In a message dated 2008/08/23, paleoartifact@......... writes:

> Hi Gary and all,
> Amateur verticals (the more popular published ones), as you've probably 
> noticed, are quite diverse in their final published suggested sensor pickup use. 
> Obviously, one can build the selected mechanical equivalent and substitute 
> whatever output sensor you really know / feel comfortable making / building / 
> using and henceforth go from there inasfar as electronic amplification, period 
> extension, feed back and so on.... at least in what is available for such 
> choice.
> 
> If one really gets down to the nitty-gritty... it's going to be the old 
> coil-magnet models that 95% of the amateurs will actually build in my opinion. 
> Complicated sensor electronics is for those that already possess the ability, 
> tools, material and  time to devote in the building for doing so. Its too bad 
> that the electronic circuits for some can't be bought.

Hi Meredith,

    Following a proven circuit design should not be too difficult, surely? 

> Although I've not yet tried to duplicate any of the published models; their 
> is one in particular that (mechanically) really draws my interest and it 
> offers a variety of improvements that I think is worth paying A LOT more 
> attention to.  Its the Randall Peters vertical at: 
> http://physics.mercer.edu/hpage/new-zhtml

       NB: That should be http://physics.mercer.edu/hpage/new-z.html

> The web site details offer up very significant pivot friction lowering 
> points; but doesn't delve into any amateur construction details.
> If one mechanically "thinks outside the box", its likely that a wide variety 
> of some substitutions of material could be arrived at.
> 
> Note the front two leveling screws; that setup will help give more 
> mechanically stability.  A square frame overall, can help for stability and for fixing 
> whatever sensor arrangement that is desired.  The spring "looks" like a 
> common hardware store door spring; but it may have been cut to a certain length 
> (?).
> 
> How would it be with a coil-magnet sensor (?), I really don't know; but I'am 
> sure it will be much better in overall mass enertia sensitivity and the 
> length of the period will be noteably longer than any other low cost model thats 
> presently around.

        A coil + magnet sensor would likely give a very inferior performance. 
You can extend the period below the resonant frequency by x10 fairly easily, 
but it still has a fairly high inherent noise level and the f^2 drop off in 
response. It is this square law relationship which limits the period extension, 
due to circuit noise. 

       A capacitative linear position sensor is fitted to a vertical 
mechanical system which has a relatively short period. This allows the mechanical 
period to be extended by x100 or more fairly easily. Capacitative detectors also 
normally have MUCH lower noise levels than magnet + coil systems. And you CAN 
buy a specialist chip to drive one. See the Analog Devices AD7745 & AD7746 
types. There are OTHER capacitative designs on the www using the LTC1043.
       
       The only other chip which might be useful for LVDTs is the AD598AD, 
but Digikey charge $61.82 each. You would have to add efficient low pass 
filters, since the output is quite noisy. AD don't seem to quote the inherent circuit 
noise. The max drift is +/- 50 ppm / C Deg, typically 7 ppm, but the 
excitation drift is 600 ppm / C Deg.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2008/08/23, paleoartifact@......... writes:



Hi Gary and all,

Amateur verticals (the more popular published ones), as you've probably noti=
ced, are quite diverse in their final published suggested sensor pickup use.=
 Obviously, one can build the selected mechanical equivalent and substitute=20=
whatever output sensor you really know / feel comfortable making / building=20=
/ using and henceforth go from there inasfar as electronic amplification, pe=
riod extension, feed back and so on.... at least in what is available for su=
ch choice.



If one really gets down to the nitty-gritty... it's going to be the old coil=
-magnet models that 95% of the amateurs will actually build in my opinion. C=
omplicated sensor electronics is for those that already possess the ability,=
 tools, material and  time to devote in the building for doing so. Its=20=
too bad that the electronic circuits for some can't be bought.



Hi Meredith,



    Following a proven circuit design should not be too diffi=
cult, surely? 



Although I've not yet tried to=20=
duplicate any of the published models; their is one in particular that (mech=
anically) really draws my interest and it offers a variety of improvements t=
hat I think is worth paying A LOT more attention to.  Its the Randall P=
eters vertical at: htt=
p://physics.mercer.edu/hpage/new-zhtml




       NB: That should be http://physics.merc=
er.edu/hpage/new-z.html



The web site details offer up v=
ery significant pivot friction lowering points; but doesn't delve into any a=
mateur construction details.

If one mechanically "thinks outside the box", its likely that a wide variety=
 of some substitutions of material could be arrived at.



Note the front two leveling screws; that setup will help give more mechanica=
lly stability.  A square frame overall, can help for stability and for=20=
fixing whatever sensor arrangement that is desired.  The spring "looks"=
 like a common hardware store door spring; but it may have been cut to a cer=
tain length (?).



How would it be with a coil-magnet sensor (?), I really don't know; but I'am=
 sure it will be much better in overall mass enertia sensitivity and the len=
gth of the period will be noteably longer than any other low cost model that=
s presently around.




        A coil + magnet sensor would like=
ly give a very inferior performance. You can extend the period below the res=
onant frequency by x10 fairly easily, but it still has a fairly high inheren=
t noise level and the f^2 drop off in response. It is this square law relati=
onship which limits the period extension, due to circuit noise. 



       A capacitative linear position sensor i=
s fitted to a vertical mechanical system which has a relatively short period=
.. This allows the mechanical period to be extended by x100 or more fairly ea=
sily. Capacitative detectors also normally have MUCH lower noise levels than=
 magnet + coil systems. And you CAN buy a specialist chip to drive one. See=20=
the Analog Devices AD7745 & AD7746 types. There are OTHER capacitative d=
esigns on the www using the LTC1043.

       

       The only other chip which might be usef=
ul for LVDTs is the AD598AD, but Digikey charge $61.82 each. You would have=20=
to add efficient low pass filters, since the output is quite noisy. AD don't=
 seem to quote the inherent circuit noise. The max drift is +/- 50 ppm / C D=
eg, typically 7 ppm, but the excitation drift is 600 ppm / C Deg.



       Regards,



       Chris Chapman