What is jerk?



  • Hi,

    I don't knok what is jerk... and what impact has in prints... i have changed.... and seems movment is less continous, but i dont know waht is....

    I have search in Google translator.... and... Jerk = "Imbecil", synonymous is fool...

    This cant help me... someone can explain it, or say me where i can find info of Jerk in 3d printer.

    Thanks



  • @peirof As applied to 3D printers, jerk is instantaneous speed change.

    What happens is this, using circles or arcs as an example. These type of moves are created by doing lots of very small straight line segments rather than one continuous curve. If the segments are small enough, then the angle between them will be very small so the result will look like a smooth curve.

    Without "jerk" (in the 3D printer sense) for each small segment, the print head would start from standstill, accelerate up to speed, then decelerate down to a stop at the end of the segment before starting the next segment. This would make the print very, very slow as the print head would never reach a high speed before it had to slow down again. So, we use an instantaneous speed change (jerk). What then happens is that instead of the print head slowing right down to a stop, it only slows down until it is travelling at the jerk speed. Then it will change direction instantly and continue with the next segment.



  • in this case, it is defined as the amount of speed change that the motors can do "instantaneously" without having to fall back to the acceleration settings.

    https://www.dictionary.com/browse/jerk
    In this case:
    noun: a quick, sharp pull, thrust, twist, throw, or the like; a sudden movement:
    The train started with a jerk.

    The definition that you got from Google is perhaps more consistent with the slang definitions a little further down that dictionary.com page, or perhaps like the Steve Martin movie "The Jerk"

    Say you have jerk set to 10mm/s, acceleration set to 20mm/s/s and your print head is stopped.

    If your gcode says to move at 7.5mm/s, it will just start going that fast immediately, but if your gcode says to move at 15mm/s, it will gradually increase speed over the span of 750ms until the motors are moving at that speed. This allows things like curves to go smoothly without needing to be constantly calculating acceleration. If the print head is moving at 71mm/s in X and 71mm/s in Y, (45 degrees at ~100mm/s) and you want it to change to 68mm/s in X and 74mm/s in Y, it could be taxing on the CPU to recalculate the acceleration values, plus determine the change in print head travel speed and vary the extrusion rate over the course of that (possibly very short) move, so we tell it that it's okay not to bother.



  • @supraguy Actually, jerk (as used in 3D printers) is not applied to moves starting from rest. That would be very bad. "Jerk (I hate that term) is only applied when the head is already in motion and a change of direction occurs.



  • Thanks for that. I knew that, and set that up for the later example, but the first one from a dead stop was indeed wrong.



  • @deckingman. That makes me confused. I'm working with servo drives and then the use of jerk is a non linear acceleration. The speed profile has a S shape and in that way it is possible to have higher acceleration and reach target speed faster.
    So for 3d printers it is something different?

    Units:
    Speed mm/s
    Acc mm/s2
    Jerk mm/s3

    Jerk



  • Thank to all, for answers....

    If I am not in wrong... Jerk its an instantaneus change of speed.... In opossite to aceleraciĆ³n What its a change of speed a long a time...

    No?


  • administrators

    @falkia said in What is jerk?:

    @deckingman. That makes me confused. I'm working with servo drives and then the use of jerk is a non linear acceleration. The speed profile has a S shape and in that way it is possible to have higher acceleration and reach target speed faster.
    So for 3d printers it is something different?

    Units:
    Speed mm/s
    Acc mm/s2
    Jerk mm/s3

    Jerk

    In 3D printing the term "jerk" is usually used to mean instantaneous speed change commanded, so it has units mm/min in GCode like other speeds in 3D printing, although mm/sec is easier to think about. See https://duet3d.dozuki.com/Wiki/Gcode#Section_M566_Set_allowable_instantaneous_speed_change.

    However, in physics and engineering, "jerk" more usually means rate of change of acceleration, so it has units such as mm/sec^3.



  • @falkia Just about to answer this but DC got there first. So yes, in physics, Jerk is indeed the third derivative of position with respect to time. But in 3D printers the term Jerk often used when referring to an instantaneous speed change having units mm/sec (or mm/min) rather than mm/sec^3.

    Reprap firmware usually uses the correct term which is instantaneous speed change but for reason that completely escapes me, the 3D printing community in general call that same thing "Jerk". It's one of my pet hates and no wonder why a lot of people get confused.


 

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