RFID Research
A substantial amount of research was done towards implementing an RFID localization system. The main advantage of this type of system is that there is no need for powered hardware on the vehicle - the RFID tags are completely passive. They simply need to be mounted in a known location relative to the secondary coil.
RFID systems involve three components: a reader, an antenna, and one or many tags. The reader drives a precisely controlled AC current through the antenna, which induces a current in nearby tags. The tags are designed to have an electrical resonance at the frequency supplied by the reader. This resonant current then induces another current in the antenna, which is detected by the reader.
There are three standard frequencies for RFID systems: low (LF), high (HF - 13.56MHz) and ultrahigh (UHF - 858 to 930 MHz). Higher frequencies generally have longer range, but are more susceptible to interference from nearby metals. HF systems can achieve ranges up to about 1 meter. UHF systems can achieve ranges up to about 10 meters. As methods of mitigating the interference of nearby metals become better, UHF is becoming more popular even for short-range applications, but currently HF is still the best choice for operating under a car.
Many RFID readers give the user access to a measurement of the received signal strength indicator (RSSI). This value is used by the reader to determine whether or not the signal from a tag is good enough to report a successful transmission to the user. The initial RFID strategy planned to estimate distances from antennae to tags based on the RSSI value. There were two problems with this approach:
The alternative to measuring distances from antenna(e) to tags is using only binary information, and moving the antenna(e). By recording the positions where tags were seen and not seen, and having some knowledge of the area over which an antenna would read a tag when they are both parallel to one another, information about the location of the tags can be extracted. Several raster-style strategies with multiple antennae were considered, but we chose the method described in Overview/Fine Alignment because it takes much less time to complete and it uses only one antenna. Using multiple antennae with one reader should be possible, but accounting for the additional impedance of the necessary multiplexer is non-trivial.
All of the RFID components described below were purchased from www.fastrfid.com. Their founder, Patrick, was very helpful in selecting these devices and developing this solution.
RFID systems involve three components: a reader, an antenna, and one or many tags. The reader drives a precisely controlled AC current through the antenna, which induces a current in nearby tags. The tags are designed to have an electrical resonance at the frequency supplied by the reader. This resonant current then induces another current in the antenna, which is detected by the reader.
There are three standard frequencies for RFID systems: low (LF), high (HF - 13.56MHz) and ultrahigh (UHF - 858 to 930 MHz). Higher frequencies generally have longer range, but are more susceptible to interference from nearby metals. HF systems can achieve ranges up to about 1 meter. UHF systems can achieve ranges up to about 10 meters. As methods of mitigating the interference of nearby metals become better, UHF is becoming more popular even for short-range applications, but currently HF is still the best choice for operating under a car.
Many RFID readers give the user access to a measurement of the received signal strength indicator (RSSI). This value is used by the reader to determine whether or not the signal from a tag is good enough to report a successful transmission to the user. The initial RFID strategy planned to estimate distances from antennae to tags based on the RSSI value. There were two problems with this approach:
- Although signal strength does generally decrease with distance, it is not simply a 1/r^2 fall off from the antenna as if it were a point source. The shape of the signal strength function around the antenna is quite complicated, and it also depends significantly on the relative orientation of the tag and antenna.
- Most readers that give users access to the RSSI value only report 3 bits of resolution. This would not be sufficient to obtain the accuracy we were aiming for.
The alternative to measuring distances from antenna(e) to tags is using only binary information, and moving the antenna(e). By recording the positions where tags were seen and not seen, and having some knowledge of the area over which an antenna would read a tag when they are both parallel to one another, information about the location of the tags can be extracted. Several raster-style strategies with multiple antennae were considered, but we chose the method described in Overview/Fine Alignment because it takes much less time to complete and it uses only one antenna. Using multiple antennae with one reader should be possible, but accounting for the additional impedance of the necessary multiplexer is non-trivial.
All of the RFID components described below were purchased from www.fastrfid.com. Their founder, Patrick, was very helpful in selecting these devices and developing this solution.
RFID Reader
The reader used was a Feig MR102A. It allowed UART serial communication (with an RS232 converter) with our microcontroller. The manual can be found below. In the manual you can learn how to change the configuration parameters of the reader, although the process is confusing and at times the language is ambiguous. However, once the reader settings have been set appropriately, the settings can be dumped to EEPROM and will be used as the defaults from that point forward. It may help to look at the RFID driver source code to understand the configuration process. We chose to use the device in scan mode, where it searches for and reports nearby tags at some specified rate. Although the manual implies this rate can be set much lower, it seems as though the fastest rate at which it can reliably perform this process is 3 Hz. This was the rate used, and it should be fast enough for the application if the antenna moves sufficiently slowly. We eventually ran into problems with overheating the reader, and had to put a fan inside the case to keep it working. It is possible that running the scan process at this high rate caused the temperature issue. We powered the reader with 24v to maximize read range. It could also be powered with 12v.
RFID Antenna and Tags
The antenna used was a FEIG ANT 340/240. It is compatible with the MR102A. Its dimensions are approximately 20cmx30cm. It has a read range of approximately one foot when the tag is held directly above. HF RFID antennae generally have a read range approximately equal to their diagonal, although the range also depends on the reader power and the orientation of the tag. We used Secura Key Contactless RFID Tags.
MR102A.pdf | |
File Size: | 3008 kb |
File Type: |