Hi Guys:
Now that a test program is released for visualising velocities ( see previous topic ), we can discuss
the limitations or tradeoff's involved in planning strategies.
When you run the tempest velocity analyser, youll see its preset for a square. Playing with CV , you can see the effect
( on the position screen) of various CV's and motions. Youll note how the velocity in the corners slows or speeds up
dependend on how much cv there is, and the angle of the next line.
First, lets talk about how Mach3, EMC and most other planners do their job. With the standard preset conditions, ( the square)
select accel and note how the acceleration looks with a jerk of 1000. This is probablky the smoothest of all planners in this example.
TO see how MAch3 or EMC woudl look, select a jerk limit of 100,000. Note the square waves, thats typical bang-bang acelleration.
The curvy parts are the only difference from Mach3's typical planning. They represent a complex blend and are unaffected by jerk selection.
Tradeoff #1. - Speed.
This example then shows Mach3 with better . more controllable CV blends. Note the time of the 4 moves. ( the last move to 0,0,0,0 is automatic.).
Its about 3.7 seconds. That woudl be close to Mach3's time to do this square. Now set to 1000 jerk and note the time. Youll see its now
7.4 seconds. SO we can see that SCurves take longer. This figures as it takes longer to drive somewhere if you dont hammer the gas pedal.
With a jerk of 1000, its about 40% longer. Set for 10,000, the time is now 3.94 seconds. Thats probably about where most woudl use such a planner.
Its about 7-10% longer. Thats Tradeoff #1, smoothness takes longer. On the other hand, the corners are better defined, and much more controllable.
Not bad for a 3% loss in iverall velocity. ( in this example anyway..).
Tradeoff #2 - Angular accuracy.
Change the second waypoint move ( set back jerk to 1000 for visual clarity ), so it has an A of 50. Notice how the A starts in the middle of the blend from
motion 1 to motion 2. This is because the rotary motion is commaned to match up with motion #2, but #2 is blended with #1. Orientation motion accuracy will depend
on a few things. In this case, the rotary motion WILL start as the motion #2 starts, ( in the middle of the blend) and end as motion #2 stops. However, for accuracy sake
the planner decided not to blend the next motion, but to stop and allow the rotary motion to catch up. The difference in speeds will produce a non-accuracy in the
lineup of the cartesion motion and the rotary motion. How much depends on speed, and time of motion. Changing to a 10,000 jerk will make it line up better.
In terms of accuracy, an Scurve planner will be slightly less acccurate, and speeds must be taken into account. This is a tradeoff. Speed, smoothness, and complexity combine to
make rotary motion more difficult, but better at not being disjointed. Foir example, you needednt consider slowing the feedrate request for such a motion, they automatically are
fit into the main cartesian motion. Now try changin the waypoint #2 move from 100,100,0,50 to 100, 5, 0, 50. In this case the rotary just cannot fit in the short time it takes to move from
100,0,0 to 100,5,0. Notice what happens, the line is no longer blended to the first, ther eis no time, so the motion is now dead accurate, and the A move will match the Y move to 5, but blending
is prohibbited during this move. Again, this is automatic and done because the math processor decided the motion is so complex that the solution is intractable mathmatically except to
simply do the motion by itself. This is tradeoff #2, the fact that you must consider such impossibel motions, and how they are handled. I call this a CNC singularity, and one should
understand how a planner handles such things. Only real-time experience will tell how how this impacts CNC cutting in a general sense. ( SO far, this is all entirely math based theory. )
Getting late, more later. This at least has given you something to play with and consider in terms of how a future planner may do its job. And hopefully, has increased your
understanding of motion math, velocity, jerk, acceleration, and rotary blending in moves.
Thanks,
Art