Tony Williams wrote...
Plan B is to see if you can move the mechanical design to
the classic guarded ring construction, which is calculable.
___________|__________
|______________________|
__________ _ ___ _ __________
0v|__________||_||_|_||_||__________|0v
/ |
/ |___Connection to SJ(at 0v)
/
Insulating ring
Might even be a pcb layout......
Not a pcb in my case, but yes that's a good plan B. If necessary
I could project the 0.1pF electrode plug into my chamber, closer
to the pickup electrode:
.. insulating signal
.. ring | outside world
.. 0V __________ \_ _|_ _ __________ _
.. XXXXXX || || || || 0V shield,
.. XXX_________||_|| ||_||_________ _ 0.125 thick
.. |___|
.. __________________________________ _ 0.125 space
.. XXXXXX
.. XXXX______________________________ _ 0.125 ring
.. / |
.. / |___Connection to SJ(at 0v)
.. pickup electrode
Apparently at these scales fringing-fields have a *huge* effect.
Sloggett, et.al., J Physics A, 19, p2725 (1986), has an accurate
formula with fringing-field correction terms (see a.b.s.e.). **
Using the terms in their equation 36, the correction factor is a
huge 3.466 times! for disks spaced with a separation of half the
disc diameter (considered a fairly-distant separation, enhancing
the fringing-field effects). Wow! Actually, I suspect a typo in
the equation; so perhaps the correct ratio is 2.447 times. Hmm.
At any rate these big numbers raise my eyebrows, and I'll try to
check them out with some bench measurements later today, and Mike
here at the Institute will probably look at them with his FEA.
Bartlett & Corle, J Physics A, 18, p1337 (1985) and another fellow,
J Cooke (1958) have the brute-force numerical calculations that the
Sloggett team used to evaluate the various formulas in their paper.
** In a followup article Sloggett points out their formula is the
same as one published by Shaw in 1970, Phys Fluids, 13, p1935.
BTW, the 0.1pF capacitor at issue is the feedback element for a
sensitive five-axis capacitive position-measuring system.
