Barry,

The main problem that I saw with using inductive sensors in a feedback 
design is that their excitation frequency tended to be relatively 
low.  After demodulating there is a lot of residual 2xcarrier which needs 
to be filtered.  To filter it out adds a great amount of phase shift in the 
loop, right near the gain crossover frequency, (the instrument's 
high-frequency corner) that annoys the feedback loop no end and makes it 
difficult or impossible to crank up the loop to really do its job.  If you 
try, it oscillates.

Capacitive sensors operate at much higher frequencies and so the residue is 
much easier to filter in a way that allows the feedback loop to work really 
well without oscillating.  Ours uses a quasi-square wave drive, so the 
filtering is also less than it might otherwise have to be.  I sort of agree 
with Chris, that a sine-wave drive seems like it would work much better, 
but Dave's design really works--a lot better than I could have imagined.

Capacitive sensors can work well with very small capacitors, ours is 
roughly 48pF and 2.5" x 3",  which is probably significantly larger than 
necessary.  When I look at the Nanometrics Trillium compact, which packs 
three force-balance sensors into a can slightly over 5" high and 3.5" dia, 
I have the feeling that good capacitive sensors can be made quite small.

Regards,
Brett


At 02:39 PM 8/22/2009 -0700, you wrote:
>Brett et al
>I think this discussion has been made before but... I was trying to decide 
>the best displacement sensor style. What was the problem with inductive 
>style sensors? I agree LVDT's can have clearance issues. VRTD's - I like, 
>but someone had a problem with it but I can't remember what the reason 
>was. 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.
>Barry
>

Watch our wiggles
http://bnordgren.org/seismo/gif_images.htm

or watch some very very good wiggles
http://aslwww.cr.usgs.gov/Seismic_Data/telemetry_data/ANMO_24hr.html


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