Input shaping vs belt ringing

mikeabuilder

I have a belt ringing problem on my CoreXY printer. When I run at a constant speed of ~26mm/sec in X or Y, I get some serious belt ringing. I also see this in I'm running at a 45 degree angle at 18.5mm/sec. These both make sense since in both cases the motors are turning at the same speed. With my gt2 belts, this means I'm seeing about 36 tooth-transitions per second.

This correlates pretty well with my accelerometer analysis saying I have vibrations at 31Hz.

My first question is about how IS is supposed to work. My understanding is that it is applied during acceleration (positive or negative) events, like making a turn at a corner, but during a constant velocity segment, there would be not correction applied. Is my understanding correct?

My second question is whether there is any function in RRF that would allow me to designate a band of velocities to avoid? For each motor, I might set this as a range of velocities - in my case, I'd designate 24-28 mm/sec as "velocity non-grata" and the fw would shift any constant velocity movements in this range down to the lower limit (24mm/sec in this case). Of course, short bursts in this range would have to be allowed to ramp up or down through the range.

dc42

@mikeabuilder the purpose of input shaping is to suppress ringing of resonances that are excited by acceleration. Uneven movement at a constant speed is not ringing, it is better described as cogging. Input shaping does not address it.

Here are some possible causes of cogging:

Microstepping set too low. Unlikely unless you have made a mistake with the configuration.
Toothed side of the belt passing over smooth idlers. If the idlers have small diameter, this will cause cogging. Use toothed idlers instead.
Belt teeth not fitting the pulley well. Should not occur if the belt and idler both have the same tooth pitch and profile (e.g. GT2) and are of sufficient quality.
Motor current set much too low. At low motor currents, the detent torque of the motor makes fractional steps less even.
Motor not designed for microstepping. Motors can be optimised for maximum torque or for smooth microstepping, or a compromise between the two.

RogerPodacter

@dc42 I would suspect it is the extruder gear teeth. I experienced that using 8mm diameter cog gears. Now i use the 18mm LGX large gears, and i have much less artifacts, they are spaced out much further apart. i could be wrong though.

mikeabuilder

Thanks for the thoughts. I think this matches my observations. @dc42 - Thanks for passing on the knowledge of IS functionality. I thought it unlikely as a root cause, but I still wondered. I'll look again at the other items you list. I'm not expecting much since I've been running these parameters for a while, but I always hope for pilot error - it's the easiest thing to fix. I do have the toothed side of the belts running over some idlers, but I'm using 22mm flanged bearings as idlers, so I think they are not my culprit.

@RogerPodacter - I know this is not related to the extruder because I can make this happen without the extruder running.

My workaround has been to set my speed values to not have any of the main speeds set to run in the "vibration zone". Now I get a little belt movement as I accelerate through, but it isn't too annoying.

deckingman

@mikeabuilder I would add to the last two comments by @dc42, that too high belt tension can have the same effect. The bearings in most steppers are not really designed for high side loads and so can increase the torque required. This can mean that if the belt tension is too high, the motor might miss a few micro steps then jump to the next full step giving the cogging effect that you see. That's one of the reasons that I changed from Nema17s to NEMA 23s. The 23s have much bigger bearings and a larger diameter shaft which alow for higher belt tension.
That might not be the cause of your problem but it's a simple thing to slacken the belt tension a bit to see if it has an effect.

mikeabuilder

Thanks @deckingman. My printer already has NEMA23's, but stress on the bearings is a possiblity. Belt tension is also on my list. My belt tensioners do allow me to put significant tension into the belts.

The actual "problem" I have is best characterized as an annoyance. At low speed, there is a significant increase in audible noise and I can see belt segments vibrating. What I'm not sure about is whether the sound is caused by the vibrating belts, or the belt vibration is being caused by whatever is making the sound. And the bulk of my printing is done much faster, over 100mm/sec, so I'm not seeing issues with part quality. As I said, it's really just an annoyance.

cosmowave

@mikeabuilder To check if it is the belt causing the sound you can damping the belt with your hand...