Antenna location, angle

[Sergii Petrovych]

Hi there.
Perhaps I don't fully understand how it works.
The antenna has circular polarization and a radiation angle of about 30 degrees. How do I set the tags to trigger on the finish line?
Will the tags trigger earlier? marked in green.
Do I need to use narrow beam antennas? Yagi antenna?

photo by Edward Sitarski

[Jonathan Rosen]

CrossMgrImpinj has different read modes.  The First Read mode will pickup the earliest read of the chip, resulting in a chip read before perpendicular to the antenna.  The QR mode will accumulate multiple reads of the chip, using math to calculate the position of the chip in front of the antenna. Combined with CrossMgrVideo, you can see where the chip is when it is read. The QR feature was released in May 2018.

CrossMgr - CrossMgrImpinj 2.21.11: Greatly improved RFID accuracy (google.com)

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[Jonathan Rosen]

Regarding antennas, when using QR mode the widest field antenna is the best.  I always use two antennas; one linear and one circular.  The linear antenna has a wider field than the circular and is more sensitive, so it usually captures 66% of the reads.  The circular antenna has a narrower field but will read chips in any position, so it catches the 33% that are not aligned (matching polarization) with the linear antenna. I also have a Yagi antenna but its very large (over 1 meter) and has an extremely narrow field so it doesn't provide many reads with QR.  I also use an Impinj R700 reader that is faster than the R1000 and R420, so it catches more reads from each chip.

[Sergii Petrovych]

Thank you. The QR function is perfect for me and explains everything.

неділя, 13 жовтня 2024 р. о 19:30:09 UTC+3 jonrose...@gmail.com пише:

[stuart lynne]

When using a polarized antenna is to match the tag. If your tags are mounted horizontally, you must mount your antenna to match that. 

That somewhat dictates your antenna choice. You want an antenna that has smallest beam spread that matches the polarization.

We used Superpass SPC12 (sadly no longer available) antennas with 34 degrees H Beam Width to match H-linear polarization (vs. 62 for the non-optimized V-linear). And tried to get people to mount their frame plates so that the tags were horizontal.

The issue with passive RFID tags is powering them. Once powered they will almost always provide a read. The more RF power you can get into the read zone aligned with the tag the higher the success rate is. 

I also prefer to use two readers each with a single antenna. If you put multiple antennas on a single reader, it will cycle through the antennas. If you have two antennas, then each antenna is only used 50% of the time. Three antennas 33%, etc. If you have a single antenna, it is in use 100%. 

Two antennas on opposite side of the road with 50% duty cycle, will see (from memory for R420) tags moving about 5-10cm between each attempted inventory for high-speed finishes (as well as 5-10 cm for the actual inventory.)

Whereas at 100% each antenna is doing continuous inventories. Depending on your reader and software, you can use sessions to get tags to "alternate" between sessions to get the multiple reads that the QR algorithm uses. If I recall correctly the best solution uses an Impinj session mode to get best results.

RFID tags are generally used for counting things. And once you get a read from a tag (e.g. as it is going through an RFID reader arch on a pallet) you don't want to hear from it again. One read per tag for the possibly thousand tags on the pallet is sufficient etc.

We have the opposite problem of a small number of tags that we want to read and get the time of the read closest to the finish line. So continuous reading, and QR to do the heavy lifting to try and determine the best one. And if you have multiple antennas, then separate QR for each and then some guessing as to which result appears to be best.

The good news is that typically the best reads are for the leading tags in a bunch. The problem is getting the middle of 40-50 tags passing through in a large pack at 50km+.

Then you need FinishLynx. And we use tags to help them identify people :-)

To view this discussion on the web visit https://groups.google.com/d/msgid/crossmgrsoftware/CAC7BV2xZeXEqF5QXZ%2B8jmDB1zsDrNSmU43tX7H_w6-ZAU32s%2BA%40mail.gmail.com.

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[Sergii Petrovych]

stuart lynne Thanks for the extended answer.
Yes, I am going to use 1 reader and 2 (4) antennas.
I will be providing finishes for running and cycling events. In fact, I rarely have more than 20 tags in 3-10 seconds. In parallel, I use a photo / video finish system. In case of disputes, priority is given to the photo finish.
Orientation of tags is a headache for the organizer...

нд, 13 жовт. 2024 р. о 21:03 stuart lynne <stuart...@gmail.com> пише:

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[Edward Sitarski]

Thanks for the question.

Your understanding and conclusions are correct.

See here for even more details.

QR puts the reader into a mode where it returns tag reads, and their signal strength, as frequently as possible.

Each antenna generates a series of tuples consisting of [tagID, timestamp, signalStrength].

CrossMgrImpinj first chooses the antenna with the highest signalStrength in its series for further analysis.

It then does outlier detection to eliminate values that are "likely" outside a reasonable range.

Outliers have many causes.  Reflections are most common.

Imagine a piece of flat metal (eg. car door), that the tag return bounces off.

If the distance of the tag-metal-antenna reflection is in phase with the wavelength, it could theoretically cause a return that is 2x the normal signal strength.

If perfectly out of phase, it could cancel the signal strength entirely.

It is critical to eliminate these outliers because the Quadratic Regression formula is very sensitive to them.  This can lead to wonky results.

Quadratic Regression uses the signalStrength to compute a best-fit quadratic, then returns the time at the apex

The calculation is not limited by the time resolution of the reader (usually about 1/200 of a second).

Sometimes the data is too "squirrelly" and outlier detection fails.

The system then falls back to choosing the time at the highest signal strength.

This behavior can also be forced by choosing the "Strongest Read" option in CrossMgrImpinj, however, QR is usually a better option.

And, Strongest Read times are limited to the reader's read frequency 

I really liked your diagram. Itt shows "First Read" logic, which is also supported by CrossMgrImpinj.

As you point out, First Read is useless for bunch finishes (better for TTs with a narrow finish line, but there are many problems here too).

Unfortunately, some cheap readers do not support the mode required for QR and first-read is the only option.

Some really cheap Chinese readers don;t return signal strength at all.

It is frustrating that I get requests for this a few times a month.

After years of experience with Criterium bunch finishes, we feel QR has a resolution of 100-200mm, that is, if riders are separated by more than 100-200mm, they will be recorded in the correct order.

It's not perfect, and in a bunch sprint we might have to swap a few finishers based on the camera (which we take with cell phones).

This saves a lot of time as you don't even have to look at riders more than 200mm apart.

This is competitive with active chip technology, and within the error of bike mounting variability.

Of course, you also get chip timing benefits like real-time results and identification beyond the bib numbers.

In CX and MTB, corrections are rare.

To view this discussion on the web visit https://groups.google.com/d/msgid/crossmgrsoftware/CAHhPCq-q_Keex_ckbKdErTnxkLr2d7Xv8%3DrUgsApnmm6N%3D47Bw%40mail.gmail.com.

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Edward Sitarski

[Sergii Petrovych]

Thank you Edward. A lot of knowledge for me.

I intend to use the Zebra FX9500 or Alien ALR-F800 reader. I think there are no problems with the signal level data.
Reflected surfaces are also excluded.
Edward, how effective is the use of floor mats (antennas) in running competitions? The chip is attached to the bib.
There is an option to install side antennas of circular radiation.
Will the floor antenna be useful if the chip is installed as poorly as possible? Edge to the antenna 90 degrees.

понеділок, 14 жовтня 2024 р. о 17:48:50 UTC+3 Edward Sitarski пише:

[Edward Sitarski]

Unfortunately, I am not 100% sure that QR works on Zebra or Alien as those implementations of LLRP are incomplete.

My only direct experience is with Impinj readers.

I suggest asking for more experiences on https://groups.google.com/g/crossmgrsoftware to confirm.

To view this discussion on the web visit https://groups.google.com/d/msgid/crossmgrsoftware/8ae50d08-3f48-428b-a801-53ed2b539617n%40googlegroups.com.

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Edward Sitarski

[Sergii Petrovych]

 No problem here.

 LLRP is supported on Alien ALR-F800 and Alien ALR-9900/+ readers only.  

This is stated in the documentation.

вівторок, 15 жовтня 2024 р. о 10:52:50 UTC+3 Edward Sitarski пише: