Pressure Advance killing extruder motor



  • I'm using a 40:1 gearing on my extruder. So the motor is setup at 400mA, 8x micro-stepping, and jerk 600. When I turn on PA, and I try to print at 80mm/sec, my extruder motor just grinds and doesn't move. As soon as I turn off PA, everything works normally.

    M201
    Accelerations: X: 2000.0, Y: 2000.0, Z: 300.0, E: 120.0:120.0

    M203
    Maximum feedrates: X: 48000.0, Y: 48000.0, Z: 1200.0, E: 1800.0:1800.0

    M566
    Maximum jerk rates: X: 750.0, Y: 750.0, Z: 150.0, E: 600.0:600.0

    M572 D0:1 S0.1

    M906
    Motor current (mA) - X:1800, Y:1800, Z:1400, E:400:400, idle factor 60%

    M350
    Microstepping - X:64, Y:64, Z:16(on), E:8:8

    [0_1553910034110_LOW_RES_-_CRUSHED.zip](Uploading 100%) 0_1553910076310_LOW_RES_-_CRUSHED.zip.gcode



  • I don´t know if this is related but 750 jerk seems kinda high. I have it set at 15



  • @nitrofreak makes sense. When I reduce the jerk, the print goes sooooooo sloooooow. drawing a single, short line, it goes, woooooooOOOooOOOooOOOOOOOooooOOoooOOoooOOoooooooo. My print times would easily, literally be 50-100x longer.



  • 750 is mm/min, so that isn't too high for XY. 600 might be too high for your extruder, though. It's jumping straight to 10 mm/s. Lower that to 120, 180, or thereabouts.



  • @bot the weird thing is, without PA, it works fine. With PA, it just squeals and whines.



  • I have a good, large printer. I want to be able to print fast. The prints are perfect slow and without PA. But at any kind of speed, the ghosting at the corners is terrible. PA usually takes care of it. But with this new extruder, I can't find the right combination of settings to allow PA to work AND not slow down the print tremendously.



  • @gnydick What are your steps per mm with that 40:1 gearing at the 8x micro stepping you are using? Also, what value do you have PA set to?



  • whats your value for pressure advance?

    with a ratio of 40:1 your extruder prob cant move fast enough for a E jerk of 600 try going down on that.



  • If you use too low microstepping the motors can also start skipping steps, but, yeah, 40:1... I'd say 10:1 is the practical limit or you need to switch to 48V or higher.



  • I have a zesty nimble. They have 30:1 gearing. I have done extensive testing with PETG, and jerk seems best in the range 50 - 70 ( or around M205 0.8 to 1.2)

    Acceleration of 350 to 420.

    Works fine with M350 E16

    I am using a 0.6 nozzle. 0.4 may require a lower jerk and acceleration than a 0.6.

    Pressure Advance around 0.02 +/- Retraction 0.7



  • @garis The firmware jerk actually increases when you enable PA in the current implementation. It is computed as

    pa_jerk = jerk / pa_time

    so, in your case the firmware attempts to wiggle the motor 50 times faster than you've set it. At 8 microsteps (assuming TMC 2660) without interpolation you get loads of resonance fast. Plus at 40:1 it's a stretch to get it spinning as fast as 30mm/s with all the wiggling going on.

    Have you checked if the EMF is too high on https://reprapfirmware.org/ ?



  • I have Nimbles which use 30:1 and solved the jerk problem by using these 3.6 low inertia disc magnet steppers:

    https://www.portescap.com/products/disc-magnet-motors/p430-disc-magnet-high-speed-step-motor

    They are extremely expensive new but I lucked into a sweet deal on used ones which didn't break the bank.

    Here is an 80mm/s retraction test that shows how fast that stepper can change direction:

    https://youtu.be/sBAlXLfjx78



  • Perhaps I haven't used a sufficiently high magnifying glass to understand what defects I should be observing or tolerating. My printing experience is limited and just now I am reprinting a few parts to upgrade the originals I printed for my Hypercube now that it is working.

    As I understand it during the jerk phase the driver is asking the stepper to accelerate as fast as possible limited by the the motor current, up to the jerk setting leading to resonance in the more controlled velocity ramp during the acceleration phase. For the XY motors this is of concern but for the extruder I could imagine some/much of this resonance would be absorbed by the plastic, and perhaps some under extrusion if steps were missed. For a stepper with 2,600 steps that is likely to be of minor concern???

    I am using a pancake stepper with lower inertia and it seems to work fine. (My original stepper was a recycled cheap one that did not move - it just growled at me - clearly the EMF was way too high on that one.)

    The general print quality with PA of 0.02 was better than no PA. Using retraction speeds of 30 mm/s seems to be the sweet spot. The other variable is the print jerk with
    the extruder being the slave but with the extruder able to limit the print speed - I am using XY jerk of 1,200 but higher values up to 2,000 + also seem to mechanically work ok, although print quality may be another matter.

    I also don't know how far to go down this burrow given it seems to work ok and others don't seem have a problem - another limiting factor is the rotational flexibility of the nimble cable with the high performance magnetic motors just mentioned - curious about them.



  • I don't have any data on the rotational flex of the Nimble cable. What I do know is that with the recommended Nema 14 and or the recommended Nema 17 pancake I could only increase extruder jerk to 60 and coupled with pressure advance made for really slow prints. With the disc steppers I can go to "normal" jerk and acceleration settings. Out of town so I can't see what they are right now.



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  • I can't explain the mystery as to why PA is causing this problem, very strange that everything works when it is turned off. PA is such a good feature, its worth trying to get it to work. I am recommending that you consider increasing your extruder motor current. 400mA seems low to me, as I am running 800mA on my Ormerod 2 and 700mA on my Delta.



  • @ayudtee The motor current is what the stepper motor can tolerate and a good rule of thumb is to 85% of the rated maximum. So increasing the current to 800mA would not be a good idea if the Op's motors are rated at (say) 500 mA.



  • @deckingman they're definitely rated for more. I think the logic is, at 40:1 gearing, anything much more than 400-500mA doesn't work well. I've tried it, it's not happy.



  • @gnydick said in Pressure Advance killing extruder motor:

    @deckingman they're definitely rated for more. I think the logic is, at 40:1 gearing, anything much more than 400-500mA doesn't work well. I've tried it, it's not happy.

    Fair enough - there is nothing in this thread to state what the rated current of your motors are so I had to revet to using a crystal ball.

    Referring back to my earlier post of 30th March @ 05:51, I might have some ideas but you haven't answered the questions I asked.



  • It was explained to me this way. At such high steps per time frame required with high gear ratio the current doesn't have much time to decay and so higher current exasperates the situation. With 3.6 degree steppers I run normal current levels but couldn't with 1.8 degree steppers.


 

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