Re: microstep accuracy was what comprises as step

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.


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.

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.

Mariss

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> "Scott M. Thomas" wrote:
>
> > Accuracy may be the wrong term here, but you would get finer
steps of
> > positioning, correct? So if you were using feedback, let's say
from a DRO
> > unit, couldn't we then achieve higher accuracy?
>
> Not necessarily. A stepper guarantees it will move when issued a
half
> step, under almost all circumstances. Much finer than a half step,
and
> there's no guarantee of any movement at all, and it most likely
will not
> move a proportionate amount. If you have X10 microstepping, and you
> slowly click in steps, the motor may only move every 3 or 4 steps,
> erratically. If you use encoder feedback, without deadband, you
> guarantee hunting will occur. it might be rather innocuous, or it
could
> cause a really annoying vibration.
>
> Jon