Cura: Jerk is labeled in wrong units?

[gnarf]

Sorry to blow up the issue template, but this is in the current version and I imagine spans all platforms/printers.

Jerk is labeled in mm/s which is the wrong unit for the setting as far as I can tell.

• Speed (or velocity) is mm/s.
• Acceleration is mm/s² - or "speed change per second"
• Jerk is mm/s³ - or "acceleration change per second"

[fieldOfView]

"jerk" in Cura is 3d printer jargon, and should not be confused with the "jerk" we know from physics. The "jerk" in Cura is actually a speed(-change), so the unit is correct:

Jerk: The maximum instantaneous velocity change of the print head

"_instantaneous_ velocity change" means this is not "over time", so it is the difference between two speeds.

[gnarf]

Thanks! Sorry, I'm still a bit getting into the 3d print jargon, and was confused by my physics ;)

Does the info you quoted go any more into this? I'm trying to understand what this setting means now, it made more sense to me when jerk was a "max change in acceleration". Instant velocity change just sounds like an impossible measurement?

[fieldOfView]

A 3d printer is bound by physics, but the controller is digital. So it is very possible for a controller to "instruct" the motor to change speed instantaneously. The motor would not manage doing that especially if the difference is big (and it would create actual "jerk" and "jolt" trying to do it), so firmwares implement a sort of threshold for the maximum difference in current and new speed they are making the motors turn. And this threshold was (improperly?) named "jerk". Confusing, but a fact of life.

At least, that's how I understood it.

[gnarf]

Thanks for the education! This makes a lot more sense now.

On Thu, Apr 19, 2018, 11:51 AM Aldo Hoeben [e-mail] wrote:

A 3d printer is bound by physics, but the controller is digital. So it is
very possible for a controller to "instruct" the motor to change speed
instantaneously. The motor would not manage doing that (and would create
actual "jerk" and "jolt" trying to do it), so firmwares implement a sort of
threshold for the maximum difference in current and new speed they are
making the motors turn. And this threshold was (improperly?) named "jerk".
Confusing, but a fact of life.

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