PSN-L Email List Message

Subject: Re: Seismic detector with AIS326DQ
From: ChrisAtUpw@.......
Date: Wed, 22 Apr 2009 23:39:09 EDT


In a message dated 22/04/2009, alexmirabal2000@........ writes:

I need to build a device that alarms when a seism takes place, preferably  
in an early state, let's say, for being used in public buildings or schools. 
 It is not intended to measure and record the magnitude of the earth 
movement  at all, just detect once it reaches certain level in any of the 3 axis 
and  activate a sound/visual indication.
 
As a first approximation, I was taking a look to this IC: AIS326DQ 
_http://www.st.com/stonline/products/literature/ds/14956/ais326dq.pdf_ 
(http://www.st.com/stonline/products/literature/ds/14956/ais326dq.pdf) ,  but honestly I 
have no practical criteria to determine in advance if it  sensitive enough 
for my purpose.  The manufacturer web page mentions it  is for automotive 
applications, but my feeling is that the accelerations  changes in such 
environments are far beyond the ones I need to deal  with.

Hi Alexander,
 
    If you look at the data sheet, the minimum  detection level is 1 milli 
g and you only have a 12 bit ADC. These MEMs devices  do not have the high 
sensitivity, low noise and resolution that I would normally  consider for 
seismic applications. They use 3.3V supplies and low level digital  outputs, 
not 5V. There is NO low pass filter to remove noise.
 
    You still have to take measurements in the real  world, with all the 
environmental background noise that this entails. 
 
    See the comments about noise and drift at 
_http://psn.quake.net/strongmotion.html_ (http://psn.quake.net/strongmotion.html) 
 
    Larry does offer a triaxial sensor board 
_http://psn.quake.net/psnaccel/_ (http://psn.quake.net/psnaccel/) 

In case you consider it not suitable for this application, could any of  
you, please, recommend some other chip?

    Not really. However, you can make a sensitive  detector using a 44 mm 
OD piezo disk from _www.digikey.com_ (http://www.digikey.com)  such as 
102-1170-ND @ $1.62  each with added load weights, but you do have to produce all 
the electronics,  filters and an ADC. This is quite a lot of work. How good 
are you at designing /  building electronics? 
 
    I use similar discs in flex bending mode, not as a  loaded drum. You 
can also buy high capacity disks already mounted in plastic  cases, but their 
output is nearly square law, not linear like a simple flex  disk. The flex 
disk that I use gives over 5x the voltage output of a 4.5 Hz L15B  geophone 
see _http://psn.quake.net/geophone/index.html_ 
(http://psn.quake.net/geophone/index.html)  and  the setup can go down to 10 seconds period. Note that you 
may want to detect P  and S waves from 0.5 Hz to 5 Hz. 
 
    Let me know if you want any further help?

I am located in Mexico City.


    Regards,
 
    Chris Chapman in UK





In a message dated 22/04/2009, alexmirabal2000@........ writes:
I need to build a device that alarms when a seism takes place, pref= erably in an early state, let's say, for being used in public buildings or scho= ols. It is not intended to measure and record the magnitude of the earth move= ment at all, just detect once it reaches certain level in any of the 3 axis= and activate a sound/visual indication.
 
As a first approximation, I was taking a look to this IC: AIS326DQ= http://www.st.com/stonline/products/literature/ds/14956/ais326dq.pdf= , but honestly I have no practical criteria to determine in advance i= f it sensitive enough for my purpose.  The manufacturer web page mention= s it is for automotive applications, but my feeling is that the accelerations= changes in such environments are far beyond the ones I need to deal with.
Hi Alexander,
 
    If you look at the data sheet, the minimum detection level is 1 milli g and you only have a 12 bit ADC. These MEMs de= vices do not have the high sensitivity, low noise and resolution that I would no= rmally consider for seismic applications. They use 3.3V supplies and low level di= gital outputs, not 5V. There is NO low pass filter to remove noise.
 
    You still have to take measurements in the re= al world, with all the environmental background noise that this entails.=
 
    See the comments about noise and drift at http://psn.quake.net/stron= gmotion.html
 
    Larry does offer a triaxial sensor board http://psn.quake.net/psnaccel/=
In case you consider it not suitable for this application, could an= y of you, please, recommend some other chip?
    Not really. However, you can make a sensitive= detector using a 44 mm OD piezo disk from www.digikey.com such as 102-1170-ND @= $1.62 each with added load weights, but you do have to produce all the electroni= cs, filters and an ADC. This is quite a lot of work. How good are you at desig= ning / building electronics?
 
    I use similar discs in flex bending mode, not= as a loaded drum. You can also buy high capacity disks already mounted in plast= ic cases, but their output is nearly square law, not linear like a simple fle= x disk. The flex disk that I use gives over 5x the voltage output of a 4.5= Hz L15B geophone see http://psn.quake.net/geo= phone/index.html and the setup can go down to 10 seconds period. Note that you may want to dete= ct P and S waves from 0.5 Hz to 5 Hz.
 
    Let me know if you want any further help?
I am located in Mexico City.
    Regards,
 
    Chris Chapman in UK

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