In a message dated 15/10/05, rene@........... writes:

>   Is there any documentation on using capacitive measurements instead of 
> the coil based ones? So far i saw all lehman-typed instruments equipped with a 
> pickup coil. With my testing equipment i used two capacitor plates, each 
> about 30cm2 large, at about 3-5 mm of each other. On that is a variable 
> oscillator attached, with pulse counting i determine position of the arm with 
> reasonable success but no hard data on resolution. I am wondering if there are any 
> known disadvantages of such setup, i think it is suitable to build a 
> Shackleford-Gundersen type this 
> way as well.

Hi Rene,

       The easiest capacitative sensor to use is described at 
http://physics.mercer.edu/petepag/tutorial.html The drive and sensor plates can be fixed to 
the frame and the moving plate does not need to be earthed. Since the  armature 
plate moves parallel to the other plates, there is no problem due to variable 
air damping, which you get with variable gap devices when plates approach 
eachother. 

      Have a look at the LTC1043 data sheet and applications AN3, AN45 from 
www.linear.com  Jeff Witt's capacitative bridge circuit using the LTC1043 is 
described at www.linear.com Application Note AN87 p87 These types should give 
resolutions down to tens of nano metres, if not less. You want to allow at least 
+/-1 cm movement on a Lehman, possibly more.

       Have a look at Allan's great work 
http://physics.mercer.edu/petepag/mkxx.pdf and http://physics.mercer.edu/petepag/MKXVIII.pdf 
and http://physics.mercer.edu/petepag/MKXVII.pdf

       I think that you can still buy the LX1358 LVDT kits from 
www.nuovaelettronica.it which were designed for use with a Lehman. The overall travel is 
+/-15 mm with +/-6mm being highly linear.

       Regards,

       Chris Chapman
In a message=20=
dated 15/10/05, rene@........... writes:



  Is there any documen=
tation on using capacitive measurements instead of the coil based ones? So f=
ar i saw all lehman-typed instruments equipped with a pickup coil. With my t=
esting equipment i used two capacitor plates, each about 30cm2 large, at abo=
ut 3-5 mm of each other. On that is a variable oscillator attached, with pul=
se counting i determine position of the arm with reasonable success but no h=
ard data on resolution. I am wondering if there are any known disadvantages=20=
of such setup, i think it is suitable to build a Shackleford-Gundersen type=20=
this=20

way as well.




Hi Rene,



       The easiest capacitative sensor to=20=
use is described at http://physics.mercer.edu/petepag/tutorial.html The drive and sensor=
 plates can be fixed to the frame and the moving plate does not need to be e=
arthed. Since the  armature plate moves parallel to the other plates, t=
here is no problem due to variable air damping, which you get with variable=20=
gap devices when plates approach eachother.=20



      Have a look at the LTC1043 data sheet and=
 applications AN3, AN45 from www.linear.com  Jeff Witt's capacitative b=
ridge circuit using the LTC1043 is described at www.linear.com Application Note AN87 p87 These types should give=20=
resolutions down to tens of nano metres, if not less. You want to allow at l=
east +/-1 cm movement on a Lehman, possibly more.



       Have a look at Allan's great work h=
ttp://physics.mercer.edu/petepag/mkxx.pdf and http://physics.mercer.edu/pete=
pag/MKXVIII.pdf=20

and http://physics.mercer.edu/petepag/MKXVII.pdf



       I think that you can still buy the=20=
LX1358 LVDT kits from www.nuovaelettronica.it which were designed for use wi=
th a Lehman. The overall travel is +/-15 mm with +/-6mm being highly linear.



       Regards,



       Chris Chapman