In a message dated 13/04/2006, barry_lotz@............. writes:

Hi Brett
  STM wrote as a solution to LVDTs "The common solution is to use a =20
capacitive bridge transducer, where a moving vane moves between two fixed  p=
lates that=20
form a capacitive bridge that is unbalanced by the movement of the  vane.=20
With proper geometry, these can be very sensitive and linear over a  range o=
f=20
several hundred microns.

Hi Barry,
=20
    The capacitance in pF =3D 0.0885xAxK / d, where A is  the plate area in=20=
sq=20
cm, K is the dielectric constant and d is the plate  separation in cm.=20
    With proper design, sensors can be linear over +/-5  mm or more. It=20
depends on what you need. It may be difficult to get the noise  below 1 ppm=20=
x range=20
for 10 Hz bandwidth, but it can be done.

However, they are difficult to construct and have a working gap of less  tha=
n=20
a millimetre, and are prone to off axis sensitivity.=20

    This is largely incorrect for amateur applications.  If you demand 1nm=20
resolution or better, you do need special materials and  construction -> Sil=
ver=20
coated Invar / Platinum coated Quartz electrodes.=20
=20
    There are three basic types of capacitor sensor.  You can have a pair of=
=20
parallel plates excited by sine or square waves with a  central sensor plate=
=20
which moves perpendicular to the plane. This needs  voltage detection to be=20
highly linear and has a range limited by  the separation of the outer plates=
 -=20
likely to be quite limited. If you use  charge sensing, the linear range is=20
reduced to maybe 1/4. You are likely to  have to bore holes in the plates to=
 allow=20
adequate airflow as they move.
    You can use two pairs of parallel plates with a  central sensor plate=20
moving parallel to the plane - a split stator  variable capacitor. The excit=
ation=20
is applied between the pairs of plates on  opposite sides of the sensor=20
plate. Charge detection is usually used with an  electrical connection to ce=
ntral=20
plate. The sensor range depends on  the width of the moving plate, which is=20
half the overall stator width.  There is no air flow problem with plate move=
ment.
    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 on=20
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 to=20
be wired up, which is a considerable  advantage. The maximum movement is hal=
f=20
the square 'cell size'. Again  this can be quite large. See Randall Peters'=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 make=20
the shadow plate out of etched double  sided glass board. The 'electrical=20
thickness' is the actual thickness divided by  the dielectric constant. Havi=
ng all=20
three boards made from the same material  greatly reduces any thermal drift.=
=20
There is no air flow problem with plate  movement.

They usually operate at relatively high frequencies, from khz to mhz,  which=
=20
complicates design and implementation problems"=20

    Operating at 10 to 50 k Hz is just fine. You can  use digital to sine=20
weighted step down counters which give very good sine waves  - see=20
_http://www.eas.slu.edu/People/STMorrissey/index.html_=20
(http://www.eas.slu.edu/People/STMorrissey/index.html)    Alternatively, you=
 can use a LTC1043 quad chopper switch=20
/ oscillator with  either sine or square waves. Using sine waves allows you=20=
to=20
get a higher  S/N ratio. XR8038 & especially XR2206 function generators can=20
give quite a  good performance, as can a FET stabilised Wein Bridge oscillat=
or=20
- see _http://www.keckec.com/seismo/_ (http://www.keckec.com/seismo/) . Two=20=
=20
stages of RC bandpass filter are used in=20
_http://psn.quake.net/info/bb13OperManual.pdf_ (http://psn.quake.net/info/bb=
13OperManual.pdf)  starting  with the=20
square wave from a quartz oscillator. For low drift, avoid  resonant circuit=
s=20
and diode rectification.

It looks like the AD device would solve some of these  concerns.=20

    The concerns seem to be largely illusory in  practice.
=20
=20
In a message dated 13/04/2006 14:48:47 GMT Daylight Time, =20
Brett3kg@............. writes:=20

Biggest  VRDT problem seems to be its low drive frequency. In a feedback =20
design the large demod filters are prime contributors to loop oscillation =20
problems.
    So reduce the filtration and apply a DC + pulsed  feedback? Use another=20
method?
    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 1=20=
=20
>=B1 nm range?

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

=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 sen=
sor =20
gaps. I'm not sure about the noise. Does the VBB measure displacements in  t=
he=20
1 =B1 nm range?   --- Just thinking out loud. I think it greatly  depends on=
=20
what type of sensing one wants to do local, regional or  teleseismic.

      Amateur seismometers are usually  limited by either microseisms or by=20
environmental noise - we can't usually  choose a quiet remote site. I manage=
d=20
to reduce the noise of my LVDT to about 7  nm for a 6 mm range at 10 Hz, but=
 my=20
environmental noise is much greater  than this.=20
=20


It would  be great to be able to use this with feedback.  No question that=20
you  could use it for integral feedback, which mainly works below the low =20
frequency roll off (say, below 0.011 Hz).  You could make an awesome =20
integrator by digital summing, then feed back with a D/A.  Mid  frequency=20
range (derivative feedback) may not be practical.  Although  you could keep=20
your beam well centred with integral feedback, without  derivative feedback=20
at higher frequencies, you're limited by the +/- 1.0mm  (+/- 0.5 mm max, for=
=20
linearity) sensor gap.  I'm suspecting that  clipping levels in the=20
mid-frequency range are going to be the biggest  limitation.




So, we may need some 'lateral thinking'  here! There are 'problems that you=20
do not need to have' - like:-
=20
    The velocity feedback damping does not need to be  generated that way!
=20
    Neither do we need to use that troublesome design  of capacitative senso=
r!
=20
 ***   You can use JUST position +  integral current / coil feedback if you=20
ALSO have a quad magnet +  Cu plate for the velocity damping! Trying to prov=
ide=20
velocity damping by  differentiation and coil feedback is likely to very=20
significantly increase the  overall circuit noise!  ***
=20
    Regards,
=20
    Chris Chapman






In a message dated 13/04/2006, barry_lotz@............. writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
  
Hi Brett
  
  STM wrote as a solution to LVDTs "The common solution is to us=
e a=20
  capacitive bridge transducer, where a moving vane moves between two fixed=20
  plates that form a capacitive bridge that is unbalanced by the movement of=
 the=20
  vane. With proper geometry, these can be very sensitive and linear over a=20
  range of several hundred microns.
Hi Barry,
 
    The capacitance in pF =3D 0.0885xAxK / d, where=
 A is=20
the plate area in sq cm, K is the dielectric constant and d is the plate=20
separation in cm. 
    With proper design, sensors can be linear over=20=
+/-5=20
mm or more. It depends on what you need. It may be difficult to get the nois=
e=20
below 1 ppm x range for 10 Hz bandwidth, but it can be done.

  
However, they are difficult to construct and have a working gap o=
f less=20
  than a millimetre, and are prone to off axis sensitivity.=20

    This is largely incorrect for amateur applicati=
ons.=20
If you demand 1nm resolution or better, you do need special materials and=20
construction -> Silver coated Invar / Platinum coated Quartz electrodes.=20

 
    There are three basic types of capacitor sensor=
..=20
You can have a pair of parallel plates excited by sine or square waves with=20=
a=20
central sensor plate which moves perpendicular to the plane. This nee=
ds=20
voltage detection to be highly linear and has a range limited by=20
the separation of the outer plates - likely to be quite limited. If you=
 use=20
charge sensing, the linear range is reduced to maybe 1/4. You are likel=
y to=20
have to bore holes in the plates to allow adequate airflow as they move.
    You can use two pairs of parallel plates with a=
=20
central sensor plate moving parallel to the plane - a split stator=20
variable capacitor. The excitation is applied between the pairs of plates on=
=20
opposite sides of the sensor plate. Charge detection is usually used with an=
=20
electrical connection to central plate. The sensor range depends on=20
the width of the moving plate, which is half the overall stator wi=
dth.=20
There is no air flow problem with plate movement.
    You can use basically parallel circuit board pl=
ates=20
with a pair of excitation strips on one side, a cross coupled square /=20
rectangular sense array on the other and a plate with vertical 'shadow strip=
s'=20
moving parallel in between. It is easy to make these out of double si=
ded=20
glass circuit board. The central shadow plate does not need to be earth=
ed.=20
Only the fixed plates need to be wired up, which is a considerable=20
advantage. The maximum movement is half the square 'cell size'. Aga=
in=20
this can be quite large. See Randall Peters' SDC sensor at http://physics.mercer.ed=
u/petepag/sens.htm Charge=20
detection is usually used. An array of coupled cells can be used to increase=
 the=20
sensitivity. It is an advantage to make the shadow plate out of etched doubl=
e=20
sided glass board. The 'electrical thickness' is the actual thickness divide=
d by=20
the dielectric constant. Having all three boards made from the same material=
=20
greatly reduces any thermal drift. There is no air flow problem with plate=20
movement.

  
They usually operate at relatively high frequencies, from khz to=20=
mhz,=20
  which complicates design and implementation problems" 
    Operating at 10 to 50 k Hz is just fine. You ca=
n=20
use digital to sine weighted step down counters which give very good sine wa=
ves=20
- see http://www.eas=
..slu.edu/People/STMorrissey/index.html =20
Alternatively, you can use a LTC1043 quad chopper switch / oscillator w=
ith=20
either sine or square waves. Using sine waves allows you to get a highe=
r=20
S/N ratio. XR8038 & especially XR2206 function generators can give quite=
 a=20
good performance, as can a FET stabilised Wein Bridge oscillator - see http://www.keckec.com/seismo/. Tw=
o=20
stages of RC bandpass filter are used in http://psn.quake.net/i=
nfo/bb13OperManual.pdf starting=20
with the square wave from a quartz oscillator. For low drift, avoid=20
resonant circuits and diode rectification.

  
It looks like the AD device would solve some of these=20
  concerns. 
    The concerns seem to be largely illusory in=20
practice.
 
 
In a message dated 13/04/2006 14:48:47 GMT Daylight Time,=20
Brett3kg@............. writes: 
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Biggest=20
  VRDT problem seems to be its low drive frequency. In a feedback=20
  
design the large demod filters are prime contributors to loop oscillat=
ion=20
  problems.
    So reduce the filtration and apply a DC + pulse=
d=20
feedback? Use another method?
    The feedback phase delay is only a problem if y=
ou=20
do it this way!
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>>I'm=20
  not sure about the noise. Does the VBB measure displacements in the 1=20
  
>=B1 nm range?

With the sensor plates above, 1LSB=3D0.08nm.&=
nbsp;=20
  But I think noise is what 
determines the useful resolution.  Howe=
ver=20
  0.3nm / sqrt-Hz and 2.1nm RMS at 
50 SPS isn't too shabby.  It wou=
ld=20
  be interesting to assume a seismic-mass 
system and model how this woul=
d=20
  compare with commercial instruments and 
earth-noise models.  I'm=20
  betting it won't look so bad.
    Have you measured your environmental noise leve=
l?=20
Is 2.1 nm a realistic target? The amplitude of the 6 second ocean microseism=
s=20
may be from 500 to 15,000 nm!


<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2> He=20
  then goes on to describe the VRDT. I suppose for the VBB sensor this would=
=20
  greatly simplify the electronic design if one can deal with small sen=
sor=20
  gaps. I'm not sure about the noise. Does the VBB measure displacement=
s in=20
  the 1 =B1 nm range?   --- Just thinking out loud. I think it gre=
atly=20
  depends on what type of sensing one wants to do local, regional or=20
  teleseismic.
      Amateur seismometers are usual=
ly=20
limited by either microseisms or by environmental noise - we can't usually=20
choose a quiet remote site. I managed to reduce the noise of my LVDT to abou=
t 7=20
nm for a 6 mm range at 10 Hz, but my environmental noise is much greater=20
than this. 

 
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>It would=20
  be great to be able to use this with feedback.  No question that 
=
you=20
  could use it for integral feedback, which mainly works below the low=20
  
frequency roll off (say, below 0.011 Hz).  You could make an awes=
ome=20
  
integrator by digital summing, then feed back with a D/A.  Mid=20
  frequency 
range (derivative feedback) may not be practical.  Alth=
ough=20
  you could keep 
your beam well centred with integral feedback, without=20
  derivative feedback 
at higher frequencies, you're limited by the +/- 1=
..0mm=20
  (+/- 0.5 mm max, for 
linearity) sensor gap.  I'm suspecting that=20
  clipping levels in the 
mid-frequency range are going to be the biggest=
=20
  limitation.

 

    So, we may need some 'lateral thinking'=20
here! There are 'problems that you do not need to have' - like:-
 
    The velocity feedback damping does not need to=20=
be=20
generated that way!
 
    Neither do we need to use that troublesome desi=
gn=20
of capacitative sensor!
 
 ***   You can use JUST position +=20
integral current / coil feedback if you ALSO have a quad magne=
t +=20
Cu plate for the velocity damping! Trying to provide velocity damping by=20
differentiation and coil feedback is likely to very significantly increase t=
he=20
overall circuit noise!  ***
 
    Regards,
 
    Chris Chapman