Info wrote:
> So if I ask a favour I cant question the suggestions?

It's more that you argue why it would not work without having tried it and
not following the directions in your analysis.  I said several times that
the best defense against scratchy pots is to keep the wiper unloaded.
Instead of buffering the wiper signal you explain that it goes into a A/D
with 100Kohm input impedence, which is definitely loaded in this context.

For normal pots, scratchiness comes from intermittent contact between the
wiper and the pot body, not from jitter sliding in the normal direction.
Electrically this looks like a varying resistance in series with the wiper.
Therefore you don't want current thru the wiper since that times the noisy
resistor is the noise voltage.  This is also why a cap helps.  If the
voltage coming from the pot is steady but the resistance to this voltage
varying, the cap voltage will stay constant once it's been charged to the
pot output voltage.  That takes care of the steady state case.  There will
be unavoidable scratching as the pot is moved, which is what the low pass
filter after the cap is for.  There is always some lag time you can tolerate
during motion, so you adjust the filter to be as aggressive as possible
without exceeding this lag time to some settling fraction.

I would have gone into all this detail and shown real numbers if you hadn't
kept trying to tell me it wouldn't work without having tried it and without
understanding the problem.

However, this was all before we knew that the mechanical coupling to the
slider was magnetic.  That adds a whole host of problems, including
overshoot and more scratchiness since the contact force is probably less to
keep the moving resistance low.

Slide pots are well known to suffer from these problems because they are
difficult to seal compared to rotary pots.  The magnetic coupling achieves
good sealing at high cost in dollars, ringing, overshoot, scratchiness, and
vibration tolerance.  None of what you are observing should be a big
surprise.  Basically, you're using the wrong tool for the job.

Why not convert the linear motion into rotary motion and use a rotary pot or
quadrature shaft encoder?


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