RE: [CAD_CAM_EDM_DRO] Re: Higher performance systems

Mariss,

The topic is near and dear to me as well as we are quite
hard at work trying to implement FUNCTIONAL third order
splining in the EMC trajectory planner.

And yes I would describe the math as "a little busy"!

I will get to your group and read about your system.
If it is all digital that moving average must be
a rectangular convolution that might be done with
an FIR digital filter with constant coefficients-
otherwise known as a nested loop.

Your method must go beyond the second order (arc, parabola,
or ellipse) rounding that automatically happens when
acceleration (torque) is limited to a particular value as
it of course always is.

Integrating velocity over a finite (moving)interval would seem
to make accel dV/dt tend to look more like the original
unfiltered V(t) since it would be the integral of a derivative.

Anyway, interesting. I will follow up on your group to
avoid too much math gibberish here.

A note also for Josh... Doing 3-d contouring can be very machine time
intensive so yes routers and such typically can have quite high
feed rates and correspondingly high accelerations so corners can be
sharp at speed. In the big machines cutting at several FEET per second
is common.

But to use those speeds you have to have a LOT of spindle horsepower at
high rpms. Many of those fast machines have 25 HP+. If you are using
a small router motor you may not need such high travel speeds simply because
you can't push that router through the work that fast. An exception might be
cutting foam or something that requires little power.

High rapid speeds might not be such a big issue either if your code
is not cutting a lot of air.

You might try some manual speed/feed tests to see how fast you can really
go with the spindle you use. If you can go a lot faster than your
current stepper tables then it would justify something with higher
performance.

Very high speeds and accelerations tend to be very expensive though!

Les

Leslie M.Watts
L M Watts Furniture
Tiger Georgia USA
http://home.alltel.net/leswatts/wf.html
Engineering page:
http://home.alltel.net/leswatts/shop.html
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-----Original Message-----
From: Mariss Freimanis [mailto:mariss92705@...]
Sent: Thursday, August 28, 2003 7:25 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: Higher performance systems


Leslie,

Ah, a topic near and dear to me just right now! The conundrum is
vector path fidelity and a constant vector velocity are mutually
exclusive.

To accurately follow a path means you have to decelerate to a stop at
the end of every line segment which is irritatingly slow and noisy.

You can "round" line segment nodes to keep a constant contouring
velocity. It is a computational nightmare though, having to insert
arcs between nodes whose start and end angles and radii depend on a
given axis set acceleration and velocity. The "on the fly"
trignometry and path look-ahead schemes get real ugly real fast.

I have come up with a method that neatly sidesteps those problems
entirely; it is computationally trivial and acts automatically. In
effect, the faster you go, the "rounder" the nodes are.

The method outputs a moving average of velocity, effectively
integrating velocity with time. A step change in velocity gives a
ramped change, a ramped velocity results in an S-shaped velocity
profile.

As you probably know the G2002 is up and running using this
algorithm, now on 3-D moves (XYZ). The results are very pretty indeed.

Mariss