Extruder velocity profile during retract or prime



  • I realise that I could get the answer to this question by looking at the code but I hope it's a quick question to answer.

    When doing a retract or prime with no axis movement does the extruder velocity ramp up and down at the extruder acceleration rate with possibly a constant velocity segment in the middle depending on whether the extruder velocity is reached? If not, what is the profile? Let's assume the extruder jerk is 0.

    Thanks


  • administrators

    If pressure advance is set to zero, it does exactly what you say.

    Edit: pressure advance is not applied to retract or prime moves, so it always does exactly what you say.



  • Thanks.



  • Just as an exercise, I put some numbers to this. The time to reach a given speed is (Vf-Vi)/a where Vf is the final velocity, Vi is the initial velocity (zero in the case) and a is the acceleration. So in my case I have extruder acceleration set to 2,000mm/sec^2 and have firmware retraction set to 2400 mm/min = 40mm/sec. Putting those numbers into the formula reveals that the time to reach 40mm/sec is 0.02 secs. To calculate the distance moved while accelerating s=1/2at^2 = 0.4mm. So if the retraction amount is less than 0.8mm, it'll be either accelerating or decelerating. Hmm, never thought about that before….......



  • Just coming at this the other way if you want the extruder to retract at 2000 mm/min set this as your e jerk value. I appreciate this might cause a stall depending on the extruder/gearing and you might have to use a lower value but at least you have control over it. I'm using a cable driven extruder on one machine and have to use low jerk and acceleration to get the motor to reverse and turn at high rpm (40:1 gearing) so I expect it never reaches the intended retract speed. It's irrelevant as it retracts enough to prevent blobs and strings.


 

Looks like your connection to Duet3D was lost, please wait while we try to reconnect.