RF-ID Technology Overview
The core of any RF-ID system is the 'Tag' or 'Transponder' which can be
attached to or embedded within objects.
A RF-ID reader sends out a radio frequency wave to the 'Tag' and the 'Tag' broadcasts back its stored data to the reader. The system works basically as two seperate antennas, one on the 'Tag' and the other on the reader.
data collected from the 'Tag' can either be sent directly to a host computer
through standard interfaces, or it can be stored in a portable reader and
later uploaded to the computer for data processing.
The automatic reading and direct use of the 'Tag' data is often called 'automatic data capture' and with a RF-ID tag system, which works just as effectively in environments with excessive dirt, dust, moisture and poor visability, you can be assured that it overcomes the limitations of other automatic identification approaches.
How the Low Frequency Tag system worksWhen the transponder, which is battery free, is to be read, the reader sends out a 134.2KHz power pulse to the antenna lasting approximately 50ms. The magnetic field generated is 'collected' by the antenna in the transponder that is tuned to the same frequency. This received AC energy is rectified and stored on a small capacitor within the transponder. When the power pulse has finished the transponder immediately transmits back its data, using the energy stored within its capacitor as its power source.
In total 128 bits are transmitted (including error detection information) over a period of 20ms. This data is picked up by the receiving antenna and decoded by the reader unit. Once all the data has been transmitted the storage capacitor is discharged resetting the transponder to make it ready for the next read cycle. The period between transmission pulses is known as the 'sync time' and lasts between 20ms and 50ms depending on the system setup.
The transmission technique used between the transponder and the reader is Frequency Shift Keying (FSK) with transmissions between 134.2kHz and 124.2kHz. This approach has comparatively good resistance to noise while also being very cost effective to implement.
System PerformanceReading Distance - one of the key benefits of the TIRIS system is its superior reading range with low power consumption. The actual reading distance that can be acheived depends on many factors, transponder type, electromagnetic noise, transponder orientation, antenna type, and government regulations. In general a standard 32mm glass capsule can be read with a stationary reader and a gate antenna from a distance of upto 1 meter Larger transponders can acheive ranges upto 2 meters with handheld readers offering lower ranges upto 250mm.
Data Accuracy The TIRIS system uses a 16-bit Cyclic Redundancy Check algorithim (CRC-CCITT) which ensures that only 'valid' data is sent from the reader to its associated controller. To date there have been no cases of an incorrect identification number being read from a transponder.
Antenna Selection Of the two standard antenna types (ferrite rod and gate) the larger gate antennas give the best reading range. Although in some environments an overall better performance can be acheived with smaller antennas.
Each antenna has its own specific 'readout pattern' i.e. the electro magnetic field emanating from the transponder during its reply stage. The shape and size of this pattern depends on the specific readout antenna selected and a country's government regulations that define the amount of electromagnetic field strength that can be generated. Electromagnetic noise in the environment can also affect readout ranges.
Transponder Orientation The orientation of the transponder with respect to the antenna also impacts the reading range. For maximum range the orientation of the antenna with respect to the transponder must be optimised to acheive maximum coupling.
As you will see to the left diagram orientations in line with a ferrite antenna produce the longest read ranges from the 32mm glass transponders.
Whereas as you can see on the right diagram orientations at right angles to a gate antenna produce the longest read ranges from the 32mm glass transponders.
Reading Speed Many applications require that transponders attached to objects be read while travelling at specific speeds by a readout antenna. Since a standard stationary reader completes one read in approximately 120ms transponders must remain in the boundaries of a readout pattern for at least that amount of time. As a guide 32mm transponders can be read at typically 3m/s using the appropriate reader and antenna. With larger antennas designed for Automatic Vehicle Identification (AVI) TIRIS has perfomed successfully at read speeds of 65m/s (about 150mph, 240km/h)