RE: EMC minor quirk

["Dan Blouse" <dblouse-at-golden.net>]

Hi,

I have been following the EMC mail list for a couple of weeks now, and I am
admittedly a newbie at this, so if I am asking a stupid question I hope you
will excuse me.

I am involved in programming CNC machines in the woodworking industry, and I
have noticed that straight line moves that lead into an arc always run
better if the line is tangent to the arc.

Is this the case of the situation you describe below.

If the line is not tangent to the arc,  I wonder if you notice the same
acceleration when it is.

I have chalked up this behavior on our machines to the guess that it is
easier for the controller to calculate the tool path if lines and arcs are
tangent since no matter how much the tool dia. compensation offset is the
end points of the line and arc will always meet if they are tangent.

Any comment on my guess would be welcome.

Thanks for letting me comment,

Dan Blouse

-----Original Message-----
From: emc-at-nist.gov [emc-at-nist.gov]On Behalf Of Jon Elson
Sent: February 12, 2000 8:41 PM
To: Multiple recipients of list
Subject: EMC minor quirk


This is one for Fred Proctor or Will Shackleford;

When EMC is moving in a rounded-off rectangular
path of alternating linear and arc moves, with cutter
diameter compensation turned on, I noticed that at the
end of each straight line, the machine would accelerate
monentarily, before starting into the arc move.
It is not serious, but seems like there is a quirk in the
calculation of velocity on the offset path to get the
desired velocity on the actual cutting line at the perimeter
of the tool.  Since the velocities on the linear parts
are the same (offset and actual) it must have something
to do with the calculation for the offset velocity for the
arc.  But, the arc seems to be done right, it may be the
software is wrongly calculating arc velocity when it
attempts to match velocity at the end of the line with
the beginning of the arc.  This is the best that I can
interpret the behavior.  If a stepper system will exhibit
this same behavior, the audible steps might be very
obvious.  It is a little less obvious with servos, but
I can clearly detect the accelerations.  The RS-274
program has all of this movement programmed at a
constant speed of 10 IPM.  The tool is set to a
diameter of 0.5".

My setup is using the 20-DEC-1999 EMC, with
servos driven by the Servo-to-Go card.

This is not a serious problem, but if I have guessed
the origin of the problem correctly (a long shot) then
it might get more serious the larger the tool diameter
is set to.  There is no evidence of this speed variation
when cutter diameter compensation is not active.

Thanks,

Jon