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Trouble with backlash compensation and fast movements
« on: August 04, 2009, 03:33:29 PM »
Hi all, I am having some trouble with backlash compensation (BLC) and was wondering if anyone had seen this issue. The short version is that if I turn on BLC and try to move in more than one axis at > 15 IPM one of the axes will seize up. The exact same movements with BLC off work fine. I made these two videos that show the issue pretty well:

BLC OFF: http://www.youtube.com/watch?v=vrF1-n6oWqU

BLC ON: http://www.youtube.com/watch?v=PMa6wtscvog

I found this line in the manual "Mach3 is not able to fully honor the axis acceleration parameters when compensating for backlash,
so stepper systems will generally have to be detuned to avoid risk of lost steps." and that sounds dangerously like what I am seeing. Can someone explain this further, and is there any way around it?

I've tried exact stop mode but that didn't seem to make any difference.

A few other things: The mill is a Taig 2018. I am using Gecko 201 drives with 258 oz steppers. The controller is a custom job by Dan Mauch of Camtronics and has a 36v power supply rated for 20 amps. The drives are set to about 3.5 - 4.0 amps. I've checked voltages on the power supply and the 5v computer bus. The 36v is solid and does not dip during hard use. The 5v is coming from the USB port and is 5.02v and looks solid. I'm running the current version of Mach 3 as of today; upgraded this morning and it didn't solve anything.

Any ideas?

Thanks,
Jason von Nieda

Offline Hood

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Re: Trouble with backlash compensation and fast movements
« Reply #1 on: August 04, 2009, 03:56:06 PM »
What settings do you have for Backlash and Shuttle?
I have never used backlash myself but sounds like you may have something wrong.
Hood
Re: Trouble with backlash compensation and fast movements
« Reply #2 on: August 04, 2009, 04:04:09 PM »
What settings do you have for Backlash and Shuttle?
I have never used backlash myself but sounds like you may have something wrong.
Hood

I'm at work, so I will get the exact settings when I get home and post them here. Off the top of my head, I think it's something like:
X 0.0012
Y 0.004
Z 0.003

Shuttle I don't remember, but I recently set it to what "Hoss" recommends in his video about backlash compensation, and that's .005 so I think that's where it still is.

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Re: Trouble with backlash compensation and fast movements
« Reply #3 on: August 04, 2009, 04:11:59 PM »
Ah ok, think Hoss has it se pretty good from what I hear so hopefully someone else may be able to help you. As I said I have never used it and cant even mess with it to test as I use the smoothstepper on all my machines.
Hood

Offline RICH

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Re: Trouble with backlash compensation and fast movements
« Reply #4 on: August 04, 2009, 04:37:33 PM »
Jason,
You shouldn't have any problems at 15 ipm, if you were up at 80 to 100 the backlash can seen a little strange
but it still works.

What is the Max IPM you can do before you start skipping steps?
Set your Accel and ipm down 50% of were you start to skip. Also set your backlash to 50%.
Work your way up from those settings. You can also slow down the shuttle setting from .005 to .05 ( it determines how fast the backlash is implemented ) and you may not even notice the difference.

RICH
Re: Trouble with backlash compensation and fast movements
« Reply #5 on: August 04, 2009, 04:45:13 PM »
Jason,
You shouldn't have any problems at 15 ipm, if you were up at 80 to 100 the backlash can seen a little strange
but it still works.

What is the Max IPM you can do before you start skipping steps?
Set your Accel and ipm down 50% of were you start to skip. Also set your backlash to 50%.
Work your way up from those settings. You can also slow down the shuttle setting from .005 to .05 ( it determines how fast the backlash is implemented ) and you may not even notice the difference.

RICH

I'll need to double check when I get home tonight but I am pretty sure that going from 15 IPM to 16 IPM causes what you see in that video above. It's definitely not gradual where I start losing a few steps here and there. It goes from working fine to that awful grind / seize in the video. I'll try the settings you mentioned there and see if I get any changes. Thanks!

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Re: Trouble with backlash compensation and fast movements
« Reply #6 on: August 04, 2009, 04:50:59 PM »
Jayson,
Axis is moving say - X and then will change direction. The axis must decelerate to a stop, then the stepper rotates to take out the non-movement ( backlash ), then the axis must accelerate up to whatever speed commanded.
Can't do that instantly. You wouldn't put the car in reverse when going forward !
The same is going on when there are two axis in motion. One slows down and the other does the motion changes and needs to catch up while both are addressing the moves commanded.

RICH

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Re: Trouble with backlash compensation and fast movements
« Reply #7 on: August 04, 2009, 04:56:03 PM »
When backlash is not implemented how fast IPM can you run the axis before it skips?
Post a picture of the curve from MACH. The ramp to max speed should not be a vertical line. You need time to accelerate / decelerate.
RICH
Re: Trouble with backlash compensation and fast movements
« Reply #8 on: August 04, 2009, 05:02:14 PM »
Jayson,
Axis is moving say - X and then will change direction. The axis must decelerate to a stop, then the stepper rotates to take out the non-movement ( backlash ), then the axis must accelerate up to whatever speed commanded.
Can't do that instantly. You wouldn't put the car in reverse when going forward !
The same is going on when there are two axis in motion. One slows down and the other does the motion changes and needs to catch up while both are addressing the moves commanded.

RICH
When backlash is not implemented how fast IPM can you run the axis before it skips?
Post a picture of the curve from MACH. The ramp to max speed should not be a vertical line. You need time to accelerate / decelerate.
RICH

HI Rich, thanks for the replies.

I currently have the acceleration set to 2 and velocity set to 50. That's how the two videos were taken. With BLC off it runs fine; nice and smooth. With BLC on I get the grinding you hear in the second video. I think 50 is as high as I have tried. I can try higher tonight. I'll also post a picture of the ramp. I know it's not straight. It's about a 45 degree angle both up and down.

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Re: Trouble with backlash compensation and fast movements
« Reply #9 on: August 04, 2009, 05:32:35 PM »
So leave your accel at 2 and drop the vel to 35, backlash at 50%  and test.
You want to have  " torque head room" such that the velocity is not to high for a change in cutter / material / etc
 and still have some extra left for backlash implementation.  Sort of like a v8 engine in a small car but you only
do speed limit but have a reserve should it be needed. In the case of the steppers your just running them below
an upper limit so they won't skip if you go out of your normal operation too far.

RICH