How to turn off Constant Jerk?

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  • December 15, 2013 at 7:56 pm #5077
    tomking505
    Member

    At first I thought constant jerk control was a good thing. Now I’m not sure.

    The whole idea with feed and speed is getting the correct chip size. You do that by adjusting the spindle RPMs and the linear speed, so that each tooth bites off an appropriate sized chip. If the spindle is too fast or the feed is too slow, chips are too slow and the tool rubs. slow spindle or fast feed makes chips too big, lowering tool life.

    Constant jerk, correct me if I’m wrong, slows the feed at the beginning and end, while making it faster in the middle. Right?

    So, wouldn’t that mean, with constant jerk, that I would spend almost no time in the sweet spot, and always getting rubbing at the beginning and end, and oversize chips in the middle? Also, wouldn’t chip size go through the roof in the middle of long moves, where the machine can really get moving? And rubbing is virtually guaranteed on very short moves?

    Is there a way to turn constant jerk off, or turn it way down so that each move spends more time in the sweet spot, and just some smoothing at the beginning and end?

    I’m no expert. What am I missing?

    December 16, 2013 at 3:05 pm #5079
    alden
    Member

    It doesn’t work like that. All machine tools need to manage acceleration and deceleration. The tool cannot go from zero to the feed rate (F) instantaneously. It must go through an acceleration at the start and a deceleration at the end. Constant jerk acceleration management just does that faster than constant acceleration. You spend less time in accel/decel this way and therefore more time at the selected feed rate.

    If you are spending too much time in accel / decel your jerk setting is probably too low. You would have the same problem in a constant acceleration system if your acceleration was set too low.

    The other thing to realize is that jerk is the impact imparted to the machine. It happens in start and stop, and around corners. This can cause all kinds of issues like oscillation (bad surface finish), chatter, and even misalignment. Trying to control jerk by controlling acceleration means giving up a lot of ultimate speed relative to controlling jerk directly, as TinyG does.

    • This reply was modified 10 years, 11 months ago by alden.
    January 11, 2014 at 1:42 pm #5130
    tomking505
    Member

    Well, okay.

    Then for example, how would I lay down a uniform extruder bead if the speed isn’t constant? I’d have to extrude less while the machine was coming up to speed, and extrude more when it was moving fastest.

    If my jerk setting is too low, is there a video that shows what the motion should look like? Or does that depend on the mass of the moving part?

    How would I know when I have the jerk setting right?

    January 12, 2014 at 2:03 pm #5151
    alden
    Member

    You correctly point out the challenges. If you start with the goal that you want to motor motion to adhere as closely as possible to the physics – which dictate that jerk (and even further derivatives) be taken into account, it creates other challenges as you suggest. Laying down a uniform bead being just one of them.

    It’s interesting to note that the early reprap machines (including the makerbot) did not have acceleration management at all. They just turned the motors on and off, so the velocity was always constant. This made extrusion computation very easy, but the motors had to run very slowly as they were unable to accelerate to their full speed. They could only move as fast as the initial step would “catch”.

    So you are right, you have to change your rate of extrusion.

    Guidance on setting the jerk can be found here:
    https://github.com/synthetos/TinyG/wiki/TinyG-Tuning

    Basically you just crank it up as much as you can until it starts failing and back off. You’ll get a feel for it once you play with it a bit.

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