In a message dated 24/09/2006 22:06:39 GMT Daylight Time,  
gmvoeth@........... writes:

>  Why  can't a seismic sensor be built into a microchip somehow  ?
>    You can't get the low noise and the high sensitivity,  partly due to gas 
> interactions in very small spaces and partly due to  intrinsic material 
noise. 
> You also have Brownian noise / kT /  frequency considerations. This may 
limit 
> you to weights of about an  ounce.

I'm not sure this is true, You can create a vacuum
and  eliminate all brownian motion I think

Hi Geoff,
 
    You need some gas present to cool the silicon  wafers and you may also 
need to ~damp the response. The Brownian motion is  caused by the seismometer 
being at room temperature. You are 'stuck' with it. To  get rid of it, you would 
need to cool the seismometer to absolute zero  -273C. 
    I understand that trying to minaturise  accelerometers down to silicon 
chip size has several problems, one of which is  that their properties don't 
quite 'scale' as simple calculations.

>    So we make a highly sensitive vertical sensor,   where the mass is 
> balanced by a spring force. The dimensions of  the  apparatus are 
temperature 
> dependant and the spring constant  is temperature  dependant, but neither 
are strictly 
> linear, or  of comparable magnitude. In  general, it may not be too 
difficult  
> to reduce temperature effects by a factor  of 10, but any  further 
improvement 
> gets progressively much more difficult. Throw in  the fact that springs do 
not 
> behave truly elastically and the whole  problem gets quite difficult. 
Springs 
> with a very low temperature  coefficient are  inherently magnetic, which 
can 
> add other  sources of noise. The STS1 probably represents about the best 
that 
>  can be done commercially.

All we are looking for here is an  unquestionable mark
for first time of arrival and not the whole seismic  signal.
    No, we need to be able to record the whole seismic  signal. I am just 
pointing out that the 'simple' mechanical properties are not  quite linear, so 
when you try to compensate them out, the non linear bits become  of major 
importance.

>    The move from coil springs of the LaCoste type  to  leaf springs of the 
> Streckheisen type enabled the 'parasitic  vibration'  responses to be 
reduced. A 
> bent sheet of copper  plate close to a leaf spring may  be used to 
stabilise 
> the  temperature. Small NdFeB magnets may be stuck to the  leaf spring to  
provide 
> inductive damping with the copper plate. 

You are  getting way too complex for what I call amateurs here.
Amateurs do not make  money off their sport and may be only a layman
with an interest in whatever  area.
    Some amateurs have tried and succeeded very well!  Have a look at 
_http://www.bryantlabs.net/seismo.html_ (http://www.bryantlabs.net/seismo.html)  and  
_http://www.eas.slu.edu/People/STMorrissey/index.html_ 
(http://www.eas.slu.edu/People/STMorrissey/index.html) 

Whatever  happened to that scientific idea that simpler is better and more 
reliable  ?
    Alive and well, but we have to take the world as we  find it, not as we 
would wish it to be! Mathematicians may describe simple  harmonic motion quite 
clearly, but your average pendulum seems to be rather  deaf!  

Regards,
 
    Chris Chapman






In a message dated 24/09/2006 22:06:39 GMT Daylight Time,=20
gmvoeth@........... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>>=20
  Why  can't a seismic sensor be built into a microchip somehow=20
  ?
>    You can't get the low noise and the high sensitivit=
y,=20
  partly due to gas 
> interactions in very small spaces and partly du=
e to=20
  intrinsic material noise. 
> You also have Brownian noise / kT /=20
  frequency considerations. This may limit 
> you to weights of about=20=
an=20
  ounce.

I'm not sure this is true, You can create a vacuum
and=20
  eliminate all brownian motion I think

Hi Geoff,
 
    You need some gas present to cool the sili=
con=20
wafers and you may also need to ~damp the response. The Brownian motion=
 is=20
caused by the seismometer being at room temperature. You are 'stuck' with it=
.. To=20
get rid of it, you would need to cool the seismometer to absolute=20=
zero=20
-273C. 
    I understand that trying to minaturise=20
accelerometers down to silicon chip size has several problems, one of which=20=
is=20
that their properties don't quite 'scale' as simple calculations.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>>    So we make a highly sensitive vertical sensor,&=
nbsp;=20
  where the mass is 
> balanced by a spring force. The dimensions of=20
  the  apparatus are temperature 
> dependant and the spring cons=
tant=20
  is temperature  dependant, but neither are strictly 
> linear,=20=
or=20
  of comparable magnitude. In  general, it may not be too difficult=20
  
> to reduce temperature effects by a factor  of 10, but any=20
  further improvement 
> gets progressively much more difficult. Throw=
 in=20
  the fact that springs do not 
> behave truly elastically and the who=
le=20
  problem gets quite difficult. Springs 
> with a very low temperature=
=20
  coefficient are  inherently magnetic, which can 
> add other=20
  sources of noise. The STS1 probably represents about the best that 
>=
;=20
  can be done commercially.

All we are looking for here is an=20
  unquestionable mark
for first time of arrival and not the whole seismic=
=20
  signal.
    No, we need to be able to record the whole seis=
mic=20
signal. I am just pointing out that the 'simple' mechanical properties are n=
ot=20
quite linear, so when you try to compensate them out, the non linear bits be=
come=20
of major importance.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>>    The move from coil springs of the LaCoste type=20
  to  leaf springs of the 
> Streckheisen type enabled the 'paras=
itic=20
  vibration'  responses to be reduced. A 
> bent sheet of copper=20
  plate close to a leaf spring may  be used to stabilise 
> the=20
  temperature. Small NdFeB magnets may be stuck to the  leaf spring to=20
  provide 
> inductive damping with the copper plate. 

You are=20
  getting way too complex for what I call amateurs here.
Amateurs do not=20=
make=20
  money off their sport and may be only a layman
with an interest in what=
ever=20
  area.
    Some amateurs have tried and succeeded very wel=
l!=20
Have a look at http://www.bryantlabs.net/sei=
smo.html and=20
http://www.eas=
..slu.edu/People/STMorrissey/index.html
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Whatever=20
  happened to that scientific idea that simpler is better and more reliable=20
  ?
    Alive and well, but we have to take the world a=
s we=20
find it, not as we would wish it to be! Mathematicians may describe simple=20
harmonic motion quite clearly, but your average pendulum seems to be rather=20
deaf!=20
 
    Regards,
 
    Chris Chapman