This is a really old idea. It's funny how things keep coming back around. In
1991 Bob Ogborn of San Jose and later Texas built a bunch of these types of
devices. They are best for short periods and local events. His design was
simple. He used the Piezo butterfly fan from the Apple computer and encased
it in a short length of PVC pipe with two PVC caps. He drilled a hole in one
cap for the wires to exit and filled the device with oil for damping and
then sealed it up with glue. They worked well enough but they can generate a
lot of current if they are tapped too hard so he had to protect the circuit
with two clamping diodes or the current would fry the electronics.
Regards, Steve Hammond PSN San Jose, Aptos CA.
From: psn-l-request@.............. [mailto:psn-l-request@............... On
Behalf Of ChrisAtUpw@.......
Sent: Saturday, May 30, 2009 8:50 PM
To: psn-l@..............
Subject: Re: Geophones / Piezo sensors
In a message dated 30/05/2009, DSaum@............ writes:
Hi Chris
Sounds interesting in theory, but how well does it actually work?
Do you have any data / spectra from your piezo device that shows it
picks up teleseismic events as well as a my horizontal 4.5 Hz geophone?
Dave
Hi Dave,
There are more sensor construction details and circuits at
http://www.jclahr.com/science/psn/chapman/piezo/index.html Please Note that
the stated circuit components and their values are critical to success.
The device acts as an accelerometer, but the internal noise of capacitor
is very low and the voltage sensitivity to bending stresses is high.
I initially extended the period of my 4.5 Hz geophones by x10 to 0.5 Hz
using a modified Roberts circuit and it worked fine. See
http://www.jclahr.com/science/psn/roberts/index.html The increased response
to teleseismic P and S waves is very evident. The modifications were to
effectively add a high pass function at about 0.3 Hz to remove most of the
1/f VLF noise, that the original circuit displays only too well. You can
use CAZ opamps for the input stages ($$), but a filter also reduces 6 second
microseism noise, which is also quite obvious. I do not find microseisms
particularly interesting.
Lennartz put a negative input impedance amplifier onto 4.5 or 2 Hz
geophones and amplify the current required to keep the mass stationary. They
can increase the response period to 20 seconds this way, but you need
careful temperature compensation and a very low noise amplifier. See
http://www.lennartz-electronic.de/Pages/Seismology/Seismometers/Seismometers
..html The NoeMax triaxial sensor used in French schools appears to be
similar but they are not exactly cheap either. See
http://www.agecodagis.com/WebData/Documentation/Commercial/NoeMax/NoeMax.pdf
The apparent output response below 0.05 Hz concerns me and it crosses the
NHNM high ground noise plot. Extending 4.5 Hz geophones to 20 seconds is a
factor of x90. This would be below the noise level of the 'standard' seismic
amplifier that I used.
I first made a very simple piezo detector by sticking a disk onto two
ridges of adhesive silicone rubber spaced 1.25" apart on a backing plate.
This silicone adhesive does NOT smell of acetic acid / vinegar. I then stuck
a brass rod parallel to the ridges on the top centre of the disk. I used a
unity gain FET amplifier to provide a signal for the same amplifier used for
observing the geophone signal. The piezo signal for the background noise was
about 5x that of the geophone, measured on an oscilloscpe. I then tried to
optimise the design and extend the period as far as practicable. My sensors
do work fine to well below 1 Hz. I do not know about yours, Dave.
Regards,
Chris Chapman
This is a really old idea. It’s funny how things =
keep coming
back around. In 1991 Bob Ogborn of San Jose and later Texas built a =
bunch of
these types of devices. They are best for short periods and local =
events. His
design was simple. He used the Piezo butterfly fan from the Apple =
computer and
encased it in a short length of PVC pipe with two PVC caps. He drilled a =
hole
in one cap for the wires to exit and filled the device with oil for =
damping and
then sealed it up with glue. They worked well enough but they can =
generate a
lot of current if they are tapped too hard so he had to protect the =
circuit
with two clamping diodes or the current would fry the electronics. =
Regards, Steve Hammond PSN San Jose, Aptos CA. =
From:=
psn-l-request@.............. [mailto:psn-l-request@............... On =
Behalf
Of ChrisAtUpw@.......
Sent: Saturday, May 30, 2009 8:50 PM
To: psn-l@..............
Subject: Re: Geophones / Piezo sensors
In a message dated 30/05/2009, DSaum@............ =
writes:
Hi Chris
Sounds interesting in theory, but how well does it actually work?
Do you have any data / spectra from your piezo device that shows it
picks up teleseismic events as well as a my horizontal 4.5 Hz =
geophone?
Dave
Hi Dave,
There are more sensor construction =
details
and circuits at http:=
//www.jclahr.com/science/psn/chapman/piezo/index.html Please
Note that the stated circuit components and their values are =
critical to
success.
The device acts as an =
accelerometer, but
the internal noise of capacitor is very low and the voltage sensitivity =
to
bending stresses is high.
I initially extended the period of =
my 4.5
Hz geophones by x10 to 0.5 Hz using a modified Roberts circuit and it =
worked
fine. See http://www.=
jclahr.com/science/psn/roberts/index.html The
increased response to teleseismic P and S waves is very evident. The
modifications were to effectively add a high pass function at about =
0.3 Hz
to remove most of the 1/f VLF noise, that the original circuit =
displays
only too well. You can use CAZ opamps for the input stages ($$), but a =
filter
also reduces 6 second microseism noise, which is also quite obvious. I =
do not
find microseisms particularly interesting.
Lennartz put a negative input =
impedance
amplifier onto 4.5 or 2 Hz geophones and amplify the current required to =
keep
the mass stationary. They can increase the response period to 20 =
seconds
this way, but you need careful temperature compensation and a very =
low
noise amplifier. See http://www.lennartz-electronic.de/Pages/Seismology/Seis=
mometers/Seismometers.html The
NoeMax triaxial sensor used in French schools appears to =
be similar
but they are not exactly cheap either. See http://www.agecodagis.com/WebData/Documentation/Commercial/N=
oeMax/NoeMax.pdf
The apparent output response below 0.05 Hz concerns me and it crosses =
the NHNM
high ground noise plot. Extending 4.5 Hz geophones to 20 seconds is a =
factor of
x90. This would be below the noise level of the 'standard' seismic =
amplifier
that I used.
I first made a very simple piezo =
detector
by sticking a disk onto two ridges of adhesive silicone rubber spaced
1.25" apart on a backing plate. This silicone adhesive does NOT =
smell of
acetic acid / vinegar. I then stuck a brass rod parallel to the ridges =
on the
top centre of the disk. I used a unity gain FET amplifier to provide a =
signal
for the same amplifier used for observing the geophone signal. The piezo =
signal
for the background noise was about 5x that of the geophone, measured on =
an
oscilloscpe. I then tried to optimise the design and extend the =
period as
far as practicable. My sensors do work fine to well below 1 =
Hz. I do
not know about yours, Dave.
Regards,
Chris Chapman