Revision as of 10:38, 29 January 2023

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• For an introduction of what a RepRec system is and
• for what peculiar design constraints it comes with

see main page: RepRec Pick & Place Robots

Possible Designs for the Poser subsystem of a RepRec

Design suggestion: parallel SCARA robot + 3+1-DOF rotation head

The FANUC M-1iA/0.5A robot has an interesting head serial mechanics 3-DOF rotating head that can turn all three hinges the full 360°. The internal workings of the head are very fine-grained and will by far not meet the "avoidance of small or special material parts" requirement for the RepRec. With major redesign it should be possible to meet this requirement. The head will become quite big and heavy though (wild guess: a little less that 20cm in diameter)

Beside the nice head there are problems with the delta-bot design though.

• There are a lot of non-planar hinges which are difficult to make compact with the design restriction of small component avoidance.
• A self replicating robot needs a quite big working volume to build its copies. To make it bigger the FANUC rotative arm drives in the top cupola could be changed to a linear rail design. But then coupling the three rods that drive the rotative head with statically mounted drive motors at the top will become very space wasting (long overstanding torsion-drive-rods). Thin torsion rods are a bad choice for low stiffness plastic anyway.

Parallel SCARA instead: What kind of robot design would lend itself well if one wants to transmit all the mechanical motions exclusively via chains to the top entrance side of the FANUC-style serial mechanics rotating head?

One possibility is a parallel SCARA design (similar to RepRap Morgan). Unlike a delta robot only hinges in two dimensions are present. It is easy to thread a chain drive through such a 2D-hinge by double-sprockets (to further hinges or like here to the head). Actually arbitrary many chains can just by stacked with just one design. Also with a parallel SCARA robot there's no need for chains to do two-dimensional translations via statically mounted motors. The parallel SCARA arms can be driven directly.

It is natural to supply the rotative head with a fourth chain (two chains of each side of the parallel SCARA bot). The one additional rotation available at the tip of the serial mechanics rotative head can be used for both clamping action and screwing action.

Adding a fifth or even a sixth chain should probably avoided since the head will get way to big, heavy, hard to assemble, unstable and whatnot. (Each DOF adds an onion-like shell to the head). Instead statically mounted simplistic counter actuators that are reachable from within the work-space (a simple possibly mechanically multiplexed clamp-board) should help a lot.

The hinges of a SCARA and the thereon stacked gear-bearings for chain sprockets can be made from split-sun-post-assembly-gear-bearings. By tensioning the whole bearing stack (clamping all the suns wit a common axle) the gap-backlashes of the bearings can be fully reduced to zero.

Compared to a delta-bot a SCARA design may have some minor shortcomings like:

• lower stiffness
• the scissor mechanism may be more in the way that a delta platform
• the z-axis must be handled separately (could be a good thing)

The double tripod

A design similar to Ralph Merkles double-tripod - just with linear drives
It can be found here: Zyvex nanotech 6dof there's a vrml 3D model available This is a compromise between:

• the steward plattform (high stiffness low range of motion) and
• the serial robotic arm (low stiffness high range of motion)

FANUC style robot

A design similar to the "FANUC M-1iA/0.5A" robot - just with linear drives.
That is a chain driven linear rail delta-bot for bigger working area (instead of rotative direct stepper drive)

Maybe with the differences of a chain driven rotating head instead of the torsion rods??

Comparison of goals with other Robots

This is not a judgement of the projects - they have different goals.
This is a judgement about how suitable design decisions in those projects might be for RepRec projects.

The Dollo printer

• (--) mounting is done by wedges that are not locked by springs against slideout => relyance on friction self holding
• (--) one wedge per connection => tension is not collected to viewer points as in the ReChain Frame System
• (-) motors mounted on moving parts
• (++) No vitamins in frame (no screws, no threaded rods, no extruded aluminum profiles)
• (+) modularity (a bit too coarse grained for RepRecs)
• (+) modular herringbone racks

Links: Main project website; kickstarter site

The RepRap Morgan printer – by Quentin Harley

The 2D parallel SCARA design seems to be well suitable to ...

• (+) transmit motion serially over the revolute joints via chains (for head rotation and gripper actuation)
• (+) provide stiffness against sideward deflection (should be a bit better than a serial arm design)
• (+) Motors are static i.e. non-moving & factored out into the base – good.

RepRap Wally – by Nicholas Seward

Just like the RepRap Morgan this is also a 2D parallel SCARA design.
But with some differences which are:

• (?) the bases of the two arms not co-located
• (?) the arms driven at the first joint rather than the base; meaning possibly some less trivial math
• (+) Motors are still are static i.e. non-moving & factored out into the base – good.

(-/0) The string drive is nice cheap and efficient but it ...

• violates the coarseness design constraint and thus
• seems difficult in automated assembly with limited means

So it would need to be replaced by a chain drive (lower performing, since heavier and less stiff).

Side-note: The z-axis on this machine is done also rotatively.
It's done by a parallelogram mechanism.

RepRap Helios – by Nicholas Seward

A 2D serial SCARA design.

• (++) High buildspace reach
• (-) less pathways to thread motion parallelly
• (-) perhaps a bit lower structural stiffness
• (+) Motors are static i.e. non-moving & factored out into the base – good.

RepRap Wheelios – by Nicholas Seward

RepRap Wheelios is basically almost the same as RepRap Helios. The only obvious difference is that

• (+?) the vertical axis at the tip got replaced by horizontal movement of the base
• (-?) the SCARA part got slanted such that the coordinates remain linearly independent
• (?) Motors factored out into the now moving base – still good?

Motion on one linear axis is akin to the design in Moses2014 which may make it interesting.
Though using wheels not affixed to rails violates the clingyness constraint.
With wheels slips need to be detected by some feedback loop. Not what we want.
Easy to fix by switching to geared (rails or qualitatively similar solution).

The industrial robot "FANUC M-1iA/0.5A"

The serial mechanics rotating head seems promising.
For bulky vitamin free overhang free plastic parts a major and very challenging redesign will be necessary.
So I don't think the patent they hold:
patent: Parallel robot with a wrist section with three degrees of freedom
would apply for that.

The BrickRap printer

The BrickRap is mostly composed of lots of small Bitbeam parts. A working RepRec could be made capable to assemble such LEGO like structures but it would do so only as a secondary product. A RepRec in contrast to the BrickRap does not depend on friction fittings to lock its own parts together. A RepRec also tries to concentrate the clip lockings (energy barriers) and and minimizes their number.