CoreXYUVWA - 3rd gantry homing SOLVED
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@deckingman said in CoreXYUVWA:
This also shifts the weight of the spools (up to about 6Kgs depending on how full they are) down by about 300mm, so lowering the centre of gravity, which will help with stability.
I don't think so. The moving mass should be as low as possible (ie close to the ground, which can be seen as the pivot of your 'shaking' machine), but static mass should be as far as your can from this pivot. This way, you increase the moment of inertia, and it becomes more difficult to move (shake) the machine
This is how tightrope walkers don't fall: they use a large stick, to increase their moment of inertia. Same, it is easier to keep a stick standing on your finger if it has a mass on its top. You can make the test with a simple M8x300mm threaded rod. With and without nuts at the top. You'll easily see the difference.
Frédéric
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@fma Moving the centre of gravity downwards inherently helps with stability, making the machine less top heavy and reducing it's tendency to "sway". I'm also moving the mass away from the centre line towards the sides. And I'm reducing the overall height of the machine. As well as aiding stability, moving some of the mass lower down will also reduce any flexing of the upper part of the frame, compared to having more mass at the top.
Don't forget that once it starts to "sway" the entire machine becomes a moving mass and is no longer static. So moving some of that mass lower to the floor will reduce it's tendency to sway.
What you describe with the nuts on a threaded rod and tightrope walkers applies when the contact point (tightrope or finger) moves. In this case, the floor doesn't move, so maybe a better analogy would be a low centre of gravity car vs a high sided SUV. Which one do you think can go faster round corners before it rolls over?
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About the height of the machine, only the distance from the floor to the moving mass matters. If the moving mass is still at the same height, total height does not matter.
And moving rolls lower will make them easier to energize, so what you gain on one side is lost on the other.
This is not a trivial problem!
Good luck
PS: your car example is different, and the maths are much easier (this is more or less a static problem).
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@fma The car example is just fine. Take two vehicles of the same mass, with the same friction and apply the same force. The one with the high centre of gravity will roll more and topple more easily than the one with the low centre of gravity. But I'm not going to argue. If you think that raising the centre of gravity makes a more stable structure than lowering it, that's fine by me. You believe whatever you want. I'll do what I normally do and ignore the people who say it won't work and build it anyway. It's been a pretty good strategy so far.
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I don't say that the car with the lower center of gravity is better in this case (of course it is not!), but this is not the same problem with your printer. In the printer case, this is a mode excitation problem; in the car example, this is a static problem.
What you really need to do, is ensure your rolls don't move (except turning). If they do, you will have troubles, whatever the position you put them.
Another (and I hope) better example: a door. Is it easier to push it close to the lids, or close to the handle? Changing the position where you push it is the same as moving the mass away from the point where you push.
The only issue of all this is resonances: you need to stay away from them, whatever you do.
And putting a mass at the rotation center (bottom of the printer) don't help at all regarding the rotation!
You believe whatever you want.
It work in both directions. I'm just trying to point some common problems with excitation and mass positions, that's all. I also have a tall printer, and I already experimented this. And tightrope walkers too.
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In fact, I think you agree with me, because intuitively, you plan to put your dynamic system (third gantry) on the top, not on the bottom
And his system is great: it has been used for years on buildings, in japan, to stabilize them during earthquakes. And the mass is on the roof, not in the basement! They have to measure the earthquake vibration, as they don't know it. In your case, you know the vibration: it is the movements of the lower gantries. Moving this mass in the exact opposite direction as you plan to do is enough, and the gain will be very important.
About resonances. For a given total mass, the resonance will only depend on the relative position of the moving mass and the static mass (center of gravity) regarding the rotation center. Again, this is a simple level arm problem (door): the closer to the rotation point the static mass is, the higher the resonance frequency is. The further to the rotation point the static mass is, the lower the resonance frequency is.
Frédéric
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you can look how japanese skyscrapers pretect against earthquakes, this may give additional ideas. For example Abeno Harukas:
- strong supporting pillars (we already discussed concrete sockets...)
- knots in the structure reduce vibrations
- hydraulic shock absorbers in the lower floors
- corrugates steel plates in the middle
- pendulum dampers hanging from the top
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@fma haha we had the same idea with japanese skyscrapers!!!
But I would add the masses at the bottom...
Because this minimizes risk that the hole building is falling. Japanese building are protected against the earthquake, but not against falling of the building as a whole.There is a difference between the skyscraper and Ian's printer: whether it is fixed at the bottom. (The size is simlar
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Another silly idea, more for David: what about dynamically analysing the vibration of the structure, and reduce accelerations/speeds, as you do for reducing ringing?
It maybe be complicated, and CPU consuming: maybe just a calibration tool, to find the limits for the config file? Not that silly, after all... Such tools exists on industrial brushless motors drivers, to automatically find the correct PID: they move back and forth, from slow speed to high speed. I guess they use encoders and back EMF. Here, we need an external sensor.
Saying that, Ian, I think you should completely remove the spools from the printer: as their weigh change, it will be more difficult to stay away from resonance, as its frequency will vary
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...the resonance will only depend on the relative position of the moving mass and the static mass (center of gravity) regarding the rotation center. Again, this is a simple level arm problem (door)
There are two points away from the rotation center: up or low of it, so both of you are right.
The resonance frequency is a good point: a CNC constructor said that mixing steel and aluminium has the advantage that the resonance frequencies are different and the combination of the two materials lower total vibration. Adding connections between the extrusions with a different material may be a good method to lower vibrations also.
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@joergs5 said in CoreXYUVWA:
But I would add the masses at the bottom...
Because this minimizes risk that the hole building is falling. Japanese building are protected against the earthquake, but not against falling of the building as a whole.Yes, if the system fails, them it is worth. But this system does exist, I saw some many years ago. They may have drop it...
There is a difference between the skyscraper and Ian's printer: whether it is fixed at the bottom. (The size is simlar
)I took as principle that the printer does not move on the floor. It is easy to do, using EPDM discs or so. But it is maybe better to let it move on the floor? If it can't, the frame will twist, leading to geometrical issues. If it can roll on the floor, it may not twist as much? Problem: it may cross the room!
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@joergs5 said in CoreXYUVWA:
There are two points away from the rotation center: up or low of it, so both of you are right.
No! The rotation center is on the floor, as the feet don't move... The rotation center is not the center of gravity!
Frédéric
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@fma Thanks, I understand. I thought you mean the moving mass (hotend level) as rotation center.
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@fma Exactly! The feet don't move. But in all your examples, the walker on a tightrope, the rod on a finger, and the skyscraper in an earthquake, it is the floor that moves.
So in my case, moving the centre of gravity down will make the structure inherently more stable.
In the case of skyscrapers, the tuned mass damper swings on a pendulum. It isn't rigidly fixed to the building because if it was, it would make matters worse.
In reality, the entire printer weighs about 50 Kgs, and I'm planning on moving the filament spools down from 1.6 metres to 1.3 metres so it won't make all that much difference.Edit. And in all you examples, the rotation centre is not at the base. That is where the movement occurs. For a rod placed on your finger, to stay upright when you move your finger from side to side, the top of the rod needs to stay more or less in the same place. So the pivot point is actually at or near the top of the rod, not at the base. Likewise the tightrope walker - his head needs to stay more or less in the same place so the pivot point is close to his head. So in all your examples, you are adding mass close to the pivot point, not away from it.
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@deckingman said in CoreXYUVWA:
@fma Exactly! The feet don't move. But in all your examples, the walker on a tightrope, the rod on a finger, and the skyscraper in an earthquake, it is the floor that moves.
Maybe, but the physics remain the same, and the momentum of inertia acts in the same way. Do you argue the door example ? It is exactly the same system as your printer.
So in my case, moving the centre of gravity down will make the structure inherently more stable.
Statically more stable, yes. But regarding dynamic (oscillations), absolute position of the center of gravity doesn't matter: only relative position of the center of gravity and the exciting force applying point, regarding the rotation center, matters.
I know you don't trust anyone before you experiment things: this is great, experimenting is one of the best things in the life. I love your blog for that; kids should be inspired. So, do this simple experimentation: take a big wooden stick (2kg, 2m long), press it in the mud for 2cm, so you have a rotation point. Put it vertical (balanced) and start to shake it (small movements, so it remains almost vertical). Change the vertical position of your hand on the stick to shake it. Tell me if it is harder to shake it when you are close to the rotation center, or close to the top? Or, at a given exciting distance from the rotation center (say middle), try to add mass at the top, at the bottom, and see what need a higher energy to excite the stick.
Anyway, let us know about the third active gantry progress: this is a very good solution, and fun to build!
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All arguing aside here.
I would love to see what solution Ian will come up with and how the results are -
@deckingman said in CoreXYUVWA:
Edit. And in all you examples, the rotation centre is not at the base. That is where the movement occurs. For a rod placed on your finger, to stay upright when you move your finger from side to side, the top of the rod needs to stay more or less in the same place. So the pivot point is actually at or near the top of the rod, not at the base. Likewise the tightrope walker - his head needs to stay more or less in the same place so the pivot point is close to his head. So in all your examples, you are adding mass close to the pivot point, not away from it.
In the rod example, the hand is the compensation force, not the initial force which makes the rod fall. The physics is the same, it is just more complicated to understand.
Let's focus on the door, or on the big stick experiment I described above.
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@fma said in CoreXYUVWA:
In the rod example, the hand is the compensation force, not the initial force which makes the rod fall........
So if the rod is initially upright and balanced, what is the initial force that makes it fall if not the movement of the hand?
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Almost balanced; a tiny tiny angle and it starts to rotate. This is a mathematical point of view; of course, this is because the hand shakes (at our ages, it is normal
). Then, you really move it to compensate.Whatever the total weight of the rod, is will fall at the same speed (no friction in the air), so what makes the difference is the momentum of inertia, which is higher with the mass at the top rather than at the bottom. Try with a big screwdriver, it works very well.
But this example is not as close to the printer problem as the door one... So, forget it.
