I've not used _linear_ motorized pots, but the rotary ones I have used were
quite slow. Inertia and friction were basically non-issues.
The other detail is they had slip clutches so the knob could be moved
without the motor, this also acted to slip when the motor "hit" the end
stop.

Yes, an H-bridge is a necessary driver. You can get these as a driver chip
(TI's DRVxxx series comes to mind) or do it with discrete transistors.

For my money, PID is complete overkill for this application. i drive lots
of resistive feedback linear actuators with full ON-OFF control and a
deadband of +-2 ADC counts, never seen a hunting problem with that.

Good luck.
-Denny


On Tue, May 29, 2012 at 10:10 AM, Josh Koffman <joshybear@gmail.com> wrote:

> Hi all.
>
> Second email! As generally happens when I'm about to travel, I'm
> struck with all sorts of ideas I want to try out, and no time to do
> them. For now this will remain a thought experiment, but I hope to do
> some of the design while traveling, then build a prototype when I
> return.
>
> I've had an idea for a project that would use motorized faders. These
> are linear (slide) potentiometers with a DC motor attached. I've
> started reading about these, and apparently driving them can be a bit
> tricky. They have two resistance tracks, originally one was used to
> control your signal (audio or otherwise), and the second was used as
> positional feedback for your servo control. In essence these were
> linear servos.
>
> I'd like to drive mine digitally, as I'm going to be connecting the
> unit to an ADC input anyways. What's tricky is that there's a
> mechanical component to the design that needs to be taken into
> consideration, otherwise you stand the chance of having the fader
> yo-yo around as it "homes in" on the correct value it should be at.
> This is due to inertia and friction. In addition, you need to be
> careful about not slamming the unit into the end stops repeatedly, or
> trying to drive the fader if it's stuck (lest you burn out the motor).
>
> I'm thinking about an H bridge to drive the motor, and software to
> help with the positioning. I will also need to come up with some sort
> of capacitive touch sensor, as the unit has a touch path from the
> slider, and I'd like to know when it's being touched so as to disable
> the motor drive.
>
> I've never done any PID work, and I'm wondering if that's the correct
> route to go for the control. I've done some searching, but haven't
> come up with much for this particular application. Does anyone have
> any tips, or links to other documentation? I've been sent an
> application note from the North American rep for Penny and Giles, one
> of the most respected fader manufacturers. I'm working through it, but
> it's mainly based on analog control schemes. Still, useful stuff.
>
> Thanks!
>
> Josh
> --
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> completely foolproof is to underestimate the ingenuity of complete
> fools.
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>
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