Re: [CAD_CAM_EDM_DRO] stepper motor accuracy

palrajas@... wrote:

> Hi list,
>
> Trying to figure out what is involved in determining the accuracy of
> a stepper motor.
>
> Some extracts from Gecko's "ACCURACY AND RESOLUTION" for stepper
> motors.
>
> 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.
>
> When torque sufficient to overcome the holding torque is applied to a
> step motor, the shaft will jump to the next stable location, which is
> 4 full steps ahead or behind the original one, depending on which
> direction the load is applied. Peak restoring torque occurs a full
> step ahead or behind the original location, beyond which it weakens
> and reverses at the 2 full step ahead position to attract the shaft
> to a full step location ahead or behind the original one.
>
> QUESTION:
> What is "a break that has the same holding torque"?

> What is the above trying to explain?

What he is trying to say is that the stepper motor in holding mode is much
more like a stiff detent that an absolutely locked shaft. The comment
about a "brake" is referring to a mechanical device for holding a shaft
from turning up to some limiting torque, at which point it slips.
While no brake is absolutely without deflection, the stepper motor
is very "loose" around the center of the position it is holding in.
As the shaft is forced farther from the center, it resists deflection
with more force, until the max. holding torque is reached, and the
shaft jumps to the next "detent".

> What relevance has the above to, say, a step motor controlling the
> movement of the x-axis of a milling machine? The torque mentioned
> above originate from where in this milling machine - the frictional
> force that prevent the tool from moving in the x-direction or the
> turning force from the current applied to the coils of the stepper
> motor?

There are a huge number of forces that all are applied back to the
stepper motor. Residual fluid drag of the way lubricant, springiness
of mechanical linkage parts, "memory' in the drive belts, leadscrew
"windup" (torsional spring effect of leadscrew), reaction to cutting
force of tool, weight, inertia of decelerating loads, and of course, just
plain friction. Some of these forces are quite unpredictable, others
are generally resisting the last increment of motion to the desired
position. the stepper's torque has to resist all of these, but they cause
the stepper to fail to reach the commanded position. If the total of
these forces is very much less than the rated holding torque, then
the error is small. If these forces reach 10 - 15 % of the rated holding
torque, then the error can approach 1/2 of a step, the point where the
motor starts to develop a lot of reaction torque.

If you are not sure these effects really exist, I suggest clamping a
lever (visegrips may mar the shaft, but work well with a bit of thin
sheet metal to protect the shaft) onto a stepper and connect to a
driver. the bigger the stepper, the better, so you can really feel
the effect. A dial indicator would also be good for measuring
displacement. Apply force until the stepper jumps 4 steps, and
measure displacement. Now, you can go back to the original
position, and see how little force it takes to move 1/8 of that 4-step
movement. 1/8 of 4 steps is half a full step. You will generally
find a region around the center of the step where significant
movement can be made with very little reaction force from the
stepper.

Jon