Thoughts about the 'arms' of a 'parallel' delta robot

Is it possible to "contract" and "extend" the length of the arms of the delta robot with a linear motor, as opposed to changing the angle using a servomotor? This would surely make the robot a little more compact. Just remember to add 'moveable joints' so the 'arms' can still bend around.

Two inital thoughts on implement this:

1. Pnumantic style like [builders.reprap.org] . Sounds messy, and expensive to aquire the parts.
2. long metal screw linear motor. Is a good idea but the motor will have to be located at the 'tool platform', making it heavy. Also there would be metal rods sticking out of the system.

However I got one other idea as well. This one sounds like the best idea for a compact delta bot.
Still has problems, but I'm sure there are strategies to fix it:

3. If you remember your humble metal tape measure, it creates a long straight tape that can be rolled back up. What if we can hook a motor to a 'spool' of flexible metal tape and use that as our 'adjustable arm' for the delta robot. The problems I can already imagine is that 'flexible tape' is not as resistant to vibration compaired to 'hard rod arm'. Perhaps one way to fix this is to make each arm consist of 'two metal tape spool', so that there is more support against bending.

Do a google image search for "parallel robot" and you ight get something like your first idea

Heres a link to a springband design as per your second idea

Delta robot designs

Interesting, however again, I think penumatics is out of the questions for reprap (as i doubt we want it to use fluids. As for the springband idea, its getting there, but I think theirs are more for 'pick and place' in a 2d platform.

Anyway have you guys seen FESTO's new delta design? [www.youtube.com] . Frankly I think it looks pretty good, only problem I can imagine for their design, is that it is restricted in material volumn, compaired to rostock's delta design. [www.youtube.com]

Frankly when I think about the springband idea I got, I think vibration is going to be a massive problem. Something like the FESTO delta and the rostock delta design shown in the youtube video links above would be much more stable.

Looking back at the spingband idea again. I think the design in [www.iwf.tu-bs.de] is actually not that bad of an idea for reprap, as long as the platform itself can move up and down. It seems the design in the picture above, only allows for movement in the xy plain, but it could still be very stable due to tension on 4 sides. My only problem with implementing the springband form, is that it would require 4 motors in total, and also requires 'strips of flexible metal' which is hard to source (Maybe we can replace it with a system of pulleys?).

However I think if you are going for the '3 motors' for xy plain, might as well make the 'three' arms stiff, and use timing belts. Which in that case... might as well switch back to the rostock's design. Bugger.

Anyway thanks for the feedback, I say this idea is now dead for impraticality. Have a look at [forums.reprap.org] instead, which I think is a more useful idea for improving the mendel by borrowing the delta concept.

Possibly you could replace the third motor of the spring band design with just a tensioning system, perhaps a pulley and a weight or spring. It only takes two motors to define the position in 2D.

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[www.hydraraptor.blogspot.com]

Good point. Going to need a strong motor to overcome the tensioning system. Also the extrusion nozzle can't be too heavy that it overcomes the tensioning system. Since the tensioning arm is not active, I wonder if vibration is going to be an issue.