In a message dated 02/02/2007, mckimzey@........... writes:

I have  continued to modify my initial Lehman prototype.  I had been getting  
noise so I tried several things... 1) boom now made of aluminum, lead and  
stainless steel.  No iron present.  2) Magnet on ground and coil  on boom.  
3) Aluminum dampening.

These  seem to have removed some noise but some still remained, 
especially when  little feet were jumping above the crawl space.  Looking at 
a design  by Chris Chapman, he oriented the poles of the magnet up and down, 
as  opposed to sideways.  I was wondering: is the up-down orientation better, 
 
as any up-down motion of the ground (probably noise) would _not_ induce a  
current because movement of the coil would then be parallel to the field  
lines, while side-to-side movement (in the axis of the pendulum) would  still 
be perpendicular to the field lines and induce a current.  Any  thoughts?
Hi Mike,
 
    I have used both orientations. Can you check to see  if up and down 
motions of the mass produce any signal? Tap / thump the top of  the frame? Some 
seismometer constructions / suspensions are more rigid than  others. If this is a 
problem, it can be damped in two planes, using four square  NdFeB magnets on 
both backing plates with a vertical orientation.
 
    You can also get problems if the centre of the  damping force is at a 
different height to the line joining the centre of mass to  the bottom suspension 
- eg rocking of the seismometer arm about it's long axis.  I can't remember 
your construction. Is there a photo anywhere? Twin wire V  suspensions were 
used on commercial instruments to avoid this.
 
    The pounding of little (or big) feet is a real  vibration.
 
    I suggest that you view the background noise and  type over 24 hr periods 
to check for times of increased activity.

A second  question - some have reported that the SG has some limitation 
because of  electronics. With some other seismometer designs using 
capacitance  displacement (Allan Coleman's designs, volksmeter), has anyone 
thought  about changing the design of the SG to using capacitance 
displacement, or  would that be unfeasible?
    It should be relatively easy with Allan's circuit.  You would probably 
use a higher audio frequency to drive the sensor and phase  sensitive detection. 
The original SG circuit used two tuned resonant circuits  and diode 
rectification for the position sensor. Both are temperature sensitive  and can cause 
drifts, but a good thermally insulated case should minimise any  problems. 
Silicon diodes drift by ~2.5 mV / C Deg and you are interested in  micro volt 
resolution.
 
    Regards,
 
    Chris Chapman

 





In a message dated 02/02/2007, mckimzey@........... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I have=20
  continued to modify my initial Lehman prototype.  I had been getting=20
  
noise so I tried several things... 1) boom now made of aluminum, lead=20=
and=20
  
stainless steel.  No iron present.  2) Magnet on ground and=20=
coil=20
  on boom.  
3) Aluminum dampening.

     These=
=20
  seem to have removed some noise but some still remained, 
especially wh=
en=20
  little feet were jumping above the crawl space.  Looking at 
a des=
ign=20
  by Chris Chapman, he oriented the poles of the magnet up and down, 
as=20
  opposed to sideways.  I was wondering: is the up-down orientation bet=
ter,=20
  
as any up-down motion of the ground (probably noise) would _not_ induc=
e a=20
  
current because movement of the coil would then be parallel to the fie=
ld=20
  
lines, while side-to-side movement (in the axis of the pendulum) would=
=20
  still 
be perpendicular to the field lines and induce a current. =20=
Any=20
  thoughts?
Hi Mike,
 
    I have used both orientations. Can you check to=
 see=20
if up and down motions of the mass produce any signal? Tap / thump the top o=
f=20
the frame? Some seismometer constructions / suspensions are more rigid=20=
than=20
others. If this is a problem, it can be damped in two planes, using four squ=
are=20
NdFeB magnets on both backing plates with a vertical orientation.
 
    You can also get problems if the centre of the=20
damping force is at a different height to the line joining the centre of mas=
s to=20
the bottom suspension - eg rocking of the seismometer arm about it's long ax=
is.=20
I can't remember your construction. Is there a photo anywhere? Twin wire V=20
suspensions were used on commercial instruments to avoid this.
 
    The pounding of little (or big) feet is a real=20
vibration.
 
    I suggest that you view the background noise an=
d=20
type over 24 hr periods to check for times of increased activity.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>A second=20
  question - some have reported that the SG has some limitation 
because=20=
of=20
  electronics. With some other seismometer designs using 
capacitanc=
e=20
  displacement (Allan Coleman's designs, volksmeter), has anyone 
thought=
=20
  about changing the design of the SG to using capacitance 
displacement,=
 or=20
  would that be unfeasible?
    It should be relatively easy with Allan's circu=
it.=20
You would probably use a higher audio frequency to drive the sensor and phas=
e=20
sensitive detection. The original SG circuit used two tuned resonant circuit=
s=20
and diode rectification for the position sensor. Both are temperature sensit=
ive=20
and can cause drifts, but a good thermally insulated case should minimise an=
y=20
problems. Silicon diodes drift by ~2.5 mV / C Deg and you are interested in=20
micro volt resolution.
 
    Regards,
 
    Chris Chapman