James Williams wrote:
>
> Hello,
>
> I have a project in which I am making a CNC machine.  I am using stepper
> motors, however, I am having problems making the
> acceleration/decleration timer values work.

> First, my question is how can I determine the constant acceleration
> value given an acceleration rate of 30"/sec^2.


I suggest a re-think on your whole acceleration
design. I did what you are proposing with my first
XY machine, with smoothed acceleration, and as soon
as it was installed I had to scrap most of it.

The load may like being accelerated mathematically,
but the motors don't. There are resonance speeds and
a load friction speed where the load stops moving in
"jerks" and starts sliding at a constant speed. Motor
resonances are predictable but there are system
resonances that change with length of travel along
beams.

What I suggest is to start the motor at about half of
its max starting speed, ie where starting torque is
still strong and reliable. Then quickly accelerate
based on step speed, ie 1% less time each step until
the correct top speed is reached. This will need to be
done in stages, as you "snatch" through resonances
with higher rates of accleration during these narrow
freq bands. You need control points for what freq each
stage starts at, minimum.

Where your load will tolerate it, probably the best
system is to start the motor as fast a speed as reliable,
get it to full speed in half a rev, and move at a
constant speed not near a resonance freq.

Yours steps/inch sounds like 16 motor revs/inch or a
threaded rod linear axis. If this is right, you will
probably have max speed of maybe 5 or 10 RPS? Have you
made decisons whether you are going to run in the sub
resonance bands only <4 RPS etc?
-Roman

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