larry d clark wrote:

i have always wanted to build a step motor driven
balanced tuner. i have acquired an excellent tx air capacitor
and 2 matched 28 uh roller inductors. next step is
to acquire 2 step motors with some kind of positioning
indicator, either optical or a pot.

any have any recommendations for a company site, or for
anyting else related, pulleys, belts, encoders etc

larry
kd5foy

Be careful sizing your steppers. The biggest drawback to designing with
steppers is that if you exceed their available torque they'll quietly
fail, and your system will do Very Bad Things. So you want to
over-specify your stepper motors by quite a bit.

You'll need to consider two things when doing this: the amount of
torque it takes to turn your inductors, and their rotational inertia
(which will determine how fast you can accelerate them without causing
grief). You can determine your necessary torque experimentally with a
lever arm and weights, and you can determine your torque available from
a 'mystery motor' in a similar manner -- there's even a web page out
there to tell you how to do it, if you look for it.

Once you have selected steppers that are big enough, you'll know their
other drawback: they take up a lot of space compared to DC motors with
feedback.

But it's easy to understand how to control them, and you don't need to
figure out complicated position feedback schemes, so maybe steppers are
the right choice for you. If I were you I'd just use a stepper and a
pair of home switches -- one to tell me that the roller is at one
extreme of travel, and another to tell me that the drum has passed by an
index mark. If you 'and' these two signals together you'll get a
reliable home. Then each time your system starts up you just run the
steppers down to the home position, and back to where you want them.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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