PSN-L Email List Message
Subject: Re: Real time traces
From: Brett Nordgren brett3nt@.............
Date: Sat, 22 Aug 2009 20:30:35 -0400
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.
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.
Watch our wiggles
or watch some very very good wiggles
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