> A cute trick I've seen is to use just a stepper motor.  When the user
> turns it you get pulses out the windings, and it can act like a shaft
> encoder without the zero reference.  Of course it's a motor, so you
> can make the shaft move electronically too.  It can be done, but it's
> not trivial and will take some development

That would work. I wrote it up for PIClist last year

http://www.piclist.com/techref/io/sensor/pos/enc/stepper_as_encoder.htm

That particular implementation was incorporated into a reasonably
low-tech tracking device and worked well

I do have enough steppers, and have seriously considered it. Also
magnetic or optical encoders. But you'll appreciate when you have
a cheap pot and a cheap motor, it is tempting to put them together
to make a new useful component

> How long does it really have to last, and how many operations
> before it's allowed to break?

Well, I'm thinking perhaps more about the durability of the clutch
material itself, and its natural degradation over time. eg rubber
breaking down, plastic hardening etc. Number of shaft turns is a
little hard to estimate but I'd say generally infrequent (at least for
one application I have right now). Maybe I should look around
here and see how various materials I know the approximate age
of have fared over the years

Joe

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