Re: lookahead / blending

[Jon Elson <jmelson-at-artsci.wustl.edu>]

Hello, all,

I was working with a 3-D contouring program created (badly) by
Bobcad/CAM over the weekend.  (The badly is another problem,
not EMC's.  But, one thing I noticed is that there was a very jerky
motion when I had a high feed rate override set, and the program
section contained a number of very short 3-D moves which had
significant curvature in them.  If I set the feedrate override to slow
things down, the motion became smooth, but when I sped it up,
the machine was executing each vector move and then stopping.

I'm pretty sure these were all G01 (linear) moves, not arc segments.
I am using a 100 MHz Pentium classic, and the servo update rate
is 1 KHz.  I have the trajectory planning running at 2 x the default
rate.

My guess is that the trajectory planning is not blending these moves
due to the discontinuity at the endpoints being greater than some
threshold.  I noted that there is a section that is nearly straight, and
the blending does allow these segments to be done smoothly.
(This isn't quite consistant with the change in behavior due to
the feed rate override, though.)

Is this the correct hypothesis?  Is there a parameter that sets
the threshold for the blending of linear moves?

Thanks for any insight on this,

Jon