Re: stepper accuracy compared to servos...

Dave,

It's the end of the month, the latest numbers are in and it was a
killer kick-off for 2004.

We shipped 2,612 drives this month; 534 were G320/G340 drives. Servo
drives consituted about 20% of units shipped, which is a little on
the low side. Typically they fall between 25% to 30% of total volume.

I'm not sure if our numbers are representative of what would
correctly reflect the choices of this group though, because nearly
80% of our sales are for imbedded OEM applications. OEMs generally
avoid brush-type servomotors, favoring DC-brushless or AC servos. For
economic reasons and reliability, the stepper is the motor of choice
with them if an application does not absolutely require a servo.

Anectdotal observations are Europeans seem to prefer servos over
steppers more than any other sales region on earth. Meanwhile
Canadians and Chinese disproportionally favor steppers 20:1 over
servos. Go figure, I'm sure sales data would have other surprising
little gems were it further analyized.

I think what biases the numbers are extraneous considerations. Step
motors are largely a standardized commodity both mechanically and
electrically while servo motors are not. You can buy the ubiquitous
NEMA-23, 4.7A, 6-wire motor from a dozen different mfgs. and know it
will function nearly the same. Try doing that with a DC brush-type PM
servo motor.

Why that is so remains one of the great mysteries of life, like why
are surface-mount resistors marked with a value while capacitors bear
no markings at all.

The G204V prototype is slowly comming alive; I was able to run
preliminary overdrive test data on it today. I got an astounding 80:1
out of it versus the conventional 20:1 for a normalized thermal
equilibrium. This means a stepper could more closely resemble a servo
for short bursts of peak power. This is all thanks to a completely
novel switching topology the G204V uses.

Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "turbulatordude"
<davemucha@j...> wrote:

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Paul <no.spam27@n...> wrote:
> >
> > At first read, the original post looked like B.S., however, if

the

> servo
> > system in question is a G320/340 step/dir type drive, then yes,

the

> dynamic
> > accuacy is going to be suspect. The one point I would disagree

with

> is the
> > "you are guaranteed to have a dynamic accuracy of much better

than

> one full
> > step" statement. Your accuracy is only going to be plus or minus
> one step,
> > never "better than".
>
>
> I read that as if one is microstepping.....
>
> That means 8, 10 or more microsteps which may be off by some error,
> but unless a step is lost, will be much better than one full step.
>
> Even a half step error reduces the possible error.
>
> I think the bottom line is that most people start out with

steppers,

> and then either build another machine and use servo's, or swap out
> the steppers and use them in another machine.
>
> There is a HUGE established base for steppers and demand is
> constant. I would not be surprized if the installed base on here

is

> more than 10:1 in favor of steppers.
>
>
> I wonder if Mariss would tell us the percentages of each style he
> ships a month ?
>
> Dave