Peter L. Peres wrote:
>
> On Fri, 14 Jun 2002, Roman Black wrote:
>
> >Peter L. Peres wrote:
> >> I have researched
> >> variable capacitor position and angle sensing thoroughly enough to be able
> >> to say that it works iff you find a way to reduce the sensing input
> >> capacitance 10 to 20 times (from what is a normal IC input cap. of 5pF).
> >> It turns out that this is VERY HARD to do lacking specialised parts. A
> >
> >Again I say use the cap to drive an osc, divide freq
> >via hardware (counter chip) or via PIC prescaler etc
> >and test freq against the PIC clock. All system
> >capacitances are negated provided you can calibrate
> >the system degrees/freq which will have to be done
> >anyway. :o)
>
> Say again, starting at 'negated' please, I've lost you there. A system
> with a self capacitance in the 5pF range will induce a potential error of
> 50% when measuring something in the say 3-12pF range (depending on which
> end of the range). All the systems that use low value capacitors as
> sensors (f.ex. many low cost scales) use the differential system and use
> low frequency and effective capacitance measurement. There are exceptions
> to this of course. Of course you already know that an air capacitor is
> also a thermometer, hygrometer, and manometer all in one.


I assumed from the start that the system would need
software calibration as the variable caps are not
perfectly linear vs rotation. The solution I suggested
using the cap as timing element in an osc is quite
valid. For example if system capacitance is 30pF,
and variable cap is 10pF to 60pF, the total result
is 40pF to 90pF through the range. Calibrate using
a software table etc (which had to be done anyway)
and it works. :o)

I fail to see why this is "very hard to do without
specialised parts" as the cap can drive a simple
one-inverter osc which the PIC can freq-divide using
one of the counters. Total parts count one inverter,
one resistor and the variable cap. I have successfully
built one-inverter oscillators using very low value caps
and they are a useful way of measuring changes in low
value caps.

I take your point re moisture etc, but i'm not a fan
of using resistive pots as position sensors, especially
in high-use environments. Being able to use a variable
cap gives benefits from it's zero-wear contactless
construction, especially at speeds. :o)
-Roman

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