I am working on a project involving RFID in a defense application. At the moment, we are installing small UHF RFID tags into the selector of an Armalite-style rifle, programmed with "Safe," "Semi," etc., based on the selector's position. That is then read by an antenna hidden in the detent and spring, which keep the selector indexed, running down into the weapon's pistol grip.
The next project we were thinking about was to perhaps use RFID tags in some part of the weapon's magazine to somehow ascertain, when interrogated, how many rounds are left in the magazine. The magazine has a follower, a spring, a floorplate retainer (which cups the compressed spring) and a floorplate. A time-of-flight sensor reading the change in upward ascent of the follower when queried (as the follower's position changes for every round stripped from the magazine by the bolt) would require some very fast and precise computations.
We thought perhaps we could use two RFID tags out of phase with each other, then somehow determine position via phase relationship, but this would be complex. Then we thought of using a compression sensor on the floorplate retainer, acting sort of like an RFID tire pressure sensor, sensing spring tension—how compressed or loose the spring would be at empty, at full capacity, and at every variation in between.
Perhaps we could use some kind of magnetically reactive or coded strip embedded in the inside track of the magazine that the follower rides in. When interrogated, an RFID sensor in the follower could report its position back to the reader, indicating where it currently is positioned, respective to the magnetic strip. Is there a simpler way to go about reading the position of the magazine follower as it moves, vs. a sensor in a static position, and thus determine how many rounds are left in the magazine? I'm beginning to think that I am over-complicating the situation.
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Robert, congratulations for being innovative. That's the kind of thing the RFID industry benefits from. I went to our friends at Advantech for their thoughts on your project, and they said this application might work best with Bluetooth Low Energy (BLE) technology, though you do have several options. Here are their thoughts:
"The solutions you mentioned all carry with them advantages and disadvantages. The most pressing, though, is what form factors you would be using in these solutions. A passive system presents one set of technical concerns, and an active system presents a different set.
"From a capabilities perspective, the time-of-flight sensor would be the most practical, with the least number of technical hurdles to overcome in our opinion. This could be implemented in a new smart magazine with a laser sensor and reflective point within the magazine gauging the distance from the bottom of the magazine to the bottom of the follower. Position measurements would be sent to an RF device on the bottom of the magazine and connected wirelessly to a receiving unit for the servicemember.
"This is an existing technology and would require the least amount of modification to build a prototype. Having said that, it would still face multiple technical hurdles in an implementation, including interference caused by the ammo itself, interference from the internal structures of the magazines, vibrational impact from firing, powering the interrogator/RF device, maintaining this new smart magazine, and ensuring that the servicemember is only seeing those magazines that they possess.
"The spring tension idea is an interesting one, though this seems like more of a solution suited to a piezoelectric sensor and a wireless sending/receiving system versus an RFID application. Keep in mind though, environmental factors could play a role in the effectiveness of this solution. Springs wear over time, either from extended use, environmental factors, ambient temperatures during operations, etc. You would have to correct for the tension variations that will be seen in normal operations, making the solution overly complex.
"In all of the designs you mentioned, there are significant hurdles that you would need to overcome, regardless of the technology used. RFID, particularly a passive system, is elegant in its simplicity. It has an encoded identifier and will respond with that information when interrogated and that's all it will do. It would be extremely difficult to implement your solution with a passive system that can only send a fixed EPC. While we too enjoy pushing the limits of RFID technology, this use-case may be something better suited to Bluetooth."
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