Re: [CAD_CAM_EDM_DRO] stepper motor accuracy

> Finally, the static or frictional load applied to the motor affects
> accuracy. A stopped step motor, which has 100 oz/in of holding
> torque, is fundamentally different than a break that has the same
> holding torque.
>
> The break will not turn at all until its holding torque is exceed.
> However a step motor only generates restoring torque if it is
> displaced from its rest position Using the brake analogy, think of
> the output shaft being connected to the break with a torsional
> spring. Now when applying a load, the output shaft has to be radially
> displaced to apply torque to the break.

A break (brake?) is simply a friction device that resists being turned
up to some torque, at which it starts to slip. They are comparing it
to a stepper motor that is more spring-like.

However, I wouldn't worry about it on a machine tool, and here's why.
To get decent torque and small increments, you are going to use
substantial gear and screw reduction. For example, 1:4 reduction
driving a 5 tpi screw. That's effectively 20 tpi or .050 per
revolution. Dividing into 400 half steps gives increments of .000125.
That's the best you can hope for in terms of positioning, so this is
clearly not a high precision machine. If you apply a huge load to the
table, or a moderate torque to the handweel, you will be able to move
it some. But you won't be able to move it very far before it snaps
over to the next stable position - at which point an open loop stepper
system is hopelessly confused.

In other words, you have to design a stepper machine such that the
loads that will be seen won't overcome the restoring force anyway, or
you will loose steps. And then accept that at loads which are a large
fraction of this limit, you may temporarily introduce an error of a
half step increment or two.

Servo machines are not immune to this kind of problem either. If I
grab one of the handwheels on the eztrak bridgeport at work I can
twist it rather far. The servo loop fights this, trying to put it
back, but I really have no trouble holding it .005 or so out of
position without straining my hand, but by applying a lot more torque
than I would want to when cutting on a manual bridgeport. More
powerfull sevos and higher gains in the servo loop would make it
stiffer, but there will always be some play. On the other hand, the
servo machine will always recover when the external torque is removed,
unless you twist it so far out of position that you cause an error
condition shutdown (something I doubt I am strong enough to do).

Acme screws may actually have a bit of an advantage over ballscrews in
that a force applied to the table is much less likely to backdrive the
screw since it has a lot more friction to overcome. But of course
acme screw mean backlash...

Chris Stratton