Understanding RFID Part 3: Passive Tags
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discussed a classification of RFID tag called the active tag. Even
though the active tag has many applications, it does not get as much
press as its sister, the passive tag. It isn't hard to understand why
this is the case. Passive tags are the darling of the retail industry
and the U.S. Department of Defense (DoD) because they are cheaper than
active tags and disposable. Passive tags cost less because they do not
require a battery to operate and are generally less expensive to
manufacture. The most inexpensive passive tag is used in electronic
article surveillance (EAS) systems. EAS tags are found in books,
attached to clothes, and sealed inside DVD and compact disc cases,
among many other applications. These types of tags only transmit an "I
am here" signal when they are activated. They do not have the unique
identifier that is usually associated with RFID technology but they do
discourage would be shoplifters.
The EPC specification classifies passive tags as Classes 0 through 3
because they communicate via a method called backscattering. EPC Class
3 tags do have an onboard battery, but the battery is not used for
active transmission. The battery is only used to power the tag's
circuitry and any onboard sensors or peripherals. Class 3 tags are
also known as semi-passive tags. The vast majority of passive tags on
the market today are the second generation EPC Class 1 tags.
Basics of passive tagging
Passive tags do not require a battery to operate because they can
extract energy from the electro-magnetic radiation with which they
come in contact. In 1948, Harry Stockman discovered that radio waves
can be used to power a remote transmitter. This is the principle of
operation for passive tags. Passive tag manufacturers design their
tags to be efficient energy collectors. This requires the
manufacturers to become very creative with the antenna designs that
they attach to the tag's central processing circuitry. The antenna is
the key component in the physics behind how energy is collected from
the electro-magnetic radiation generated by the antenna connected to a
reader. This interaction between the tag and the reader's antenna is
also known as coupling. Our next article will delve into the details
of different antenna types and configurations. For now, it is
important to know that the antenna is a key component in the function
of the passive tag.
Most passive tags use a method called backscattering to communicate
with the reader. Backscattering refers to the way in which the tag
communicates with the reader through a carrier wave that originated
from the reader's antenna. The tag "tugs" on the carrier wave to
create minute fluctuations in the wave's amplitude. The amplitude
changes are used to encode digital information to transmit to the
reader. The reader must be able to detect these tiny changes while, at
the same time, provide enough energy to power the tag. Backscattering
works much like a transformer does except that it occurs in free
space. The reader and tag play the parts of two coils in the
transformer. As the tag shunts the coil through a transistor, the
reader's side will detect a tiny drop in voltage. The tag simply
shunts the coil to encode the data that is to be transmitted.
Passive tags can be purchased in many different form factors. The
naked tag is called the inlay consisting of the integrated circuit and
antenna only. Usually, the inlay is deposited onto a substrate using a
chemical process (but, not electroplated). The substrate can be paper,
polystyrene, or some other material. Most "slap and ship" RFID tags
are nothing more than the inlay in a paper envelope. The paper
provides some protection and is ideal for tags that have a short life
expectancy. The paper is usually printed with a barcode that contains
the RFID tag's unique identifier so that the item can be identified
with an RFID reader, barcode scanner, or by simply reading the number
printed below the barcode. If a more durable passive tag is required,
the tag may be encased in rubber or enclosed in plastic.
Passive RFID tags come in a wide range of sizes. Some tags' dimensions
may be measured in millimeters while others could be measured in feet.
The selection of antenna affects the size of the tag. Larger antennas
allow tags to be read at a greater distance.
Once the passive tag is powered and the coupling between the tag and
reader has occurred, the transponder, the tag, and the reader, can now
have a conversation as long as the tag stays in close enough proximity
to the reader. The conversation that the two components have is known
as the air interface protocol. There are several organizations that
are in the process of standardizing RFID protocols. Some of the more
notable standards organizations are GS1/GS1 US and the International
Organization for Standardization (ISO). Many of these organizations
work closely with each other so their standards are compatible at some
level. In fact, it is not uncommon for these organizations' members to
actively participate in more than one standards organization. Some tag
standards of interest are the ISO 18000 series of air interface
standards. The ISO 18000 series also includes amendments that ratify
other standards such as EPC Class 1 Generation 2 (ISO 18000-6C) air
interface protocol.
Key commands for passive tags
No matter which standard the tag implements there are some basic
commands that all passive tags must implement to be effective. Readers
must be able to control the tag population in an organized manner. If
all tags transmitted at the same time and without any order, the
reader may never be able to receive a single uninterrupted
transmission from a tag when the tag population is high. Tags are
usually selected by the reader and given directions on when they are
allowed to transmit. The EPC standard refers to a Q value which
represents a seed number used to select subsets of the tag's
population for interrogation. Other protocols have other methods to
reach the same end.
Once a tag has been selected, the reader must be able to read the data
from the tag. If the tag has onboard memory, the reader can request
that the tag transmit a certain number of bytes from a given address
in the tag's memory. Some tags do not have extra memory, so they only
transmit their unique identifier. Some protocols have access
restrictions for different address regions in the tag. To access these
restricted regions, the reader must authenticate with the tag.
Authentication may be associated with only certain commands, or may be
required of them all.
Some tags allow readers to write to the tag's onboard memory. If the
tag supports writing, the reader transmits the starting memory
address, the number of bytes to write, and the data to write beginning
with the starting memory address.
The 'kill' command can be issued by the reader to stop the tag from
ever transmitting again. This command usually has multiple parts
because the tag manufacturers want to make sure that the request to
kill the tag is deliberate. These are the basic types of commands that
can be found in most standardized air interface protocols.
Interrogating tags in real world environments
The deployment of a solution using passive tags has some interesting
considerations. As mentioned in our first article, passive tag
antennas are usually set up in a portal type configuration. In this
configuration reader antennas are placed on each side of the path
through which a tag may travel. In loading dock scenarios, the
antennas may be placed above and/or below the path in addition to the
sides. This configuration ensures that no matter which way the tag is
oriented, there will most likely be an antenna that can power it. In
some cases, tags at the very center of a palette or container of goods
may not be able to receive enough energy from the antennas in any
configuration.
Misreads may require changes in how and when the RFID tags are
interrogated. Most RFID enabled systems with these types of issues
require the tag to be read at multiple locations and in different
configurations. For example, an attempt may be made to read all the
tags in a container when the container is taken off of the truck, and
a second attempt is made when the container is unpacked. If there are
boxes in the container, a third attempt may be made when the items are
removed from each box. When these interrogation attempts are correctly
integrated into the inventory process, it is possible to obtain almost
100% read rates using passive tag technology.
Assembly lines have an added advantage because they can control the
orientation of the tag and placement of the tag on the goods being
tracked. Real time location systems (RTLSs) place portals at each
entry and exit way into a zone that is to be tracked. As tags move in
and out of these portals, the system can assume the tag's current
physical location.
Passive tag prices have not yet reached the US$0.05 per tag goal set
by the retail industry, but there is still a great return on
investment based on today's prices. We may not be able to tag every
can of soup, but container and pallet tagging is a valid goal with
real monetary rewards.
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