How RFID Works
SageData is based in Ottawa, Ontario, Canada
RFID - Radio Frequency Identification. This technology works by using a radio signal to communicate with a small tag, and, by extension, the item to which that tag is attached. Originally developed by the military to distinguish between friendly and hostile aircraft, it is now commonly used to refer to the tags used to identify a wide variety of items. These are commonly small items, goods in transit, fixed assets or file folders, but use can extend up to larger items, vehicles and agricultural or construction machinery.
Active or Passive RFID Tags
The distinction is the source of the power for the signal transmitted by the tag. Passive tags have no internal energy source. They collect energy from the reader, and remain dormant when not energized by the reader. Active RFID tags have an internal battery. They transmit continuously to a passive reader, which picks up and processes the transmitted signal.
Passive RFID Tags
Passive tags have an antennae or receiving coil, which collects and stores energy
transmitted by the
reader.
The tag then emits a short burst of energy, encoded with the critical
information it is communicating. Passive tags only transmit when they have been energized by
a
reader.
Passive tags tend to be cheaper and smaller, and their life is only limited by the
reliability
of
their
components.
They can be expected to function for decades. The range at which the tags
can be read depends on their frequency of operation, and the power of the reader. Range can
vary
from less than
a centimeter to several meters.
The ability to read a tag is affected greatly by the reader. Readers can be fixed, or
integrated with
a portable handheld (mobile) computer.
Antennae types
The two different formats of antenna result in different reading capabilities.
The linear antenna can give a longer read range, but requires a specific alignment for the
tag
to
be read. If the reader is held on its side
, then range is dramatically diminished.
For a
circular
antenna,
the orientation of the reader with respect to the tag does not affect read range.
As an example, a typical mobile circular antennae may have a read range of six feet,
regardless
of
orientation,
compared to a similar linear reader, which might have a read range of twelve
feet when properly aligned, but only twelve inches when either the tag or the reader is
rotated.
Frequency Range - passive tags
Tags can today be broken down by frequency into three categories. Tags for each of
these
three
frequency ranges were developed sequentially, so RFID tag technology has moved from LF to HF
to
UHF.
Each
has
specific
benefits and drawbacks, and choosing the best
tag depends very much on the
application.
LF RFID Tags
Low Frequency tags operate in the range 125 KHz. These were the first tags to be developed, and are frequently used in animal tracking. Their physical design makes them thicker, and more expensive to manufacture. Range is of the order of an inch or so. Format can be cylindrical - think of one inch cut from the end of a drinking straw, or bobbin shaped - think of several large coins in a pile.
HF RFID Tags
High Frequency RFID tags operate in the range 10-15 MHz. These are frequently used
for
access
controls.
You may see them at your local gas pump. These tags are often manufactured using the same
techniques that were
developed
for printing wiring boards. These
tags are thinner, but typically occupy a similar surface area to a credit card, though
smaller
button versions
are available.
Read range is typically up to twelve inches. Applications include shelf readers - which can
give
you
a real time
view of
your inventory. HF tags are also available as encapsulated buttons,
which can be sewn into the hem of a garment. These buttons can survive the rigors of a
laundry,
and
are often
used to
track uniforms.
UHF RFID Tags
Ultra High Frequency tags operate in two bands, around 900 MHz and 2.4 GHz. These
are
the
most
recently
developed tags, and are used in unit tracking and the supply chain applications.
At these frequencies, the wavelength of the transmitted signal is much shorter, so the tags
use
a
simple
antennae. This makes the
design much simpler, though the antennae still needs to be long enough to give a reasonable
read
reliability.
Often, also, the tags are encapsulated, and may be artificially thick, to
enable the tag to be mounted on a metal surface. In this case the actual tag mechanism is
stood
off
from the
metal
by the thickness of the tag.
Read ranges can be up to twenty feet of more, but the tags are more affected by metal or
liquid.
Placing your hand over a tag can reduce its readability.
These tags often have the ability to store information, typically to identify a specific
item by
using codes
to identify company, model, and serial number.
Applications include supply chain and asset management.
Writing to passive RFID tags
Most LF tags are used as read only items. They are manufactured with a fixed
identity,
and the reading system uses that ID, either directly, for data collection, or as a key to
information stored in a database.
HF and UHF tags can generally be written to. The volume of data changes from tag to tag.
Write
range
is
typically half or less the read range.
Points to ponder when writing to an RFID tag
First, and most important, are you sure you have to? Just because you can
is
not
a
good
reason.
In most cases, data is best stored in the central database.
Second, and perhaps most important, you have to be sure you know which tag you are writing
to.
Without careful
thought,
you can end up writing to the wrong tag. Contact SageData for more information on writing to
tags.
Active RFID Tags
Active tags get their power from a small internal battery. They can be read at greater distances, but they tend to be slightly larger and more expensive. The life of an active tag is limited by the life of the battery, which generally cannot be replaced. The life of an active tag is of the order of several years, though this can be modified as the tag is set up, with speed of response being traded for battery life.
Active Tag transmissions
Active tags are often transmit only. From the time of manufacture, until the battery expires, these tags regularly transmit a small message which uniquely identifies them. They do this whether in range of a reader or not. A typical cycle would be once per second. There is a trade off between how ofter the transmission occurs and battery life, but even with one burst of transmission per second, several years of battery life can be expected.
Writing to Active RFID Tags
Most basic active RFID tags are read only. Most do not even have a receiving circuit. They have an identity which is set during manufacture, and that cannot be changed. More advanced RFID tags have many additional functions, including read / write capability and data storage.
Follow these links for more information on
- active RFID tags
- RFID myths
- RFID limitations
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