On Tue, 18 Jun 2002, Roman Black wrote:

>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

It's not just perfectly nonlinear, it is a C ~= 1/x^2 curve, where x is
the axle angle, because they are designed to produce linear scale
graduations when used with a coil, as in radio (1/x^2 comes from the
Thomson formula for the LC tuned circuit). VERY linear variable capacitors
are easy to make and available. But they are not the ones you refer to.

>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 agree but an air cap with 10 pF is about 12 square centimeters per
plate, at 1 mm separation. Two plates and you still need 6 cm^2. And, this
is the larger value, the smaller one is much lower.

>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.

Try your measuring scheme against changes in temperature, supply voltage
and oscillator parts (ic). With some work you will make one behave. Ten
will be do-able. Make a hundred and you'll have a full time job doing
adjustments.

>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)

Look into linear Hall sensor potentiometers (both rotary and linear). Very
good. You can dunk one in goo and it will work as if it was new.

Peter

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