Monday, August 11, 2008
Larger scale positioning system
There is a very simple plotter design based on hanging a pen by two threads. The threads are rolled up or down through pulleys. I won't explain more, just look at these video's:
Another nice spritesmod project project page.
More video's of hektor, here.
The reason this system works (no upward rigidity) is because the write head is pretty light.
A big advantage is that you can work on a large canvas, or, tatada daaa, a LARGE BUILD AREA! Because this approach is not limited to two axes, you could also put a write head on three strings. I know it wouldn't have the precision you'd like, but often the tolerances for larger scale objects are also bigger. I can imagine that someone could pretty easily make a 3 string version of hektor with instead of spray paint a PUR spraycan (Polyurethane)
It would not compete with RepRap's resolution, but it will be manufacturing on larger scale. I can imagine a nice playful version to investigate this method:
You have a three axis system and put the pulleys on three poles, on a beach. There, you connect, besides the threads, a hose to the write head that transfers sand that is colloid. This makes it possible to make 3D sand-sculptures. Nice?
I was also thinking of automated construction sites (being reminded of a patent to print buildings). If you make it VERY big the poles would need to be able to handle a lot of force. This positioning system wouldn't work in that case.
- The entire device would be pretty mobile. As with hektor (video 2), there would be a case that controls the machine. Apart from that, three motor/pulley assemblies and a write head. (less chance of TSA molesting your precious child :S )
- easy to construct
- could be controlled with the Arduino, RepRap stepper drivers and steppers. The most radical changes are in software and the frame. Victor has dealt with the 3D positioning based on non-orthogonal axes and will probably have pointers.
- Low resolution, which isn't necessarily very low: it also depends on how fast you go, room conditions (even becomes sensitive to wind) or perhaps a movement model that deals with inertia of the write head.
- Probably more I didn't think of yet...
Erik de Bruijn
My RepRap Blog
Labels: 3D positioning
my gut feeling is that the heaver your print head, the more stable this system would be.
i want to try hacking one together sometime!
I like the idea of a RepRap inspired sand castle building machine. Imagine the crowd that would draw (no pun intended).
A heavier print head might indeed reduce any jitter - maybe some kind of dampening / self correcting logic could be built in also, if there is feedback on the actual position of the print head - but this would get complicated.
It also has problems near the left, right (and presumably upper) edges, where the weight is very unevenly divided on the motors. This could be corrected by restricting the working area to the center region.
last year we've already had some talk about string-robots - look here through the linked threads:
Especially Alex Joni's toy is a simple 3-string-thingy ...
Somewhere there too i mentioned a 'top-down' system, where a string-robot is fixed upside-down to the ceiling and three motorized and synchronized blinds (for the stability) around elevates the building-bed ...
A free hanging head wouldn't be well constrained by only 3 cables though; it would rock and twist a bit. That whiteboard head is constrained by the whiteboard to only move in 2 dimensions, so two strings work great in that case.
6 cables might work, but it gets messy quickly.
you need six wires to fully constrain the print head. adding mass will just slow the system down overall, shifting resonances to lower frequencies.
I think with the bowden extruder (light extrusion head) this type of mechanical bot would become feasible!
The lighter the head, the faster the moves can be.
Dae Kyung Ahn:
He implements this string-based system to create a ultra portable cutting-plotter.
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