Determine RFID Tag Distance

[James_W]

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@Tolgahan_ss wrote:

Hi fadihajj ,

As you know  there are two methods  that;

One is measuring time interval, another one is signal magnitude. 

Measuring time is not possible with low speed DAQ devices that lower clock rate than Ghz..Because signal will move at speed of sound and that means 3 ns for 1 meter distance. So if you use Arduino as example that has 16 Mhz clock rate, it means 60ns. In other words, you can not measure small distances at that rate. Maybe if your tag is so far away like km from receiver it can work for it 🙂

Only measuring signal magnitude can work but you need a receiver that measures magnitude of signal . You can use spectrum analyzer that has more than 60db resolution.

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Tolgahan, this has already been explained, but let me try another way.

Signal moves at Speed of Light with radio waves (which will be used for Radio Frequency IDentification - RFID) which is over 874030 times greater than Speed of Sound.

Signal Magnitude is the ONLY feasible method of determining position accurately when Windows host clock only updates every 1ms.

Speed of Light is 299 792 458 m/s

This means that in 1ms, a radiowave travels (299 792 458 /1000m = 299 792 . 458 m = 299. 792 458 km,

lets call it 300km in 1ms

or to put it another way, in 1ms, the radiowave goes 1/3 distance Paris-Berlin, and this is your smallest margin of error for a distance accuracy - not very helpful for detecting office equipment for calibration.

The response time of the serial coms was 20ms - so we are looking at a 20*300km accuracy = 6000km

so the equipment location is accurate to +/-6000km

of to put it another way, if your office is in Paris, you could "accurately" say it was somewhere between you and New York! (plus a bit extra)

[fadihajj]

Hello guys,

sorry for being late. 

I am going to try a different approach, i can control the power transmitted by the reader ( from 0 to 30dbm). So i am going to try to set the power at 1dbm and measure the distance that allows the reader to capture the tag then increment it by 1 and measure the distance again. This goes until i read 30dbm, so i can develop a relationship and use an iterative process to get the distance. This isn't very efficient but i will be able to get an approximate location.

[linda1988]

good

[JÞB]

Why don't you just add a cheap distance measuring device?  The air gapped RF budget is a fairly poor indication of distance anyhow.   Reflections, retractions dust, water vapor.... all impact that RSSI as much as distance already.  Which is why the measurement is RSSI not distance anyway.  

Radars work for distance only because there is no variable latency with a reflection.  IFF transponders have a highly accurate latency in response to interrogation  RFID is designed only to detect What the stored object is not where it is.   The specific implementations cannot replace the lack of support within the design for what you're looking for.   

LIDAR cares where things are and any radial velocity component.  But does not care what it is.  Again, by design! Who cares what it is,  the purpose it is to avoid occupying the same point at the same time (generally resulting in a delta V that is damaging)

Wrong technology!  Like using a stopwatch to measure luminosity