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motion control (was: standing waves)

Quoting Bernard Cleyet <anngeorg@PACBELL.NET>:
... after taking an inordinate time to determine the internal
resistance of the P/S and the impedance ** of the driver

BC is 100% correct to worry about the funky amplitude response
of this sort of driver.

But I'm not sure that doing a good job of characterizing the
driver is the optimal solution to the overall problem. So
let's take a step back and inventory our options.

a) Voice-coil ("speaker") driver: excellent control of frequency,
but questionable flatness of the amplitude-versus-frequency curve.
Readily available from Pasco and others --- or just roll your own
using the audio output from your PC.

b) DC motor + crank: excellent control of amplitude, but
questionable control of frequency under conditions of variable
load, especially at the low end of the speed range. Readily
available from various sources:
-- variable-speed electric drill or moto-tool.
-- electric egg-beater
-- "lego" motors => http://www.philohome.com/motors/motorcomp.htm

Some but not all speed/frequency issues can be overcome by
using an encoder to measure the speed... so you can apply feedback.

c) Stepping motor + crank. Absolute control of amplitude and
absolute control of frequency, right down to zero frequency,
independent of load and everything else over a wide range.

Tutorial:
http://www.ams2000.com/stepping101.html
Amusing info and link farm:
http://www.doc.ic.ac.uk/~ih/doc/stepper/

I've been using stepping motors for various things ever since I
was a sorcerer's apprentice. There are lots of situations where
a stepper is The Right Thing (tm) and nothing else will do. If
you haven't yet made friends with such creatures, now would be
a good time. Note that in general you need a motor AND a driver
AND a controller ... although it is possible get a starter kit
that has them all together, e.g.
http://www.herbach.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=HAR&Product_Code=TM93KIT2421&Category_Code=STEPMTRCTL