RFID is a means of storing and retrieving data through radio signals. Instead of a scanner that optically reads a code, a device sends and receives radio signals from a small, reusable tag attached to the assembly itself or to the pallet or tote holding the assembly.
The added ability to add and subtract information from identification tags became important to assemblers of cars, computers, appliances and other high-value, high-mix products. An RFID tag traveling with a car chassis can tell an assembler which parts to install--just like a bar code. But, unlike other identification technologies, the assembler can also input information back to the tag. For example, he/she can report that certain parts are installed and successfully tested.
With this background in perspective, let us talk about system components that we defined yesterday.
A typical RFID system implementation will need an antenna, a reader, a controller interface and a number of tags.
Each RF tag (transponder or data carrier, ) consists of a solid-state memory chip, a substrate or circuit board, and an antenna, all of which is encapsulated in epoxy and plastic. The memory chip, usually EEPROM, can be programmed to hold ASCII, hex or decimal characters. The chip can have "read only," "write-once, read-many," or "read-write" memory.
The size, shape and cost attributes of the tag depends on how much memory it has, how far it can send and receive data, and how it will be used. Tags can be shaped like a watch battery, a flat disk, a thin cylinder, a cracker, a credit card or a cigarette pack. A tag the size and shape of a US quarter (suit button) can store 128 bytes of information. A credit card sized tag can store 8 kilobytes.
Unlike printed codes, RFID tags can be reused and they can withstand harsh manufacturing environments. Depending on the tag and its environment, each memory address can be overwritten hundreds of thousands of times. Some tags are unaffected by acids, detergents and other chemicals, and some tags can operate in temperatures of -40 degrees to 350 degrees Fahrenheit.
Tags cost vary from $1 for "off the shelf" ones to $200 each for highly customized ones.
Tags are either passive or active. Passive tags derive their electrical power from the radio waves generated by the reader and antenna. Active tags are powered by a battery. Both tags have their pros and cons and are used depending on the application. Passive tags are less expensive and are long lasting. On other hand, active tags have more memory and a longer radio range than passive tags.
The antenna (interrogator or read-write head) is a coil of copper wire that emits a radio signal at a certain frequency. It constantly broadcasts the signal and waits for a tag's reply. Like RFID tags, the antenna comes in various shapes. It can be a flat, rectangular box or a small cylinder similar to a proximity sensor. Antenna can be mounted above or alongside a conveyor, or it can be a handheld wand.
Either way, the antenna's maximum transmission distance is directly proportional to its size and signal frequency. A large antenna broadcasting a high-frequency signal will have a longer range than a small antenna broadcasting a low-frequency signal.
When first introduced in the 1980s, the first RFID systems were low-frequency devices that broadcast at 125 to 150 kilohertz. The maximum range of these systems is approximately 2 feet for passive tags and 10 feet for active tags.
Medium-frequency systems emerged in the 1990s. These systems broadcast at a standard frequency of 13.56 megahertz. The advantage of this frequency is that tags need fewer coil windings and thus are less expensive. The broadcast range is also slightly larger.
Some recent RFID systems broadcast at 915 megahertz, or the microwave spectrum. These systems have an even longer range than 13.56-megahertz systems. However, because liquids can interfere with microwave signals, this system is not suitable for all applications.
High-frequency systems that broadcast at gigahertz frequencies are available, but are rarely needed for manufacturing. With a broadcast range of several hundred feet, these systems are used in automatic toll collection systems.
For assembly line applications, the distance between the antenna and the tag is usually less than 18 inches and rarely exceeds 6 feet. Tags do not have to be directly in sight of the antenna, but they must be within the antenna's broadcast range. To avoid interference, large metal objects should not be placed between the antenna and the tag.
The reader powers the antenna. It receives tag data from the antenna, then filters, boosts and transmits the data to the controller interface. The interface translates the signals from the reader into a computer language and transfers that information to a PC or a programmable logic controller. A reader can control one or two antennas, and one interface can control several readers. Some RFID suppliers offer the reader and the interface as a single unit.