The fact that water absorbs RF bothers me. Having entry and exit points designed in such a way to ensure single file movement, as well as multiple antennas, should hopefully allow for accurate readings. Would having multiple readers in close proximity to each other cause any problems? I assume that the system could determine who has entered and left the plant, based on the antennas that identified the tag. If the antennas were close together, could there be confusion or collisions of any sort? Also, what would you suggest we use to identify people who have gathered in our emergency assembly area? It's an open garden space with multiple entry points. I initially thought pole-mounted UHF antennas, angled like flood lights, would identify groups of people, but if RF energy were absorbed, then this method wouldn't be foolproof.
Passive RFID tags and UHF antennas could indeed track people entering and exiting a factory, but you would need to design the system in such a way as to guarantee that you could read all of the transponders. Passive UHF RFID systems operate from 868 MHz to 956 MHz. In this frequency range, water absorbs RF energy, and the human body is mostly composed of water. Therefore, if a person were standing between another worker's tag and the reader antenna, his or her tag would most likely not be read.
I have seen companies avoid this issue by placing tags on helmets worn by workers. If your employees do not wear helmets, you might need to set up the system so that they would walk past the reader antenna in single-file, in order to ensure that no tags were blocked by coworkers' tags. In addition, if a person were to carry another worker's badge with an RFID tag in it past the antenna, that tag would be read as well, and that employee would thus be marked present. It is possible to us video analytics or other technologies to ensure that the number of people passing the RFID portal equaled the number of tags interrogated; a good systems integrator should be able to set that up for you at a reasonable cost.
Antennas should be angled away from each other, if possible. Antennas controlled by a single reader would emit energy in sequence, rather than at the same time. Antenna #1 would send out a brief burst of energy, then antenna #2, then antenna #3 and so on. But the energy wouldn't dissipate the moment the antenna stopped firing, so if two reader antennas were pointed directly at one another, there would be a brief period during which the reader antenna might prevent another antenna from picking up the tag. Angling the antennas away from one another should eliminate this problem.
The locations of the reader antennas in the assembly area would depend on the locations of the antennas on your personnel. Having them mounted on poles would probably be the best approach, but you would need to conduct some testing first to determine whether you could read all of the tags. You wouldn't need to read all of them all of the time—you would just need to read a tag once to know someone was there.
You might want to look into using battery-assisted tags, which would reflect back a stronger signal to the reader. Convergence Systems Ltd. (CSL) makes tags that cost approximately $1 apiece. I have heard that they work well near the human body, and they might eliminate some of the challenges inherent in using passive tags to track your employees.
—Mark Roberti, Founder and Editor, RFID Journal
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