Re: stepper vs servo Chicken & Egg

Hi,
Not to burst anybodys bubble, But! DC motors have dead
spots with less torq. The only difference between a great
stepper and a great servo is! Speed! DC motors also step!
They are not linear outputs like voltage. At slow speeds
they jump. How many POLES does the DC motor have? 8-16-32?
Maybe 6? Servo Systems have just as many problems as
steppers. In the real world they both work great! The
answer is "The Rooster".

Bob

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> cadcamcenter@y... wrote:
>
> > Hi,
> >
> > Wonder if this describe the difference between a stepper and a
servo:
> >
> > Stepper: A ship where the OPTIMIST captain set a course, and
doen't
> > touch the steering wheel thereafter, assuming everything will be
> > perfect. Everything OK as long as there are no unsurmountable
> > obstacles, otherwise course will be off by an offset caused by the
> > obstacle. Therefore a straight line will be a straight as long as
> > there are no insurmountable obstacles
> >
> > A _______________________ B no obstacle
> >
> > A _________
> > \_____________ B obstacle
> >
> > Servo: A ship with a PERFECTIONIST captain always trying to
maintain
> > a theoritically correct course, making macro/micro (more likely
> > micro) adjustments to the steering wheel continuously. (anyone has
> > compared steering a car and an ocean going liner?)
> > _ ___
> > A ___/ \ /\ / \_____ B
> > \/ \/
> >
> > Final destination alway exactly, +- micron, at where it should be,
> > course corrections may be very micro, but path will ALWAYS be wavy
> > due to numerous course (micro) corrections.
> >
> > Theoritically then, as long as steppers are sufficiently powerful,
> > maybe with very generous margins, the path cut will be more true
to
> > planned cut, whereas servos will alway be (microscopically)
cutting
> > a "wavy" path because of continuous (micron) course corrections.
>
> No, absolutely wrong! Steppers have discrete steps, and can't
position
> in between them, without microstepping drives. Servos DO, indeed,
cruise
> smoothly between encoder points (at least true velocity servos with
DC
> tachs)! My servo system is totally smooth all the way down to .01
IPM!
> That is about 3 encoder ticks per second! Because of the discrete
steps
> of a stepper motor, it is absolutely impossible to get smooth
motion on the
> slow axis of a coordinated move with plain steppers. Due to the
sluggish
> performance of microstepped steppers at very slow speeds, they can
only
> do a little bit better. The reason is that the torque produced for
small
> sub-step
> moves is very small, and the friction and other dragging forces are
high.
> The stepper, when loaded with the typical machine tool, will
develop more
> and
> more torque and then suddenly jump a substantial fraction of a step.
>
> Typical servo systems have a digital to analog converter that
produces
> quite precise velocity commands to the servo amp. Typically, these
go from
> 12 to 16 bits, or 1 part in 2048 up to one part in 65,536. This
allows
> extremely
> fine adjustments in machine velocity, to make minute corrections in
the path
>
> being taken. My particular setup has encoder resolution of .00005"
on the X
> and Y
> axes. Yes, that is NOT a typo, it is 50 Micro-Inches.
>
> > Therefore, as long as there is sufficient margin on torque, a
stepper
> > more precise (pathwise) than a servo?
>
> I can't believe these ill thought-out rumors are still going
around! This
> is total
> FANTASY! If you want accuracy, talk to the guys who build the wafer
> steppers
> that print the patterns onto IC wafers. They use servos with air
bearing
> stages
> and laser interferometers as their linear encoders. They position
to
> NANO-METER
> resolution over a 12" travel, and while they don't do anything
while moving,
> they
> could, as they are extremely smooth drives. (That's the high end.)
>
> Don't ever think you are going to get something for nothing (except
by
> dumpster diving, which can be quite interesting). First, think of
the
> accuracy
> of the magnet alignment in a stepper. The stated accuracy is
generally +/-
> 5%.
> Therefore, the positioning of any step is within 5% of a full
rotation, or
> 18 DEGREES!
> Assuming one full step = .001" with a 5 TPI leadscrew, then an 18
degree
> error
> equals .010" error. this will be a cyclical error that will build
and wane
> each .200"
> of table travel, assuming a worst case stepper motor. Now, of
course, some
> of
> them will be much better, but right there, you can see how much
error you
> are
> getting, with no way to correct for it with an open-loop stepper.
> Microstepping
> will not be any help with this, without an encoder for feedback.
Again,
> variations
> in the magnets and stator cores prevent the microstepping from
finely
> subdividing
> the steps accurately.
>
> Now, a servo system with a machine-tool quality encoder will have
much
> higher
> accuracy of the encoder transitions. Good encoders are usually
spec'd such
> that
> a transition is within +/- 1 cycle of its correct position all the
way
> around the
> circle. For a 1000 cycle/rev encoder, where you get 4000
transitions
> (encoder
> counts) per turn, that is no worse than 1 part per thousand or .1
%, 50
> TIMES
> more accurate than the stepper. Here, my cyclical error will be no
worse
> than
> 4 counts, or .0002"!
>
> With the velocity feedback from the DC tach to keep motion smooth,
the high
> encoder resolution and the CPU making 'steering' adjustments at
1000 times
> per second, these errors are going to be MIGHTY small. I routinely
bore
> holes
> with an end mill, making the machine's table follow a circular
orbiting
> path, and
> the hole will come out VERY smooth! I can get it just a little
smoother and
> rounder
> with a boring head, but milling it out is a LOT faster and more
flexible.
>
> > Hair splitter.
>
> Well, it depends on what you are doing, but this is NOT splitting
hairs.
> For extremely
> fine machining, which I sometimes do, the added precision and
smoothness of
> a
> servo system CAN NOT BE BEAT! This is why ALL high end machine
tools
> use some form of servo, and NOT steppers.
>
> Jon