Re: microstep accuracy was what comprises as step

Ok,

I'm seeing that the micro step is not just making the stepper more
accurate just based on the number of micro-steps. ie: a 200 step
stepper does not become a 2,000 step stepper. Although it does need
a controller pulse for each step (or micro-step)

If I have a 10 TPI lead screw and drive it with a 200 step per
rotation stepper, I theoretically will have an positional accuracy of
a half a thou. A concern I have had was speed. 200 steps per
rotation is 2,000 steps per inch, and at 10 micro-steps per full
step, that is 20,000 pulses from the controller per inch. Using
Master5 and expecting 5,000 pulses per second that is 4 seconds per
inch, or 15 inches per minute. That is pleanty fast for my needs.
EMC and Turbo are still options if I need more speed.

Accepting lead screw slop, and other mechanical innaccuracies, I
think I will be within my targeted 0.005" accuracy, over 7 inches.

What I am doing is drilling one hole, 5/16 spotting drill, moving the
table 7 inches then drilling my thru hole 1/16 dia. if I am within a
few thou, the 1/16 will just find center of the spotting hole and not
have any problems.

My test set-up using a 12 VDC power, NEMA23, 50oz and serial
controlled PIC based drive is getting me close.

The problem I will address in the future if it really is a problem is
the slight movement. The machine vibration from the twin heads will
pose more pressure on the leadscrew/stepper than the 1/16 drill.
Since I clamp the part before each drill, the table is locked in
place. This means there is no pressure on the posistioning system
during the drilling process. Not that the pressure of drilling a
1/16 drill is all that much, but it does prevent the occasional
broken drill.


Dave

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> mariss92705 wrote:
>
> > Jon,
> >
> > I think you are confusing servoes with steppers here. A microstep
> > drive outputs sine and cosine currents to the motor. Between steps
> > these currents are steady (unchanging), so the motor position does
> > not dither.
>
> No, I was referring to steppers systems with encoder feedback.
Unless
> the stepper positions are aligned well with the encoder
transitions, I
> would expect some hunting in a closed loop stepper system, but
> not in an open loop stepper. This was just an aside at the end of
> my comment.
>
> > Accuracy depends on the ratio between the motor's holding torque
and
> > the mechanical system's friction. As an example, if your motor
has a
> > holding torque of 150 oz-in and the system friction is less than
15
> > oz-in, you can pretty well be guaranteed an accuracy of 1/10 of a
> > full step.
> >
> > There is nothing special about a half-step versus a microstep.
Half-
> > stepping is the equivalent of 2 microsteps per step. Going to a
finer
> > resolution of 10 microsteps per step simply means your resolution
now
> > equals the motor's native accuracy.
> >
> > Anything less than that and you are not getting the full potential
> > from the motor. Anything much more than that buys you "empty"
> > resolution since accuracy pretty much 1 part out of 2,000 (1/10
step)
> > on an average step motor.
>
> Right. This is really what I was trying to get at, that a X1000
microstepping
> drive will not give you positioning of 200,000 steps/revolution. A
lot
> of people apparently think they really can move the motor reliably
and
> repeatably in micro-arcseconds, just because the computer can
command
> such a microstep.
>
> >
> > In practical terms this accuracy can be depended on. As the load
on
> > the motor relaxes, it moves into the correct (accurate) position.
> > When you are cutting metal, the inital load may be high (>15 oz-
in).
> > As the metal is removed, the load lessens and the motor responds
by
> > moving into its accurate location. All this takes milliseconds.
> >
> > You mention "may move 3 or 4 steps, erratically", "no guarantee of
> > any movement at all" and "cause really annoying vibration". This
is a
> > phenomena I have never observed. Can you please elucidate? None
of my
> > instrumentation has ever even hinted at this "effect" when using
step
> > motors. Perhaps you have taken data that you would consider
sharing
> > to shed light on this behavior? It would help everyone.
>
> What I meant, there, was 3 or 4 MICRO-steps, erratically, in a
microstepping
> drive with, say x10 or x16 microstepping. Every single microstep
may not
> cause actual movement of the motor, depending on load.
>
> The really annoying vibration was ONLY in reference to closed-loop
> stepper drive systems with insufficient deadband. There are lots of
> servo systems that jitter, too.
>
> Jon