ebclr, thank you for the suggestion, unfortunately that device is limited to under 14k samples per second. I was thinking of using a cascode (great isolation) oscillator where the capacitor is in a tank (with an inductor) - oscillating say a few MHz to give very fast update rate. That would go into a psd (4046) whose other input is the DDS (which are normally 32bits, 28 bit resolution go to 0.05Hz) to a PID and then audio amplifier.
Suppose you go for 1/2 octave change from 7.5-10MHz, even with a 28 bit DDS, you get 2,500,000 / 0.05 = 50,000,000 gradations of 200mm would be 4 nano meters - well into detecting the imperfections of the edge of the tapered capacitor. Clearly the range has to be more than 200mm (that's just the size of the board) but the figures tell you that it's well within reason.
For a turntable type axis you only need half the distance as the business end only needs to go (precisely) through the middle. You can turn the turntable to get to the other side.
What use is that - you need an accurate posiiton. Ballpart isn;t useful
It is if you want 'belt and braces' - "My position system is no-where near where my visual feed back says it should be - yipes! now would be a good time to stop" - before it impales itself into the opposite wall
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How is that useful ? You need the fiducial positions.
If someone removes the board, or something lands on it, or someone moves their arm nearby - belt, braces & safety pin(s)
I agree Brumby,
Maybe one issue is lack of information about my character. I am not Meredith Perry, I don't go blindly into things and think I can do it just because I think I can. I am more cautious than that. I am confident about the position measurement because I have used it many times. One version was used at Leeds university to measure the amount piezoelectric devices move when stimulated - that could resolve well under a 100nM.
I have built CNC machines that are rigid - none of this gantry stuff, real "built in a box" devices. I am fully aware of backlash which is why a servo system offers so many advantages.
Somehow text doesn't convey the necessary information - but I am trying (very trying
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