Hello Chris;
question below inline like in news groups.

Maybe psn should have a public news group
no registration is necessary like alt.amateur.seismic.psn ??
maybe I could investigate the creation of such a thing ???
  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Monday, February 09, 2009 9:24 PM
  Subject: Re: HAS ANYONE TRIED THIS b4...


  In a message dated 10/02/2009, gmvoeth@........... writes:
    build the mechanics to any type of seismometer
    then use several different sensors to see which
    produces the best results?
  Hi Geoff,

      That is not necessary.
    does anyone know what the best would be?
      Yes, but they are also sample time and design / range dependant.

      In sequence of best first
      Capacitative. LCDT 10^-12 m
      Inductive. LVDT 10^-10 m
      Optical. 10^-8 m
      Hall Magnetic. 3x10^-8 m

      Wavelength optical is sometimes used, but it gets excessively =
expensive below 5x10^-7 m

      The absolute stability may only be better than ~10^-8 m.

      Velocity sensors tend to be grouped around Inductive, but are =
frequency dependant. They are not so effective at long periods.

      I am excluding the operation of superconducting sensors. They can =
have extremely high stability.
    are there mechanical ways to damp (remove energy) that might work as =
well as magnetic?
      Viscosity methods are difficult to use and strongly temperature =
dependant.
      You can use phased electrostatic feedback as well as =
electromagnetic.=20
    It might be nice to have a chord for a pendulum that will damp as it =
flexes?
      Not usually necessary. Springs sometimes give problems, =
particuarly coil springs. It is easier to apply magnetic damping to leaf =
springs.
    is it possible to levitate two magnets against each other then =
measure changes in the field as the two magnets bounce south against =
south or vice-versa?
      Yes, but it is non linear and temperature dependant. Solenoid + =
magnet can be used, but there is still a temperature dependance and a =
highly stabilised current requirement.
    it seems to me that whatever sensor you use you need two identical =
for a proper input into a differential amplifier?
    that alone would increase your output by 6Dbv or X2?
      That may not be necessary. A single sensor can often drive a =
differential amplifier OK. Single sensors drive non differential =
amplifiers just fine.

  *****************************
  Hello Chris:
  Yes I understand what you say here but I have learned through =
experiences that a differential amp is about the only way to achieve =
stability in that you have a phantom null point that lies within a (lets =
say here) coil. This is also the best way to get common mode rejection =
of unwanted noise. If you use the diff amp as it seems to be intended =
with two identical sensors symmetrical about ground the best results =
seem to be obtained. you can gang multiple sensors together and enhance =
the signal while cancelling 50% of the noise through summation or =
correlation of all the individual sensors. Noise is somewhat random but =
the signal will affect all sensors identically within a small area like =
1ftsquare (1ft^2).
  can you further comment on this idea ?
  (e.g. n^2 (2,4,8) sensors in series or parallel or both all within a =
small area together like on a single slab of concrete bonded and =
shielded and underground all together)
  Best regards
  geoff
  ********************************

      Regards,








Hello =
Chris;
question below inline =
like in news=20
groups.
 
Maybe psn should have a =
public news=20
group
no registration is =
necessary like=20
alt.amateur.seismic.psn ??
maybe I could =
investigate the=20
creation of such a thing ???

  
----- Original Message ----- 
  From:=20
  ChrisAtUpw@....... 
  
To: psn-l@.............. 
  
Sent: Monday, February 09, 2009 =
9:24=20
  PM
  
Subject: Re: HAS ANYONE TRIED =
THIS=20
  b4...
  


  
In a message dated 10/02/2009, gmvoeth@........... =
writes:
  
    
build the mechanics =
to any type=20
    of seismometer
    
then use several =
different=20
    sensors to see which
    
produces the best=20
    results?
  
Hi Geoff,
  
 
  
    That is not necessary.
  
    
does anyone know =
what the best=20
    would be?
  
    Yes, but they are also sample time and =
design /=20
  range dependant.
  
 
  
    In sequence of best first
  
    Capacitative. LCDT 10^-12 m
  
    Inductive. LVDT 10^-10 m
  
    Optical. 10^-8 m
  
    Hall Magnetic. 3x10^-8 m
  
 
  
    Wavelength optical is sometimes used, but =
it gets=20
  excessively expensive below 5x10^-7 m
  
 
  
    The absolute stability may only be better =
than=20
  ~10^-8 m.
  
 
  
    Velocity sensors tend to be grouped =
around=20
  Inductive, but are frequency dependant. They are not so effective at =
long=20
  periods.
  
 
  
    I am excluding the operation of =
superconducting=20
  sensors. They can have extremely high stability.
  
    
are there =
mechanical ways to=20
    damp (remove energy) that might work as well as=20
  magnetic?
  
    Viscosity methods are difficult to use =
and=20
  strongly temperature dependant.
  
    You can use phased electrostatic feedback =
as well=20
  as electromagnetic. 
  
    
It might be nice to =
have a=20
    chord for a pendulum that will damp as it=20
  flexes?
  
    Not usually necessary. Springs sometimes =
give=20
  problems, particuarly coil springs. It is easier to apply magnetic =
damping to=20
  leaf springs.
  
    
is it possible to =
levitate two=20
    magnets against each other then measure changes in the field as the two magnets=20
    bounce =
south against=20
    south or vice-versa?
  
    Yes, but it is non linear and temperature =

  dependant. Solenoid + magnet can be used, but there is still a =
temperature=20
  dependance and a highly stabilised current requirement.
  
    
it seems to me that =
whatever=20
    sensor you use you need two identical for a proper input into a differential=20
    amplifier?
    
that alone would =
increase your=20
    output by 6Dbv or X2?
  
    That may not be necessary. A single =
sensor can=20
  often drive a differential amplifier OK. Single sensors drive non =
differential=20
  amplifiers just fine.
  
 
  
*****************************
  
Hello=20
Chris:
  
Yes I understand what =
you say=20
  here but I have learned through experiences that a differential amp is =
about=20
  the only way to achieve stability in that you have a phantom null =
point that=20
  lies within a (lets say here) coil. This is also the best way to get =
common=20
  mode rejection of unwanted noise. If you use the diff amp as it seems =
to be=20
  intended with two identical sensors symmetrical about ground the best =
results=20
  seem to be obtained. you can gang multiple sensors together and =
enhance the=20
  signal while cancelling 50% of the noise through summation or =
correlation of=20
  all the individual sensors. Noise is somewhat random but the signal =
will=20
  affect all sensors identically within a small area like 1ftsquare=20
  (1ft^2).
  
can you further =
comment on this=20
  idea ?
  
(e.g. n^2 (2,4,8) =
sensors in=20
  series or parallel or both all within a small area together like on a =
single=20
  slab of concrete bonded and shielded and underground all=20
  together)
  
Best=20
regards
  
geoff
  
********************************
  
 
  
    Regards,
  
 
  
    Chris=20
Chapman