3. Active tags are battery-powered devices that have an active transmitter onboard. Unlike passive tags, these generate RF energy and apply it to the antenna. This autonomy from the reader means that they can communicate from distances of over several kilometres.
HF and UHF are best suited to the supply chain. UHF, due to its superior read range, will become the dominant frequency. LF and microwave will not be used in certain cases.
RFID tag ICs are designed and manufactured using some of the most advanced and smallest-geometry silicon processes available. The result is impressive, when you consider that the size of a UHF tag chip is around 0.3 mm2.
In terms of computational power, RFID tags are quite dumb, containing only basic logic and state machines capable of decoding simple instructions. This does not mean that they are simple to design. In fact, very real challenges exist such as achieving very low power consumption, managing noisy RF signals and keeping within strict emission regulations.
Other important circuits allow the chip to transfer power from the reader signal field, and convert it via a rectifier into a supply voltage. The chip clock is also normally extracted from the reader signal.
The amount of data stored on a tag depends on the chip specifications, and can range from just simple identifier numbers of around 96 bits to more information about the product containing up to 32 kbits. However, greater data capacity and storage (memory size) leads to larger chip sizes and hence more expensive tags.
In 1999, the AUTO-ID Center (now EPC Global) based at the Massachusetts Institute of Technology in the US, together with a number of leading companies, developed the idea of a unique electronic identifier code called the electronic product code (EPC). The EPC is similar in concept to the universal product code used in barcodes today.
Having just a simple code of up to 256 bits would lead to smaller chip size and hence lower tag costs, which is recognised as the key factor for widespread adoption of RFID in the supply chain. Tags that store just an ID number are often called licence plate tags.
One of the main ways of categorising RFID tags is by their capability to read and write data. This leads to the following four classes:
Class 0 (read-only, factory-programmed). These are the simplest type of tags, where the data, which is usually a simple ID number (EPC), is written only once into the tag during manufacture. The memory is then disabled from any further updates. Class 0 is also used to define a category of tags called electronic article surveillance or anti-theft devices, which have no ID and announce their presence only when passing through an antenna field.
Class 1 (write-once read-only, factory- or user-programmed). In this case, the tag is manufactured with no data written into the memory. Data can then either be written by the tag manufacturer or by the user one time. Following this no further writes are allowed and the tag can only be read. Tags of this type usually act as simple identifiers.
Class 2 (read-write). These are the most flexible type of tags, where users have access to read and write data into the tag’s memory. They are typically used as data loggers and therefore contain larger memory space than what is needed for just a simple ID number.
Class 3 (read-write with on-board sensors). These tags contain on-board sensors for recording parameters like temperature, pressure and motion by writing into the tag’s memory. As sensor readings must be taken in the absence of a reader, the tags are either semi-passive or active.
Class 4 (read-write with integrated transmitters). These are like miniature radio devices which can communicate with other tags and devices without the presence of a reader. This means that they are completely active with their own battery power source.
Selecting a tag
Choosing the right tag for a particular RFID application is an important consideration, and should take into ac-count many of the factors listed below:
1. Size and form factor—where does the tag have to fit?
2. How close will the tags be to each other?
3. Durability—does the tag need to have a strong outer protection against regular wear and tear?
4. Is the tag reusable?
5. Resistance to harsh (corrosive, steamy, etc) environments
6. Polarisation—the tag’s orientation with respect to the reader field
7. Exposure to different temperature ranges
8. Communication distance
9. Influence of materials such as metals and liquids
10. Environment (electrical noise, other radio devices and equipment)
11. Operating frequency (LF, HF or UHF)
12. Supported communication standards and protocols (ISO, EPC)
13. Regional (US, European and Asian) regulations
14. Will the tag need to store more than just an ID number like an EPC?
15. Anti-collision—how many tags in the field must be detected at the same time and how quickly?
16. How fast will the tags move through the reader field?
17. Reader support—which reader products are able to read the tag?
18. Does the tag need to have security?
How tags communicate
In order to receive energy and communicate with a reader, passive tags use one of the two following methods shown in Fig. 6. These are near-field, which employs inductive coupling of the tag to the magnetic field circulating around the reader antenna (like a transformer), and far-field, which uses techniques similar to radar (backscatter reflection) by coupling with the electric field.
The near-field is generally used by RFID systems operating in the LF and HF bands, and the far field is used for longer-read-range UHF and microwave RFID systems. The theoretical boundary between the two fields depends on the frequency used, and is in fact directly proportional to l/2p, where ’l’ is wavelength. This gives, for example, around 3.5 metres for a HF system and 5 cm for UHF, both of which are further reduced when other factors are taken into account.
Tag orientation (polarisation)
How tags are placed with respect to the polarisation of the reader’s field can have a significant impact on the communication distance for both HF and UHF tags. This can result in a 50 per cent reduction of the operating range and, in the case of the tag being displaced by 90° (see Fig. 7), inability to read the tag.
The optimal orientation of HF tags is when the two antenna coils (reader and tag) are parallel to each other as shown in Fig. 7. UHF tags are even more sensitive to polarisation due to the directional nature of the dipole fields. The problem of polarisation can be overcome to a large extent by different techniques implemented either at the reader or tag as shown in Table IV.
Developments in RFID technology continue to yield larger memory capacities, wider reading ranges and faster processing. However, it is highly unlikely that the technology will ultimately replace barcode. Even with the inevitable reduction in raw materials coupled with economies of scale, the integrated circuit in an RF tag will never be as cost-effective as a barcode label. RFID, though, will continue to grow in its established niches where barcode or other optical technologies are ineffective, such as in the chemical container and livestock industries.