1 Introduction
1.1 Brief History of RFID
The concept of Radio Frequency Identification ( RFID) has been around since the development of Radiocommunications and RADAR in the 1930s and 1940s, and early applications of the idea are to be found in the Identification Friend or Foe ( IFF) transponder systems developed in the UK and used in the Second World for aircraft identification. The transponder fitted to the aircraft was interrogated by a radio signal and transmitted a reply identifying the aircraft as friendly.
A patent for a rewriteable, passive RFID transponder was submitted by Mario Cardullo in 1973 and throughout the 1970's and 1980's, the theory and development of RFID systems was refined through research by companies and institutions, for example, the Los Alamos Scientific Laboratory and the Microwave Institute Foundation in Sweden amongst others.
Electronic technology also advanced significantly during this period, particularly in cost, which made the widespread application RFID a commercial reality.
Early applications of RFID systems were as simple security devices, e.g. in libraries, to prevent theft of books. This is known as Electronic Article Surveillance ( EAS) where the presence of a tag is detected without necessarily any transfer of data. Governments and large organisations were quick to realise the huge potential of RFID and it entered widespread use in toll collecting systems and in product (including animal) tracking.
Today, RFID systems are ubiquitous wherever large numbers of items are required to be tracked, inventoried or controlled, from supermarkets and stores, worldwide distribution networks to governmental departments.
1.2 What is RFID?
In general terms, RFID refers to a system whereby a transponder ( e.g. tag or label) can be interrogated using a radio frequency signal by a reader and respond with data which identifies the tag.
Figure 1 Reader interrogating tag
1.3 How does it work - a basic guide
Transponders come in three variations; passive, semi passive and active. The frequency of operation also varies with the type of application as detailed in the next few sections.
1.3.1 Passive
A passive tag, Figure 2, has no internal power source; a built-in aerial uses the radio frequency signal from the reader to generate enough voltage to power the internal circuitry and transmit the stored data back to the reader to decode.
Figure 2 Flat RFID tag showing antenna coil (Generic type)
These tags are used where low cost is an important consideration. Tags can be produced on a thin, flexible material in a similar way to production of a printed circuit board. This makes them ideally suited to self-adhesive label applications, e.g. packaging, books, boxes, etc. They are not normally reusable.
For agricultural use, RFID ear tags, Figure 3, are commonly used. The coil and chip is contained within a sealed plastic housing making it robust enough to withstand adverse conditions. As well as the data stored in the tag, the housing also carries the code in print, as a visible 'backup' should the tag fail. They are easily visible and can be recovered at slaughter, if required.
Figure 3 Round ear tag typical of livestock applications1
Another method for agricultural use is the bolus, Figure 4. The RFID transponder is sealed in a dense, non-toxic casing, usually ceramic, and the bolus inserted into the animal via its mouth using an applicator. The bolus lodges in the animal's rumen and its weight ensures that it stays in place and not passed through the gut. A disadvantage in their use is that there is no visibility and the only way of testing for the presence of a tag is by checking with a reader.
Figure 4 Bolus tag for livestock applications2
For other animal applications, in particular pets and small animals, an implantable transponder, Figure 5, can be inserted under the skin using a hypodermic applicator. These are not normally used in farm animal applications because of the risk of them not being recovered and finding their way into meat products.
Figure 5 Implantable transponder (Generic Type)
1.3.2 Semi Passive
Similar to the passive tag, this variant contains a small internal battery to maintain power to the tag allowing it to respond more quickly to the reader.
1.3.3 Active
Active tags contain an internal power source. They can be used as beacon tags which broadcast the transponder information at pre-defined intervals or as response tags which reply to an interrogating signal.
In general, active tags are used in applications where data must be accessible at long range. They may also include sensors that can record biometric data for remote monitoring.
1.4 Passive (inductive) tag operation
In terms of cost and ease of use, passive tags are the preferred choice for livestock tagging. For animal tagging, both agricultural and domestic, the 134.2 kHz frequency band is the standard.
There are two standards that set out how the tags must operate;
- ISO 11784 Radio frequency identification of animals - Code structure
- ISO 11785 Radio frequency identification of animals - Technical concept
ISO 11784 defines the way the data is stored within the 64 bits available on this type of tag.
ISO 11875 more accurately defines the frequency of operation at 134.2 kHz and the technical concept of collecting the data from the tags in terms of activation frequency, half and full duplex operation, error correction, modulation, etc.
The ISO documents were drawn up to try to produce a common standard in the way tags store and transfer data so that any readers operating to the same standard are able to decode the information they contain.
At this frequency of operation, the electromagnetic waves are able to penetrate materials such as water and living tissue with minimal attenuation, making them suitable for use in animal tagging. There is a trade-off in the speed and volume of data that can be transferred and the range at which the systems can reliably operate. A typical read range for this type of system is approximately 1m although the actual range depends on the system design and power. Hand held readers require closer proximity, approximately 10-25cm, for accurate reading, whilst panel readers with a larger antenna and more power available have a greater range.
In passive tags, the tag power is generated inductively by the reader. At the frequency used, the distance between the reader and the tag is much less than the wavelength of the RF signal; at 135 kHz the wavelength is greater than 2000m. This is known as the 'near field' and the RF field can be considered as predominately magnetic due to the characteristics of the wave impedance.
Power is induced in the tag through an effect similar in operation to that of a transformer. The tag is tuned to the reader frequency to maximise energy transfer. A simple diagram of this method of powering tags is shown in Figure 6.
The coupling of the magnetic field into the tag coil is used to produce the required voltage for the tag circuit to operate. When the tag operates, the stored data is used to switch the tag load, and data is transferred to the reader by a process known as load modulation. This is the usual method of data transfer for inductively coupled (low frequency) tags.
Systems can operate in either half duplex or full duplex mode. Both modes transfer data in both directions (duplex), but in half duplex, data can only be sent in one direction at a time, whereas in full duplex data can be transferred in both directions simultaneously. This can be likened to the difference between a walkie-talkie and a telephone: in a walkie-talkie only one person can speak while the other listens, in a telephone both can communicate at the same time.
In solely full-duplex systems, the reader emits a continuous RF field at a constant frequency and the tag produces a modulation signal while energized by the reader field. Continuous reader operation allows tags to be activated at any time they enter the reader's field and to be decoded in the minimum possible time
In half duplex systems the reader emits a pulsed field to send energy to the tag, and the tag sends back its message in the interval between reader field pulses.
There are arguments in favour of both types of tag, in terms of read range and speed of operation, with manufacturers championing their own type. However, to comply with the ISO 11784 and 11785 standards, a reader must be able to read both these types.
Figure 6 Induced power in an RFID tag
1.5 Processing of data
The data received from the tag must be read and processed correctly for identification of the tag. The standard ISO 11784 defines that, so that all makes of tag complying with the standard will carry the same code structure. It has to be a requirement that each tag carries a unique ID traceable to the tagged animal.
The possibility of an incorrect reading can occur for various reasons;
i) Decoding errors in reader
ii) The range of the reader is limited and correct tag interrogation depends on the distance and orientation of the tag in the reader's field. For animal use, the animals can be guided through a race or narrow lane so that they are guaranteed to pass through the field at an acceptable distance.
iii) The reader field can be distorted or absorbed by local structures. This is less of a problem with the low frequency tags defined by the ISO standards.
iv) Speed of the tag passing through the field. For animal use and with large fixed panel antennae where the field volume can be large this is unlikely to be a problem.
v) Two tags in the field simultaneously. Manufacturers can include 'anti-collision' technology in the systems so that tags trying to communicate with the reader do not interfere with one another. In animal use, animals can also be restricted to pass in single file through the field.
vi) Electromagnetic Interference ( EMI) from other equipment disrupting the operation of the system.
vii) Tag failure or loss.
Apart from total loss or failure if the tag, the other points can be addressed by careful design and installation of the systems. The systems would also, of course have to recognise either bad or no data so that manual intervention can take place. RFID manufacturers have demonstrated systems whereby the gate will only open on a good read to let an animal through. On bad or no data, the animal is diverted to a side pen where manual recording can take place. Tags normally also carry a printed identification number for manual cross checking.
Another problem is duplication of data, especially if two readers are operating in the same area. In a single reader this is overcome by software 'recognising' the same tag but not recording a duplicate. If two readers are operating in the same area, they can be synchronised so that data is shared and duplicates recognised by each reader.
This data must be transferred to a computer running a suitable program which will correlate the data for transfer to a central database either directly, if connected via a LAN/ WAN or internet link, or for non-networked PCs at a later time when network access is available. Manufacturers of the tag readers normally supply this type of software with the systems.