Re: positional resolution

On 18 Oct, CAD_CAM_EDM_DRO@yahoogroups.com wrote:

> A leadscrew with 20tpi and 200step/rev should have a positional
> resolution of 1/(20x200) = 0.00025 in.
>
> Have I missed something in the above calculation because I read on
> the website of a system vendor listing the positional resolution as
> 0.000125 in.
>
> Further, if in the above example, one uses 10x microstepping, is it
> presumptous to claim a positional resolution of 0.000025in?

Yes, it is presumptuous.

One cannot easily get a stepper motor to assume an accurate and
predictable "intermediate" position. There is almost always some
cyclic ( but non cumulative ) error in position from that expected when
windings are fed current in the classic sin(a) cos(b) relationship.

In those cases where extreme intermediate positional accuracy IS called
for, individually tailored current ratios for each position is
employed, sometimes going so far as to take into account the direction
of rotation.

These caveats aside, where microstepping does shine and is very nice
to have is in the ability to provide a smoother rotation than a lower
"resolution" mode AND in the ability to run a motor faster than it
would in single step or half step mode.

Alan

--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta