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What is RFID?
The Radio Frequency Identification (RFID) technology was developed during World War II as a tool to identify friend and foes. It has come a long way in the last 50 years and is now making inroads into everyday life. RFID is the use of radio frequencies to read information on a small device known as a tag. RFID can be usually read electronically even when obscured or disoriented. The object of any RFID system is to carry data in suitable transponders, generally known as tags, and to retrieve data, by machine-readable means, at a suitable time and place to satisfy particular application needs.
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From a functional point of view, an RFID system consists of the following four components:
Tags: into which identification data can be embedded. These are devices that identify the item to which they are attached. RFID tags are also called transponders.
Readers: are devices that communicate wirelessly to the tags.
Writers: are used to write data to the tags. They come as separate units or inbuilt with the readers.
Middleware reads/writes data to/from tags through the reader. The middleware (in a workstation or pc) initiates all communications between the reader and the tags on one hand while effectively communicating with various ERPs on the other.
RFID Readers
An RFID reader accomplishes two tasks:
It receives commands from the middleware
It communicates with tags
Readers may be handheld or mounted on a particular object. In the case of handheld readers, the workstation, the reader and the antenna are all part of one device. Data exchanged with tags may be stored and transferred to a main processing unit at a later stage depending on the application.
An RFID reader is practically a bridge between the middleware and the antenna that radiates radio waves towards the tags. The radio waves emitted by the antenna propagate in the surrounding space. As a result, data travels wirelessly towards the tags that are in the vicinity of the antenna.
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How does a reader work?
Readers are either inductively coupled or use propagation coupling to communicate with the tags. They both do the same thing but use different methods to generate the required radio waves.
In practice, readers that use propagation coupling can generally read over longer distances. Both the reader and the tag, have antenna that are tuned to the same frequency to allow communications to take place.
The range that can be achieved by a reader is a function of:
Reader power (typically measured in watts)
The "form factor" of the transponder (the bigger the tag's antenna the better the read distance.)
Environmental factors : these vary according to frequency, for example at 13.56 MHz the presence of metal can detune both the tag and the antenna whilst at 2.45 GHz line of site is a pre-requisite.
In theory, if you provide enough power you can read a tag at any distance. In practice, too much power will cause certain materials (like humans for example) to heat up and possibly become damaged. For this reason, most countries set limits on frequency and maximum power that can be used. The UHF band has one major advantage over 13.56 MHz - under US regulations it can achieve reading distances of two or three meters, suitable for applications such as container identification. However reading distance is reduced to 0.7 meters under European regulations and in Japan, for example, UHF cannot be used at all.
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RFID Tags
RFID tags are devices made up of an electronic circuit and an antenna integrated all into one. The electronic circuit of an RFID tag has a memory where data may be stored. The memory is physically and logically divided into cells. Some of these cells store data that may be read only, such as unique serial numbers written at the production stage. Other cells of an RFID tag may be both written to and read repeatedly. RFID tags may be active or passive depending on whether they have an on-board power source or not.
How a Tag works
A schematic of a typical tag construction is shown below:
The antenna (shown in the diagram as a black line surrounding the tag) is used to communicate with a reader that transmits or receives data to and from the outside world.
Analog circuitry is required for the tag to communicate with the outside world and to provide power for it to do this. The digital circuitry (which is in reality a silicon chip) is the brains of the tag. This is similar to the chip in the PC and carries out a variety of processing functions. It is also responsible for managing data transfer both internally and with the outside world.
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Depending on the type of tag it may have three different types of memory:
EEPROM (Electrically Erasable Programmable Read Only Memory) this is where any changeable data is stored and isn't found in a read only tag.
ROM (Read Only Memory) this contains the operating system of the tag and other instructions necessary for its operation.
RAM (Random Access Memory) this stores the unique serial number of the tag and any security mechanisms such as passwords.
Typically a tag will possess the following features:
- Small, physically robust and chemically inert
- Long operating life (>100,000 read / write cycles)
- Resistant to extremes of temperature and immersion in water / chemicals
- Read / Write operations unaffected by non-metallic materials. May be read at distances from a few centimeters up to several meters
- Memory capabilities up to 2048 bits of information
Unique identification number (normally based on time of manufacture)
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Active tags reduce the power requirements of the reader and can transmit information over relatively far ranges. They possess a battery that can last generally from two to seven years. The downsides of such tags are their cost and their size, which reflects their complexity. Generally speaking, the more functions an RFID tag can perform, the more complex and bigger it will be.
Passive tags are less complex than active tags, because the reader provides them with their operating power. They are small, light, and in-expensive and can last up to 20 years. Their range of transmission is relatively short and RFID systems with passive tags require a much higher-powered set of readers.
Frequency
An important factor to be taken into account is the operating frequency of an RFID system. This is the frequency used to enable the reader to communicate wirelessly with the tags. There are several available frequency bands for the deployment of an RFID system. Generally speaking, RFID systems may be classified as low and high-frequency systems. The choice of operating frequency affects the reading distance, interference with other radio systems, communication data speed, and antenna size. Low frequency systems typically use passive tags whereas higher frequency systems operate with active tags.
As a rule of thumb, at lower frequencies, passive tags are not capable of transmitting their data further than a few feet, due to power limitations. However, wireless communications between the reader and the tags may occur through a non-line of sight path, i.e. traveling along a wide range of materials, with some exceptions.
At higher frequencies the reading distance between the active tags equipped with internal batteries and the reader generally increases, although not dramatically because of power limitations imposed by international bodies that govern the frequency spectrum allocation. Electromagnetic signals at higher frequencies also suffer more signal attenuation when tags are covered with ice or water. In the worst-case scenario, a tagged object covered by a closed metal shield is invisible to any reader outside the shield.
If electrical interference is high then readers must be placed well apart one from each other and noise sources can degrade the communication of an RFID reader with tags. If data rate is low the amount of data that can be transferred between the reader and tags over a fixed time interval may not be satisfactory for specific applications.
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Tag Mounting
Typical attachment methods include:
- Rivets, screws, keys, self-adhesive, cable-tie, heat-shrink cover, sewing, bonding, etc.
- Tough waterproof encapsulated in PVC-nitrile and/or silicone rubber
- Can be read through any non-metallic material - plastics, grease, dirt
- Can be mounted inside plastic enclosures
- Appropriate tags can be mounted directly on metals
Tags come in a wide variety of physical shapes, sizes and can be further processed in a variety of protective housings (i.e. converted into paper labels and injected in plastic molds). Most commonly used shapes are:
- Cylindrical or screwed type
- Plastic ear studs (for animals)
- Circular pill
- Paper thin bare tags (for integration into paper brochures, cupboard boxes or library books)
- ISO card (similar to credit cards), with and without a magnetic stripe
- Polystyrene and epoxy discs, with diameters from several mm to several cm
- Injection molded plastic buttons/casings with reinforce metal frames (for mounting on freight containers, gas cylinders or beer kegs
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