Hi Steve, I can picture what you are describing, I think.........Do =
you know of any drawings, sketches, which shows this? Is it just a =
"butterfly fan" which looks like tongue depressor and vibrates on one =
end?
Did he add a mass to the end?
Thanks, Ted
----- Original Message -----=20
From: Steve Hammond=20
To: psn-l@.................
Sent: Saturday, May 30, 2009 10:31 PM
Subject: RE: Geophones / Piezo sensors
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.=20
Regards, Steve Hammond PSN San Jose, Aptos CA. =20
=20
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
=20
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,
=20
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.
=20
The device acts as an accelerometer, but the internal noise of =
capacitor is very low and the voltage sensitivity to bending stresses is =
high.
=20
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.
=20
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/Seismomet=
ers.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.=20
=20
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.
=20
Regards,
=20
Chris Chapman
=20
=20
Hi Steve, I can picture =
what you are=20
describing, I think.........Do you know of any drawings, sketches, which =
shows=20
this? Is it just a "butterfly fan" which looks like tongue =
depressor=20
and vibrates on one end?
Did he add a mass to the =
end?
Thanks, Ted
----- Original Message -----
From:=20
Steve=20
Hammond
Sent: Saturday, May 30, 2009 =
10:31=20
PM
Subject: RE: Geophones / Piezo=20
sensors
This=20
is a really old idea. It=92s funny how things keep coming back around. =
In 1991=20
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=20
simple. He used the Piezo butterfly fan from the Apple computer and =
encased it=20
in a short length of PVC pipe with two PVC caps. He drilled a hole in =
one cap=20
for the wires to exit and filled the device with oil for damping and =
then=20
sealed it up with glue. They worked well enough but they can generate =
a lot of=20
current if they are tapped too hard so he had to protect the circuit =
with two=20
clamping diodes or the current would fry the electronics.=20
Regards,=20
Steve Hammond PSN San Jose, Aptos CA. =
From: =20
psn-l-request@.............. [mailto:psn-l-request@............... =
On=20
Behalf Of ChrisAtUpw@.......
Sent: Saturday, May 30, =
2009 8:50=20
PM
To: psn-l@..............
Subject: Re: Geophones =
/ Piezo=20
sensors
In a=20
message dated 30/05/2009, DSaum@...............
writes:
Hi=20
Chris
Sounds interesting in theory, but how well does it actually =
work?
Do you have any data / spectra from your piezo device that =
shows=20
it
picks up teleseismic events as well as a my horizontal 4.5 Hz=20
geophone?
Dave
Hi=20
Dave,
There=20
are more sensor construction details and circuits at http:=
//www.jclahr.com/science/psn/chapman/piezo/index.html Please=20
Note that the stated circuit components and their values are =
critical to=20
success.
The=20
device acts as an accelerometer, but the internal noise of capacitor =
is very=20
low and the voltage sensitivity to bending stresses is=20
high.
I=20
initially extended the period of my 4.5 Hz geophones by x10 to 0.5 Hz =
using a=20
modified Roberts circuit and it worked fine. See http://www.=
jclahr.com/science/psn/roberts/index.html The=20
increased response to teleseismic P and S waves is very evident. The=20
modifications were to effectively add a high pass function at =
about 0.3=20
Hz to remove most of the 1/f VLF noise, that the original =
circuit=20
displays only too well. You can use CAZ opamps for the input stages =
($$), but=20
a filter also reduces 6 second microseism noise, which is also quite =
obvious.=20
I do not find microseisms particularly=20
interesting.
Lennartz=20
put a negative input impedance amplifier onto 4.5 or 2 Hz geophones =
and=20
amplify the current required to keep the mass stationary. They can =
increase=20
the response period to 20 seconds this way, but you =
need careful=20
temperature compensation and a very low noise amplifier. See http://www.lennartz-electronic.de/Pages/Seismology/Seis=
mometers/Seismometers.html The=20
NoeMax triaxial sensor used in French schools appears to =
be similar=20
but they are not exactly cheap either. See http://www.agecodagis.com/WebData/Documentation/Commercial/N=
oeMax/NoeMax.pdf =20
The apparent output response below 0.05 Hz concerns me and it crosses =
the NHNM=20
high ground noise plot. Extending 4.5 Hz geophones to 20 seconds is a =
factor=20
of x90. This would be below the noise level of the 'standard' seismic=20
amplifier that I used.
I=20
first made a very simple piezo detector by sticking a disk onto two =
ridges of=20
adhesive silicone rubber spaced 1.25" apart on a backing plate. This =
silicone=20
adhesive does NOT smell of acetic acid / vinegar. I then stuck a brass =
rod=20
parallel to the ridges on the top centre of the disk. I used a unity =
gain FET=20
amplifier to provide a signal for the same amplifier used for =
observing the=20
geophone signal. The piezo signal for the background noise was about =
5x that=20
of the geophone, measured on an oscilloscpe. I then tried to =
optimise the=20
design and extend the period as far as practicable. My sensors do =
work=20
fine to well below 1 Hz. I do not know about yours,=20
Dave.
Regards,
Chris=20
Chapman
=