PSN-L Email List Message

Subject: Re: AD7745 Capacitative sensors
From: Brett Nordgren Brett3kg@.............
Date: Fri, 14 Apr 2006 10:23:55 -0400

Hi Chris,

I was hoping you'd get in on this thread.  I enjoy hearing your ideas.

At 08:30 PM 4/13/2006 -0400, you wrote:

>           However, they are difficult to construct and have a working gap=
>           less than a millimetre, and are prone to off axis sensitivity.
>    This is largely incorrect for amateur applications. If you demand 1nm=
> resolution or better, you do need special materials and construction ->=20
> Silver coated Invar / Platinum coated Quartz electrodes.

I'm not sure pure resolution is that hard to come by.  Noise & temperature=
sensitivity are another matter, though.

>      There are three basic types of capacitor sensor.
>You can have a pair of parallel plates excited by sine or square waves=20
>with a central sensor plate which moves perpendicular to the plane. 
>     You can use two pairs of parallel plates with a central sensor plate=
> moving parallel to the plane 
>     You can use basically parallel circuit board plates with a pair of=20
> excitation strips on one side, a cross coupled square / rectangular=20
> sense     array on the other and a plate with vertical 'shadow strips'=20
> moving parallel in between. 

I'd always assumed that only the first type had adequate displacement=20
sensitivity, a couple of orders of magnitude greater than the=20
others.  Sounds like I need to go back and check the numbers.
>They usually operate at relatively high frequencies, from khz to mhz,=20
>which complicates design and implementation problems"
>     Operating at 10 to 50 k Hz is just fine.
>     The concerns seem to be largely illusory in practice.

Absolutely.  There's nothing that difficult with managing high audio=20

>   In a message dated 13/04/2006 14:48:47 GMT Daylight Time,=20
> Brett3kg@............. writes:
>Biggest VRDT problem seems to be its low drive frequency. In a feedback
>design the large demod filters are prime contributors to loop oscillation=
>     So reduce the filtration and apply a DC + pulsed feedback? Use=20
> another method?

Can you amplify on this?  Not exactly sure what your'e proposing, but it=20
sounds interesting.

>     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
> >=B1 nm range?
>With the sensor plates above, 1LSB=3D0.08nm.  But I think noise is what
>determines the useful resolution.  However 0.3nm / sqrt-Hz and 2.1nm RMS at
>50 SPS isn't too shabby.  It would be interesting to assume a seismic-mass
>system and model how this would compare with commercial instruments and
>earth-noise models.  I'm betting it won't look so bad.
>     Have you measured your environmental noise level? Is 2.1 nm a=20
> realistic target? The amplitude of the 6 second ocean microseisms may be=
> from 500 to 15,000 nm!

Actually 2.1nm was no target.  That's just what I calculated you could get=
using the AD7745.  I agree that it is a good bit better than a typical home=
site would justify, which is why it looked so interesting.
>  He then goes on to describe the VRDT. I suppose for the VBB sensor this=
> would greatly simplify the electronic design if one can deal with small=20
> sensor gaps. I'm not sure about the noise. Does the VBB measure=20
> displacements in the 1 =B1 nm range?   --- Just thinking out loud. I think=
> it greatly depends on what type of sensing one wants to do local,=20
> regional or teleseismic.
>       Amateur seismometers are usually limited by either microseisms or=20
> by environmental noise - we can't usually choose a quiet remote site. I=20
> managed to reduce the noise of my LVDT to about 7 nm for a 6 mm range at=
> 10 Hz, but my environmental noise is much greater than this.
>    It would be great to be able to use this with feedback.  
>     So, we may need some 'lateral thinking' here! There are 'problems=20
> that you do not need to have' - like:-
>     The velocity feedback damping does not need to be generated that way!
>     Neither do we need to use that troublesome design of capacitative=
>  ***   You can use JUST position + integral current / coil feedback if=20
> you ALSO have a quad magnet + Cu plate for the velocity damping! Trying=20
> to provide velocity damping by differentiation and coil feedback is=20
> likely to very significantly increase the overall circuit noise!  ***

I'm now thinking that's where I was heading, except for retaining the=20
"perpendicular" capacitance sensor.  Since the AD7746 goes directly from=20
capacitance to digital, I was hoping to use either a PC or commercial DSP=20
chip or FPGA to do most of the Position-Velocity derivative and other=20
shaping.  I like that because that needs minimal analog circuitry and what=
you do need (integral current feedback) is working at virtually DC.  And,=20
yes, for that to work you would need a well-damped and stable spring=20
mass.  Don't know how easy that is to do with a vertical, but assume=20
there's an answer.  First-order displacement linearization could be done=20
digitally.  Also you'd need to be sure that your displacement sensor range=
was adequate.

Thanks for your thoughts,



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