re:Re: mill threading, Acceleration, Axis

Alan,

Jon already gave you some good advice. I'll add in a different
direction...

The "best" acceleration "curve" for a stepper motor is generally
considered to be the "S" curve. This is where we start acceleration
slowly, then accelerate the acceleration (yep, read that one twice!),
and then do the reverse to slowly move into the desired step rate.

It's two exponential curves back-to-back. The benefit here is that
the acceleration RATE is low at the beginning and end of
the "ramp"(curve), where the inertia of the load is against us. And
the acceleration RATE is HIGH in the "middle" of the ramp, so we get
to our desired rate as quickly as possible.

We all do this when we drive our cars. As the car starts to move we
give it more and more "gas", since we are now moving; and then we
take a lot "off the gas" when we are near our desired speed,
and "fine-tune"(small gas pedal adjustment) INTO the desired speed.

More and more CNC Controls use this form of accel/decel ramping
nowadays, since the computer power is available to do the necessary
calc's fast enough. I have a friend with a Fadal VMC (vertical
machining center) who likes to balance nickels (borrowed!) on the Z
axis of the mill and watch it (the mill) zip thru parts with the
nickel still standing on edge! Impressive to watch.

The "other" common accel "curve" (ramp) is linear. In this type of
accel, the RATE of acceleration is CONSTANT throughout the ramp. Easy
to program (in the CNC control software code), the only variable here
is to change the RATE. This has the effect of "shortening" the time
to desired speed (if the rate is increased), but at the expense of
requiring the system to "keep up". MaxNC, CncPRO, DeskNC, and most
others in the "low-end" use this form of acceleration.

Think of driving again. The example will not be exact, but I think it
will get the point across :-) . Imagine that you could set the pedal
at ANY position; BUT were then REQUIRED to hold it there UNTIL you
reached the desired speed, at which point you would instantly move it
to maintain this speed. So, you can set it for a long/slow
acceleration which will work almost always, but take a lot of time.
Or you could set it for a fast accel. and hope you don't break
something in the drivetrain!

What your question is really asking IMO, is "is there any way to
know "where to set the pedal" based on the motor/machine. As Jon
said, Yes! BUT...

Hope this helps.

Ballendo

>In a related topic, is there a way to determine the "best"
>acceleration curve for a stepper motor? I have an acceleration
>table that I built based on my "understanding" of Cybernetic
>Microsystems' CY545 chip,<s>
>Alan