In a message dated 03/02/2007, lehmancj@........... writes:

You mention your design has the coil on the boom, and  magnet on the
ground.  That is quite ok I guess, but I have never  understood the advantage
over the coil on the ground - with the coil on the  ground, the boom is free
of any attachments..... Maybe I am missing  something---



Hi Jim,
 
    There are several different situations if you mount  a magnet on the 
boom, use an iron mass, or make the boom from steel, some  of which are worse than 
others. You are very likely to be sensitive to magnetic  field changes, or to 
changes in the field gradient, all of which can result  in unwanted noise. 
You don't have to use magnetic components! It is advisable to  'design out' such 
problems in the first place where possible, rather than having  to 'find and 
eliminate' them later on. Commercial mechanical seismometers  are usually 
designed with integral magnetic shielding - even the Ni-SpanC  springs are 
magnetic. 
 
    The principle effects are due to changes in the  Earth's field, to the 
magnetic attraction / field changes produced by cars,  trucks, trains, kids 
cycles, lawn mowers and especially to pulses on the utility  power wiring in the 
house. TVs, refrigerators, cookers and central heating  systems are common 
culprits. A seismometer is extremely sensitive - it doesn't  take much force to 
move the arm by over 50 nano metres!
 
    You may pick up RF noise / static with the sensor  coil / onto the 
wiring. I favour the quad NdFeB magnet arrangement on mild steel  backing plates for 
producing the sensor field. This construction shields the  pick up coil from 
both external magnetic and electric fields and you don't need  very large 
coils. 
    Co-axial microphone cable gives good screening. I  use the sort which has 
a black plastic conducting layer in between the woven  screen and the central 
polythene insulation. This reduces signals due to  vibration / cable movement 
/ thermal expansion. 
 
    Regards,
 
    Chris Chapman






In a message dated 03/02/2007, lehmancj@........... writes:
<=
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  size=3D2>   You mention your design has the coil on the boom, an=
d=20
  magnet on the
ground.  That is quite ok I guess, but I have never=20
  understood the advantage
over the coil on the ground - with the coil on=
 the=20
  ground, the boom is free
of any attachments..... Maybe I am missing=20
  something---


Hi Jim,
 
    There are several different situations if you m=
ount=20
a magnet on the boom, use an iron mass, or make the boom from steel, so=
me=20
of which are worse than others. You are very likely to be sensitive to magne=
tic=20
field changes, or to changes in the field gradient, all of which can re=
sult=20
in unwanted noise. You don't have to use magnetic components! It is advisabl=
e to=20
'design out' such problems in the first place where possible, rather than ha=
ving=20
to 'find and eliminate' them later on. Commercial mechanical seismomete=
rs=20
are usually designed with integral magnetic shielding - even the Ni-SpanC=20
springs are magnetic. 
 
    The principle effects are due to changes in the=
=20
Earth's field, to the magnetic attraction / field changes produced by c=
ars,=20
trucks, trains, kids cycles, lawn mowers and especially to pulses on the uti=
lity=20
power wiring in the house. TVs, refrigerators, cookers and central heat=
ing=20
systems are common culprits. A seismometer is extremely sensitive - it doesn=
't=20
take much force to move the arm by over 50 nano metres!
 
    You may pick up RF noise / static with the sens=
or=20
coil / onto the wiring. I favour the quad NdFeB magnet arrangement on mild s=
teel=20
backing plates for producing the sensor field. This construction shields the=
=20
pick up coil from both external magnetic and electric fields and you don't n=
eed=20
very large coils. 
    Co-axial microphone cable gives good screening.=
 I=20
use the sort which has a black plastic conducting layer in between the woven=
=20
screen and the central polythene insulation. This reduces signals due t=
o=20
vibration / cable movement / thermal expansion. 
 
    Regards,
 
    Chris Chapman