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Layer shift on Y axis

Posted by thecrazy 
Layer shift on Y axis
December 11, 2012 10:53AM
Hi,

My printer used to work well but since I added a peice glass on top of my dibond printbed I sometimes have a layer shift on the Y axis, usualy in the middle of the print. Im guessing its because of the added weight.

What do I need to adjust exactly to fix this? speed, jerk, accel, stepper voltage??? im kinda lost.

I tried to fix it myself and I just made things worst, could use some pointers.

Thank you.
Re: Layer shift on Y axis
December 11, 2012 11:10AM
Turn up the stepper driver slightly, if it still happens reduce the acceleration setting in the firmware.
Re: Layer shift on Y axis
December 11, 2012 01:29PM
Thank you I will try that.

If anyone else drops by, could someone explain to me what the acceleration and jerk settings do exactly?

Tried to find some kind of marlin fw manual but wasnt lucky.
Re: Layer shift on Y axis
December 11, 2012 02:06PM
Jerk is the instantaneous change in velocity, i.e. it's the initial speed the printer starts to move at.
The acceleration is the rate of change of velocity above that point, how quickly it speeds up to the requested velocity.
Re: Layer shift on Y axis
December 12, 2012 02:05AM
The term Jerk is actually supposed to refer to the rate of change of acceleration. I donk know if it is implemented in firmware that way though.
Re: Layer shift on Y axis
December 12, 2012 09:46AM
I agree what's called Jerk ought to be named Impulse.
I haven't looked at the code either, but it's in mm/s which to me implies instantaneous change.
Re: Layer shift on Y axis
December 12, 2012 10:38AM
Thanks for clarifying that.

But if im reading this properly does this mean our printers don't slow down before changing direction? They just interrupt the move they were doing with another move at "jerk" speed and then accelerates from there?
Re: Layer shift on Y axis
December 12, 2012 11:16AM
It's not that simple the firmware maintains a buffer of commands, and there is a path planner reads ahead in the buffer to determines how fast the head should be moving at any point. As far as I can tell just watching the printers, the planners don't use exact position, but rather allow some epsilon and try to maintain velocity.

Stops aren't instantaneous, for a long move you can see the print head both accelerate and decelerate, I assume it starts at the jerk velocity and ends at it.

The source code is all ion git-hub if your really interested.
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