current Dev version, Jerk values?

I started playing with the Dev version for the first time today, so I apologize if this has been asked and answered but I couldn't find mention of it.

This version seems to want to set high (28.3) numbers for my XY jerk...I see it in the startup communications and confirm it is now set on the printer even though I specified 15 in the EEProm.  Is there a reason for this the serial stream indicated the Jerk is too low....I never realized there was a lower is this determined?

P.S. this is a delta printer



  • Yes, there is a lower limit to prevent a miscomputation of first step duration. If that happens the first step takes much to long and then it continuous with a much too high speed. The min jerk prevents that. It depends on steps per mm and acceleration. Here is the computation:

     float accel = RMath::max(maxAccelerationMMPerSquareSecond[X_AXIS], maxTravelAccelerationMMPerSquareSecond[X_AXIS]);
        float minimumSpeed = accel * sqrt(2.0f / (axisStepsPerMM[X_AXIS]*accel));

    so reduce acceleration to lower jerk. That was always there, only writing it to console is new.
  • So is the jerk value computed on the firmware set Mac acceleration just once or is it updated as new Accel numbers are specified in the G-Code?  Is the jerk number I see in the controls on the printer some maximum jerk or the one the printer will actually use all time?

    If it has always been there, are you saying always in the 1.0 versions?  In the .9x versions I have set the jerk in the EEPROM and it never seemed to change?  I used the same Max accelerations on the 1.0 version as the .9x but my set jerk was changed from the number I input to the new calculated value in 1.0 but possible I never noticed in .9x
  • edited February 2017
    I just did the math but something still doesn't compute for me....

    I have 80 steps per MM on all axis, I have the firmware acceleration set at 8000 (although I lower that to either 500 or 6000 during most parts of the print)

    so if I did the math right:

    accel = 8000
    minimumSpeed = 8000 * sqrt(2.0/ (80*8000)) = 14.14

    This is pretty close to what I wanted (15) but the firmware seems to set basically 2x this number (28.3) on startup.
  • That is because jerk is 2*minimumSpeed since start speed is 50% of jerk.
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