Continuous vertical lines on shell
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@diamondback said in Continuous vertical lines on shell:
Is it possible that my motors are just crap?
Sort of, yes. How tight are your belts? The reason I ask is that as a general rule, stepper motor bearings aren't realy designed to take the high lateral loading that we tend to apply. There is a real temptation to apply the highest belt tension that we think we can get away with, or that the belt manufacturer recommends. However, with this sort of loading, it is eminently possible that some degree of "stiction" (for want of a better word) of the shaft can occur. It may not be much but it could be enough that the requirement for higher torque, coupled with the lower torque available for partial full steps (due to micro-stepping) might mean that the motor will have a tendency to jump to the nearest full step. i.e. small (say single digit) micro steps might not necessary translate into any physical movement of the motor.
This is more a theory on part than any sort of scientific fact. Although, when I first built my printer, I was a bit too enthusiastic with the belt tension and had to use very high motor currents to get any sort of reasonable speed.
But it would be easy enough to slacken the belt tension a bit just to see if makes any difference. Worth a try I'd have thought.
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@deckingman So do You think that it's worth trying higher motor current? Which should fix this problem on slower moves?
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@deckingman said in Continuous vertical lines on shell:
@diamondback said in Continuous vertical lines on shell:
Is it possible that my motors are just crap?
Sort of, yes. How tight are your belts? The reason I ask is that as a general rule, stepper motor bearings aren't realy designed to take the high lateral loading that we tend to apply. There is a real temptation to apply the highest belt tension that we think we can get away with, or that the belt manufacturer recommends. However, with this sort of loading, it is eminently possible that some degree of "stiction" (for want of a better word) of the shaft can occur. It may not be much but it could be enough that the requirement for higher torque, coupled with the lower torque available for partial full steps (due to micro-stepping) might mean that the motor will have a tendency to jump to the nearest full step. i.e. small (say single digit) micro steps might not necessary translate into any physical movement of the motor.
This is more a theory on part than any sort of scientific fact. Although, when I first built my printer, I was a bit too enthusiastic with the belt tension and had to use very high motor currents to get any sort of reasonable speed.
But it would be easy enough to slacken the belt tension a bit just to see if makes any difference. Worth a try I'd have thought.
Just tested this, I made the belts as loose as possible while still not skipping teeth and while that added a bunch of ringing as expected, it didn't really affect the lines...
@petrzmax said in Continuous vertical lines on shell:
@deckingman So do You think that it's worth trying higher motor current? Which should fix this problem on slower moves?
I actually had them fairly high (housing got a bit too toasty probably) I lowered it a bit yesterday so that the housing is only handwarm...
I think I'll try different printing speeds next...
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@diamondback said in Continuous vertical lines on shell:
.......................Just tested this, I made the belts as loose as possible while still not skipping teeth and while that added a bunch of ringing as expected, it didn't really affect the lines...
Oh well, it was worth a try..........(and bang goes that theory
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@deckingman said in Continuous vertical lines on shell:
coupled with the lower torque available for partial full steps (due to micro-stepping) might mean that the motor will have a tendency to jump to the nearest full step. i.e. small (say single digit) micro steps might not necessary translate into any physical movement of the motor.
This is a common fallacy / misconception that is all over the web. The torque available when microstepping is the same as full steps because the vector sum of the two coil currents is always the same. I.e. the peak current when only one coil is on is sqrt(2) times the current when both coils are on. So at any stop position the holding torque is the same. It isn't less at intermediate microstep positions.
It is true that the torque is always proportional to displacement from the rest position, so if you only move a very small step the torque applied is very low and may not overcome stiction. However that is only relevant at the start. If the the load does not move the displacement quickly increases and so does torque until it does move.
Once moving at constant speed the rotor lags the rest position by an amount that generates enough torque to overcome the dynamic friction. If that lag exceeds two full steps the motors stalls because the torque is sinusoidal so reaches a maximum. During acceleration it lags more and during deceleration it leads.
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@nophead Thank you for the explanation - (actually I think I had read that before but had forgotten).
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New results: slowing things down significantly (ie just 25mm/s for the outer walls) seems to almost get rid of the problem, pitch of the lines seem unchanged but the amplitude is much smaller.
I'll try the other way and speed things up to see what happens -
Do you think you could post a few photos of the printer, particularly the XY arrangement, and maybe a video of it in action? I'm a visual person, so it helps me to see what's going on in motion. Perhaps something will become apparent.
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I was thinking if bad quality pulleys, for example misaligned center of the hole can create effect like this.
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@diamondback It would be interesting to see the stepper motor current waveform to see if there is some distortion at higher speed. Back EMF does normally have an effect but I am not familiar with these drivers as I haven't attached motors to my DUET yet.
What supply voltage are you using?
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I had fought this problem on my CoreXY and in my case it was misaligned X Y linear rod bearings which created vibration (it's a custom contraption that I overtightened by mistake so it didn't run smooth). Plus, the motors were resonating and the vibration of the axis added to the resonance. I ended up going for 0.9deg motors and smoother rods and now it's 99% gone.
On the topic of belt tension, I observed that no matter how tight you make them, the plastic parts on the axes (and possibly the belt) will give in until equilibrium is reached. Yeah, it might seem a bit tighter at first, but try checking again in a year.
