit radio signals a few inches to 100 feet or more based upon its power output and radio frequency. When the tag enters this electromagnetic field it becomes activated and transmits it data to the reader where it is decoded and forwarded to the host computer to be processed.

Transponders/Tags

Transponders come in a wide spectrum of sizes and shapes. The hard plastic tags that are attached to goods in stores and used as anti-theft devices are RFID tags. Small animal identification RFID tags are injected into the skin and are no bigger than the eraser in a 5 mm mechanical pencil. They can be credit card shaped for access control applications and even screw shaped for insertion into trees or wood objects.

The word transponder comes from TRANSmitter/resPONDER. The tag responds to an activation signal from the reader requesting its data and then transmits its data to the reader. Tags are basically low power integrated circuits (ICs) made to interface to external coils or made using "coil-on-chip" technology. Transponders can hold as little as a single bit of data for presence/absence applications such as electronic article surveillance (EAS), or as much as 128kB or even 1 MB. They are classified as either active or passive and can be read-only, read/write or write-once read-many.

Active transponders are powered by a miniature, on-board battery and are usually read/write devices. They use battery power to detect and send much stronger electromagnetic signals than passive transponders, giving them much greater range of up to 100 feet or more. Because they use batteries they have limited lifetimes, but when set-up correctly for a given application they can last ten years or longer. The basic trade-off for the added performance of larger memory capacity and transmission range is greater size, greater cost, and limited life.

Passive transponders do not have batteries so they derive their power from the electromagnetic field produced by the interrogator. When the transponder comes into the electromagnetic field of the interrogator, the antenna in the transponder gathers the energy from the field and converts it into electrical energy for the IC in the transponder. This energy is then used to transmit the memory of the transponder back to the interrogator. The interrogator then demodulates and verifies the signal from the transponder. It is then decoded and sent to the host computer. Because passive transducers do not use batteries, they have indefinite lifetimes and are much less expensive than active transponders. Again there is a tradeoff – passive transponders have limited range and data storage capacity, require high-powered readers, and are more easily affected by electromagnetically noisy environments.

Depending on the application, transponders bought in large quantities (tens of thousands) can range in price from tenths of a penny to tens of dollars. Things that directly affect the cost of transponders are complexity of circuit function, packaging, and memory.

Interrogators

Readers/interrogators are available in a wide range of complexity depending upon the tags they are used with and the functions they are to perform. Regardless, the main function of the interrogator is to communicate and exchange data with the tags. The signal transmitted by the tags is picked up by the antenna of the interrogator and converted to electrical signals. Sensitive receiving systems are incorporated into interrogators that communicate with passive transponders whose signals are very weak. From there interrogators can perform some very sophisticated functions such as signal conditioning and parity error checking and correction. Once the signal has been validated, the information is decoded, restructured, and forwarded to the host computer for processing. Although all these steps happen in an extremely short period of time, it is important to realize that it does take time. If a tag moves too quickly through a readers' zone, a successful read will not be obtained. The reader may have a long range, but due to the given configuration, the tag may not be in the read zone long enough. By changing the configuration, the tag can be made to travel towards the reader, keeping it in the read zone longer. Therefore, in designing an RFID system travel speed, read area and read range must all be considered.

Some of the most notable applications of RFID include:4

1) Electronic article surveillance – retail clothing outlets

2) Automatic Vehicle Identification (AVI) and Electronic Toll & Traffic Management (ETTM)

3) Automobile anti-theft systems – transponders mounted on ignition keys where engine will not start without receiving signal from transponder

4) Automated purchasing of gasoline where key ring transponders communicate with pumps to debit preapproved credit cars (loyalty cards)

5) Fleet maintenance – fuel dispensing, security, tire and equipment tracking

6) Livestock management

7) Manufacturing, processing and distribution – tracking of materials, inventory and final products from production through shipping

8) Security and access control

9) Time recording of sporting events

10) Waste collection and recycling – tags are used to track waste for automated customer billing

11) Time and attendance

12) Automatic identification of tools in CNC machines – condition monitoring of tools, managing tool usage and minimizing excessive tool wear

13) House arrest – electronic monitoring of offenders at their residence

Passive RFID Tags

Passive tags have been getting a lot of attention lately due to new developments that have created inexpensive, essentially disposable tags. Several manufacturers are promoting prices of under $1 and this price is expected to go as low as $0.25 for high volume orders. This pricing has generated new interest in RFID in applications where it has traditionally been cost prohibitive to use. Some of the new applications using disposable RFID tags include baggage handling, parcel tracking, pallet tracking and warehouse management.5

In some applications the tags are laminated into labels and have bar codes printed on them, essentially creating "smart" labels. This allows the tag to work in both bar code and RFID systems, which may help generate wider acceptance of the technology by allowing it to be used at various locations in a supply chain. These tags capitalize on RFIDs advantages, such as greater memory capacity, without alienating others in the supply chain that are not RFID enabled by allowing them to access necessary information from the bar code printed on the exterior.

Other types of tags and related products that have recently been introduced are creating a family of inexpensive and flexible systems that should allow this technology to be leveraged by many new markets.7

Sources
1 Cooke, James A. March 1, 1999. Tool Time. Logistics Management & Distribution Report.
2 Jones, Lorne. August 1, 1999. Working Without Wires Industrial Distribution. V88. I8. PM6.
3 Staff. July 24, 1998. Radio Frequency Identification (RFID; A Basic Primer. AIM White Paper. Automatic Identification Manufacturers International Inc.
4 Ibid.
5 Werb, Jay. March 1999. Seven Ways to Track Your Assetts. ID Systems.
6 Andrews, David. May 1999. RFID Vendors Get Smart(er). ID Systems.
7 Staff. May 1999. Wireless Smart Label Technology. Material Handling Engineering. V54. I5. P13.
8 Staff. May 1999. Report: Steady Growth Predicted for RFID. ID Systems.

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