Why does it move jerky when powered?

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This topic contains 19 replies, has 6 voices, and was last updated by  grishien 6 months ago.

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  • #94223

    grishien
    Participant

    Hi all,

    I wonder, will problems ever come to an end?…

    Ok, here you can see how smooth the gantry moves without belts:

    Now I rig belts, switch on, trying to move and see that gantry moves in awful jerks like if burly joint is kind of all on rusty pipe or whatever:

    You not see it clearly here but believe me, jerks are very noticeable, like I cannot ignore it.

    I tried everything: tightened and relaxed screws 1,2 and 3 in main joint; tightened and relaxed belts; tried using homing (dual endstops I have here) and without homing.

    Even when I manually move the gantry unpowered (with belts on) holding by the steppers’ cases – main joint also jerks like it is super-tight on a pipe.

    Anybody who had such experience or have ideas, please help out…

    #94237

    Ryan
    Keymaster

    Lots of things here.

    What board? What steppers? What is your stepper current set to?

    How many teeth are on the pulleys?

    With the belts off do both sides turn with equal force (test with your fingers)?

    Series, parallel, or individual?

    1.8 degree or 0.9?

    It looks as if you might also be using steel belted belts, is that true? If so you need to replace them they will fail quickly.

    Are your belts so tight you are causing strain?

    You have to remember we can not see much and you are not giving hardly any information, information overkill will solve this quickly, making us ask all the questions will take much longer. If you did not buy it from me, we have no clue to what you are using for any part whatsoever.

    #94290

    grishien
    Participant

    Ok,

    it’s arduino mega, RAMPS1.4

    drv8825 (set to 16 steps)

    nema17  steppers 42BYGHM809

    Steppers current is set to 1.7A (0.750 V between minus and driver’s resistor’s handle)

    0.9 degrees per step

    Pulleys are 16 teeth, GT2

    Belts are 2mm tooth, not causing strain – they are loose enough, the have metal strings inside

    With belts off I move with equal force on each finger – yes

    I am doing dual endstop layout where each motor is fed by its own driver.

    Thanks.

    #94291

    grishien
    Participant

    There are no jerks when it moves fast (like on G0 with 1000-2000 mm/min) but when it moves slowly (like 250-400 mm/min) – jerks are present all the time.

    #94307

    Ryan
    Keymaster

    drv8825 (set to 16 steps)

    0.9 degrees per step

    the have metal strings inside

    You have to get the fiber belts, the metal will not last long. 0.9 means you have a slower top speed, and 16th stepping combined with that means you need to edit the firmware. Does a 100mm move =100mm of actual movement?

    As for the jerks you could be a bit tight on the tension bolts, or rough galvanizing with some use the galvanizing should smooth out.

    #94311

    MegaDad3D
    Participant

    There are no jerks when it moves fast (like on G0 with 1000-2000 mm/min) but when it moves slowly (like 250-400 mm/min) – jerks are present all the time.

    I’ve been playing with controlling stepper motors in Arduino (totally different project) and have found this to be true, generally. There is a trade-off between precision and speed, and running higher precision at slow speeds is inherently jerky.

    I don’t know the best approach for setting the firmware, but I think that since you’re using .9 degree steppers instead of 1.8 degree steppers that you’re going to have to tune this in to get it to run smoothly.   As Ryan pointed out, I’d start with measuring whether your distances are correct. My hunch is that you’re going to have to adjust the numbers somewhere to get 2x the number of pulses per each unit of time (or distance). I just don’t know the most elegant place to do this.

    #94391

    grishien
    Participant

    Hi,

    Distances are totally fine. I already edited Marlin to fit my 0.9 degree steppers. The software sends twice more step signals for a distance than it was by default.

    I assume that I cannot increase to even more steps per distance because I’m at maximum now (all jumpers are in place on RAMPS under drives currently).

    So do you mean that I have to play with jumpers RAMPS in order to reduce amount of steps for a turn of a stepper and thus reduce precision and increase smoothness?

    #94419

    grishien
    Participant

    Edit. Sorry puzzling you. I am using 32 steps jumpers layout. During writing previous posts I mistakenly thought that 16 is the smallest division.

    Either way, theoretical 10 mm of movements = 10 mm on practice in my case.

    #94580

    grishien
    Participant

    I tried to decrease amount of microsteps steps to 16, 8, 4. Almost no changes – slow speeds make it move jerky.

    My next router will never use nema17 or belts… only ballscrews.

    #94589

    Dui, ni shuo de dui
    Participant

    I tried to decrease amount of microsteps steps to 16, 8, 4. Almost no changes – slow speeds make it move jerky.

    My next router will never use nema17 or belts… only ballscrews.

    This has absolutely nothing to do with belts or steppers. I have a router using ballscrews and so I can tell you: it doesn’t make any difference. It’s even harder to get right because everything needs to be perfectly aligned, and I mean perfectly.

    You say that it jerks but that is quite a vague description: what kind of jerk? Does it stop moving for a tiny amount of time then goes forward again? is it just noise and vibrations?

    More importantly, does it causes any issue while actually doing something with the machine? Does this jerkiness translates to something whenever milling some wood or using the machine as a plotter?

    I don’t see anything wrong in the video you posted, it seems to move smoothly without much noise.

    If what you’re referring as jerk is something like “the printer moves, then stops, then moves again, then stops, etc.”, I think I saw something similar occured before and the cause was the refresh rate of the screen. Basically, the jerks occured each time the screen was refreshing. But that was with a Delta printer, those machines have a different version of the firmware which creates high loads on the CPU because of trigonometry operations.

    Maybe you could check that, could give you a lead.

    Less likely, but if the jerkiness is not what I just described but more like an inconsistent speed (the CNC moving slightly faster, then slower, then faster, etc.), it could be a defective pulley, where the hole and the belt contact surface are not concentric.

    #95306

    grishien
    Participant

    You say that it jerks but that is quite a vague description: what kind of jerk? Does it stop moving for a tiny amount of time then goes forward again? is it just noise and vibrations?

    Hi. As you can see in 1st video, it moves super smooth when unarmed (no belts).

    But when I rig belts and move under the power – then it’s just as you describe. It stops for a tiny amount and then goes again.

    Steppers don’t stop. It’s just like something is holding the gantry and it tries to overpower such invisible obstacle in order to continue moving.

    I don’t see anything wrong in the video you posted, it seems to move smoothly without much noise.

    If you mean second video then yes  – I suspected that it will not show much. But believe me, it’s not moving well.

    the cause was the refresh rate of the screen

    Well, I disconnected LCD screen without any effect.

     

    Now, I did a research. I removed another axis’s pipe leaving only the pipe with most obvious effect. Movement was to some extent smoother but not much. Jerking was evident.

    Back to fully rigged state: when moving slowly and jerking – I hear irregular sound of steppers which in it’s irregularity totally correlates with jerking pattern. Increasing speen to somewhat 1000mm/min effects in smooth sound from steppers and smooth movement of all parts.

    So far I didn’t understand whether it’s a problem coming from steppers or from friction of moving parts.

    #95331

    Barry
    Participant

    Okay.  Pull the belts off the steppers and attach tape flags to them.  Then do a movement test and watch the flags.  They should be pretty smooth.  If they are, it’s a mechanical issue, if they stutter, then it’s either electrical or a stepper issue.

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    #95577

    grishien
    Participant

    I have found where jerks come from. In order to do that I had to remove belts, hold steppers and move them super slowly, like 5mm/second.

    I felt that bearings go along pipes not smoothly when slow. Like they are facing some little obstacles that obstruct their movement. I checked the pipes (stainless steel) – they are perfectly polished. But still somehow the bearings find some bumps on their way.

    It’s like when you drive car slowly you feel all the small cavities of the road but when you will speed up – you will feel nothing.

    I highly doubt that it’s only me with such thing. Taking into account that most of you don’t have dust suction systems attached – and dust/chips land on pipes in front of bearings making even more obstacles for them.

    So is it that nobody cares about jerks coming from it or you run it on high speed or is it I’m missing something?

    #95600

    Ryan
    Keymaster

    If that is the case your tension on the bearings is extremely too high, you will damage your rails. I brought some damaged stainless rails to MRRF.

    #95611

    Bill
    Participant

    It’s also possible the bearing themselves are bad. Did you get the sealed bearings? If you hold a bearing in your hand and move the center around, can you feel any wiggle or grinding?

    #95647

    grishien
    Participant

    I will check that and report.

    Ryan, I don’t quite understand second sentence, what is MRRF?

    I suspect that pipes are a little wide because its harder to insert them into bearings than old aluminium pipes that I installed initially. I will try to enlarge the holes for screws that act as bearings’ axis in order to allow bearings to stay wider from each other. Hope it will help.

    BTW, Ryan’s advise to install non-steel cored belts helped in easiness of movement of gantry – steel core belts provide something similar to friction (which is obviously not friction but energy needed to bend steel in the area of pulley).

    #95648

    Ryan
    Keymaster

    BTW, Ryan’s advise to install non-steel cored belts helped in easiness of movement of gantry

    The steel fails and the belts do not function properly.

    #95684

    Barry
    Participant

    You shouldn’t open up those bearing holes.  It will make it almost impossible to get everything squared up.  The bolts that go through bearings with plastic on both sides should be loose.  The nut is there to keep the bolt from falling out.

    #95896

    grishien
    Participant

    Well, Barry, your comment was to late, I already did it )

    I did 8.5mm diameter instead of 8mm. I had to tune the force of screws tightening very good this time to avoid the backlashes between pipe and bearing-plastic parts.

    And yesss! Now it moves smoothly on all speeds!

    Now another problem – innertion vibration on start of moving or at the moment of direction change. It is not new problem but it was not so obviois in the background of other problems or maybe steel belts prevented it.

    #96991

    grishien
    Participant

    I went further and updated into setup from BraunCNC (you can youtube or thingiverse it) (but with 6mm belt still). Now a lot better. Can finally focus on other problems.

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