Re: stepper vs. servo

A step motor's ultimate accuracy is 2,000 locations (steps) per
revolution. This is based on the "+/-5% non-accumulative tolerance"
spec.

Any microstep setting past 10 gives empty resolution.

I can think of only two reasons for using a higher resolution than 10:

1) Need smooth rotation at speeds <<1 rev per second.
2) The motor is used closed loop with a high resolution encoder.

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> turbulatordude wrote:
>
> >
> >But that opens the question of how others can get 256 microsteps
or
> >more. And THAT really is a more general motion control questions
as
> >even though we tend to think our interests are 98% of the
universe,
> >CNC machining applications of motion control are probably more in
the
> >2% range. I'll bet Hewlett Packard (one supplier) sells more
Laser
> >Printers in one year using motion control than all the home brew
CNC
> >stuff ever built.
> >
> >
> If you look at the deflection vs. torque curves for stepper motors,
it
> becomes
> obvious that for most motors, the friction in the motors OWN
bearings, only,
> is enough that you can't move the rotor in increments smaller than
about
> 1/10th of a full step. Any moves smaller than this don't produce
any motion
> until you have accumulated some deflection between commanded and
> actual position, then the motor jumps. Thinking that a steper
motor can
> actually
> move in 200 * 256= 51200 microsteps/rev is ludicrous. That comes
out to 25
> arc seconds, I think.
>
> Some of the problem in this stepper/servo debate is caused by the
makers of
> microstepping drives who make drivers that do microstepping down to
> thousandths of steps, without declaring that no motor can be
expected to
> follow such fine steps. There is some maker that has a drive that
> divides down
> to 144,000 steps/rev, or is it 144,000 microsteps per full step?
Either
> way,
> there is no motor that can take advantage of such fine
microstepping, and
> anyone who thinks their machine can be commanded to move in
nanometer
> increments because of this fine microstepping is, I'm sorry to have
to
> say this,
> a fool. Everything is flexible, of course, and the torsional
spring
> rate and
> compressibility of the leadscrew would make this impossible, even
IF the
> motor could actually do it. I get a bit fed up with people saying
their
> steppers
> are more accurate than any servo system, because they have
microsteps equal
> to microinches. But, of course, they have NOT measured the
performance, and
> in fact they may have .010" of backlash, .002" of cyclical
leadscrew error,
> .003"/Foot of leadscrew pitch error, .001" of static friction, and
a
> table that
> is .025"/foot out of square with the supposedly orthogonal axis!
And, they
> have absolutely NO IDEA that any of these huge errors are present
in their
> machine.
>
> I KNOW what these errors in my machine are, because I have measured
> them with the best equipment I have available (no Renishaw ball-bar
> testers or
> laser interferometers, just gauge blocks, precision squares, dial
test
> indicators,
> etc.)
>
> Jon