wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

8-6 Industrial Communication Systems
8.5 Proximity Card
Since a smart card draws current via direct contact, it is not contactless. With the unprecedented success
of cellular phones, the spoilt public wants that every electronic device be wireless. As the use of smart
cards for daily transactions is ever increasing, more and more people are annoyed by the need to look
for an appropriate one out of numerous cards in a wallet or a purse, pull it out, and insert it into a reader
for every business deal.
Addressing public demand, current loops are added in both reader and smart card, such that power
could be drawn from the reader by aligning the coils in series without physical contact, as if it were a
mutual inductor. Via energy stored in a capacitor, the built-in microprocessor of a smart card could
feed back signals through the mutual inductor. Unlike the 50.Hz power supply at home (60.Hz in North
America), the frequency used is 125.kHz for a stronger coupling. Due to the lack of space, the number of
coils in a smart card is very small; thus, the effectiveness of this inductive coupling is very weak and the
coil separation is correspondingly small, in millimeters. Reflecting that a contactless smart card must be
very close to a reader, it is called a proximity card. Other than the reduction in wear off, the millimeter
separation does not bring us significant changes because the card must be pulled out of our wallet for
every transaction. Having said that, the proximity card finds extensive applications in hotels, gasoline
stations, dry cleaners, and toys (Figure 8.6).
8.6 HF RFID
Based on fundamental electromagnetic theories, the coupling coefficient of a mutual inductor could
be enhanced via a frequency increase. However, it cannot be increased indefinitely because a higher
coupling is obtained at the expense of higher energy dissipation. In fact, the frequency cannot be chosen
arbitrarily as it must fall within one of the (industrial, scientific, and medical) ISM frequency bands [10],
the spectrum open for public use without license. Incidentally, the frequency adopted for enhancing the
performance of a proximity card is 13.56.MHz, one of the frequencies used in radio broadcast in the
high-frequency band. Thus, it is called a radio frequency identification card with the acronym HF RFID.
The name is actually attributable to Charles Walton who is perhaps the first person who filed a US patent
using a “radio frequency” and an “identifier” in 1980 [11].
Another gain obtained from using a higher frequency is size reduction because it makes placing a
ferrimagnetic disk inside the primary coil economically feasible. With the magnetic coupling, the range
of operation could be increased more than tenfold to 200.mm.
FIGURE 8.6
HF RFID pair for toys.
© 2011 by Taylor and Francis Group, LLC