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How often can a passive RFID be polled?

Posted on 2016-11-07
Medium Priority
Last Modified: 2016-11-13
I need to measure vibration using RFID, or some other technology. (Feel free to say if there is a better choice that RFID passive)

I need very low cost per chip and very low profile, hence RFID passive. Also, I can not justify the difficulty of batteries needed to be replaced.

Given that the "energy" comes from the message received, how often can a passive RFID be polled? I expect these vibrations to be short lived, and therefore, easily missed.

Is it crazy to poll the chip every second? Or every half second?

It sounds crazy to me, but this may be the only way I can create a solution cheap enough and useful.

Question by:newbieweb
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LVL 21

Expert Comment

ID: 41877591
We need to know at which RFID tag do you refer exactly.

Author Comment

ID: 41877595
Well, I have not chosen a chip and was asking a general question. How about this one?

What is the most often a passive RFID chip can be polled? And is it possible to poll hundreds of them in a small area with the same frequency?

If so, which chip can do this?
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Assisted Solution

viki2000 earned 1000 total points
ID: 41877648
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Assisted Solution

viki2000 earned 1000 total points
ID: 41878212
When you said above "How about this one?", it is not clear at which one do you refer. Can you be more specific?

"What is the most often a passive RFID chip can be polled?"
Does that refer general at any passive RFID tag or at vibration tags?
But 1st time you mention a vibration RFID tag. Is that correct? Because I have seen only active vibration tags.

"And is it possible to poll hundreds of them in a small area with the same frequency?"
Yes, it is possible, because each one has a different digital ID code even if they use the same frequency.
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Assisted Solution

viki2000 earned 1000 total points
ID: 41878236
Here are few more explanations from my side.
The passive tag means there is no battery in it. The active means there is a battery which may last up to some years. How long it will last? Depends how much energy you draw from it, meaning how many times the chip becomes active, waking up from sleep mode low energy state, and records the activity/vibration + how many times you interrogate the tag, because some of energy is then needed for transmission via RF. When you decide over a certain tag, then the datasheet from manufacturer must be readed and eventually additional questions posed to that company, so you would know then the lifetime of the battery under certain working/setup conditions as you need.
There is also know so called passive tags with assisted battery, which is a kind of mix between passive and active, but are more active then passive. The idea is to use a part of energy from RF field sent by receiver and only the needed supplement to be used from internal battery.
Here are some explanations:
The only passive orientation tag (so a bit of vibration) that I found is next:
but is expenisve:
and as all passive tags, because there is no energy from a battery, then range is short, for sure under 1m.

You did not mention the range, the distance from receiver to the tags, what has to be covered.
I just guess you need some meters around the receiver. In such case you need definitely active tags or passive-battery assisted tags. Then you can go up to 100m range and with a smart receiver you set up, change the reading range between 5-100m whenever you want.

A passive tag receive 1st energy wireless from receiver/reader then send back the answer, but in short range because is not too much energy in RF.
An active tag can respond faster, because the energy is there and can send the energy on longer distance.

How many times per second can you interrogate a tag? It depends by tag. It can be each couple of seconds or many times per second. The communication protocol can be for example 50Kb/s up to 500Kb/s. Then you can ask over and over again, but remember each pool implies sucking energy from battery. So if you intend to interrogate each 1s or each 0.5s then the battery may be empty sooner then you expect.
In your case I suggest to go through next steps:
-      Define the range of the rfid tags. That will tell you if you need passive or active.
-      Define the type of the rfid tag. Do you need one to record vibration? Then I am afraid you need an active one, because there is no way to record something in the internal memory of the rfid tag microcontroller without energy and the vibration itself must have a certain  level in order to be used as energy harvest field.
-      Find a specific rfid active vibration tag manufacturer and ask for datasheet, receiver compatible with that tga, communication protocol details. Then you can see or ask how fast can be interrogated and what is recommendation with regards to lifetime of internal battery based on specific number of interrogations in a given period of time.
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Assisted Solution

viki2000 earned 1000 total points
ID: 41878278
Let’s take for example next one:
The communication speed is 1Mbps, that means 1 Million bits per second.
I do not know that data word format, if its 8bit or 16bit or whatever and how many words are sent at one interrogation, but even if we consider 100bits used, that means you can read 10.000 times per second. And if we just assume even lower times as 1000 times per second or 100 times per second, is more than enough from communication speed protocol point of view.
It seems there is no problem to read/update info 100 tags or more each 1s.
The distance is up to 100m, but you can set from receiver a lower reading range if you want.
They say the battery life id 4years, but you must ask them under what conditions:
-      Temperature is important
-      How often can you read the tag in order to get those 4 years.
LVL 47

Accepted Solution

Craig Beck earned 1000 total points
ID: 41879807
You can't usually poll passive tags.  You need an 'exciter' device to read the tag when it passes.  If you have vibrations moving a tag a couple of mm the tag may only ever be read once.

Why do you think RFID would be good for this?  There are vibration sensors designed to do this.

Author Comment

ID: 41879898
>  If you have vibrations moving a tag a couple of mm the tag may only ever be read once.

I am not sure I understand.

My needs are a very low cost and low profile tag I can attach to a widget. I do not care if it's RFID or some other technology. I need to get able to get vibrations from that chip.

What do you suggest?

Author Closing Comment

ID: 41885594
Thanks, I need to learn much more about:

vibration sensors piezoelectric

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