JERK !!!

[jsnbrgg]

Hello,

Can someone please explain to me the settings on my Rostock Max v2 under the Acceleration menu on the LCD? I assume Jerk reduces the amount of jerk the printer does when moving from one direction to another but should I raise the setting to decrease jerk or should I lower the setting to decrease jerk? What are the other two settings for? One for print and one for movement or travel (can't remember exactly) but it's set at 3000.

I'm getting some strange noises that I don't like when the printer is jumping about from one spot to another really quickly. It almost sounds like the nozzle is raking across the plastic on the top of the build, but it's not. The noise is coming from the towers. I'm assuming it's jerk, but I'm in the middle of a large print and I can't stop to adjust my slicer settings. I was hoping these LCD settings might help. I tried reducing the print speed from 100% to 70% and I'm still getting it but not quite as loud.. This is printing at 60mms, infill at 80mms, travel at 300 (which was default). Maybe it's normal? The print looks good! Straight edges and all.. Maybe I need to tighten something in the towers? My cheapskates are all tight with no wiggle in any direction.

I've attached a screen shot of my slicer settings, and here is a short video of the noise: (Issue Resolved - Video Removed)

[bot]

Acceleration is the rate that the feedrate is increased in mm per second per second. Jerk is the speed to which the feedrate will immediately jump to without applying any acceleration. Keep in mind that this also applies for slowing the nozzle down.

So for example with a setting of Jerk: 35 and Acceleration: 2400, given a line segment lone enough, and instructed to make the move at 100 mm/s, the nozzle will immediately jump to 35mm/s [edit: this is wrong, it actually starts at jerk/2; see my post below], increase the speed by 2400mm/s/s until it reaches 100mm/s, then it will slow down at 2400mm/s/s to 35mm/s before turning/stopping/moving on to the next instruction.

The settings in the photo you show are simply the slicer settings, giving the g-code generator the desired speed to ramp up to (the 100mm/s value from my example).

[jsnbrgg]

This issue is resolved. Thanks!

[Neptune]

So what was the issue, you could end up helping someone else down the road...

[jsnbrgg]

My belts were too loose. They were tight enough to work and not skip, but loose enough that it caused a strange noise.

Thanks,
J.

[Neptune]

Thanks, that's good to know!

[Eaglezsoar]

Acceleration is the rate that the feedrate is increased in mm per second per second. Jerk is the speed to which the feedrate will immediately jump to without applying any acceleration. Keep in mind that this also applies for slowing the nozzle down.

So for example with a setting of Jerk: 35 and Acceleration: 2400, given a line segment lone enough, and instructed to make the move at 100 mm/s, the nozzle will immediately jump to 35mm/s, increase the speed by 2400mm/s/s until it reaches 100mm/s, then it will slow down at 2400mm/s/s to 35mm/s before turning/stopping/moving on to the next instruction.

The settings in the photo you show are simply the slicer settings, giving the g-code generator the desired speed to ramp up to (the 100mm/s value from my example).

A very good explanation of what the jerk setting does, Thank You!

[lbarger]

Acceleration is the rate that the feedrate is increased in mm per second per second. Jerk is the speed to which the feedrate will immediately jump to without applying any acceleration. Keep in mind that this also applies for slowing the nozzle down.

So for example with a setting of Jerk: 35 and Acceleration: 2400, given a line segment lone enough, and instructed to make the move at 100 mm/s, the nozzle will immediately jump to 35mm/s, increase the speed by 2400mm/s/s until it reaches 100mm/s, then it will slow down at 2400mm/s/s to 35mm/s before turning/stopping/moving on to the next instruction.

The settings in the photo you show are simply the slicer settings, giving the g-code generator the desired speed to ramp up to (the 100mm/s value from my example).

Thanks for the information. This is considerably different from the classical engineering definition of "Jerk", but does seem to be what the printer is actually doing. In engineering, "Jerk" is defined the the rate of change in acceleration. So given your parameters, the nozzle would start at 0, with an acceleration of just 35mm/s/s, and ramp up the acceleration until it reaches 2400mm/s/s or the velocity reaches 100mm/s. The classical definition would require about 2.4 seconds to reach velocity while your scenario only takes 0.02 seconds. I don't hear it changing pitch much, so you must be right!

[Jimustanguitar]

I also thought that the Jerk settings referred to the physics definition, i.e. the rate of change in acceleration.

So... Which one is it really?

[Polygonhell]

The term jerk is very confusing in all the GRBL derivatives (Marlin/Repetier etc.), it's the maximum instantaneous change in velocity or more usefully the Junction speed, which is what it was introduced to resolve.
It's more confusing if your used to the guts of any other CNC software, or any of the academic research in the subject where they use Jerk in the physics sense of the word.

[bot]

So, this text from the firmware source code:

Code: Select all

Caution: This is no real jerk in a physical meaning.
The jerk determines your start speed and the maximum speed at the join of two segments.
Its unit is mm/s. If the printer is standing still, the start speed is jerk/2. At the
join of two segments, the speed difference is limited to the jerk value.

and a Thomas Sanladerer video on the topic:

[youtube]http://www.youtube.com/watch?v=7HsIZuj9vOs[/youtube]
are all I know about how it works. I was slightly mistaken.

[lbarger]

So, this text from the firmware source code:

Code: Select all

Caution: This is no real jerk in a physical meaning.
The jerk determines your start speed and the maximum speed at the join of two segments.
Its unit is mm/s. If the printer is standing still, the start speed is jerk/2. At the
join of two segments, the speed difference is limited to the jerk value.

and a Thomas Sanladerer video on the topic:

[youtube]http://www.youtube.com/watch?v=7HsIZuj9vOs[/youtube]
are all I know about how it works. I was slightly mistaken.

Thanks "bot" for the definitive answer. I'm a mechanical guy. I don't go digging in the source code for fear of really messing things up!