System Components
In a basic RFID system, four fundamental components are required:
- A transponder (tag) that is programmed with information that uniquely identifies itself, thus the concept of "automatic identification".
- A transceiver (reader) to handle radio communication through the antennas and pass tag information to the outside world.
- An antenna attached to the reader to communicate with transponders.
- A reader interface layer, or middleware, which compresses thousands of tag signals into a single identification and also acts as a conduit between the RFID hardware elements to the client's application software systems, such as inventory, accounts receivable, shipping, logistics, and so on
Figure below shows an overview of how a passive RFID system works:
1. The tag is activated when it passes through a radio frequency field, which has
been generated by an antenna attached to a reader.
2. The tag sends out a programmed response.
3. The antenna that generated the field originally and is attached to the reader
detects that response.
4. The transceiver (or reader) sends the data to the middleware.
5. The middleware sends the information contained in the tags to whatever
systems need that information.
TAGS
Tag Components
An RFID tag comprises a microchip mounted on a substrate with an attached antenna:
Passive Tag Components
DATA STORAGE
There are two basic types of chips available on RFID tags, Read-Only and Read-Write. Read only chips are programmed with unique information stored on them during the chip manufacturing process. The information on read-only chips can never be changed.
With Read-Write chips, the user can add information to the tag or write over existing
information when the tag is within range of the reader. Read-Write chips are more
expensive that Read Only chips.
Another method used is called a WORM chip (Write Once Read Many). It can be written once and then becomes "Read Only" afterwards. Chips can also vary widely in the data storage capacity of the chip.
POWER SOURCE
A tag can harvest power from the signal received from the reader, or it can have its
own power source (a battery). Depending on the source of the power, tags are
classified as passive, active or semi-passive tags.
ACTIVE TAGS
Active tags have a power source that is used to run the microchip’s circuitry and to
broadcast a signal to the reader. This allows the active tags to be read from large distances and they are also able to respond to lower-level signals compared to passive tags. Some of the more powerful active tags can communicate up to 1 kilometer. Active tags can also support larger memory and processing functions. For example, they can carry sensors (such as humidity, temperature, motion…) and communicate with each other as well as with the readers.
PASSIVE TAGS
Passive tags have no internal power source. They draw their power from the
electromagnetic field created by the signal from the reader. Because they rely on this
energy for both power and communication, they are restricted in their read/write range (up to 5 meters). They have a smaller memory capacity and are considerably lower in cost (0.50 € or less) making them ideal for tracking lower cost items.
SEMI-PASSIVE TAGS
Semi-passive tags use a battery to run the microchip’s circuitry but communicate by
drawing power from the reader. This design increases its read range and due to their
power source, semi-passive tags can utilize a larger memory capacity and include
processing capabilities.
Table showing summary of different tag types
FREQUENCIES
Four frequencies are used in tag design: LF (125-133 KHz), HF(13.56 MHz), UHF(862- 870 MHz) and SHF(2.5 Ghz/5.8 GHz).
RFID waves behave differently at each of these frequencies, which means the different frequencies are suitable for different applications.
Low-frequency tags are ideal for applications where the tag needs to be read through
material or water at close range. As you increase the frequency of radio waves they
start to behave more like light. They can't penetrate materials as well and tend to
bounce off many objects. Waves in the UHF band are also absorbed by water. The big challenge facing companies using UHF systems is being able to read RFID tags on
cases in the centre of a pallet, or on materials made of metal or water.
RFID frequencies
MATERIAL TO BE TAGGED
Some common materials affect performance of the tag attached to them:
- Water and liquids: Absorb RF signals. Material with high moisture also absorbs RF signals.
- Metals reflect radio frequency. However this can be overcome by providing
small air gap between the tag and the material.
- Metals in very close proximity of tag detune it so that it cannot be read.
- Carbon fibers absorb RF signal
The friendliest materials are cardboard, clothe and plastic.
READERS
A reader uses its antenna to send digital information encoded in amplitude or pulse modulated waveform. A receiver circuit on the tag is able to detect the modulated
fields, decode the information, and use its own antenna to send a weaker signal
response.
Readers are available as handheld devices, mounted (forklift or cart) or fixed. Typical
configurations consist of portals, arrays and tunnels.
Reader examples (from left to right: hand-held, fixed, tunnel and portal)
The capability of a reader to communicate successfully with a tag is heavily dependent
on two factors:
- Dwell time: The time a tag is in the reader’s field.
- Read range: The distance between the reader and the tag at which the signals from the tag can be read properly.
INTERROGATION ZONE CONSIDERATIONS
Special considerations should be addressed when setting up an RFID system with
multiple readers that have overlapping interrogation zones.
DENSE READER MODE
Dense reader mode, provides each reader the capability to operate at a slightly
different frequency, which helps reduce the radio interference between readers.
LISTEN BEFORE TALK
Using this technique, a reader tries to "listen or hear" whether another reader is using a channel. If it learns that another reader operates on that channel, it rolls to another channel to avoid interfering with the other reader.
FREQUENCY HOPPING
Reader signals hop between channels within a certain frequency spectrum. In Europe, they can hop between 15 channels from 865 MHz to 868 MHz, and they can be required to listen for a signal before using a channel.
READER SYNCHRONIZATION
In certain applications that require multiple readers operating at the same time and in the same proximity, it is necessary to coordinate their transmitting and receiving
functions. The radio transmissions from the reader's antennas may interfere with other readers, so much so that the tags are unable to completely understand the information being read or written and the reader may misread the tag.
The main synchronization methods used:
- Software synchronization: when multiple readers are connected to the same communication bus, the controlling (host) computer is able to command each reader to transmit at a separate time so that it is not possible for more than one reader to be transmitting at the same time.
- Multiplexing: In the multiplexing method, a single reader is connected through a switching box (MUX) to multiple antennas. The reader output is directed to each antenna in turn, again ensuring that only one antenna is transmitting at a time.
- Shielding: prevents interference between readers. It also prevents tags that are passing outside the interrogating system from being interrogated by an adjacent system, and when antennas are close together, shielding prevents the same tag from being interrogated by an adjacent antenna.
COLLISIONS AND ANTICOLLISION METHODS
When two or more tags respond simultaneously, this is known as a collision. Anti-collision processing is the means by which the reader distinguishes one tag from the others so only one tag is processed at a time.
Some algorithms used, are ALOHA, FM0.
ANTENNAS
RFID antennas are connected to a reader. Typical parameters are:
MONOSTATIC/BI-STATIC
Depending on the design they can be mono-static (a single antenna transmits as well
as receives) or bi-static (one antenna transmits and other one receives).
POLARIZATION
If the reader’s antenna and the tag antenna do not have the same polarization, a
severe loss in signal is experienced. Antenna’s polarization can be linear or circular.
A linear polarized antenna radiates wholly in one plane containing the direction of
propagation. Can be also either horizontally polarized (propagate parallel to the earth) or vertically polarized (propagate perpendicular to the earth). Linear polarized
antennas must be used when the tag’s orientation can be assured.
Left: Linear vertical polarization. Right: circular polarization
In a circular polarized antenna, the plane of polarization rotates in a circle making one complete revolution during one period of the wave. Circular polarized antennas can receive signals from both horizontal and vertical planes but there is a loss of signal. Circular polarized antennas must be used when the tag’s orientation cannot be assured.
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