How about using a car or better yet a truck windshield wiper motor
with it's speed control?
If the travel of the arm would not be enough for your requirements you
could tap into it's rotary motion at the gearbox.
Engineman

On Feb 13, 11:50*am, Rich Grise wrote:
Karl Townsend wrote:
On Sun, 13 Feb 2011 09:49:54 -0600, "Pete C."

PWM DC motor drive

like this one?
http://www.electronickits.com/kit/co.../CANUK1133.htm

Looks perfect, assuming it's guaranteed to operate at your battery voltage.

I can vouch for PWM DC motor control - you can maintain full torque
down to zero RPM - with a permanent magnet DC motor (which I'm ass-u-me-ing
you have), the torque is proportional to the current. (not too sure about
a series-wound, or "universal" motor, but I'm guessing it still applies.)
I've designed and built a PWM motor speed controller, and it's pretty
awesome to watch it maintain full torque at any RPM. This was only a 1.5
amp motor, but the same principle applies; buying one that somebody else
has designed, built, and tested is obviously better than trying to klooge
up something on your own, unless *you're on a learning mission. :-)

Cheers!
Rich