Gantry resonant mode and VFA artifacts
Hi all,
I'm one of the unlucky ones with pretty noticeable VFA artifacts. I think consensus is that there is a belt-driven resonance that's to blame. I think most of the focus has been on the excitation side, that is, avoiding driving the resonance. I admit I didn't look very hard, but I didn't see any posts on the exact nature of the resonance itself. I've got some experience in tracking down modal resonances from my day job, so I wanted to take a crack at it.
First, I drove the unit into resonance by printing horizontal lines at the known most-problematic speed. Then, with the case sides off, I started pushing, holding, or otherwise stressing different parts of the frame and gantry. I noticed the resonance completely disappeared when I pushed hard on the left and right sides of the gantry itself. I then tried mass-loaded it as well as I could and saw the same result.
If you completely loosen the belts you'll find that there's quite a lot of play in the gantry. The only thing that really seems to be keeping it square is the belt tension itself. So my best guess as to the actual mode is that it's a spring-supported torsional vibration of the gantry itself, rotating about the the vertical axis, as shown below.
The flimsy L brackets form a torsional spring and the belts act like another set of springs. The resonant frequency should be related to those spring constants and the moment of inertia of the bar about that axis.
I did a few test prints to verify. I swept over speed starting at 40mm/s and increasing in 20 mm/s increments. Harmonics at 80mm/s and ~160mm/s so fundamental is going to be around 80mm. I then mass-loaded the gantry by attaching masses (~0.3kg) to the ends of the gantry and ran again. Harmonics are visible 40, 80, 120, and 160 mm/s, so that would put the fundamental lower at 40 mm/s, exactly what you would expect from this mode. I then stuffed some plastic shims in the gap between the L bracket and the linear rail. This increased the first resonance higher to 140 mm/s with no visible harmonics.
Still could be wrong, but this is as close to a smoking gun as I ever get in my field.
Speculation: I think the reason there's such a wide variation in individual printers' behavior is that the L brackets have some variation in the bend. Some people get close to 90 degrees while others are quite different than square. I think what that does is if the bracket is out of square, it effectively acts as a preloaded spring, so the resonance changes (maybe pushing it out far enough that it's unnoticeable).
In any case, it's clear to me the gantry bar just needs to be dramatically stiffened up in order to squash this resonance. I realize that (somewhat) overconstrains the gantry, but my alignment is already pretty good; I honestly don't think it would be much of a problem.
The bigger problem is that there really isn't any room to make substantial changes to the gantry without reducing the print area or having to redesign a lot of parts. It'd be great if I had 5cm of clearance on all sides but it just isn't there... The best stopgap approach I can think of would be to change the steel bar to a slightly longer (5mm longer) solid bar with screw holes on the ends, then the bracket could be screwed to it T style as well as from the side of the bar like it is now. Maaaybe if the L bracket were made out of much thicker steel, that could also help.
RE: Gantry resonant mode and VFA artifacts
I've never thought much about the difference between the brackets, only that the gantry is square without belt influence. It's essential that be the case, and that belt tension does not deflect it out of square. You never want to correct an out of square condition with belt tension. That makes me wonder, what if you went the other way? What if the brackets were very flexible, plastic even, and the belts were the only controlling force? IMO, mass increase is bad and any increase should be avoided, or it will cause response speed problems. That may be why brand-B uses carbon fiber rods instead of steel. At any rate, I think if the brackets are both square and the gantry is dead square without belt tension, those artifacts don't seem to show up.
RE:
i agree, mass increase is bad. it was only to show the resonance shifted. i believe stiffer is better, e.g. compare to the voron's gantry design (SUPER stiff), but that's testable. My next steps are to shim with something more rigid and repeat. I suppose i could completely loosen the brackets to test the other case.
For what it's worth, my gantry is super square and i've gotten quite good at tuning the belts now, right at their new recommended targets.
RE:
Awesome - I feel this is going somewhere...
>>is that the L brackets have some variation in the bend
Yes - The process for building kits involves bending the gantry, or in other words said L brackets. We control through one variable - twisting the gantry either way - but there are two degrees of freedom, namely the angle offset left and right. The sum shows as gantry misalignment, the difference as internal force that also stresses the bearings (some printers are very noisy and evidence points at the bearings).
The idea with the plastic shims is interesting. I wonder whether there is any possibility of putting absorbing material e.g. TPU in as part of the spring, to de-Q the resonance. I doubt that's effective but maybe there is a way.
Thinking aloud (this isn't practical but maybe someone else improves it):
If I'd extend the L bracket on either side with a stiff beam whose endpoints have clear "line-of-sight" in front of and behind the print head. Then string a piece of TPU filament - as absorbing spring - between opposite points and pull it really tight - TPU is tough. The idea is that the rotational mode works into this "lossy spring" at double frequency. Whether it's effective, can't say but it could be fairly lightweight.