RFID coil question

Thread Starter

rjjenkins

Joined Apr 16, 2011
187
I have an application which uses animal tags and a RFID reader operating at 134.2KHz. The tags are embedded in wooden objects that are passed through the coil. The coil at the moment is about 110 mm in diameter. I have wound it to the specified inductance and have also tuned it with an oscilloscope. All is fine except that about 5 per cent of tags fail to be read. This is probably because I can't control the orientation of the tag as they pass through the coil. I made another coil slightly smaller at 100mm, same inductance, and the performance is slightly worse. I'm not sure whether size matters - my instinct was that a bigger coil is likely to have a dead spot in the middle, and that using a smaller one would give a greater chance that the tag will be near the side of the coil as it passes through. But maybe I'm wrong - my experiment suggests that I might get a better result with a bigger coil.

Any thoughts, please?
 

DickCappels

Joined Aug 21, 2008
8,705
You wrote: This is probably because I can't control the orientation of the tag as they pass through the coil

There is a good chance of that. Maybe you should work on that aspect of the problem. Back in high school about 100 years ago a friend found that he could smuggle books out of the library by holding them at a particular angle when walking through the detector.

Never got caught and later in life he straightened out and made a very good living as a programmer with a big computer company.
 

DickCappels

Joined Aug 21, 2008
8,705
I don't know what is inside the tags, but could magnets help get the balls aligned?

I guess your fallback position is to pass the failed balls through multiple times before scrapping or trashing them.

Also, for reference, what RFID reader are you using?
 

panic mode

Joined Oct 10, 2011
2,131
what stops you from using more than one reader with antennas oriented differently or one but with two antenna coils that are angled and together match the required inductance?
orientation matters. also how fast your reader works? I am using LF tags too (mine is 125kHz) and some of the products are way too slow and may not be able to catch the tag if it moves too fast. some can read in 10ms, some can do 60-250ms and there are some that are really slow (1+ sec). so if your tags are dropped through some chute, you may need to consider changing inclination angle to slow it down.


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Thread Starter

rjjenkins

Joined Apr 16, 2011
187
Thanks. Not many of these things are under my control. I don't have space for a second reader (and they are power hungry and not cheap). I can try two coils at different orientations but the space is limited. I can't do much to alter the read speed.

Really this was why my original question was about coils. What I need to know is whether, apart from inductance, there are coil issues that will make them more or less sensitive. Size, as I originally asked. But also, maybe, does it make a difference if I use thicker gauge wire which would have a lower resistance? Would a second coil, not connected to the first but in the same plane and the same inductance, generate some kind of useful resonance effect?
 

MisterBill2

Joined Jan 23, 2018
11,877
The diameter and length of the coil have a great effect on the shape of the magnetic flux field. So another coil the same diameter but a greater length, because the turns are spaced farther apart, will provide a larger volume with a uniform flux field density. The smaller coil will have a less uniform field, stronger but smaller.. And that sounds like an interesting application.

To save power it might work to have a photo sensor switch on the RFID device only when the coil was approached.
This sounds a bit like a animal training project.
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
187
Thanks. I have wound a longer coil (turns more widely spaced) and it seems to be better. But I'm guessing the field, although it acts over a larger volume of space, is less concentrated? So there will be diminishing returns to this approach?

I suppose I'm not giving away too much in this forum to say that this is a magic trick: the balls are painted to look like bingo balls, and are picked out of a bag. The rim of the bag contains the coil and is attached to a handle that contains the electronics and a transmitter. Everything has to look normal and unsuspicious, which limits my options considerably - i.e. nothing can be too big. And it's difficult to have a second coil in a perpendicular plane.
 

Ya’akov

Joined Jan 27, 2019
5,659
Does the magician or a confederate remove the ball from the bag, or is it a volunteer?

If the latter, since the source of the problem is probably insufficient field to power the RFID tag do to the tag’s coil‘s plane being close to perpendicular, it might be possible to use a manipulation to ensure the tag is read. If the ball is allow to rotate in the hand as it withdrawn, it is almost certain some angle of the tag will allow reading.

Just a thought.
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
187
It's a volunteer. Yes, if the volunteer can be persuaded to rotate the ball as they remove it, then that will probably work.
 

Ya’akov

Joined Jan 27, 2019
5,659
It's a volunteer. Yes, if the volunteer can be persuaded to rotate the ball as they remove it, then that will probably work.
I suppose you can use some patter about rotating the ball in their hand to “feel” what it is. That can also put the blame on them ”not doing it right” for a failure and misdirect the action if there is more than one instruction about how to “feel” it.
 

Thread Starter

rjjenkins

Joined Apr 16, 2011
187
Yes - I think something like "have a good look at the number before you take it out - you may not want that number"
A fallback is to get them to put the ball back, which gives a second chance of a reading.
 

Ya’akov

Joined Jan 27, 2019
5,659
Yes - I think something like "have a good look at the number before you take it out - you may not want that number"
A fallback is to get them to put the ball back, which gives a second chance of a reading.
You could have some patter that includes a procedure where they take the ball out, then you have them ““activate it” by rubbing it between their hands, then put their closed fist with the ball inside back into the bag so the “vibrations” from all the other balls can be cancelled out by it then take it back out.

You could use some other theory about how it works and substitute terminology, but I feel that it could be successful and has a lot of potential for misdirection about potential mechanisms for the trick. If you had some kind of annunciator that only you could detect (ear piece ? just an example of the technology, I don’t know if that particular one is any good) you could know if the reading happened so you could ask them to repeat the procedure if it doesn’t read..

Seems like a high chance of getting a reading and an excuse for why it doesn’t work sometimes (though that should be rare).
 

MisterBill2

Joined Jan 23, 2018
11,877
There are eventually diminishing returns with coils like this, and there is a lot of information about coils available. One choice would be to mark the balls such that the volunteer would be inclined to remove them in the correct orientation. This migh work with an instruction like "hold them with the number down so that nobody thinks you are cheating" or some other pretext for holding them a certain way. Maybe a green patch to go along with that old joke about "Green side up".
 
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