Accelerometer frame movement compensation

littlehobbyshop

Would accounting for printer chassis movement give better results for accelerometer based input shaping?

I ask this having only followed the conversation on input shaping, I don't have an accelerometer yet but am planning on getting one and giving input shaping a go. So if I've overlooked something, please forgive and move on with your day.

It seems to me that the current method of mounting an accelerometer to the print head covers the machine as a whole at the print head. I would think what's more important is the movement of the head vs the printer frame or more specifically the bed.

My printer definitely moves and vibrates whilst printing, and I deliberately allow it to. I don't want to pin the printer down solid and have all those vibrations transfer to my workbench which works wonderfully as an amplifier. So, the whole machine is moving by the amount measured by an accelerometer at the head. Would it not be better to take measurements both at the head and on the bed then deduct the former from the latter to get the difference?

I would still think this important on both fixed and moving bed machines where I suspect the total vibrations of the machine may change measurably with z height regardless of whether it's the bed or print head that moves in Z. I'm curious about opinions.

oliof

Frame movement usually means the whole rig moves at the same time, which shouldn't have too much of an effect on your print. I seem to remember that @deckingman decided to disable his counterweight system because while it did effectively cancel out frame movement, it had no discernible effects on the prints.

littlehobbyshop

@oliof That's exactly my point. The accelerometer will pick up that movement whether it affects the print or not and potentially make a mess of the input shaping calculation.

Edit: An extreme example: A printer hanging from a rope and swinging like a pendulum.
That swinging motion will be detected by lone accelerometer and end up included in any calculations intended for input shaping despite that pendulous motion having no bearing on the print head moving across the print surface. To compensate we would need to measure the pendulous motion separately and compensate for it by subtracting it from the acceleration measured at the print head while it's moving. IMO we ideally want to measure the motion of the print head relative to the print surface, not the print head alone in space.

oliof

@littlehobbyshop that's why you look for the largest amplitude in the accelerometer reading and build your compensation around that I believe. As I understand it, the frame based one will be milder in comparison to the actual movement in comparison and as such won't be as pronounced.

littlehobbyshop

@oliof Ah OK, that makes sense. I need to look into the procedure closer. More than that I should really just grab an accelerometer and give it a go!

oliof

Yes, sometimes just doing it is better than over thinking it.

deckingman

@oliof said in Accelerometer frame movement compensation:

................. I seem to remember that @deckingman decided to disable his counterweight system because while it did effectively cancel out frame movement, it had no discernible effects on the prints.

That's not entirely accurate. As I had gone to the trouble of fitting it, I decided to leave it in place. But if I was starting from scratch, I wouldn't bother adding a force cancelling mechanism.

It is true that the AB load balancing gantry had no discernable effect on print quality (because the fact that entire frame "rocked" had no detrimental effect on print quality). But it did stop the printer from "rocking" when doing high speed printing with my 4.5 Kgs of moving mass, which is why I decided to leave it in place.

Having said that, my printer frame is very rigid. If one had a frame that was less rigid and could "flex", then one might see some benefit (but it would be cheaper and easier to simply make the frame rigid).

oliof

Thanks for the clarification, I wasnt too sure on the details which is why I pulled you in (-: