Frequency bandwidth of modem

#1

Hi I am interested in using PR52-33N vibration sensor, where it could sample up to 25,600Hz and with a bandwidth of 8kHz. What is the bandwidth of the IoT modem that could support? It is not specified in the website.

If I intend to send out continuous vibration data at 8kHz, must I need to use one modem for each sensor node?

Thank you

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#2

This sensor is not designed to send continuous data. if you multiple sensors in the area and all of them are sending continuous data then their will be a lot of network congestion and data will get corrupt and most probably wont even make to modem.

The sensor has a time domain mode where is sends 340 data samples. You can have it send these data samples after every few seconds. Even in this case you will need one gateway for each sensor.

Thanks

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#3

Hi Bhaskar, thanks. If the time domain could only send 340 data samples, that’s not much use of sending time data since FFT of it would have a very coarse resolution.

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#4

it sends 340 data samples for each axis. due to limited bandwidth its not possible to send continuous data stream. i will recommend looking into a wired sensor.
the issue is this – the internal accelerometer has a 340 byte buffer ( for each axis) so even if we try to send more data points they wont be continuous data points because while we are sending data over the wireless in the background the buffer will keep updating and when we are done sending the old 340 samples it will have new 340 samples ready. but these 340 samples will be latest 340 samples. we will loose the intermediate data points.
in short even if we push the wireless limits and try to send more data it will end up skewing the results.

do you happen to know how many samples/data points you need in order to apply FFT.
we have a labview utility which usages 340 samples in order to perform FFT. you can take a look and see if this something could work.

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#5

I intend to use to monitor for vibration for at least 1 second with sampling rate of 1kHz. With only 340 samples, I am only monitoring for duration of 0.34 second, with FFT resolution of about 3Hz. Ok i mean the FFT resolution is still decent, but duration of time is what it is lacking.

Thank you for explaining on buffer thing. What is the wireless transmission throughput limit?

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#6

in one transmission packet we can send around 196 data bytes. here is quick math
Each axis Sample size – 340 samples
each sample size – 2 byte
total data bytes == 340x3x2 == 2040 bytes

sensor sends a number of packets one after another to send all the data. with this data it also sends few other parameters like sensor mac address, battery level, data rate and some other basic info.
sensor also makes sure all the packets makes to the gateway. because if it misses even one packet the entire data packet will be useless.
during the design we sent millions samples and made sure every single sample makes to the gateway without any corruption.

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#7

I see… thank you. In that case it doesn’t seem I could do much on transferring raw time data. May I know if reprogramming of what statistical features I want from the sensor is possible? For example, instead of sending max, min or rms value, could I send curtosis, peak-to-peak, and etc.

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