In a message dated 22/01/2007, gpayton880@....... writes:


Chris, where may I find more information on the capacitive  detectors, 
please.  And, what is  "LVDT?"



Hi Jerry,
 
    Have a look at 
_http://mysite.verizon.net/ressczez/homebuilt_seismometers/_ (http://mysite.verizon.net/ressczez/homebuilt_seismometers/)  for  a good 
capacitative sensor design. There are two basic sorts. In gap varying  sensors, 
you have a central electrode which can move between two outside pickup  
electrodes. The charge coupled to the electrodes depends on the plate  separation. 
There are direct and capacitance bridge designs. The direct  resonant design 
used on the SG sensor is moderately temperature  dependant see 
_http://psn.quake.net/sgboard.html_ (http://psn.quake.net/sgboard.html)  Other  direct designs 
are available with phase sensitive detector outputs, a high  sensitivity and 
very low drifts. Half bridge designs may be used to give a  high rejection to 
stray capacitances / external fields. The linear range is  small, maybe +/-2mm 
max. These are often used on force feedback systems. 
 
    Another arrangement uses two fixed parallel plates,  edge on, with a 
moving plate of half the total width partially overlapping them.  The capacity of 
the moving plate to the each fixed plate varies linearly with  position - a 
differential plate capacitor (also butterfly rotational type).  The linear range 
of these can be much greater. 
 
    However, you can also use linear movement /  parallel shield plate 
design. See Professor Peters' description at 
_http://physics.mercer.edu/petepag/tutorial.html_ (http://physics.mercer.edu/petepag/tutorial.html)  This  consists 
of two static outside plates and one central moving plate. One  static plate 
has two longitudinal conducting strips excited by an oscillator.  The opposite 
static plate uses four sense electrodes which are cross coupled.  The moving 
electrode has 1/2 the length of the excited strip. This  acts as an earthed 
shield electrode although no earth contact is required. As  the shield electrode 
moves, the phase of the charge coupled to the  plates varies. This is amplified 
differentially and put into a phase sensitive  detector. The length / 
movement range can be varied at will. For relatively  small movements, you can use a 
long drive electrode and several parallel  coupled four electrode sensor 
cells. An arrangement of parallel  strips is then used for the moving electrode. 
Rotational and moving  diaphragm types can also be used.
 
    A LVDT is a Linear Variable Differential  Transformer. You have an AC 
excited central coil winding and also two  outside pickup coils connected in 
series / opposition on the same axis. A soft  magnetic armature can move along the 
central axis. It is magnetised by the  central coil, but the fraction of the 
field coupled to the end pickup coils  depends upon the armature position. 
Phase sensitive detectors are usually used.  A good example is shown at 
_http://www.keckec.com/seismo/_ (http://www.keckec.com/seismo/)  You  can either wind 
or buy the sensor coils + armature. If you buy, look for a 'free  armature' 
model. My system gives a +/-6mm range with a +/-7 nano metre noise  floor. At 
+/-10 mm, it is 10% down in sensitivity. 
 
    Another inductive type uses a variable  reluctance sensor made from 
miniature audio transformers eg from Mouser. See 
_http://www.eas.slu.edu/People/STMorrissey/_ (http://www.eas.slu.edu/People/STMorrissey/)  Again,  this type has 
a high sensitivity, but a small linear range and may be used with  force 
feedback systems.
 
    Hope that this helps,
 
    Chris Chapman






In a message dated 22/01/2007, gpayton880@....... writes:
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Chris, where may I find more information on the capacitive=20
  detectors, please.  And, what is=20
"LVDT?"

Hi Jerry,
 
    Have a look at http://m=
ysite.verizon.net/ressczez/homebuilt_seismometers/ for=20
a good capacitative sensor design. There are two basic sorts. In gap varying=
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sensors, you have a central electrode which can move between two outside pic=
kup=20
electrodes. The charge coupled to the electrodes depends on the plate=20
separation. There are direct and capacitance bridge designs. The direct=20
resonant design used on the SG sensor is moderately temperature=20
dependant see http://psn.quake.net/sgboard.html=
 Other=20
direct designs are available with phase sensitive detector outputs, a high=20
sensitivity and very low drifts. Half bridge designs may be used to giv=
e a=20
high rejection to stray capacitances / external fields. The linear range is=20
small, maybe +/-2mm max. These are often used on force feedback systems. 
 
    Another arrangement uses two fixed parallel pla=
tes,=20
edge on, with a moving plate of half the total width partially overlapping t=
hem.=20
The capacity of the moving plate to the each fixed plate varies linearly wit=
h=20
position - a differential plate capacitor (also butterfly rotational type).=20
The linear range of these can be much greater. 
 
    However, you can also use linear movement=20=
/=20
parallel shield plate design. See Professor Peters' description at http://physics.merc=
er.edu/petepag/tutorial.html This=20
consists of two static outside plates and one central moving plate. One=
=20
static plate has two longitudinal conducting strips excited by an oscillator=
..=20
The opposite static plate uses four sense electrodes which are cross coupled=
..=20
The moving electrode has 1/2 the length of the excited strip. This=
=20
acts as an earthed shield electrode although no earth contact is required. A=
s=20
the shield electrode moves, the phase of the charge coupled to the=
=20
plates varies. This is amplified differentially and put into a phase sensiti=
ve=20
detector. The length / movement range can be varied at will. For relatively=20
small movements, you can use a long drive electrode and several paralle=
l=20
coupled four electrode sensor cells. An arrangement of parallel=20
strips is then used for the moving electrode. Rotational and moving=20
diaphragm types can also be used.
 
    A LVDT is a Linear Variable Differential=20
Transformer. You have an AC excited central coil winding and also=20=
two=20
outside pickup coils connected in series / opposition on the same axis. A so=
ft=20
magnetic armature can move along the central axis. It is magnetised by the=20
central coil, but the fraction of the field coupled to the end pickup coils=20
depends upon the armature position. Phase sensitive detectors are usually us=
ed.=20
A good example is shown at http://www.keckec.com/seismo/&nbs=
p;You=20
can either wind or buy the sensor coils + armature. If you buy, look for a '=
free=20
armature' model. My system gives a +/-6mm range with a +/-7 nano metre noise=
=20
floor. At +/-10 mm, it is 10% down in sensitivity. 
 
    Another inductive type uses a variable=20
reluctance sensor made from miniature audio transformers eg from Mouser. See=
 http://www.eas.slu.edu/P=
eople/STMorrissey/ Again,=20
this type has a high sensitivity, but a small linear range and may be used w=
ith=20
force feedback systems.
 
    Hope that this helps,
 
    Chris Chapman