RFID Read Distance Problems

John Burrow

I am having problems with the reading distance using the Parallax RFID reader. The tags have to be very, very, near to the reader before being read.

I have an assortment of tags. The larger tags will read when they are between 0.5 and 1.5 inches from the reader - any nearer or further away, they stop reading. But the glass tags have to be almost touching the surface of the reader (less than 0.25 inches) before being read. I am holding the glass tag perpendicular to (pointing at) the board, not parallel to it. I have 2 readers, and many glass tags, they all give the same results.

I have the single reader installed on the Home Work Board (part of the Basic Stamp Activity Kit I bought at Radio Shack) in the breadboard section. I have checked the voltage that the reader it is receiving, and it's 4.95 Volts. I have powered the board with a Radio Shack 9 Volts 1500 MA supply.

I have tried this setup in various locations, to avoid any potential interference from EM/RF sources. I have also tried with the reader on the end of 3 feet of CAT5 cable. None of this seems to help. I have tried variations in the Stamp code, but this only affects the read time, not the distance from the reader.

On another setup, in the same location, using a Phidgets reader the same glass tags will read up to a distance of 2 inches. I am pretty much locked in to using the glass tags because I need the small cross section.

I had intended to move my application from my laptop onto a Stamp, but if I can not resolve the read distance problem, it appears that I am stuck with the laptop.

I have already pulled all my hair out; does anyone have any other suggestions?

Thanks, John

Paul Baker

Hi John, Please look on the back of your reader and tell me if it is a Rev A or a Rev B board.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

John R.

Paul;

(This is a different "John")

I have two readers that are "Rev: 1" (not A or . Are you implying that newer readers that might have longer read ranges? I was working on a project where the read distances were "marginal" (also working with glass tags), and I killed the project, at least for the time being. If the newer board do indeed have a longer read distance, I may be interested in resurrecting this project again.

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John R.
Click here to see my Nomad Build Log

Paul Baker

Yes Rev B has a longer range than the first version (Rev 1 I guess is what it was called), the glass tags have the worst distance out of any tag because of simple physics, but they should have a longer range than nearly direct contact.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

John R.

Coooooolll...

At slow speed I was using glass tags (from another source) and getting about 0.5 inches read distance reliably. For the speed I was after, I needed to be at less than 0.25 inches. Once I get a couple other projects out of the way, I may get a new reader and try things out again.

Thanks for the info!

John B: When I was doing the testing, I made a "test jig" from some Legos (Great "prototyping" tools). I found that I got different performance (read distance) depending not only on the orientation of the glass tag, but also how I approached the reader (head on, or passing parallel with the antenna face), as well as what part of the antenna I passed over (the edge vs. the center, and the range in between). I don't remember where was best, and my notes are buried, but I do remember it wasn't the "obvious" path that worked out best. I know that isn't a lot of help, but there are more variables for you to look at...

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John R.
Click here to see my Nomad Build Log

John Burrow

Paul,

I have checked both my readers; they are Rev B.

I wish I could come up with some more variables, but I think I've tried them all.

I didn't use a test jig, I simply held one end of the tag between thumb and two fingers and tried all the orientations, angles and moves I could think of.

Do you have any more ideas?

Thanks, John Burrow

Paul Baker

I was afraid you'd say that, I'll discuss your problem with our lead tech support guy and see if we can shed anymore light on this.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Paul Baker

Ok we did some testing here and sorry to say but the range for the glass tags using our reader is about a half inch. The glass tags were designed as subcutaneous identification to be read using a reader in contact with the skin, the reading range was sacrificed in exchange for a small size. If you need a longer reading distance you will need to use a larger tag such as the disc sticker variety.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

parsko

Guys,

I've been working on RFID recently myself. I noticed the same problems too. I too, have a Rev 1 board (two of them, actually). I have desoldered the EM4095 chip, and tomorrow plan to wire them up with new capacitors and try to get my prop to read out numbers. In my travels, I ran through the calculations that are provided in the EM4095 datasheet. I found that the resonant frequency is VERY sensitive to changes in any of the parameters. AKA, antenna size and shape and the resonant capacitor values.

Blah blah blah.... with that said, Paul, can you comment, or do you even know, what the difference is between rev 1 and 2? I am thinking that even the difference between using 10% accurate and 5% (or 1%, not sure what flavors they come in) capacitors would have on the effective detecting range would be. My laptop won't load run MathCAD right now, so I can't confirm what difference it would make in the calculations.

BUT, you are certainly correct in that the range with the Glass Tags is dictated by the antenna built into them. From what I have read and experimented with, it won't get much more than about two inches, and that's with a tuned detector.

I would venture to guess that mass produced RFID readers won't yeild the best results. If you want better, you need to pick and choose the capacitor circuit to the specific antenna. I have been asking Parallax for details about the reader they sell, but to no avail... I have been a bit hesitant to remove the SMD stuff on the reader, other than the SOIC EM4095, for fear of permanantly harming the reader or the components.

Well, I've had my coffee, I still have bed-head, and I'm ready to start playing. I'll post my results on my post on the Propeller forum if you're interested...

-Parsko



(Luke's disclaimer: I don't want to sound like a broken record, or that I want Parallax to solve my problem. I understand that you outsourced this project to Grand designs, and/or there might be some sort of copyright or simply a lack of knowledge transfer between the two, but you guys are normally pretty good about providing detailed schematics about the circuits you produce and sell. So, please don't get annoyed that I am keeping an eye on every RFID post on your forum, I'm just trying to figure out how to get all my cats to Smile in the litter box and not have to clean the darn thing every day!!! [noparse]:)[/noparse] )

Paul Baker

The difference between Rev 1 and 2 is the resonant frequency of the antenna, Rev 2 is better matched. What this means is that on full tags (the badge type) Rev 1 can read them roughly 3-4 inches away, and Rev 2 can read them upto 6 inches away. Like you noted the RFID reader was developed by a 3rd party, so we can't provide you with all the details of design, because we don't have them.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

Tim-M

Why don't you get in touch with Joe Grand of Grand Idea Studio, he is the developer of the RFID reader. If anyone can help, it should be Joe.

www.grandideastudio.com

Tim

Post Edited (Tim-M) : 8/1/2007 7:14:23 PM GMT

parsko

Tim,

I had thought of that. I don't know the guys there, but I would gander that they may not release the info. I'm not saying they won't, I simply never got around to it. It is a good idea, and should be done. It would close the loop regarding the info.

I did get around to getting my homebrew RFID reader to (partially) work with the EM4095 chip. Partially simply means that the hardware is working, but I am not reading tags, only confirming that a tag is present by the reaction of the CLK pin to a SHD toggle.

My homebrew distance is the same as that of the Parallax reader, within millimeter's or so.

-Parsko

EDIT: I had thought of contacting them, but they only have a phone number, and I'm in the Netherlands. Not that the cost of the call is an issue, but email is easier and they don't give that as an option. Also, there is a cat playing with my soldering iron sponge right now...