PSN-L Email List Message

Subject: Re: Help on finding natural frequency of a geophone
From: Mauro Mariotti mariotti@.........
Date: Thu, 19 Apr 2012 09:24:39 +0200


Hello,

 > Im using the geophone connected directly to the ADC in a breadboard at
 > ~22bits, so i have noise in it.. also i´m limited to the 16bits from
 > AmaSeis, so some spikes are only displayed up to 16bits.

You can use a large white noise dataset to plot a response curve at low 
frequency, you can easily recognize the 2 poles behaviour corner at X Hz.


 >
 >   The geophone looks like its responding quite well at 10Hz, i did tried
 > tapping with a pencil or different objects to create some "spikes", but
 > didint reach any good result. I did not managed to get any good
 > recording lower than 5Hz..

It maybe a 10Hz geophone, but it is hard to excite correctyly with a tap 
in order to get the lower frequency reposnse because your tap contains 
basicly high frequencies.

The only absolute solution is to excite the sensor with a bias pulse 
record the signal and measure the time between crest.

best regards

-- 
Mauro Mariotti
SARA electronic instruments s.r.l.
Via A.Mercuri 4 - 06129 - Perugia
Tel. +39 075 5051014 Fax +39 075 5006315

Il 19/04/2012 01:37, Tiago Agrelos ha scritto:
> Thanks for the answers.
>
> Nice to know there are different methods for this..
>
> Im using the geophone connected directly to the ADC in a breadboard at
> ~22bits, so i have noise in it.. also i´m limited to the 16bits from
> AmaSeis, so some spikes are only displayed up to 16bits.
>
>   The geophone looks like its responding quite well at 10Hz, i did tried
> tapping with a pencil or different objects to create some "spikes", but
> didint reach any good result. I did not managed to get any good
> recording lower than 5Hz..
>
> Here is the *.sac file with some tests (small/fast taps on the floor, in
> a table, etc etc). The sample rate is at 125SPS, which give a -3dB
> filter at ~33Hz.
> http://dl.dropbox.com/u/15802004/testing.sac
>
> Is it possible anyone see the results and if some movement clearly shows
> the natural frequency of the geophone, tell me the time in the recording?
>
> Thanks.
>
> 2012/4/16 GeoffG >
>
>     On 4/16/2012 7:04 AM, Mauro Mariotti wrote:
>
>         Hi,
>
>
>     I would hook up the geophone to the WINSDR program
>     with no damping then excite it somehow then
>     convert the PSN file to a wave file and examine
>     the spectrum in the Audacity program.
>     It can show you an FFT Waterfall display
>     and is free for personal use if you
>     are not making money at what you do.
>     But also the FFT within WinQuake can also
>     show you the spike at F0.
>     Resonance.
>
>
>
>
>         also better is to bias the coil with a small battery.
>         A simple AA battery (also one with just a residual of charge).
>         Connect an oscilloscope or other suitable device to the sensor
>         output.
>         Atthe same time connect the battery - to ground of oscilloscope
>         and the
>         use the + of battery to excite the geophone.
>         Do this all with the geophone in its natural position
>         (horizontal if it
>         is horizontal sensitivity, or vertical if it is a vertical
>         sensitivity
>         one).
>         You will see a nice swing when you release the + bias from the
>         sensor.
>         The time between of the top of two sequential wave crest tell
>         you the
>         period, and its reciprocal is the freqency.
>
>         best regards
>         Mauro
>
>         Il 16/04/2012 02:23, Tiago Agrelos ha scritto:
>
>             Hi.
>
>             I扉e small geophone almost a year, but i never knew its natural
>             frequency, how can i "find" its approximate natural frequency?
>
>             The geophone doesnt have anything written on it, its smaller
>             than 4.5Hz
>             L15B geophone sold by Larry, generate a lot more output than
>             the 4.5Hz
>             one and i read 380ohm from it.
>
>             I appreciate any help.
>
>             Regards,
>             Tiago.
>
>
>
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>

-- 
Mauro Mariotti
http://www.infoeq.it
http://www.sara.pg.it

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