In a message dated 22/08/2009, barry_lotz@............. writes:

I was  trying to decide the best displacement sensor style. What was the 
problem with  inductive style sensors? 
Hi Barry,
 
    Ordinary amateur coil + magnet sensors tend to be  more limited by 
noise. You can't use them to correct position drift with  temperature. The use 
of NdFeB quad magnet blocks will give a greatly increased  output compared to 
Alnico U magnets. 

I agree  LVDT's can have clearance issues. 
    This depends on whether you use commercial sensors  or design your own. 
You can buy commercial sensors with a wide clearance.  Schaevitz produce 
them amongst others. Commercial LVDTs seem to have quite noisy  electronic 
detector circuits, but good designs are available

VRTD's -  I like, but someone had a problem with it but I can't remember 
what the reason  was. 
    I don't know of any problems / can't remember  hearing of any. You do 
need the moving plate to considerably overlap the field  coil poles.

I use it  on a small version of STM style sensor. I do get an occasional 
low frequency  oscillation which I can't nail down (~0.01 hz). I tentatively 
attribute to  maybe my triple feedback is slightly off. I would use a 
capacitive sensor but  they seem so large to get a nominal capacitance value. 
Especially when one is  trying to design a small unit. 


A capacitance of ~10 pF is commonly used. You can  get much lower overall 
noise with a capacitative system - so long as you use  sine wave excitation. 
The common problem with the variable plate separation  design, is the large 
change possible in the air damping as the plate gap  varies. The allowable 
movement is small and this is better used in a  feedback design. Plates with 
drilled array of holes may be used. A shaded  pole design with the shadow 
plate moving parallel to the other plates may  be used. These can be used in 
open circuit or feedback designs and can measure  movements from nano metres 
to metres. See also LCDTs.
 
    Could this be an undamped torsional oscillation?  Alternatively, are 
you sure that it is correctly damped? What sort of capacitors  did you use? 
Sounds more like a phase shift problem?
 
    Regards,
 
    Chris Chapman





In a message dated 22/08/2009, barry_lotz@............. writes:
I was=20
  trying to decide the best displacement sensor style. What was the proble=
m with=20
  inductive style sensors? 
Hi Barry,
 
    Ordinary amateur coil + magnet sensors tend=
 to be=20
more limited by noise. You can't use them to correct position drift with=
=20
temperature. The use of NdFeB quad magnet blocks will give a greatly incre=
ased=20
output compared to Alnico U magnets. 
I agree=20
  LVDT's can have clearance issues. 
    This depends on whether you use commercial se=
nsors=20
or design your own. You can buy commercial sensors with a wide clearance.=
=20
Schaevitz produce them amongst others. Commercial LVDTs seem to have quite=
 noisy=20
electronic detector circuits, but good designs are available
VRTD's -=20
  I like, but someone had a problem with it but I can't remember what the=
 reason=20
  was. 
    I don't know of any problems / can't remember=
=20
hearing of any. You do need the moving plate to considerably overlap the=
 field=20
coil poles.
I use it=20
  on a small version of STM style sensor. I do get an occasional low frequ=
ency=20
  oscillation which I can't nail down (~0.01 hz). I tentatively attribute=
 to=20
  maybe my triple feedback is slightly off. I would use a capacitive senso=
r but=20
  they seem so large to get a nominal capacitance value. Especially when=
 one is=20
  trying to design a small unit. 


    A capacitance of ~10 pF is commonly used. You=
 can=20
get much lower overall noise with a capacitative system - so long as you=
 use=20
sine wave excitation. The common problem with the variable plate sepa=
ration=20
design, is the large change possible in the air damping as the plate=
 gap=20
varies. The allowable movement is small and this is better used in a=
=20
feedback design. Plates with drilled array of holes may be used. A sh=
aded=20
pole design with the shadow plate moving parallel to the other plates=
 may=20
be used. These can be used in open circuit or feedback designs and can mea=
sure=20
movements from nano metres to metres. See also LCDTs.
 
    Could this be an undamped torsional oscillati=
on?=20
Alternatively, are you sure that it is correctly damped? What sort of capa=
citors=20
did you use? Sounds more like a phase shift problem?
 
    Regards,
 
    Chris Chapman