MORE DATA:

sorry for spamming my own thread haha

SO, that little jerking motion I mentioned that you can see when you slow EVERYTHING down to a crawl? thats NOT backlash compensation. It only shows up when backlash comp is turned on, but it is NOT backlash compensation.

Why do I say that?  I just paid closer attention, and the y axis DRO is incrementing during that brief jump. Then it paused and started to do its rapid. 

So mach 3 is throwing in an actual axis move right before it does its legitimate rapid, immediately after a backlash comp move.  No idea why. If there is a setting that affects this I would love to know about it, because it seems directly related to my problem. My motors are being commanded a very rapid series of moves at full speed ignoring the motor and blc settings.

OK, Your right about the .3*90=27.  But, the next position it has to be at, at a certain time. However long it takes, taking into account your accel.,  it has to make the time difference up.  If that point is a long way off, it has more time to make up the lost time.  If it is a short move it needs to be there right now (because the other axis is) pushing your accel exponentially higher in order to achieve this, hence the lost steps.  

Brett  

MORE DATA:

sorry for spamming my own thread haha

SO, that little jerking motion I mentioned that you can see when you slow EVERYTHING down to a crawl? thats NOT backlash compensation. It only shows up when backlash comp is turned on, but it is NOT backlash compensation.

Why do I say that?  I just paid closer attention, and the y axis DRO is incrementing during that brief jump. Then it paused and started to do its rapid. 

So mach 3 is throwing in an actual axis move right before it does its legitimate rapid, immediately after a backlash comp move.  No idea why. If there is a setting that affects this I would love to know about it, because it seems directly related to my problem. My motors are being commanded a very rapid series of moves at full speed ignoring the motor and blc settings.

That's interesting.

Do you know if mach 3 always does the axis backlash compensations sequentially rather than simultaneously

I do not.

Brett

 

Yes, I understand what you are saying, though I'm having trouble relating it to what is happening here, so let me put it in other words and see if your rationale makes good walking around sense.

1) I move to a certain x,y coordinate on my table. I do this at a rate of 20ipm. Nothing fancy, just good old fashioned G1 motion of 2 axis at once.  That part is, frankly, irrelevant, but it lead up to the next move.

2) Milll arrives at destination coordinates. Lookahead tells it the next move is back in the other direction on both axes.  It has to make a very small move on the x axis, and a moderate move on the y axis, and its being asked to do it at a G0 rate and using backlash compensation. It initiates a backlash compensation at a snails pace, because i have commanded it to move at 0.1% of 90ipm, which should be .9ipm. This is the meaty part of the torque curve for a stepper motor. Only thing that could be better is if the thing was just dead stopped.  Mach 3 actually commands this backlash compensation to happen FIRST in the y axis, THEN in the x axis. Again, this is happening at a snail's pace. I could paint a scene of this on a canvas its going so slow.

 NOTE: there is no RACE to get either axis to its destination.  You mention that the x axis move being a short distance being a problem becvause it doesn't have "TIME" to get there.  In reality, it has how long it takes for the y axis to make a large traverse, which is ample time to move the x axis a short distance.

3) what happens next I have no explanation for. The y axis then lurches forward at a speed far greater than either the backlash compensation speed or the acceleration rate are set for.  The x axis is already backlash compensated, so there should be no 'rush' to get anything anywhere. It just lurches forward in the already compensated direction.

4) The rapid move should now be ramping up. At anything other than a glacial pace, this will fail and the Y AXIS MOTOR will stall, so long as the aspect ratio is high.  Mind you, I can rapid all day in any direction in any combination of axis so long as the aspect ratio is not high.

The y axis then lurches forward at a speed far greater than either the backlash compensation speed or the acceleration rate are set for.  The x axis is already backlash compensated, so there should be no 'rush' to get anything anywhere. It just lurches forward in the already compensated direction.

Where is the y going when it lurches forward? Y0, Y.5, Y-.5 ?

Brett

Just went and checked it - its ultimate destination is a move from Y1.8169 to Y0.522.  The lurch advances it to Y1.801 just before it does its rapid move.
The sequence goes like this:

1)Do slow backlash comp on Y axis. No movement displayed on the MACH3 DRO
2)Lurch to Y1.801 rapidly
3)Do slow backlash comp on X axis.
4) Rapid move both axis in an interpolated move to the new X,Y coords

The x axis may well be doing an incredibly small lurch as well and I just cant detect it (maybe sub-step resolution)

Thanks again, I know the details are fine level here and its very hard to keep track of it remotely without being able to see it!

How many lines of look ahead do you have entered in general config.?  Can you try it with this set to 1?

Brett

yes sir, I've fiddled with that too, as I read a post somewhere that said the lookahead might trigger a beep which takes away interrupt cycles etc...

As it happens, all the sampling I have done up to now for the sake of todays forum posts have been with it set to 2 for lookahead. Prior to encountering this problem my nominal configuration was a lookahead of 100.

I just reran the snail's pace configuration with lookahead set to 1, it does the exact same lurch behavior.

I dont use BC anymore but when i did this was my rule of thumb which I always recomended to others. You may want to try it out.

My 50-50-50 rule which you may want to consider.
50% - The max velocity is 50% of where your steppers will start to skip
50% - Set the shuttle Wheel setting in configuration to .0050  to .050 ( .5 too slow )
50% - Backlash speed
This is a good starting point and refine upward, but always leave say 30% headroom

BC worked quite well but nothing beats getting rid of the backlash.

RICH

Also, it may be of note that CV or ES settings have no affect on the lurch.