RFID tags connect smart cars to smart highways

Radio frequency identification (RFID) tags will play a vital role in linking automobiles with smart highways. Traffic congestion has prompted experts to use electronics as a solution. A portion of the California State Route 91 will be equipped with an automated technology that will allow two-way communications between the vehicle and the highway. The highway control system transmits a query signal, which the vehicle-mounted RFID responds to by sending back the appropriate information.

Small RF identification (RFID) tags are part of the critical communication link between automobiles and electronically directed "smart" highways. Other uses for RFID are increasing rapidly.

On an average day, 225,000 vehicles travel on California State Route 91, connecting Orange and Riverside counties. Sometimes, traffic is so heavy a driver can spend two or three hours traveling 10 miles. And, within 15 years, according to estimates, traffic on SR-91 will increase by 50%.

Fortunately, highways are getting some help from electronics to deal with heavy traffic. A year from now, part of SR-91 will be a "smart" highway, maintaining automatic, two-way electronic communication with cars to help traffic flow more smoothly. Cars on SR-91 will be smart, too, thanks largely to a simple communication device called an RFID tag.

When the SR-91 control system queries them, the RFID tags act simply as transponders that send short, unique codes. As the heart of future smart highways, the tags' essential function, automatic vehicle identification (AVI), provides information that makes traffic flow more smoothly and quickly. The system can electronically collect prepaid tolls, for example, eliminating the need for motorists to stop or even slowdown at toll booths. Traffic will move more quickly, fuel economy will improve, and pollutants will decrease.

On SR-91, an RFID tag on a vehicle's dashboard will enable a motorist to travel in toll lanes, now under construction, that will be off-limits to the hordes of non-paying motorists. The transponders will communicate with the highway's control system via antennae on gantries over the roadway. A series of gantries at strategic locations will connect with the larger system via a fiber-optic backbone. MFS Network Technologies (Omaha, NE, (402) 342-2052) is designing the system.

Although RFID tags aren't new (see box, "The growing world of RFID," and Ref 1), the transponders on SR-91 will move commercial RFID technology into new areas. Until now, most RFID tags have operated over maximum ranges of only 2 to 3 ft, using frequencies around 125 kHz. The SR-91 transponders, however, must work over ranges of 20 ft or more. In addition, to work with fast-moving cars, they must receive and send information rapidly.

For longer ranges and higher transmission rates, RFID tags must operate at high frequencies. The SR-91 transponders, for example, which Texas Instruments' Registration and Information Systems (TIRIS) Division developed, use FM transmissions in a band from 902 to 928 MHz. The California Department of Transportation specified this band, largely because of plentiful and inexpensive components for use in 900-MHz consumer products.

Each SR-91 transponder is about the same length and width as--but thicker than--a 3.5-in. floppy disk. Unlike smaller, low-frequency tags, which rectify and store received RF energy to power their own transmissions, each SR-91 tag contains a long-life battery. The battery also powers 128 bits of SRAM, which contains the device ID and other information. Each tag costs about $30 to $35.

In the most basic operational step, an RFID transponder simply responds to a query from the SR-91 control system. The system asks for the transponder's identity, and the transponder replies by sending a code programmed in its memory. The system then searches its database for that identity and grants appropriate privileges--for example, tollway access to a paid-up toll customer.

A system can also write to an RFID tag's memory, although SR-91 won't initially use that capability. By modifying stored values--for example, by debiting a stored account balance--the system can perform tasks such as accepting electronic "tokens" without involving the system database. The process takes as little as 20 msec.

To work with moving vehicles, an RFID system must be fast. The transmission rate to and from SR-91 RFID transponders is 300 kbps, allowing a one-way, 128-bit transmission in less than 0.5 msec. System protocols consume additional time, however, and multiple read operations are sometimes necessary to guarantee correct operation. The specification for SR-91 requires 40 reads while a vehicle is in the antenna's footprint, or field, guaranteeing that the system will work with vehicles traveling as fast as 150 mph.

In a system as complex and dynamic as a smart highway, RFID must work reliably. It can neither allow vehicles to go undetected nor incorrectly identify vehicles. It must distinguish vehicles traveling close to each other and vehicles that are changing lanes. It must not cause significant EMI, but it must tolerate interference. And, finally, it must operate at low power levels. (Permissible power levels vary, according to requirements that regulating bodies of different countries set. Europe has plans for several smart highways.)

To achieve good noise immunity at a relatively low cost, the RFID tags for use on SR-91 use FSK transmission. Phase-shift-keying and spread-spectrum techniques offer better noise immunity, but at significantly higher prices. To guarantee data integrity, each transmission uses a 16-bit cyclic-redundancy check. Unless there is agreement in all steps of a cycle--a poll by the system, a response by the transponder, and an acknowledgement by the system--a "no-read" indication results.

Antenna orientation determines footprint and thus affects the system's ability to distinguish one vehicle among many. Instead of being oriented for maximum read range (50 to 100 ft), each SR-91 antenna has a footprint that covers only a short segment of a highway lane. An antenna on an 18-ft gantry typically has a 12- to 15-ft-wide footprint to cover a 12-ft lane. If the gantry is at a toll station, where cars may be in a line, the footprint typically is 15 to 20 ft long. On an open stretch of highway, the footprint may be as long as 25 ft.

Avoiding simultaneous responses from RFID tags on numerous vehicles requires some clever techniques. For example, after a successful cycle of poll, response, and acknowledge, an RFID tag turns off for 10 sec. While the tag is off the vehicle, it is usually identifying travels well out of the antenna's footprint; during that time, the tag does not respond to further polls.

Synchronizing antennae helps differentiate among cars in adjacent lanes. Antennae for different lanes take turns polling, and each operates at a slightly different frequency, usually 2 to 4 MHz away from the frequencies of nearby lanes. By observing the time and frequency of an RFID tag's response, the system can tell in which lane a car is traveling.

In tests, the SR-91 system has identified two vehicles traveling in separate lanes within 1 ft of each other. It can also identify two motorcycles traveling side by side in one lane, even when they are switching lanes. The SR-91 system correctly identifies 99.95% of the vehicles it encounters, according to TIRIS general manager Dave Slinger. For the few vehicles the system doesn't identify, a video system captures an image of the vehicle's license plate and invokes human intervention.

AVI with RFID not only collects tolls but also collects different toll amounts, according to the type of a vehicle and its use. RFID can also help correlate trucks' identities with their weights as they pass over weigh-in-motion systems embedded in highway pavement.

In time, smart highways will automatically direct traffic in response to accidents, construction, or inclement weather. The SR-91 project, in fact, includes programmable highway signs for that purpose. Although SR-91 is limited in scope, it establishes the critical link between highways and cars that have the beginnings of intelligence.

Reference

1. Legg, Gary, "Small rugged memories put data where it's needed," EDN, Aug 20, 1992, pg 99.

THE GROWING WORLD OF RFID

The use of battery-operated, high-frequency RFID tags on smart highways deviates from RFID's more typical applications. Most RFID tags are tiny and operate at relatively low frequencies, typically around 125 kHz. In addition, by rectifying received RF energy and storing it in a small capacitor, a low-frequency tag powers its own response transmissions without a battery. The combination of size and battery-free operation makes RFID useful just about anywhere.

Ford Motor Co, for example, prevents car theft by embedding a passive (no-battery) RFID tag in the ignition key of some of its European-made cars. When someone inserts a key in the ignition and turns it, an antenna in the steering column polls the RFID tag for its unique identity. Without the tag's response, the car's engine-control computer shuts down, and the car stops running. The system is virtually impossible to bypass, according to Ford.

In the near future, key-embedded RFID tags may even customize cars for drivers. For example, a husband and wife could have separate keys with RFID tags programmed with preferences for seat adjustment, climate control, and so forth. Simply turning on the ignition would provide the settings the driver normally wants. Ford is planning to use RFID in some of its American-made cars, but it's not saying when or what capabilities will be available.

A common use for RFID tags is in livestock and animal control. A device resembling a large hypodermic needle inserts a tag encased in a minuscule glass capsule (about 4x30 mm) under the skin of an animal's neck. The tag holds the animal's identity and perhaps additional information, such as vaccination records. To identify the animal and to store or retrieve information about it, someone need only "scan" the animal with a handheld device within a distance of about 3 ft.

In Europe, RFID tags assist in metered trash collection. When a garbage truck lifts a trash receptacle and weighs it, a device on the truck reads RFID tags on the trash receptacles. The RFID tags provide customer and trash-weight information, which the trash collector uses to bill the customer.

Mounted on security badges, RFID tags can control access to facilities. On a key fob, a tag can authorize fuel refilling for a fleet truck or taxi. Attached to assembly components in factories, tags can provide routing information and assembly documentation and instructions. Strapped to the wrist of a marathon runner, a tag can speed processing at the finish line.

Low-frequency RFID tags are also cheap. Because they don't require high-speed circuits or batteries, some sell for as little as $2 to $3 in large quantities.

The future also holds much in store for high-frequency RFID tags, however. In addition to tags already operating in the 900-MHz band, future tags will operate at 2 GHz or even higher. In the United States, several companies are lobbying the FCC for approval of tags that operate at 2.45 GHz. In Europe, plans are under way for 5.8-GHz tags.