View source for Mechanical through joint threading

{{RepRec}}

This page is about mechanically threading drive power motions through mechanical joints to drive motions further down a robotic kinematic chain.
This applies to serial robotics or parallel robotics that still have some serial robotic aspects.

Design goals for [[RepRec]] systems include: <br>
– No mobile motors, all motors as static as possible. <br>
– No electric transmission across mobile joints. <br>
– Chains rather than belts. <br>
A goal is [[Factoring Out Actuators]]. <br>

Thus there is a need for threading motion mechanically across mobile interfaces. <br>
This can be quite challenging.

Since in [[RepRec]] systems all motors are supposed to be mounted statically (with some caveats — see [[No Moving Motors]]), <br>
and since electrical threading through joints to moving parts is not allowed (no chaincableguides, no spiralcableguides, no sliding contacts), <br> 
all mechanical motion must be threaded through joints. <br>
This strongly influencing the [[RepRec Base Geometry]]

This may makes 1DOF revolute joints (simple hinges) preferable over 2DOF joints (ball joints & co).

= Macroscale =

'''Why no mobile motors?''' <br>
'''Why not electrical transmission across moving joints?''' <br>
– Moving heavy (iron core) motors around <br>
– long flexible electrical cables (and their small plugs) massively complicate automation of assembly <br>
– adds complex wire management (cable chains) & repeatedly bent cables may wear (minor point) <br>
– sliding contacts tend to corrode, wear and usually are a very bad idea with few exceptions <br>

'''Why chains rather than belts?''' <br>
– easier automated robotic handling of chain segments compared to belts <br>
– chain segments can be printed/cast avoiding [[Replicator Vitamins]] (belts need fiberglass reinforcement or similar in rubber)

== Influence on choice of robotics geometry ==

'''Parallel robotic mechanisms can:''' <br>
– 🤩 reduce the amount of necessary mechanical through joint threading (the mechanism itself carries that info on) <br>
– 🤩 distribute through joint motion threading across multiple pathways <br>
– 🙂 As a boon they are also typically often stiffer than serial mechanisms. <br>
– 🙁 As a downside some parallel mechanisms come with a much reduced range of motion, especially in rotation <br>
(like e.g. the steward platform or delta-bots) <br>

It's easier to mechanically thread motion across <br>
– 1DOF hinge type revolute joints rather than across <br>
– 2DOF+ ball joints (like e.g. the steward platform or delta-bots)

'''Given all that here's the result:''' <br>
Parallel mechanisms with 1DOF hinge type revolute joints (that are not steward platforms or dela-bots) <br>
essentially leaves parallel SCARA robotics as a natural self suggesting choice. <br>
This can only serve as the [[RepRec positioner]] though. Not as the [[RepRec orienter]]. <br>
Additionally needed for a [[RepRec orienter]] is a [[robot wrist]] with <br> 
a high range of reachable space angle (2pi or more). <br>
Preferably in parallel robotics. <br>

=== Examples ===

'''Parallel SCARA robot:''' <br> 
When the (two) arms move then the rotations transmitted through the arms <br>
(for end effector rotation and gripping) change due to the hinge angles changing. <br>
This need to be accounted for by subtraction of this motion. 
Basically just a relative drive.<br>

'''Cartesian Robot:''' <br>
Use the difference of the main drive chain to a second parallel drive chain <br>
two drive the motion on the gantry bridge (via bevel gears — to sketch). <br>
Same from the gantry to the [[Reprec poser to tool flange]]. <br>

Differential drive approach: <br>
– H-Bot … may be interesting <br>
– Core X-Y … the oddly bent drivechains may be an issue especially in assembly <br>

There are '''two issues with cartesian robot mechanisms''' though: <br>
– The build-space is always smaller than the build stage ~ this makes "self-replication" more complicated (not impossible). <br>
– [[RepRec prismatic joints]] (of some designs) can be more challenging than revolute bearings. <br>

= Nanoscale =

'''Why no mobile motors?''' <br>
'''Why not electrical transmission across moving joints?''' <br>
– Nanoscale electrostatic motors may be quite big compared to the driven mechanical structures <br>
– A change of geometry in nano-electrical cables may lead to non-trivial changesin conductivity (ballistic electron transport) <br>
– Bending graphene nanoribbons / nanotubes  tin too narrow a radius may bead to non-continuous kinking <br>

'''Why chains rather than belts?''' <br>
– dissipation modes of straining an unstrapping material 
– potential kinking of belts on narrow radii <br>
– easier automated robotic handling of chain segments compared to belts

= Misc =

== Rolling rather than sliding ==

Slide bearing solutions preferred over roller bearing solutions (See: [[RepRec prismatic joints]]) <br>
due to these being possible in a much more compact fashion. <br>
Particular examples: <br>
* the [[RepRec sprocket stacks]] in the [[RepRec poser] of a [[ParallelSCARA]] based design
* the [[Nested axles robot wrist]] in the [[RepRec orienter] of a [[ParallelSCARA]] based design

But is reason to worry about wear for a macroscale implementation. <br> 
See: [[Macroscale Mass Overhead]] section '''Macroscale'''.

= Realated =

* [[Base Truss Crawler]]
* [[Separation of Concerns]]