Using multiple RFID readers together

Wireless & IoT
1

Hi, I’ve been lurking for a while but just joined up to post. I’m working on a project that will require embedding 20 RFID readers (I was planning on using the ID-12) in the base of some shelves so that boxes with RFID tags embedded in them can have their positions tracked by a computer.

I was hoping that I’d be able to use the digital input pins on a Wiring board to input the data and then transmit it to the computer but after doing more research it looks like that might not be possible.

What are my options? Is there some way I could use the digital IO on the wiring board to simulate multiple serial interfaces? Could I store data received on the digital input pins and then transfer it some way? Or am I going to have to do make something with a lot of individual serial ports (lots of FTDI ICs on a USB hub?).

Sorry if this question is really dumb, I have lots of experience building stuff that other people have designed but next to none designing my own stuff, or working with digital systems.

thanks a lot,

Morgan

2

Are you tracking 20 boxes with 20 readers or will there be multiple boxes per reader? These RFID readers aren’t meant to differentiate between multiple tags in the area at the same time. If you are only tracking 20 boxes, 20 readers might be an overkill. Regardless, getting back to your questions…

If there is low activity (i.e., boxes aren’t moving a lot or multiple boxes moving at the same time), you could share a single serial port by isolating each reader such that a single reader is connected to the processor. As you posted this on the Wireless forum, your receiver may be able to help there, but I can’t tell from your post. If you are using XBee boards, they should be able to handle collisions between multiple readers.

Tell us a little more about your application and the workflow.

3

Another option would be to use 1 ID-2 reader (allows external antenna) with multiple multiplexed antennas. The problem with using multiple RFID readers in close proximity is that the RF fields will interreact and you will get poor reading distance.

4

Thanks for the responses.

riden:
Are you tracking 20 boxes with 20 readers or will there be multiple boxes per reader? These RFID readers aren’t meant to differentiate between multiple tags in the area at the same time. If you are only tracking 20 boxes, 20 readers might be an overkill.

Each reader will only need to track the box directly above it. The shelves will be like a set of pigeon-holes, like they use for sorting mail. Each reader will only need to detect the one box which is placed in that particular pigeon-hole.

riden:
Regardless, getting back to your questions…

If there is low activity (i.e., boxes aren’t moving a lot or multiple boxes moving at the same time), you could share a single serial port by isolating each reader such that a single reader is connected to the processor. As you posted this on the Wireless forum, your receiver may be able to help there, but I can’t tell from your post. If you are using XBee boards, they should be able to handle collisions between multiple readers.

Tell us a little more about your application and the workflow.

I put it in the Wireless/RF forum cause that seemed like the right place to put RFID questions, maybe it wasn't though. I plan on using a Wiring board as the interface to the computer, as that's what I have available to me.

Anyway, the amount of activity will be quite low and the response time needn’t be incredibly fast, the workflow will be something along the lines of:

  1. User takes empty box, scans it on external reader and places item in box. Program notes which box is used, linking virtual item with RFID tag number.

  2. User places box with item in it on shelf.

  3. When another user is using the program and comes across virtual item, the shelves are scanned to identify where the box which is linked is and then a light goes on behind the box so the user knows where it is.

After thinking about it over the weekend and digesting your advice I’m now looking at multiplexing all the readers off one serial port and resetting each one before it’s read, hopefully the readers won’t take too long after reset to become active, and then I’ll adjust my loop time around that. Does that sound like it should work?

phishguy:
Another option would be to use 1 ID-2 reader (allows external antenna) with multiple multiplexed antennas. The problem with using multiple RFID readers in close proximity is that the RF fields will interreact and you will get poor reading distance.

I had considered that option but I was concerned that having antennae of different lengths (they would need to be spread out over a range of distances from the reader) would make tuning a real headache.
5

What reader(s) were you considering? I would think that you could hold all the readers in RESET mode and then go around round-robin raising RESET and waiting for the reader to reply, then lower RESET, and move on to the next reader. If you put a diode between the TX out from the reader and the shared bus, the readers won’t interfere with each other.

It will be a bit expensive to implement, but I can see how it could pay for itself in some situations. I don’t know anything about the size of the boxes, if they’re reusable, or the nature of the bins/shelves, but there may be less expensive approaches to encode the identity of the box. For example, placing Hall-effect devices on the bottom of the shelf and creating unique patterns to represent the 20 different boxes.

6

riden:
What reader(s) were you considering? I would think that you could hold all the readers in RESET mode and then go around round-robin raising RESET and waiting for the reader to reply, then lower RESET, and move on to the next reader. If you put a diode between the TX out from the reader and the shared bus, the readers won’t interfere with each other.

Yep, that's pretty much what I'm planning now. We will be using the ID Innovations ID-12.

riden:
It will be a bit expensive to implement, but I can see how it could pay for itself in some situations. I don’t know anything about the size of the boxes, if they’re reusable, or the nature of the bins/shelves, but there may be less expensive approaches to encode the identity of the box. For example, placing Hall-effect devices on the bottom of the shelf and creating unique patterns to represent the 20 different boxes.

We have a decent budget for this project so cost isn't really the biggest issue, we're more concerned with getting something that'll work. The boxes will be around the size of a shoebox and will be reused, when not in use they will be stored flat which is why the RFID cards will be suitable.