Re: More about motors?

Smoke wrote:

>Looks like servo motors are IT then if I want an ACCURATE system and
>steppers if I want less cost.

Smoke,

I Can't let this MYTH be perpetuated!

There is no INHERANT difference beween the ACCURACY attained by
steppers vs. servos. In fact, a case could be made (strongly) that
with a stepper system you ALWAYS know the limits of your accuracy,
while with servos the accuracy can be "all over the place", depending
on system stability, etc.

There are stepper systems holding MICRON accuracy in the electronics
and optics field. Servo's too.

Accuracy is a function of the system design. It would be like saying
propeller driven planes can't fly; only jets. Both fly. Both have
pluses and minuses.

As I said in my reply earlier, this IS a tougher question than it
used to be. And it used to be tough. :-) I would rather have a solid
well designed stepper system instead of a low-end servo, most days.
Two years ago, I'd have said ANY day!

Here's why:

A stepper is a digital discrete device. As long as I work within its'
torque/ speed limits, it WILL BE where I've told it to be, within
its' step resolution. Nothing else determines this. I send a step. It
takes a step. So I will always have an accuracy within ONE step.

A servo is a feedback analog device. It is, by definition, driven by
the ERROR between where it "is", and where it "should" be. This error
is determined by MANY components of the system; encoder resolution
and accuracy, drive resolution and accuracy, "firmware"(programs that
reside on chips, like your computer BIOS) or software equations and
algorithms.

Hey, why is computer programming like a dancing man who would be
president?
.
.
.
.
They both use Al Gore Rhythms.
.
.
I can hear the groans already :-)


So, the Accuracy of a servo system is reliant on several things
working together correctly, and well. A stepper system accuracy is
dependant on the motor itself. Period. (redundant items like
leadscrews omitted, since BOTH systems are equally affected)
Also, microstepping introduces potential rotational errors if the
motor and drive aren't matched.

To sum up, servos' ARE being made to work to a higher degree of
accuracy and reliability than in years past. And so ARE becoming
a "viable" choice for lower-cost projects. Like yours.

The stepper just sits there, doing what it always has done, reliably
positioning; not as fast maybe, but ACCURATELY.

Hope this helps.

Ballendo