Omron Electronics Inc.
In an era where manufacturers want to know more about what happens in ever more detail to their products on the production line, some bar codes just don’t hold enough data. They’re also tough to read when they’re dirty. Scanners that get smudged easily don’t help the situation.
Enter radio-frequency identification (RFID). Instead of using visible light and labels, antennas communicate with transponder tags via radio waves. Besides tolerating dirt, RFID offers something bar codes can’t—two-way communication. Optical scanners are only good for reading a bar code; RFID transceivers can change data in the solid-state memory that some tags carry. And, they can communicate with several tags simultaneously. The ability to both read and write data is probably RFID’s most powerful feature. It allows companies to more easily implement flexible manufacturing schemes. It can also enhance or simplify certain control systems.
At Panasonic’s Ohio-based TV tube plant, for example, engineers boosted production by installing an RFID system. Devised by Omron Electronics Inc., Schaumburg, Ill., the Intelligent Flag system tracks product pallets as they move through automated test stations. It also tells conveyor routing controls which pallets hold good tubes and which are rejects.
The RFID setup replaced an older one which employed banks of eight photoelectric sensors at each station. The sensor arrays would read hole patterns in pallet-mounted metal plates (mechanical flag). The readings would tell conveyors where to send each pallet. But the hardware and control logic weren’t robust. If even one sensor failed or was misaligned, the system would route tubes back for unnecessary repeat tests. “These were good tubes—we were wasting valuable production time,” says Raul Villarreal, manager of equipment engineering.
Replacing the sensor arrays and special plates are antennas and off-the-shelf tags. Now, an antenna beams a confirmation signal to the pallet tag when a tube passes a test. The tag transfers the information to an onboard memory chip. Subsequent stations check tag memory for codes indicating whether the tube has passed previous tests. The RFID system has proved to be less sensitive to misalignment and more reliable than its mechanical counterpart. Moreover, it is far less complicated.
The long and the short of RFID
RFID systems are categorized by their transmission range or distance over which transceivers and tags (transponders) can communicate. Two factors that determine range are the transmission method and the operating frequency.
Long-range, high-frequency RFID systems communicate by what is referred to as wave propagation. An oscillator circuit delivers time-varying electromagnetic energy to an antenna. From the antenna, the energy radiates (propagates) outward. Tags within transmission range can intercept the signals much like a radio receiver. From basic physics, field power diminishes with the inverse square of distance. In practice, however, textbook performance may not be realized. Obstructions in and around the field path can cause power to trail off more rapidly, sometimes as the inverse fourth power of distance. Besides obstructions, the media through which the radio waves travel can affect range. Most long-range systems transmit at or near microwave frequencies (gigahertz). Water vapor or liquid water attenuates microwaves which can impact system performance. Under ideal conditions, however, some propagation-type systems can reach several hundred feet.
In contrast, short-range RFID systems generally transmit one foot or less. Instead of wave propagation, they use what is called electromagnetic inductive coupling. This mode of communication requires that tags be within the magnetic field of the transceiver’s antenna. The term antenna, usually reserved for high-frequency systems, is loosely applied here. Inductive-coupling antennas are basically loops of wire or copper plating. Transmission range is proportional to antenna size because bigger loops can generate stronger magnetic fields. Tags that contain larger antennas more effectively collect magnetic energy so they can operate at greater distances. Unlike long-range systems that use microwaves, short-range systems operate at frequencies measured in hundreds of kilohertz. The upside to lower frequencies is they aren’t affected by water and can more easily penetrate solid objects.
Yet another factor that determines range is whether tags are passive or active. Passive tags contain no onboard power source. They obtain all their operating power from the antenna field generated by the transceiver. Electromagnetic energy collected by the tag antenna is sent to a rectifier circuit which converts the ac signal to dc voltage. Active tags, by comparison, carry batteries for power. With more power available, active tags can generate stronger return signals, thereby increasing range. Still, because of their relatively low power, tags are usually what limit transmission range.
Both long and short-range RFID systems transmit and receive data by modulating a carrier signal. Modulating involves varying the carrier frequency, phase, or amplitude in concert with a data-carrying bit stream. Simply put, information is superimposed on carrier waves. One common modulating technique is called amplitude shift keying (ASK). ASK represents digital ones and zeros as analog signals with two specific amplitudes.
A technique often used in high-frequency systems is called backscattering. At the heart of these systems are special tags that contain an antenna whose impedance can be switched to one of two values. At one impedance level, the tag antenna absorbs most of the signal coming from the transceiver. At the other, it reflects a portion of the energy back (backscattering). Turning the antenna switch on and off is an oscillator circuit. This circuit gets its feedback from a data bit stream. Encoded in this stream is the information to be sent to the transceiver. So when the transceiver signals the tag, the backscattered energy returning to the transceiver is the tag talking back.
To improve performance, companies may employ several different modulation techniques. Omron, for instance, boosts noise immunity in its low-frequency, V600 series by combining ASK with two other modulation schemes. Noise can be also be reduced by raising the carrier frequency. This is one reason makers of high-end cordless phones upped communications to 900 MHz. Electronic noise can be further quelled by spreading the carrier out over a greater range of frequencies. One way to do this is called frequency-hopped spread spectrum (FHSS).
Most electronic noise sources tend to be narrowband. In other words, power is centered about one particular frequency. Systems using FHSS combat narrowband noise by rapidly changing their carrier frequency to one of several randomly-selected values within a certain bandwidth. If one of the signals gets jammed by noise, there are others at different frequencies that will get through. Signal strength is not compromised because total power equals the area under the spectral density curve. Signals with the same total power may either have a large signal concentrated in a narrow band or a small signal spread over a larger bandwidth.