Welcome, Guest. Please login or register.
Did you miss your activation email?
April 21, 2014, 07:41:31 AM

Login with username, password and session length
Search:     Advanced search
* Home Help Search Calendar Links Login Register
+  Machsupport Forum
|-+  Mach Discussion
| |-+  General Mach Discussion
| | |-+  Constant velocity
Pages: 1 2 3 4 5 »   Go Down
Print
Author Topic: Constant velocity  (Read 11677 times)
0 Members and 1 Guest are viewing this topic.
Andreas999
Holding

Offline Offline

Posts: 1


View Profile
« on: September 03, 2007, 07:36:19 AM »

I am sorry I rewarm this subject, it has been discussed in other threads already, but I think this is still not implemented right.

After several trial and errors, I reduced my G-Code to a very simple example, a square with three rounded corners and a sharp one:
G0 G49 G40  G17 G80 G50 G90
G21 (mm)
G00 X30 Y0  (start)
G1 X70 Y0 F600  (straight line ..)
G3 X100 Y30 I0 J30 (.. goes smoothly into arc which ..)
G1 X100 Y100        ( .. goes smoothly into straight line)
G1 X30 Y100          (sharp corner 90 degrees to the left) 
G3 X0 Y70 I0 J-30   
G1 X0 Y30
G3 X30 Y0 I30 J0

Note: The lines and arcs join without any change in direction. You cannot have any smoother joins. The radii are large enough so that they can be followed with the desired feed rate.

My desired behaviour in Constant Velocity mode with the "Stop CV on angle" feature would be that Mach3 follows this contour with no acceleration and decelerations for the rounded corners, but with a stop around the sharp corner.

Well, I can either find settings so that Mach3 follows the contour smoothly, but also rounds the sharp corner, or it will stop and go for each segment. 

The only way I see how I can achieve the desired behaviour today is to set up things for Constant Velocity and insert G61 and G64 commands at the appropriate places to switch to and from Exact Stop mode around the sharp corners.

These are the settings I used:
In the Config/General config screen:
- Motion Mode set to Constant Velocity
- CV Dist Tolerance checked, 10 units (10 mm is actually a nonsensical value).
- Stop CV on angle unchecked
In the Alt6 Settings screen:
- CV Distance 10 units.
- CV Feedrate 600 (the same as the actual feed rate)
For these tests I reduced the acceleration of my motors to only 20 mm/sec/sec.
With these settings, Mach3 follows the contour with constant speed, and rounds the sharp corner.

Things I noticed:
- When I reduce the "CV Dist Tolerance" to 1 unit (which is 1 mm in my case, a very high value still), Mach3 noticeably decelerates/accelerates between the rounded segments and their adjoining lines. I do not see any need for that, since these segments join with no disruption in direction. The arcs could be entered and exited at full feed rate from the straight line segments without any resulting error in position, so this could be a smooth motion even if I set this tolerance to 0.0001.

- The CV Feedrate in the Alt6 settings screen does not make sense to me. I need to set it to 600 (the actual feed rate in my example), otherwise I get decelerations between the lines and arcs.

- If I check the "Stop CV on angle" checkbox and set the angle to 90 degrees, it runs smoothly through the contour, rounding the sharp corner. If I set the angle to 89 degrees, it stops for every segment like in Exact Stop mode. (I also had the hypothesis that the entry/exit angle between lines and segments was calculated wrong,
so I split one of the 90 degrees arcs into two 45 degrees segments, but that made no difference.)

- I also tried the Quantum beta version. I could not find any setting that would follow the rounded corners smoothly. I guess the new Jerk Correction only makes things even more complicated, since I have no idea how to set reasonable values.

- If the "Stop CV on angle" feature would work like I understood it should, it could do this example shape correctly automatically. I assume this angle next to the checkbox means the deviation in direction when following from one segment to the next. In the above example, the joints between the rounded corners and the adjoining sides will not change the direction, so their angle is 0 degrees, but the sharp corner turns the direction by 90 degreed when following it. So I assumed I could enter a value of 30 degrees for example, so that the rounded corners would not be affected and be followed smoothly, but the program would detect the 90 degree corner and stop for that.

Andreas
Logged
jimpinder
Active Member

Offline Offline

Posts: 1,233


Wakefield, West Yorks, UK


View Profile
« Reply #1 on: September 03, 2007, 12:00:25 PM »

I am sorry - I do not understand your problem. I have just programmed my mill to cut windows in a miniature railway locomotive, each window had rounded corners off a straight as you have, and (as far as I could see) the machine cut the straight then went straight round the corner at the same speed.

I did not set any Constant velocity settings or anything. I used G3 as you have ( only I use X Y R instead of X Y I J ) no problem.

I did not think that constant velocity had anything to do with your axis motors, but it was to do with spindle speed on a lathe. As you cut into the work and the diameter reduces, the spindle speed increases to keep the cutting speed at a CONSTANT VELOCITY. I cannot see how you can apply constant velocity to a miller, since the feed rate will stay at what is set.

Am I wrong in this Huh
 
Logged

Not me driving the engine - I'm better looking.
Hood
Guest
« Reply #2 on: September 03, 2007, 02:12:21 PM »

Jim you are getting confused with Constant surface speed and Constant velocity I think
Have a look on your programme run page, you may find you are in CV mode.

Hood
Logged
gskinner
Active Member

Offline Offline

Posts: 14


View Profile
« Reply #3 on: September 03, 2007, 04:30:44 PM »

It would be nice if someone could do a tutorial or something explaining the CV settings as I have tried many different settings and get either rounded corners or it stops at every segment.  I still have not figured out what the settings do.

Thanks
George
Logged
jimpinder
Active Member

Offline Offline

Posts: 1,233


Wakefield, West Yorks, UK


View Profile
« Reply #4 on: September 04, 2007, 07:43:58 AM »

Thanks Hood -
That is what I meant - so, as you say I must be in constant velocity mode, because it certainly doesn't hesitate when it comes to the corners. Having said that, my feed rate is not very high - so maybe it still has planty of time to think !!!
Logged

Not me driving the engine - I'm better looking.
jimpinder
Active Member

Offline Offline

Posts: 1,233


Wakefield, West Yorks, UK


View Profile
« Reply #5 on: September 04, 2007, 08:50:50 AM »

As far as the puzzle is concerned - surely  Andreas is correct in his findings, but is trying to achieve somethings that cannot be achieved.

Either the machine is in constant velocity or it stops. The problem is that the milling head (at a right angle) must instantaneously change from one direction to another.

 You cannot do that instantaneously

The head must either decelerate to a stop in one direction, and after it has stopped accelerate up to speed in the other - OR -
as it decelerates in one direction it starts to accelerate in the other (in which case you get a rounding of the corner).

If you have set the machine to stop it will ensure it has stopped in every direction before in moves to the next one, or, in constant velocity mode there will be a smooth transition from the speed in one direction to the speed in the other.

I do not see how the software can interpret what you want, unless you tell it, therefore you can make settings for  the norm, for example, constant velocity, but you would then have to tell the machine if you required anything different for a particular corner.

I must admit, now that you have brought it up - it would be nice to have a tutorial on this aspect of how the machine functions. It would appear that it may be possible to set an angle where a change takes place between Cv and exact stop - I do not know. Try setting the "Stop CV on angle" at 91 so that it includes your 90 angle, and it may do what you want.

Logged

Not me driving the engine - I'm better looking.
stirling
Global Moderator
*
Online Online

Posts: 1,669

UK


View Profile WWW
« Reply #6 on: September 04, 2007, 10:51:57 AM »

... but is trying to achieve somethings that cannot be achieved...

Sorry Jim (can I call you Jim Smiley) - I don't agree.

First, let's assume that we'll accept blending: As you say, when it comes to the rightangle we'll actually get an arc. Let's assume that arc is of radius theta - where theta will depend on the accel capabilities of the machine and the FR we're trying to achieve.

Now let's not accept blending: The machine (as you say again) must come to a stop momentarilly at the right angle. BUT- and here's the nub - if the radius of the other arc corners are greater than theta, we already know that the system can handle an arc of theta without slowing so there is absolutely no need for any slow down for the arcs. i.e. it should be perfectly possible to cut Andreas path with a stop at the rightangle and no slowdowns for the rest (which I think is what he wants and is expecting).

I do not see how the software can interpret what you want, unless you tell it, therefore you can make settings for  the norm, for example, constant velocity, but you would then have to tell the machine if you required anything different for a particular corner.

And that is exactly what all the CV settings are for. i.e. when CV should blend and when it should not. But here I think is the problem: I've noticed that all the CV settings are different depending on what version of Mach your using - which makes it really hard for us to talk about. Also the UG hasn't been updated to reflect the latest CV settings, so this tends to make them (to me anyway and I assume also to Anreas) virtually impossible to understand and to set correctly to get the desired result. Or maybe they just don't work that well even if you can find a way to set them up - I don't know.
Logged
Chaoticone
South Carolina, US
Administrator
*
Offline Offline

Posts: 4,697


Precision Chaos



View Profile WWW
« Reply #7 on: September 04, 2007, 12:13:14 PM »

Hey Guys, check out this thread by Art.

http://www.artsoftcontrols.com/forum/index.php?topic=1724.msg10363#msg10363

Brett
Logged

Grin If you could see the things I have in my head, you would be laughing too. Grin
www.precisionchaos1.com
My guard dog is not what you need to worry about!
Brian Barker
Administrator
*
Offline Offline

Posts: 3,434



View Profile
« Reply #8 on: September 04, 2007, 12:45:47 PM »

This is a CV problem in the current version of Mach 3 and we have fixed this for the next rev Smiley

Sorry for the trouble
Brian
Logged

Fixing problems one post at a time Wink

www.newfangledsolutions.com
www.machsupport.com
jimpinder
Active Member

Offline Offline

Posts: 1,233


Wakefield, West Yorks, UK


View Profile
« Reply #9 on: September 04, 2007, 02:21:52 PM »

Brian - Thanks for that, I was begining to get the old grey matter working on the maths.

Is there any chance of a tutorial?? They do help, especially when you don't necessarily understand it fully without a bit of experimenting.

Jim
Logged

Not me driving the engine - I'm better looking.
Pages: 1 2 3 4 5 »   Go Up
Print
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.19 | SMF © 2013, Simple Machines Valid XHTML 1.0! Valid CSS!