Re: Making a rapid prototyper

"We haven't settled the stepper/servo debate yet, frankly."

Truth is, it doesn't matter, you design according to your needs. The
only essential difference is whether you're driving open-loop or
closed-loop. Once you've decided you need position feedback, the cost
advantages of steppers pretty much evaporate.

Personally, I'm partial to AC servos. I'd only use steppers in systems
where I have no other choice than to use a PC parallel port to drive
the signal. If you're microstepping, the electronics is just as
complicated as making inverters to generate three-phase to AC servos.

Of course taking microstepping to its endpoint, you're driving a
stepper with a quadrature sine wave, which only takes one more bridge
channel than the three-phase AC, and now your stepper is an AC servo.
(Bipolar Stepper -> two bridges, AC servo -> three bridges, Quadrature
Stepper -> four bridges, DC servo -> two bridges)

If your PWM is high enough frequency (100 to 500 kHz) and resolution
(12-bit or so) you can modulate the amplitude into the AC servos to
control torque. It'll also annoy the FCC so put LC filters on the PWM
output to limit its analog bandwidth to what the motors can take.

On 8/2/06, Sebastien Bailard - Dubsen <Sebastien.Bailard@...> wrote:
>
> > 3. If you're looking at motor technologies to replace your steppers,
> > there's a branch of RC airplane, um, enthusiasts building high torque
> > motors from old CDROM spindle laminations. The brushless DC controller
> > isn't all that more complicated than a stepper controller, and if you use a
> > high resolution pwm to drive them, they'd make pretty effective ac servos.
> > (Ok I take that back about the motors. You still have a bearing problem
> > though.)
>
> I've looked it up just now. It's interesting, and something to think about if
> we ever decide it would be useful to build our own motors. For now, we'll go
> with whatever we can buy from digikey, alibaba.com, or other standard
> suppliers. We'll specify three "Nema #XX 1.8 degree/step bipolar stepper
> motor" in the parts list, for example. (We haven't settled the
> stepper/servo debate yet, frankly.)