RFID based inventory management was once the most preferred mode of keeping stock numbers up to date, but in current times even these systems are proving to be inefficient due to the sheer size of warehouses and the inventory they store.
Addressing this issue is a new study by researchers at MIT who have developed a drone-based system that enables small, safe, aerial drones to read RFID tags from tens of meters away while identifying the tags’ locations with an average error of about 19 centimeters. Researchers are of the opinion their system could be used in large warehouses for continuous monitoring, to prevent inventory mismatches, and to locate individual items, so that employees can rapidly and reliably meet customer requests.
While the solution is great, developing it was a major problem in itself because industrial drones haven’t attained a size yet that would make them safe to fly near humans while carrying equipment needed for inventory management. Smaller drones used for recreational purposes – one with plastic rotors – that are safe to fly in vicinity of humans aren’t capable of carrying RFID readers thereby making them not ideal for this purpose.
Researchers solved this by using the drones to relay signals emitted by a standard RFID reader. This not only solves the safety problem but also means that drones could be deployed in conjunction with existing RFID inventory systems, without the need for new tags, readers, or reader software.
The Phase Shift Problem
Another major problem that comes up with the RFID relay solution is the one which involves transmission. Because the RFID tag is powered wirelessly by the reader, the reader and the tag transmit simultaneously at the same frequency. A relay system adds another pair of simultaneous transmissions: two between the relay and the tag and two between the relay and the reader. That’s four simultaneous transmissions at the same frequency, all interfering with each other.
If this is not it, the whole problem is compounded by the requirement that the system determine the location of the RFID tag. The location-detection — or “localization” — system uses a variation on a device called an antenna array. If several antennas are clustered together, a signal broadcast toward them at an angle will reach each antenna at a slightly different time. That means that the signals detected by the antennas will be slightly out of phase: The troughs and crests of their electromagnetic waves won’t coincide perfectly. From those phase differences, software can deduce the angle of transmission and thus the location of the transmitter.
The drone is too small to carry an array of antennas, but it is continuously moving, so readings it takes at different times are also taken at different locations, simulating the multiple antenna elements of an array.
Ordinarily, to combat interference, the drone would digitally decode the transmission it receives from the tag and re-encode it for transmission to the reader. But in this case, the delays imposed by the decoding-encoding process would change the signals’ relative phases, making it impossible to accurately gauge location.
All radio systems encode information by modulating a base transmission frequency, usually by shifting it slightly up and down. But because an RFID tag has no independent power source, its modulations are detectably smaller than those of the reader. So the MIT researchers devised an analog filter that would subtract the base transmission frequency from the signals that reach the reader and then separate the low-frequency and high-frequency components. The low-frequency component — the signal from the tag — is then added back onto the base frequency.