In a message dated 14/04/2006, Brett3kg@............. writes:

>           However, they are  difficult to construct and have a working gap=
=20
of
>       less than a millimetre, and are prone to off axis  sensitivity.
>
>    This is largely incorrect for  amateur applications. If you demand 1nm=20
> resolution or better, you do  need special materials and construction.

I'm not sure pure resolution  is that hard to come by.  Noise & temperature=20
sensitivity are  another matter, though.
Hi Brett,
=20
    There are ways of reducing the effects of thermal  expansion without=20
using exotic materials. Noise can be greatly reduced using  a dual FET front=
 end=20
on a chopper amplifier. The latest types give about 1  nV rms / Root Hz.=20

>      There are three basic types of capacitor  sensor.
>You can have a pair of parallel plates excited by sine or  square waves=20
>with a central sensor plate which moves perpendicular to  the plane. 
    There are two major disadvantages with this  type.
    Movement of the sensor plate displaces air between  the plates on both=20
sides. This damping + spring cushion effect depends  on the plate size, thei=
r=20
separation and on the plate velocity - it varies a  lot. The traditional met=
hods=20
of reducing these problems involve boring air holes  in the capacitor plates=
=20
and evacuating the seismometer case.=20
    This sort of sensor is only linear over the whole  movement range if you=
=20
use either voltage sensing or a special feedback  circuit with 'floating'=20
power supplies. However, you are critically dependant on  stray capacity eff=
ects.=20
These can be largely compensated for sine wave  excitation, but with some=20
difficulty. If you use a charge amplifier, it will  only be approximately li=
near=20
for 1/3 to 1/4 the available movement range.
    Since other designs without these problems are  available, why do things=
=20
the hard way?

>     You can use two pairs of parallel plates with a  central sensor plate=20
> moving parallel to the plane  
>     You can use basically  parallel circuit board plates with a pair of=20
excitation strips on one side, a  cross coupled square / rectangular sense a=
rray=20
on the other and a plate with  vertical 'shadow strips' moving parallel in=20
between. It is easy to make  these out of double sided glass circuit board.=20=
The=20
central shadow plate  does not need to be earthed. Only the fixed plates nee=
d=20
to be wired up,  which is a considerable advantage. The maximum movement is=20
half the  square 'cell size'. Again this can be quite large. See Randall Pet=
ers'=20
SDC  sensor at _http://physics.mercer.edu/petepag/sens.htm_=20
(http://physics.mercer.edu/petepag/sens.htm)  Charge  detection is usually u=
sed. An array of=20
coupled cells can be used to increase  the sensitivity. It is an advantage t=
o=20
make the shadow plate out of etched  double sided glass board. The 'electric=
al=20
thickness' is the actual thickness  divided by the dielectric constant. Havi=
ng=20
all three boards made from the same  material greatly reduces any thermal=20
drift. There is no air flow problem with  plate movement.  

I'd always assumed that only the  first type had adequate displacement=20
sensitivity, a couple of orders of  magnitude greater than the=20
others. Sounds like I need to go back and  check the numbers.
    They mostly come down to dL / L considerations. The  larger you make the=
=20
total range L, the more you need to amplify the signal.  It is quite possibl=
e=20
to maintain a very high sensitivity by detecting the whole  signal range and=
=20
then adding a high pass filter and more amplification. My LVDT  Lehman senso=
r=20
allows the first stage to give +/-10V for the allowed +/-10 mm  range. This=20
signal is then put through a high pass filter and the 'AC' component  is=20
amplified further. This allows resolution of a few 10s of nm on my 16 bit AD=
C  over=20
the whole 10 mm range. =20

>   In a message dated 13/04/2006, Brett3kg@.............  writes:
>Biggest VRDT problem seems to be its low drive frequency. In a  feedback
>design the large demod filters are prime contributors to loop  oscillation=20
>problems.
>     So reduce the  filtration and apply a DC + pulsed feedback? Use=20
> another  method?

Can you amplify on this?  Not exactly sure what your'e  proposing, but it=20
sounds interesting.
    If you look at the circuit in=20
_http://psn.quake.net/info/bb13OperManual.pdf_ (http://psn.quake.net/info/bb=
13OperManual.pdf)  two  stages of RC bandpass=20
filter are used starting with the square wave from a  quartz oscillator. One=
=20
filter is on the input line to the differential capacitor  sensor and the=20
other on the amplified output from the sensor. The output of the  final ampl=
ifier=20
has a DC component with a large AC ripple on it at twice the  oscillator=20
frequency. This is applied to the inductive winding of the feedback  coil th=
rough a=20
parallel RC link to give a low phase error feedback signal.  If you fully=20
smoothed the output, you would have a greater phase delay. You then  add a l=
ow=20
pass filter to smooth the signal for the A/D converter.

>     The feedback phase delay is only a problem if  you do it this way!
> >I'm not sure about the noise. Does the VBB  measure displacements in the
> >=B11 nm range?
>
>With the  sensor plates above, 1LSB=3D0.08nm.  But I think noise is  what
>determines the useful resolution.  However 0.3nm / sqrt-Hz  and 2.1nm RMS a=
t
>50 SPS isn't too shabby.  It would be interesting  to assume a seismic-mass
>system and model how this would compare with  commercial instruments and
>earth-noise models.  I'm betting it  won't look so bad.
>     Have you measured your  environmental noise level? Is 2.1 nm a=20
> realistic target? The  amplitude of the 6 second ocean microseisms may be=20
> from 500 to 15,000  nm!

Actually 2.1nm was no target.  That's just what I calculated  you could get=20
using the AD7745.  I agree that it is a good bit better  than a typical home=
=20
site would justify, which is why it looked so  interesting.
    The lower limit for LVDT measurements is about  0.1 nano m. With=20
capacitative sensors you can reduce this by 100. This is  far smaller than t=
he=20
background noise limits.=20

>  He then goes on to describe the VRDT. I suppose for the VBB  sensor this=20
> would greatly simplify the electronic design if one can  deal with small=20
> sensor gaps. I'm not sure about the noise. Does the  VBB measure=20
> displacements in the 1 =B1 nm range?   --- Just  thinking out loud. I thin=
k=20
> it greatly depends on what type of sensing  one wants to do local,=20
> regional or teleseismic.
>     Amateur seismometers are usually limited by either microseisms or =20
> by environmental noise - we can't usually choose a quiet remote site.  I=20
> managed to reduce the noise of my LVDT to about 7 nm for a 6 mm  range at=20
> 10 Hz, but my environmental noise is much greater than  this.
>
>     There are 'problems that  you do not need to have' :-
>
>     The  velocity feedback damping does not need to be generated that  way=
!
>
>     Neither do we need to use that  troublesome design of capacitative=20
sensor!
>
>   ***   You can use JUST position + integral current / coil feedback  if=20
> you ALSO have a quad magnet + Cu plate for the velocity damping!  Trying=20
> to provide velocity damping by differentiation and coil  feedback is=20
> likely to very significantly increase the overall circuit  noise!  ***

I'm now thinking that's where I was heading, except  for retaining the=20
"perpendicular" capacitance sensor. =20
    I suggest that you reconsider the capacitative  sensor design. You reall=
y=20
don't need pneumatic damping problems and it is  helpful to be able to choos=
e=20
your range. If you are trying to measure a few  parts in a 15,000 nm signal,=
=20
you need the lowest noise most linear system that  you can get. A considerab=
le=20
reduction in expansion coefficient is possible using  either conducting=20
paint, evaporated metal, or etched metal on thin pyrex sheet  glass. You can=
 stick=20
metal foil onto sheet glass with acrylic adhesive -  don't try epoxy unless=20
you prebake the glass to over 150 C. The electrode plates  can be made by ph=
oto=20
etching.
    True chopper amplifier circuits are available.  These are immune to 1/f=20
noise. The SDC type sensor offers a great constructional  advantage in ease=20=
of=20
wiring - no electrical connections are required to the  seismic mass - and y=
ou=20
can use coaxial screened cable with a charge  amplifier.  The design is not=20
too critical on electrode spacing, which can  be quite small.

Since  the AD7746 goes directly from=20
capacitance to digital, I was hoping to use  either a PC or commercial DSP=20
chip or FPGA to do most of the  Position-Velocity derivative and other=20
shaping.  I like that because  that needs minimal analog circuitry and what=20
you do need (integral current  feedback) is working at virtually DC.  And,=20
yes, for that to work you  would need a well-damped and stable spring=20
mass. First-order displacement  linearization could be done digitally.  Also=
=20
you'd need to be sure that  your displacement sensor range was adequate.


There are two problems associated with either  analogue or digital generatio=
n=20
of a velocity feedback signal from a position  signal. The generated signal=20
is inherently noisy. You can also run off the end  of the voltage or count=20
scale --> system failure. The phase errors /  delays need to be considered /=
=20
compensated.=20
    It is relatively easy to provide a very quiet  precision velocity=20
feedback signal within the stop limits of the mass  movement using either ma=
gnet /=20
coil / resistor damping or a quad magnet /  variable area damping plate. Thi=
s=20
uses NO external power OR electronics! Why 'do  things the hard way'?
    The _http://gravity.ucsd.edu/research/OFSEIS/opt_seis.html_=20
(http://gravity.ucsd.edu/research/OFSEIS/opt_seis.html)  account  claims a r=
eduction in the=20
feedback noise using magnet / coil / resistor  damping.
    24 bit DACs are available for positional feedback /  integral feedback.
=20
    Another factor which can significantly effect the  overall seismometer=20
performance lies in the design of the suspension system, as  you outlined in=
=20
your rolling foil design.=20
    The 'art' of being successful lies largely in not  making mistakes and i=
n=20
avoiding unnecessary problems and limitations.
=20
    Regards,
=20
    Chris Chapman





In a message dated 14/04/2006, Brett3kg@............. writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>>           However, they are=20
  difficult to construct and have a working gap of
>    &nbs=
p;=20
       less than a millimetre, and are prone to off axis=20
  sensitivity.
>
>    This is largely incorrect for=20
  amateur applications. If you demand 1nm 
> resolution or better, you=
 do=20
  need special materials and construction.

I'm not sure pure resoluti=
on=20
  is that hard to come by.  Noise & temperature 
sensitivity are=
=20
  another matter, though.
Hi Brett,
 
    There are ways of reducing the effects of therm=
al=20
expansion without using exotic materials. Noise can be greatly reduced using=
=20
a dual FET front end on a chopper amplifier. The latest types give abou=
t 1=20
nV rms / Root Hz. 
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>>      There are three basic types of capacitor=
=20
  sensor.
>You can have a pair of parallel plates excited by sine or=20
  square waves 
>with a central sensor plate which moves perpendicular=
 to=20
  the plane. <clip>
    There are two major disadvantages with this=20
type.
    Movement of the sensor plate displaces air betw=
een=20
the plates on both sides. This damping + spring cushion effect dep=
ends=20
on the plate size, their separation and on the plate velocity - it vari=
es a=20
lot. The traditional methods of reducing these problems involve boring air h=
oles=20
in the capacitor plates and evacuating the seismometer case. 
    This sort of sensor is only linear over the who=
le=20
movement range if you use either voltage sensing or a special feed=
back=20
circuit with 'floating' power supplies. However, you are critically dependan=
t on=20
stray capacity effects. These can be largely compensated for sine wave=20
excitation, but with some difficulty. If you use a charge amplifier, it will=
=20
only be approximately linear for 1/3 to 1/4 the available movement range.
    Since other designs without these problems are=20
available, why do things the hard way?
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>>     You can use two pairs of parallel plates=20=
with a=20
  central sensor plate 
> moving parallel to the plane=20
  <clip>
>     You can use basically=20
  parallel circuit board plates with a pair of excitation strips on one side=
, a=20
  cross coupled square / rectangular sense array on the other and a plate wi=
th=20
  vertical 'shadow strips' moving parallel in between. It is easy to=20=
make=20
  these out of double sided glass circuit board. The central shadow pla=
te=20
  does not need to be earthed. Only the fixed plates need to be wired=
 up,=20
  which is a considerable advantage. The maximum movement is half t=
he=20
  square 'cell size'. Again this can be quite large. See Randall Peters' SDC=
=20
  sensor at http://physics.mercer.=
edu/petepag/sens.htm Charge=20
  detection is usually used. An array of coupled cells can be used to increa=
se=20
  the sensitivity. It is an advantage to make the shadow plate out of etched=
=20
  double sided glass board. The 'electrical thickness' is the actual thickne=
ss=20
  divided by the dielectric constant. Having all three boards made from the=20=
same=20
  material greatly reduces any thermal drift. There is no air flow problem w=
ith=20
  plate movement.  <clip>

I'd always assumed that only the=
=20
  first type had adequate displacement 
sensitivity, a couple of orders o=
f=20
  magnitude greater than the 
others. Sounds like I need to go back=20=
and=20
  check the numbers.
    They mostly come down to dL / L considerations.=
 The=20
larger you make the total range L, the more you need to amplify the sig=
nal.=20
It is quite possible to maintain a very high sensitivity by detecting the wh=
ole=20
signal range and then adding a high pass filter and more amplification. My L=
VDT=20
Lehman sensor allows the first stage to give +/-10V for the allowed +/-10 mm=
=20
range. This signal is then put through a high pass filter and the 'AC' compo=
nent=20
is amplified further. This allows resolution of a few 10s of nm on my 16 bit=
 ADC=20
over the whole 10 mm range.  
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>>   In a message dated 13/04/2006, Brett3kg@bnordgre=
n.org=20
  writes:
>Biggest VRDT problem seems to be its low drive frequency. I=
n a=20
  feedback
>design the large demod filters are prime contributors to l=
oop=20
  oscillation 
>problems.
>     So reduce the=20
  filtration and apply a DC + pulsed feedback? Use 
> another=20
  method?

Can you amplify on this?  Not exactly sure what your'e=
=20
  proposing, but it 
sounds interesting.
    If you look at the circuit in http://psn.quake.net/i=
nfo/bb13OperManual.pdf two=20
stages of RC bandpass filter are used starting with the square wave fro=
m a=20
quartz oscillator. One filter is on the input line to the differential capac=
itor=20
sensor and the other on the amplified output from the sensor. The output of=20=
the=20
final amplifier has a DC component with a large AC ripple on it at twice the=
=20
oscillator frequency. This is applied to the inductive winding of the feedba=
ck=20
coil through a parallel RC link to give a low phase error feedback sign=
al.=20
If you fully smoothed the output, you would have a greater phase delay. You=20=
then=20
add a low pass filter to smooth the signal for the A/D converter.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>>     The feedback phase delay is only a proble=
m if=20
  you do it this way!
> >I'm not sure about the noise. Does the VBB=
=20
  measure displacements in the
> >=B11 nm range?
>
>Wit=
h the=20
  sensor plates above, 1LSB=3D0.08nm.  But I think noise is=20
  what
>determines the useful resolution.  However 0.3nm / sqrt-H=
z=20
  and 2.1nm RMS at
>50 SPS isn't too shabby.  It would be interes=
ting=20
  to assume a seismic-mass
>system and model how this would compare wi=
th=20
  commercial instruments and
>earth-noise models.  I'm betting it=
=20
  won't look so bad.
>     Have you measured your=20
  environmental noise level? Is 2.1 nm a 
> realistic target? The=20
  amplitude of the 6 second ocean microseisms may be 
> from 500 to 15=
,000=20
  nm!

Actually 2.1nm was no target.  That's just what I calculat=
ed=20
  you could get 
using the AD7745.  I agree that it is a good bit be=
tter=20
  than a typical home 
site would justify, which is why it looked so=20
  interesting.
    The lower limit for LVDT measurements is a=
bout=20
0.1 nano m. With capacitative sensors you can reduce this by 100. This=20=
is=20
far smaller than the background noise limits. 
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>>  He then goes on to describe the VRDT. I suppose for th=
e VBB=20
  sensor this 
> would greatly simplify the electronic design if one c=
an=20
  deal with small 
> sensor gaps. I'm not sure about the noise. Does t=
he=20
  VBB measure 
> displacements in the 1 =B1 nm range?   ---=20=
Just=20
  thinking out loud. I think 
> it greatly depends on what type of sen=
sing=20
  one wants to do local, 
> regional or teleseismic.
>  &nb=
sp;=20
     Amateur seismometers are usually limited by either microseism=
s or=20
  
> by environmental noise - we can't usually choose a quiet remote s=
ite.=20
  I 
> managed to reduce the noise of my LVDT to about 7 nm for a 6 mm=
=20
  range at 
> 10 Hz, but my environmental noise is much greater than=20
  this.
>
>     There are 'problems =
that=20
  you do not need to have' :-
>
>     The=20
  velocity feedback damping does not need to be generated that=20
  way!
>
>     Neither do we need to use that=20
  troublesome design of capacitative sensor!
>
> =20
  ***   You can use JUST position + integral current / coil feedba=
ck=20
  if 
> you ALSO have a quad magnet + Cu plate for the velocity dampin=
g!=20
  Trying 
> to provide velocity damping by differentiation and coil=20
  feedback is 
> likely to very significantly increase the overall cir=
cuit=20
  noise!  ***

I'm now thinking that's where I was heading, excep=
t=20
  for retaining the 
"perpendicular" capacitance sensor. =20

    I suggest that you reconsider the capacitative=20
sensor design. You really don't need pneumatic damping problems and it is=20
helpful to be able to choose your range. If you are trying to measure a few=20
parts in a 15,000 nm signal, you need the lowest noise most linear system th=
at=20
you can get. A considerable reduction in expansion coefficient is possible u=
sing=20
either conducting paint, evaporated metal, or etched metal on thin pyrex she=
et=20
glass. You can stick metal foil onto sheet glass with acrylic adhesive=20=
-=20
don't try epoxy unless you prebake the glass to over 150 C. The electrode pl=
ates=20
can be made by photo etching.
    True chopper amplifier circuits are available.=20
These are immune to 1/f noise. The SDC type sensor offers a great constructi=
onal=20
advantage in ease of wiring - no electrical connections are required to the=20
seismic mass - and you can use coaxial screened cable with a charge=20
amplifier.  The design is not too critical on electrode spacing, which=20=
can=20
be quite small.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Since=20
  the AD7746 goes directly from 
capacitance to digital, I was hoping to=20=
use=20
  either a PC or commercial DSP 
chip or FPGA to do most of the=20
  Position-Velocity derivative and other 
shaping.  I like that beca=
use=20
  that needs minimal analog circuitry and what 
you do need (integral cur=
rent=20
  feedback) is working at virtually DC.  And, 
yes, for that to work=
 you=20
  would need a well-damped and stable spring 
mass. First-order displacem=
ent=20
  linearization could be done digitally.  Also you'd need to be sure th=
at=20
  your displacement sensor range was adequate.


    There are two problems associated with either=20
analogue or digital generation of a velocity feedback signal from a position=
=20
signal. The generated signal is inherently noisy. You can also run off the e=
nd=20
of the voltage or count scale --> system failure. The phase errors /=
=20
delays need to be considered / compensated. 
    It is relatively easy to provide a very quie=
t=20
precision velocity feedback signal within the stop limits of the mass=20
movement using either magnet / coil / resistor damping or a quad magnet /=20
variable area damping plate. This uses NO external power OR electronics! Why=
 'do=20
things the hard way'?
    The http://gravit=
y.ucsd.edu/research/OFSEIS/opt_seis.html account=20
claims a reduction in the feedback noise using magnet / coil / resistor=
=20
damping.
    24 bit DACs are available for positional feedba=
ck /=20
integral feedback.
 
    Another factor which can significantly effect t=
he=20
overall seismometer performance lies in the design of the suspension system,=
 as=20
you outlined in your rolling foil design. 
    The 'art' of being successful lies largely in n=
ot=20
making mistakes and in avoiding unnecessary problems and limitations.
 
    Regards,
 
    Chris Chapman