Michael Vinson wrote:
>
> Greetings all. My kids have been begging me to build some kind of
> robot. I'm thinking of something like an 18" square platform (wood,
> initially), cruising around via radio control from my main computer
> setup. It'll have an ultrasonic ranger (that I've already built) on
> it, a PIC to talk (via RF, I've already got that part done) to the
> computer, etc.

Sounds nice!

>  The place where I'm stuck is the motors. I'm thinking
> of two drive wheels, each with its own PWM-controlled motor with a
> sensor to measure how fast and how far each one has rotated. So what
> I need are two fairly hefty, 12-Vdc motors, reversible. I have an old
> windshield wiper motor from a former car, but it seems (?) to have
> the field in series with the armature (?)... at any rate, I can't
> get it to go in reverse, which is crucial since if I use two matching
> motors, one will have to go one way (CW, say), while the other goes
> the other way (CCW). It doesn't need to go very fast...60 RPM or so
> would be fine.

The motor you have is a DC "series motor". They have
massive low speed (stall) torques and are typical
in old Fords etc from the 60s and 70s. Now most wiper
motors are smaller cheaper permanent magnet types.

You can only reverse a series motor by reversing
the field OR the armature. :o)
You will need to get 2 matched motors. Then to make
your robot go straight you may still need to add
optical disks or encoders to the motors.

A great option is to find an old HEAVY fax machine,
try begging around the business equipment shops
for an ex-trade in or old dead fax. The old ones
have 2 matched LARGE steppers, and some gears.
This will give you precise positioning and adds
a really nice sound and style of movement. Stepper
bots move like little industrial robots, where
DC motor driven bots always move a bit like
remote controlled toys, to my eye. :o)
-Roman

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