You could use a rotary pot with some kind of rack & pinion system.  If you
can eliminate non-linearities (via e.g. table lookup), then you could use a
rortary pot with a lever on the shaft, the longer the lever, the less the
linearity, but also the smaller the change in resistance.

The main thing to bear in mind is that you will need a high quality pot
rated for a lot of operations.  Typical high quality pots for proffesional
audio mixers may be up to the job, but they are anything but cheap!

Hmmm...thinking too much in terms of resistive sensors here.  Ok, how about
a hall efect sensor that measures the field strength of a small permanent
magnet that is free to move?  Or some kind of graduated translucent material
(maybe from a pair of graduated sunglasses?) and a LED/phototransistor combo
to measure posistion.

Something from my college days is ringing a bell.  The LVDT ot linear
variable differential transformer.  Basically it uses 3 fixed coils and a
moveable magnetic core.  One coil is energised by an AC signal.  Moving the
core in and out of the coils produces a differential signal in the other two
coils.  Obviously some signal conditioning will be needed, but these devices
are available commercially and pretty rugged. We built ours at college on a
simple plastic former and used a piece of ferrite rod as the core.

Hope this gives you a few ideas.

Mike Rigby-Jones


        Jon Petty wrote:

> Hi
>
> I need a cheap linear potentiometer for position feedback for a linear
> stepper motor.
> It needs to be small, stroke is 0 -.6 inches and output prefferably 0-5 v
> for
> a/d converter. Also needs to be suitable for automotive underhood
> temperatures. It also needs to slide easily to not use up motor torque.
>
> It doesn't have to be real accurate though.
>
> It could almost be a DIY system since I can calibrate the output. Maybe a
> resistive strip and a wiper, but I don't know where I can find any.
>
> Do you think one of those flex or stretch sensors could be modified for
> this?
>
> Any ideas or suggestions?
>
> You know a linear r/c servo would solve all these problems! Anyone make
> one?
>
> Thanks
>
> Jon