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Author Topic: Very odd rapids stepper motor stalling problem  (Read 25945 times)

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Offline odex

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Re: Very odd rapids stepper motor stalling problem
« Reply #30 on: February 23, 2013, 02:46:12 PM »
Just repeated experiment both ways, velocity mode doesn't seem to affect it

Offline odex

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Re: Very odd rapids stepper motor stalling problem
« Reply #31 on: February 23, 2013, 02:49:34 PM »
Forgot to mention, I also observed that mach3 is splitting the x and y axis backlash compensation moves up through each of these programs, regardless of x traverse distance, so I will assume that is the normal behavior

Offline odex

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Re: Very odd rapids stepper motor stalling problem
« Reply #32 on: February 23, 2013, 03:01:36 PM »
A) changing blc %speed to 100, but leaving the motor acceleration rate at 1, the program will still run, but will occasionally stall.

B) setting the BLC%speed to 0.1, but setting the motor acceleration to 30 IPM^2, the program will stall

C) setting the BLC %speed to 0.1, but setting the motor accleration to 10 IPM^2, the program will stall.

SO, 10 ipm^2 is pretty slow acceleration but it still wont work... and it doesn't seem strongly linked to BLC speed as changing that through its full range seems to have no affect.

Offline Chaoticone

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Re: Very odd rapids stepper motor stalling problem
« Reply #33 on: February 23, 2013, 04:26:21 PM »
OK, you have done some really good test.  Do you see why I said it is your accel. now?  Bottom line is this, backlash is not cumulative, you only see backlash when you change directions or have an intermitten tool load (think of a 2 flute cutter at a very low rpm, I mean low, 30 RPM low) imagine how that may push the gantry and then it would spring back between tool contact time.  These are the only times you will see backlash.  If you can hold .004 with your machine your positional accuracy will be .004 if you drill 1 hole or 1000 if this inaccuracy is due to backlash alone. However, inaccuracy due to missed steps can add up, they can be cumulative but most often are random.  You have done the test now to know how severely under powered your machine is to be able to hold such a tight tolerance with speed.  Now don't take this wrong, I'm not criticizing your machine or your ability, I'm pretty impressed with both.  But, the simple fact is, we haven't figured out a way to beat the laws of physics.  You see, the amount of backlash your machine has is irrelevant to distance you ask it to move.  Backlash is just that, it is lost movement.  So if your running a square for instance at 30 IPM.  What ever accel you have tuned your motors to is what time will be allowed to slow the one axis to a stop and allow the other to get to speed (in exact stop, CV blends this transition).  When you enable backlash, your requesting that motor to move your machine the distance planned by the tool path + the distance in backlash comp. in the same amount of time.  Not only that, but the amount in backlash needs to be executed almost instantly else your tool path will suffer.  Imagine you had a bus and a corvette that had to run the same road course in the same amount of time.  That would be a big task in its self.  How much harder would it be to accomplish if every corner was so tight the bus had to stop and back up to stay on the road?  You see that lowering your accel to 1 your machine almost had the guts to cut the path you plotted for it.  You also see that changing BLC speed had little effect.  You could probably run the comp speed to 2 or 3 hundred % (I don't think mach respects anything over 100)  because usually in BLC moves, you aren't actually moving very much mass, your just taking up space.  But, just the inertia of a motor rotating and instantly having to reverse can be hard to hold.  Couple that with the fact that when both axis get to the other corner, they need to be in the right place you can see why high production, high precision machines cost 100s of thousands of $ and weigh 10s of tons with no axis having over 15 inches of travel and motors big enough to match the price tag.

Brett
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Offline odex

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Re: Very odd rapids stepper motor stalling problem
« Reply #34 on: February 23, 2013, 04:43:56 PM »
So in your line of reasoning, can you explain why the x axis micro-move causes the y axis stalling? 

Bare in mind I have a masters degree in mechanical engineering, I do understand what you are trying to tell me but it doesn't entirely add up.

In other words, if what you were saying is the root cause, then this should occur regardless of x axis involvement. The fact remains that the only way to generate this problem is not with rapid movements or high speed machining, both of which I do a significant amount of, but only with high aspect ratio moves (by aspect ratio I mean the ratio between the amount of movement of either axis). Inertia is not the problem here, load is not the problem here. Load and inertia independently cannot reproduce the problem.

Can you explain the odd jerking motion generated at the beginning of a rapid move AFTER a backlash comp move?

Also, can you explain why turning backlash compensation off FIXES the problem? With or without backlash compensation, the inertial loads should be virtually identical.

With due respect I think you may not have it yet.

Offline odex

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Re: Very odd rapids stepper motor stalling problem
« Reply #35 on: February 23, 2013, 04:51:46 PM »
Quote:
"Not only that, but the amount in backlash needs to be executed almost instantly else your tool path will suffer"

wait, maybe you know something about mach3's speeds that I don't, do the adjustments to the backlash compensation settings not in any way affect the speed?  They *seem* to. If its the 'instant course correction' that is introduced by backlash compensation, why is the condition insenstive to backlash compensation speed settings?

Maybe I misunderstand what that setting does...  ???

Offline odex

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Re: Very odd rapids stepper motor stalling problem
« Reply #36 on: February 23, 2013, 05:16:39 PM »
NEW DATA:

in an effort to further address the inertia concerns, I ran the program some more.

This time I ran it with a rapid motor rate of 120 IPM and an accleration rate of 50 IPM^2.  Ran like a champ SO LONG AS BACKLASH COMP IS OFF.

with backlash componsation ON, it will not run without stalling even with the rate at 50 IPM, and 10 IPM^2 acceleration.

Now, the inertial loads on the first case are ~2x the loads of the second case, the only difference being the software backlash comp.

Offline Chaoticone

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Re: Very odd rapids stepper motor stalling problem
« Reply #37 on: February 23, 2013, 05:18:07 PM »
The backlash comp speed is the speed at which the comp move will be made.  The speed is a percentage of rapid, not your accel.  even at 100% it will still take time to make that adjustment.  The only way to make that time up is to push the accel higher, just briefly.  If the next move is as small as your amount of backlash, where will it get the extra time?  

The settings in BLC do effect the speed but to my knowledge it only effects travel speed, not accel.  I think the accel is overridden briefly so that you end at the right place at the right time.  This is why you won't see it when just working one axis, it has all the time it wants to take to end at a certain position.  If it is has to meet another axis at another point at the same time it really has to push to make up for the lost time and motion the BLC move took.

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!

Offline odex

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Re: Very odd rapids stepper motor stalling problem
« Reply #38 on: February 23, 2013, 05:22:20 PM »
Yes, Ok, thats how I thought it worked. SO at a setting of 30% with my rapids set at 90ipm, it should do the backlash comp move at .3*9=27ipm right?

which is odd because I can make that move all day at 27 ipm with backlash comp off. The only way it becomes a problem is if mach 3 is not actually doing the backlash move at the advertised speed, which it doesn't seem to be. As I mentioned it makes a sudden jump that I cant explain, grossly in excess of what it was commanded.

Still trying to understand what the software is REALLY doing, I get the physics.

Thanks for all the help

Offline odex

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Re: Very odd rapids stepper motor stalling problem
« Reply #39 on: February 23, 2013, 05:39:31 PM »
Do you know if mach 3 always does the axis backlash compensations sequentially rather than simultaneously?  I'm observing the software command first the y axis, then the x axis backlash comp before it starts into the rapid movement. So they dont seem to be trying to meet each other at the same place at the same time, they are taking turns, as it were. At least in my scenario here... maybe at grossly different aspect ratios it does the moves simultaneously.

If I just had someway to seemlessly generate all my gcode with seperate x and y rapid moves I'd never have this bug again, in fact thats how I've manually fixed problems with this particular program :\