Hi all,

  Many years ago, after reading an Amateur Scientist article (July 1957) on 
swinging gate seismometers, I built a sensor based in part on the information 
in the article.  Its design, of course, was greatly affected by what raw 
material I had around, and what could be obtained from friends and the local 
hardware store.  You can read a description of it at John Lahr's web page, 

  http://www.jjlahr.com/science/psn/mcclure/index.html

  Originally, it was fitted out with photo-optical sensing of pendulum 
position.  The image of a pilot lamp filament was projected through a short 
focal length lens on the pendulum to a pair of photocells, hooked up parallel 
in voltage opposition.  The differential voltage output was fed to a 
homebuilt potentiometric recorder.  It was plagued with drift problems.  It 
never took long for the filament image to end up totally on one of the 
photocells.  I gave up my efforts and went on to other interests.

  In the past two years, my interest was revived by a former colleague, who 
wanted me to participate in a tripartite local network he was wanting to set 
up.  He loaned me an old vertical short period Strengnether to operate.  This 
got me going very heavily into programming and signal processing, using 
Visual Basic.  We also had to solve the problem of time synchronization of 
recordings made at separated sites, and how to set up the sensors to have the 
same period and damping, so that response differences would not distort our 
time-of-arrival estimates.

  After all this, I was inspired to take my old sensor and fit it out with a 
pickup coil and magnet to convert it to velocity sensing.  The pendulum is 
short, about 12 inches to the radius of gyration, with a total mass of 66 
grams.  I had no intention of trying for a natural period of more than 12 
seconds, because in my tripartite work, I designed a digital filter that 
could extend the useful range of the sensor to as much as 50 seconds, and 
which furthermore matched the amplitude and phase response of such a long 
period sensor.  It is presently set for a natural period of 11 seconds, and 
holds its centering without any attention.  I really don't know what natural 
period could be achieved.  Certainly more than 11 seconds.  The data obtained 
from it after broadband digital filtering to emulate a 32 second period 
sensor, matches well what I see from the nearby LDEO PAL site, 19 miles from 
me, except for my higher local noise level.

  My seismo colleague and I are now motivated to build some more sensors of 
lightweight design.  Pivots are critical.  The old sensor uses steel needles 
and sapphire bearing cups.  I do not know where I could obtain more sapphire 
bearings, so I am considering the use of mini ball pivots. From experiments 
with diagmagnetic supported upright pendulums, I have discovered (following a 
suggestion from Chris Chapman) that the ball point nib from a BIC pen makes a 
very good pivot.  The ball is highly polished, about 1 mm diameter.  You just 
insert the whole little brass nib into a hole in the boom, epoxy it in place, 
and you have your pivot.  It should rest against a hard flat polished 
surface.  Glass works, but a sapphire plate would be better.  I favor a rigid 
strut (no wire) for mounting the upper pivot.  If the upper pivot plate 
normal points to the center of gravity of the pendulum, there will be no 
lateral forces at balance on the pivots.

  I encourage others build lightweight sensors, large or small, something 
like John Cole's T Max, for example.  After all, we are no longer driving 
galvanometers, just high impedance input dc amplifiers.  The only 
justification for weight that I can see is greater resistance to air 
currents.  You will also find, if you use my magnet and coil design, that you 
will not need any damping other than that obtained by a shunt resistance 
across the coil.  The resistance required will be much higher than the coil 
resistance, so that there will be no loss of signal.

  I am willing to provide more information on how to wind the 800-turn coil, 
and how to filter your data to achieve long period results.  I can write 
post-filter programs for you for enhancing any data file, but I have to know 
the file format.

Cheers,

Bob McClure 
Hi all,



  Many years ago, after reading an Amateur Scientist article (July 1957) on swinging gate seismometers, I built a sensor based in part on the information in the article.  Its design, of course, was greatly affected by what raw material I had around, and what could be obtained from friends and the local hardware store.  You can read a description of it at John Lahr's web page, 



  http://www.jjlahr.com/science/psn/mcclure/index.html



  Originally, it was fitted out with photo-optical sensing of pendulum position.  The image of a pilot lamp filament was projected through a short focal length lens on the pendulum to a pair of photocells, hooked up parallel in voltage opposition.  The differential voltage output was fed to a homebuilt potentiometric recorder.  It was plagued with drift problems.  It never took long for the filament image to end up totally on one of the photocells.  I gave up my efforts and went on to other interests.



  In the past two years, my interest was revived by a former colleague, who wanted me to participate in a tripartite local network he was wanting to set up.  He loaned me an old vertical short period Strengnether to operate.  This got me going very heavily into programming and signal processing, using Visual Basic.  We also had to solve the problem of time synchronization of recordings made at separated sites, and how to set up the sensors to have the same period and damping, so that response differences would not distort our time-of-arrival estimates.



  After all this, I was inspired to take my old sensor and fit it out with a pickup coil and magnet to convert it to velocity sensing.  The pendulum is short, about 12 inches to the radius of gyration, with a total mass of 66 grams.  I had no intention of trying for a natural period of more than 12 seconds, because in my tripartite work, I designed a digital filter that could extend the useful range of the sensor to as much as 50 seconds, and which furthermore matched the amplitude and phase response of such a long period sensor.  It is presently set for a natural period of 11 seconds, and holds its centering without any attention.  I really don't know what natural period could be achieved.  Certainly more than 11 seconds.  The data obtained from it after broadband digital filtering to emulate a 32 second period sensor, matches well what I see from the nearby LDEO PAL site, 19 miles from me, except for my higher local noise level.



  My seismo colleague and I are now motivated to build some more sensors of lightweight design.  Pivots are critical.  The old sensor uses steel needles and sapphire bearing cups.  I do not know where I could obtain more sapphire bearings, so I am considering the use of mini ball pivots. From experiments with diagmagnetic supported upright pendulums, I have discovered (following a suggestion from Chris Chapman) that the ball point nib from a BIC pen makes a very good pivot.  The ball is highly polished, about 1 mm diameter.  You just insert the whole little brass nib into a hole in the boom, epoxy it in place, and you have your pivot.  It should rest against a hard flat polished surface.  Glass works, but a sapphire plate would be better.  I favor a rigid strut (no wire) for mounting the upper pivot.  If the upper pivot plate normal points to the center of gravity of the pendulum, there will be no lateral forces at balance on the pivots.



  I encourage others build lightweight sensors, large or small, something like John Cole's T Max, for example.  After all, we are no longer driving galvanometers, just high impedance input dc amplifiers.  The only justification for weight that I can see is greater resistance to air currents.  You will also find, if you use my magnet and coil design, that you will not need any damping other than that obtained by a shunt resistance across the coil.  The resistance required will be much higher than the coil resistance, so that there will be no loss of signal.



  I am willing to provide more information on how to wind the 800-turn coil, and how to filter your data to achieve long period results.  I can write post-filter programs for you for enhancing any data file, but I have to know the file format.



Cheers,



Bob McClure