CoreXYUVWA - 3rd gantry homing SOLVED
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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
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@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.
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@whosrdaddy said in CoreXYUVWA:
All arguing aside here.
I would love to see what solution Ian will come up with and how the results areI'll be sure to do a write up on my blog and I'll post a link on these forums. The design is complete but I need to print the parts and order the hardware (motors, extrusion, Vslot wheels, etc). But I'm going away on Saturday for a week so I won't order the parts until I get back. Then I have to strip down the existing top, build up the new top and additional gantry, wire everything up and finally do some testing. Realistically, I'd say it will be mid to late October before I can publish anything.
TBH, I don't really see much in the way of print artefacts even with it rocking as it does. The frame is pretty stiff so everything rocks together. I'd guess that any problems would be more pronounced when the bed is further away from the print head and I haven't printed anything tall for a long while. Having said that I have reduced the acceleration a lot. So hopefully, I'll be able to increase the acceleration then do some high speed back to back printing. It may be that there is still no difference in print quality but we'll see. If it proves to be a waste of time I'll still publish my findings because it's all useful information (and might save someone else a ot of time and expense if it doesn't do anything).
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Sorry to resurrect this old topic but I just wanted to report back that the latest beta firmware (2.03 beta2) allows mapping of end stops which has solved the issues I had with trying to home the 3 gantry - oh and it isn't necessary to create WA axes or any new new kinematics class either.
Thanks DC42!
In summary, I've re-purposed 2 of the 5 extruder drives for now and config.g has the drives mapped thus:
M584 X0:3:6 Y1:4:9 Z2 U10 V11 E5:7:8 P3;Then to home XYU and V, the drives gets remapped thus:
M584 X0 U3 Y1 V4 P5;Note the XY and UV gantries are joined by Bowden tubes and cables so have to be course home concurrently, then fine homed separately. But maybe, with the latest firmware, this could possibly be accomplished without creating U and V - I'll have to think that through...
Anyway, once XYU and V have been homed, then the drives are mapped again thus:
M584 X6 Y9 ; map upper motors to X and Yand the end stops mapped as follows:
M574 X1 S1 C5 ; map end stop 5(E2) to X axis
M574 Y1 S1 C6 ; map end stop 6 (E3) to Y axisAt this point the upper gantry effectively becomes X and Y and is re-homed. Once that is done, the motors and end stops are all mapped back to their starting configuration making it effectively a 3 axis machine and then Z is homed.
It's quite a complicated homing file because I'm effectively doing a fast(ish) course home, followed by a slower fine home of 7 axes. Also, I heat the nozzle because I use that as probe and need to soften any plastic that may have oozed, and I lower the motor currents and do some other stuff as well. The whole thing is here if anyone is interested.
