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Offline Scott

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Re: New CV features
« Reply #10 on: November 20, 2006, 07:01:34 PM »
Hey Brian,

I was set at 780 velocity, 13 acceleration, and a feed speed of 120 in/min.  Because my screen set still had angular limit CV, I was still thinking angular limit CV and tried starting from a high of 180 down to 10 with little change and 0 gave basically a stop mode situation.  With CV feedrate on, there wasn't really much of a sweet spot to be found.

I see that CV was changed from angular limit to distance tolerance.  I found some info on the Yahoo forum about setting it to 0.10.  That's about as low as I can go before it may just as well be set to exact stop mode.  I air cut the file that I was having problems with and at 0.1 you could feel that it was on the edge of hesitating between G1's and G2/G3's that needed little or no slow down.  A angled YZ from one level of a pocket to the next level was starting to get like this, - - - - - - - (on a downward angle of course).  It would get smoother if set at 0.12 or higher, but I couldn't tell if there was corner rounding happening.  I'll have to run some more tests.

If you have a pocket that is cut from the inside out with the transition from stepover to stepover happening at the upper left corner at a 45 degree angle (Vcarve Pro pockets in this manner), and the pocket was drawn square cornered as intended and cut with a 0.375" cutter, the upper left corner will alway be a 0.1875" radius while the other three corners will be of a larger radius.  If the transition from stepover to stepover is on a side and you choose to not have a final pass, there will be a radiused bump on the pocket wall.  I will again have to try this with the CV distance tolerance set around 0.10 - 0.12 now and compare.  I've also changed my screen set to reflect the change.

On the subject of changes in the software, could it be that after an install/update, that you could be given a choice of reading a revision history document that would alert you to any changes that may not be evident if you have a custom screen set?

Thanks,

Scott
Re: New CV features
« Reply #11 on: November 20, 2006, 09:10:52 PM »
There are doc's from time to time that pop-up to show you what we have changed... The best thing to do is if you are haveing a problem is to post on the forum or wait for the doc's. John is a great guy and I have no idea how he gets all the info into the docs!
Fixing problems one post at a time ;)

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Offline Scott

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Re: New CV features
« Reply #12 on: November 21, 2006, 12:29:30 PM »
I can see that CV is going to be the bane of my existence.  As far as I can see, if you want accurate as drawn parts, turn it off and live with a machine that hesitantly steps along through each line of code, overheating the cutter and leaving dwell marks due to each stop.  Or turn it on if you want a smooth running machine (the reason for switching to a different controller) and throw accuracy out the window.

The more you adjust down to get corners better, the more you make consecutive straight line segments turn into stop and go movements as well as flowing G2/G3's.

Is it not possible to make it ramp down as it comes up to corners, make the corner as posted, and then ramp out of the corner getting back up to speed without a hesitation at other straight line segments and flowing arc segments.  I don't make this next comparison with any implied "this one's better than that one", but if the makers of my old controller could do it, I know that ArtSoft/Mach3 can do even better!

Scott

Offline chad

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Re: New CV features
« Reply #13 on: November 21, 2006, 03:34:12 PM »
Brian i agree that we could use a lesson in cv. I have started doing 3D contours in earnest and i have been struggling with the cv settings. I have found a "sweet spot" of between 180 and 220 ipm. Anything above or below and i get the hurky jerkys. I have to turn off the cv federate in the settings page or else i get jearkys but then corners are rounded. I realize that this has a lot to do with my accel - 20 but i have a 1400lb gantry and just cant whip the machine any faster than that.  Any suggestions?

Chad
 

Offline faby

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Re: New CV features
« Reply #14 on: November 21, 2006, 03:52:31 PM »
> Is it not possible to make it ramp down as it comes up to corners, make the corner as posted, and then ramp out of the corner getting back up to speed without a hesitation

I use step-four driving software (www.step-four.at) from about 6 years and it does this, not in a good way however, because in some cases it doesn't ramp down at high speed and it loose steps... oops.
It's why I passed to mach3, because it's better in cv mode, but the rounded corners are a big problem.

I know it's an hard thing to do in terms of code writing, but I think that is good time to invest in.

I don't know how it's working now but I think that a plausible way could be to look ahead about 80/100 lines and analyze the direction vector, so slow the speed down smoothly near the corner over x degrees (as Scott says), as much as the cv algorithm could maintain small round on that corner...
Ehm... I hope you can understand what I want to say, the language differences are a bad things  ;D

This is only and idea, maybe it is not a good thing  :P

Faby

Offline ger21

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Re: New CV features
« Reply #15 on: November 21, 2006, 08:02:51 PM »
One way to possibly eliminate corner rounding is to use cutter comp, which gives a toolpath with radius corners while still cutting square corners on your parts. Or create your g-code with the corners radiused the same as the tool radius, which will give square corners on your parts. This works better the larger the tool is, and for outside corners only.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: New CV features
« Reply #16 on: November 21, 2006, 10:32:07 PM »
Brian,
What would a decent acceleration be?
Regards
Fernando
Re: New CV features
« Reply #17 on: November 21, 2006, 10:53:16 PM »
I am running 65in/sec^2 with a max feed of 500 IPM :) That is a bit much but I should hope that it is about 30 or so if you are going 500+ ipm

If you are running a slow machine you cen run down to about 8-10 with a max feed of about 100IPM


That is what I have seen work for me :)
Fixing problems one post at a time ;)

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Offline ART

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Re: New CV features
« Reply #18 on: November 21, 2006, 11:31:25 PM »
Hi Guys:

  I heard of the activity here and thought Id poke in and read up a bit.
I think we may have several things going on. So lets discuss CV a bit so we're all on
the same page. Its best if we all understand what happens in the various CV modes and a bit about why a cv works at all..

First, why have CV. Well, you know that one. doing 100 small moves in a line woudl be pretty nasty if they didnt join together
into one path. The problem of course is corners. So lets discuss that first, since most problmeatic jobs involve thousands of them.
  We're fighting physics here. Plain and simple. If you drive down the street and have to make a corner at the end, there are a few ways to
do it. First, you could floor it all the way, but your likely to have a pretty round corner. Probably into the house on the corner.

  Second, we could jam on the brakes at the end of the street, and then turn thw wheel, but we'd skid quite a ways (skidding is losing pulses here.. :) )

  Or we can do what we all do, slow down, turn the wheel and quite often, anticpate the turn by turning as we slow. Mach3 kinda does this, but allows for any of the
last two really. But to show that , I need to show it to you visually.. No machine required..

First, makeup a Gcode file with

G1X10
G1Y10
g0x0y0
m30

 
  Load it up and you can see the rigth angle lon the right of the screen. So lets test some things..
First run the file with F100 in he Feedrate box. Note the amount of rounding. Now run it at F600
, more rounding? Lots more? Some of this will depend on your personal accel settings, so you may have
to use different feedrate. Turn off CV tolerance and angular check for this.

So if you experimented with a few runs of this at various speeds, you'll see exactly how Mach3 blends,
as the X starts to slow, the Y starts to accelerate. Mathmatically, this makes the best join at the feedrate
you requested.  If you didnt notice an asymetry, then your accel is the same on each axis. Use motor tuning to
slow the Y acceleration down a ways. Try it again, see how the Y is now not symetrical ? Think about the way
as one slows the other speeds up and youll see the math behind the blend. Notice the much less rounding at
various speeds. Pick two, say 100 and 600, and take special note of the difference between them.

 Now use general config to set a CV tolerance of 1.

Run the two test again, do they look the same? (They kay or may not depending on your feedrates). But youll notice
only the last 1" is rounded ont he X side, the Y will depend on its accel in the X decel time of its last inch.  The CV distance then,
is telling the system not to start blending until we are a set distance from the end of the move. This also translates to a lower
feedrate point at which it will start blneding, hence a lower amount of deceleration time in blend, thus less rounding..

 OK, now try config/general to set an angular check of 89 degrees, run the test, no rounding at all , right? Because the
right angle is more than what was set , set the angle to 91 degrees and full rounding will appear because the allowed cv angle is
greater than the 90 degree change at the end of the line. Play wth the accel settings, without changing velocities to see some of the effect.
  The thing you should notice is the faster you get in the segment with your F move, the more rounding will result unless corrected by distance tolerance.
 Since distance is a function of velcoity and tiime, another way of thinking about distance tolerance, is the speed at which blending will begin. Thats basically
what it affects..
  Running a few games with that simple file will allow you to fully visualise whats being done.

  I saw above some suggestion of analysing vectors to determine when to slow to get to full speed the fastest, well, thats exaclty whats being done, if you analyse
that to the end, youd find that basically means blending the two moves together in one way or another to smooth the transition of one vectoral velocity into another.
 Mathmatically, Mach3 does this very well, as it actually shows the vectorral change as a circle on the corner which is what such a functions waveform SHOULD look like.

   All that being said, corner rounding is a problem, very frustrating and annoying. One afflicted with it wonders why more people arent upset with it. Its a function of power.
The more power you have, the less you see it. My Mill would need micrometers to see any rounding, as the acceleration is always maxed out. This problem afflicts those with
heavy gantries, with not enough power to push them. Generally, if it takes a second to get up to speed, thats a second too long from a physics point of view, I mean if acceleration
is infinite, then no blending is necessary, the Y would be up to speed instantly as the X stops instantly, thus no blending is required. Its a sliding scale downwards from ifinite acceleration
,the lower the acceleration, the more blending of velocities is necessary. Its the difference of driving a sports care around the corner , or a mach truck.. one slows more than the other ,
and rounds the corner more..

  OK, dont get upset yet, :), I m not picking on your power. I just want to make sure we're all on the same page when it comes to CV and what it does, as well as how it does it. And
why your type of systems represent so much more of a challenge. We need to understand all this because Im getting undercurretns of perhaps other things going on with this type of
problem. Someone mentioned I think that when their motros were going in a certain range, they sounded good, in a lower range, they sound bad. Thats bad. Shouldnt happen. Its
a suspicionof resonance, and if it is resonance, it can really mess things up when your on the edge of performance already. So if its your system thats doing it, you need to run some tests..

G1X10F10
G!X0F20
G1X10F30

etc..

 Where is the sweet spot, better high, worse low? better even lower? any particualr speed bad, if so, you need to dampen some vibration, may change the pinion arrangement,somethign to steady
things up a bit, resonance can be a nasty thing that takes away smoothness no matter what you do..

 OK, but lets say it isnt resonance. Its just CV , and it drives you nuts. (It has driven me nuts before..never did recover..). Notice form the test we did above, that without changing acceleration, the rounding is
less and less the lower the feedrate the run is made at? Theres a key there thats important. Lets say for example, that we're cutting the roadrunner test file. I command 1000IPM, it runs at 60 or so. The reasonit runs at 60 or
so instea dof 1000 is that there is never an opportunity to get up to 1000. By the time it starts the accelerate, it has to decelerate. So it never reaches 1000. SO we're safe to set th efeedrate at 500 right? I mean its only
going to reach 60 in any event?  Not a good idea. What happens, is that it actually reaches higher velocities in the middle of segments so it needs to decelerate sooner than if trying to go slower, so the rounding effect grows
exponentially with the higher demande dfeedrate, even though yourr average feedrate will be the same as a F60 to run the file. I guess hwat Im trying to say, is that not only do you have a feedrate you want to maintain
for a given chip load on your tool, but you have to understand there is a phsysical based feedrate limitation based on your tables accel and vel settings, and that if you exceed that physical limit, the CV will not act very good for you at all.
   Thoise with high acceleration in their system can demand whatever they like from a file, they may get it, they may not, the file will limit the speed in the end, a feedrate is only a maximum speed, in most hard files the tools ends up
goiing much slower much of the time. So demanding a file run only to the limits of your systemns ability will sometimes make differences in orders of magnitude of the performance. Now thats for complex files with hundreds or thousands
of segments were talking about there. If you simply cutting squares in wood, then its different. Nope. Even there slowing down the requested feedrate will make big differences in the rounding. Depending on how your settings are,
it can go al ong ways to fixing it. Many play with accel, angualr, CV dist..etc.. and never slow the commanded feedrate int he file down. But feedrate has the greatest contribution to the problem. So slow down. Youll
usually find a speed where it actually seems smooth in the corners with little rounding. Thats the real, true cv limit speed of your gantry or machine base don  your velocity and acceleration. CV distance, and angular rejection
can help you command somewhat higher speeds than that, but Id find that natural spot just so you know what it is. If its 5IPM, then you know that when you go over 5ipm, your gonna have to add helpers like cv distance and such,
or add another motor, or get a bigger one for that axis... :)

   I am continuously experimenting and trying to makie better CV algorithms, and technology slowly catches up. This is a very complex area though, I wont bore you with the technical detaisl of what CV takes and such, this has been long enough,
but I hope the suggestions help, le tme know if you see any anomolies that my explanaion does seem to jibe with. Im always on the lookout for bugs in this area, but I haven tseen any so far. I hope to be able to make it better as I go,
but I cant say I have any brainstorms at the moment to make this better, other than the suggestions above. Now thats Ive posted on this one, I'll be notified of any additions.. :)

Later,
Art
 

 

 

 
 

 

  



  

  
Re: New CV features
« Reply #19 on: November 21, 2006, 11:43:39 PM »
So is this a problem that is isolated to Mach3? or do comercial controlers from big companies like fanuc have to tackle this problem too?
regards
Fernando