Different jerk/acc settings for acceleration and deceleration



  • Once more i am trying to improve quality vs speed.

    z axis
    On z moving beds, the 1st 1g downwards is for free (also on deltas for down moves, e.g. z-hop). In order to minimize the layer change time for minimizing oozing, different jerk and acceleration settings for going up or down would help a lot. It also could be used to reduce the wear by using less jerk/acceleration in the upwards direction.

    extruder
    Fast retracts are important to minimize oozing. The faster the printing speed the faster the retracts have to happen, because of the increased back pressure. Retracts are pushed back by the applied pressure. Therefore they can be done much faster than extrudes. So we need different settings for acceleration and deceleration depending on the extruder speed direction.

    print moves
    To get the most constant extrusion possible and by that no extrusion problems, the extrusion rate should be as constant as possible. A “long enough” change in speed where the real extrusion rate will vary, will not enhance the print quality. Therefore it is important to use high jerk and acceleration settings to keep the average speed as constant as possible - but this causes ringing artifacts, which diminish the print quality. But it depends.

    Jerk is like modal analyses with a small hammer. When accelerating positive jerk doesn´t cause visible print defects (at least not at used jerk values), because it doesn´t cause movements out of the move path, but when decelerating (braking) jerk causes the print head to overshoot and oscillate. These defects (ringing) can be seen.

    So there is no reason why we should limit the jerk used when accelerating positive to the same value as when braking. Braking jerk is much more sensitive than accelerating jerk, because of ringing artifacts.

    Solution:
    Add in firmware the possibility to adjust the jerk and acceleration settings per axis for acceleration and deceleration differently. It should be able to adjust the jerk & acceleration settings in relation to the M201/M566 command.

    Adjust acceleration: (% acceleration of M201 acceleration:deceleration)
    e.g. Mxyz1 X120:80 Y120:80 Z200:50 …..
    In this example when accelerating the max acceleration would be 120 % of the initial M201 value and 80 % of it when braking.

    Adjust jerk: (% jerk of M566 acceleration:deceleration)
    e.g. Mxyz2 X120:80 Y120:80 Z200:50 …..
    In this example when accelerating the allowed jerk would be 120 % of the initial M566 value and 80 % of it when braking.

    In addition, if a travel move is followed by a travel move, use always the fast acceleration settings and only at the very last end of the last travel move the slow deceleration settings.

    These firmware changes help to be able to print faster at the same quality and/or to get a better quality at the same speed.


 

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