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Can I Affix Multiple RFID Tags to a Single Object?

Posted By RFID Journal, 12.17.2018

Is there an LF, HF or UHF RFID system that can read, with a minimum amount of readers (one or two), multiple tags on an item to determine its orientation and destination, as well as what it is?

—Martin

———

Martin,

It would be difficult to answer your question without having more details about what you are trying to track. If the object was the size of a sugar cube, I would say no. If it was the size of a car, however, I would probably say yes. Since I don't know the size, the material the object is made of and other important details, I will try to answer your question by breaking it down into its components.

A single reader can support multiple antennas, so it would be possible to use one reader and put antennas in different locations, then read tags on different sides of the object. You could write a software program that knows tag ID 123456789 is associated with the left side of the object, while tag ID 987654321 is associated with the right side of the object. When tags are interrogated by a specific antenna, that information is passed on by the reader to whatever software you are using. So, for instance, if you knew that reader antenna 1 read the tag associated with the left side and reader antenna 3 read the tag associated with the right side, you could infer which way the car was facing.

The challenge would be ensuring that only one antenna read the tag on the side of the object facing the reader. If all four antennas read all four tags, there would be no way to know which way the object was facing. Isolating the tags on each side of the object would require choosing the appropriate tags and reader antennas, then placing them on the object in such a way that all the tags were not read simultaneously. If the object were small, it's likely that each of the reader antennas would pick up all the tags, making it impossible to determine the orientation. On a larger object, it would be easier to have enough distance between a tag on one side and reader antennas on the sides not facing that tag, to avoid those antennas picking up the tag.

You might be able to use a passive UHF reader with linear-polarized reader antennas, and have dipole tags in different orientations on each side to reduce the chances of a reader antenna on side 1 picking up the tags on sides 2, 3 and 4. There might be other options, but it would depend on the size of the object, the distance from which the tags were being read and other factors.

As far as indicating what the item is, the RFID tags would send serial numbers to the reader, along with other data stored in user memory. You would need to associate the tag IDs with objects in a database, or write what the object is in user memory. The system would then identify the object.

The destination question is the hardest to answer without knowing more information about what you are trying to do, so let me describe two scenarios that will explain what a passive UHF RFID system can and can't do. Let's say you have a room that measures 50 feet by 50 feet. There is a remote-controlled toy car in the center of the room and floor doors, one in the center of each wall. You want to know which door the car is heading toward. You've placed a tag on each side of the car and are able to ascertain the car's orientation.

A passive UHF RFID reader has been placed under the floor and an antenna has been placed about halfway between the car and each door. The car turns and drives toward door 1. The reader antennas read the tags and determine that the vehicle is now facing door 1. As the car moves toward the door, the strength of the signal from the tag on the front of the car increases. The strength of the signal being picked up by the antenna facing the back of the car decreases. It's clear the car is traveling toward door 1.

Now, let's say you have no doors in the room, and you want to know to which area of the room the car is traveling and where it will stop. You could detect the movements of the tags with several reader antennas, but predicting where the car will stop would be impossible. An overhead reader system, such as Impinj's XArray, might work for you. It can plot the location of a tagged object, but it would not be able to tell you the orientation of a small item, because the tags would be too close to one another to determine that one was slightly to the right or left of another. On a larger object, you would be able to determine the orientation by reading the tags on each side.

That's a pretty long-winded answer, but I wanted to give you information that will help you choose the right solution.

—Mark Roberti, Founder and Editor, RFID Journal

USER COMMENTS

Mike Maurer 2018-12-19 12:41:08 AM
Mark: We've been doing RTLS since 1995. We now use 3 RTLS technologies on single CAT6 cable (433MHz, 125KHz, and Diffused IR) combined with our patented triangulation, Intelligent Boundary Recognition, and vector maps (even on a browser). One VA can triangulate and display tags moving on their Acad floor/site plans in real-time - covers approximately 7 million SF indoors and out. Really enjoy your approach to solving the unknown with available tech. Please let me know if you have interest in discussing some neat things that are now possible with RTLS - as a teaser, we are using our intelligent boundary recognition to make CCTV proactive as opposed to reactive - cameras nearest the moving tag locations stream video at the front display. Fun stuff! Best Regards, Mike mmaurer@mgm-solutions.com P: 856-371-3754 www.SecurTRAK.com
Joe Hoerl 2018-12-21 11:23:18 AM
Martin, HF would be difficult given the limited read distance and would not be good for directionality unless you have some closed system such as a conveyor or known work station flow. There are several instances where users are using UHF RFID (RAIN) for identity, orientation, and directionality of movement. These are typically done with overhead antenna arrays (RF Controls and Impinj both have OTS systems available) which cover large areas. As you suggest, to understand orientation would require multiple tags on the object as well as either alternating the orientation of the tags or preferably having some significant distance between the tags. I hope this is helpful. Joe Hoerl joe.hoerl@confidex.com
Martin Gonzalez 2019-01-07 05:19:39 PM
Thank you for the responses! My question leads to certain fixed circumstances. For example, limited funds, was considering having the "objects" be in boxes/totes no bigger then 3ft x 3ft x 3ft (this dimension is probably an exaggeration). Also I was thinking having the objects stationary and the reader be on a DIY conveyor or screw drive (similar to garage door opener). Funding being in issue I believe having a stepper motor controlling a belt that the reader would be attached to would be best for my case. -Martin

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