More information regarding 'jerk policy'



  • I been using my Duet board for my high speed FFF printer. It can be a bit heavy on the Y axis and I always had a problem with very specific move types missing steps. Today I think I found the answer, by initiating "jerk policy" to 1. I was able to stop a move that consistently causes a Y axis stepper skip. It always seemed to be a spot that went from a print move to a very short travel and print again.

    I want to know more about what "jerk policy 1" is doing as the information that is readily available is a bit vague. From what I can gather some moves do not obey the jerk setting? I can't understand, or something is missing.

    I am still testing to make sure its not some fluke, but either way I would like to know. Thank you!



  • That would be quite the fluke. If anything, one would expect jerk policy 1 to illicit a stall more than the other way around.

    Jerk policy 0 will bring the X/Y axes and the extruder to zero speed when the next move is a travel only move (ie an XY or Z move without E values in the line).

    Jerk policy 1 will never bring the X/Y axes down to zero speed, even if the next move is a travel move. The jerk speed is always the start and stop speed for every move (unless commanded at less than jerk speed).


  • administrators

    The default jerk policy (0) anly allows XY jerk to be used between one printing move and another. So travel moves will always come to a complete stop, a printing move after a travel move will start from standstill, and a printing move that is followed by a travel move will end at standstill. This means that jerk (i.e. a commanded instantaneous speed change) is only used where it is needed to allow curves to be printed at a constant speed, where the curve is made up of short straight line segments.

    Jerk policy 1 allows XY jerk to be used between any pair of moves.



  • Sorry to follow up on this so late, but what are the pro/cons of policy 1?

    • I guess smoother overall movement because no need to accelerate from 0 after a travel move (or before)

    • Will there be any effect on the vertical seam? Not having to come to a complete stop after finishing the outermost perimeter should reduce the ozzed plastic, but it...

    • ...could possibly lead to more stringing?

    • quieter opperation?

    Additionally, how would the behavior of a delta change if this policy was applied to Z movements too? Z-hop would get faster (I guess) which helps stringing.

    I feel like this is actually a pretty powerful setting, especially for faster machines.
    I'll do some testing when I am back home


  • Moderator

    @nikscha I'm not sure of the delta settings, but your other suggestions would mostly match what I've seen. Quieter operation? Not that I've noticed. More stringing? Not that I've seen. But yes it does lead to quicker transitions since there are fewer dead stops.


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