View source for Base Truss Crawler

== Definition ==

A crawler or "assembly crawler" is a robotic pick and place manipulation platform<br>
permanently attached to a [[RepRec Base Truss]] such that<br>

– it has boundless (or at least sufficient) mobility on that [[RepRec Base Truss]] in at least one dimension<br>
– it can boundless (or at least sufficient) extend the basetruss<br>
A crawler may be dysfunctional without a [[RepRec Base Truss]] because<br>
the motion across the basetruss may be depended on for positioning parts in pick and place operations.<br>

== Why crawlers on base-truss rather than self-contained replicators? ==

The same reason as for the [[RepRec Base Truss]].  <br>
Crawlers and base-truss go together, they are mutually dependent for self-replicative capability.<br>
In essence: To have an extensible system that as a whole remains in [[GMT/MachinePhase]].<br>
Disjoint self-replicating units (crawling or swimming around) would need some fancy things <br>
(like possibly even image recognition) to seek out and dock with other disconnected parts of the system<br>
in order to make larger scale products.<br> 
Replication is only a means to the target of productivity not "the end goal".<br>
''At the nanoscale one can go for self-assembly. But that's only useful for early foldamer based systems. Very useful there though.''<br>

Crawlers on base-truss match quite well as a [[RepRec Second Assembly Level]] in the [[gemstone metamaterial on chip factory]] concept.<br>
Self contained replicators match the (now obsolete) [[Molecular Assembler]]<br>

== Example approach (Moses2014) ==

In [[Moses2014]] the crawlers have a Cartesian gantry crane geometry and<br>
move on a 2D square grid of racetracks ( a 2D [[BaseTruss]] )<br>
All crawlers are restricted to motion along a linear track in one dimension only.<br>
To get 2D extendability crawlers assemble further crawlers in a 90° rotated state on 90° rotated track seeds.<br>
Designing [[RepRec Crawlers]] for motion along a 1D track can simplify the design complexity notably.<br>
As demonstrated in [[Moses2014]].<br>

== Restriction to one DOF only, and how to deal with it ==

In the case of 3D [[RepRec base truss]], 2D-tracks are replaced by 3D channels.<br>
For reducing system complexity crawlers can be restricted to just one single base-truss channel.<br>
Just like in the 2D example case in (Moses2014) RepRec Crawlers<br>
restricted to a single base-truss channel must be able to assemble<br>
"offspring" replicas that feature rotated orientations, such that<br>
all the six orientations of the [[Octet truss channels]] (not counting sign thus not 12) are accessible<br>
either via one or several [[stage rotations via replication]].<br>
More formally: '''Replication operation rotations need to span the whole mathematical rotation group.'''<br>

'''See: [[RepRec crawler replication rotation group]]'''<br>

== Mass ==

When putting more and more complexity from the systems products, the [[BaseTruss]] and other static parts<br>
into the mobile [[#Crawler]] then mobility may be come harder and harder to justify.<br>

'''Possible alternative:'''<br>
Having the manipulator static and moving an assembly strip (like a conveyor belt) back and forth within the [[BaseGrid]]<br>
But assembling a replication of the assembly mechanism on that moving strip what one gets essentially is a mobile<br>
manipulator. One is back at the [[RepRec crawler]]<br>

== Alternative approaches ==

'''Design space parameters:'''<br>
★ Complexity balance …<br>
– Crawler can replicate in multiple orientations(places/spaces) but cannot go anywhere — vs — <br>
– Crawler can go anywhere but replicate only in one orientation(place/space)<br>
★ Locomotion … rotative — vs —  translative<br>
★ Location of the motors … [[RepRec motor backpack]] on base or on tip of [[RepRec positioner]]<br>
★ Crawler interdependence … one crawler or more than one crawler that only can replicate as a complete set (Borromean crawlers)<br>

Yes, there are also other less motion limiting approaches.<br>
– One of them is turning on the tracks (nontrivial)<br>
– One of them is rotation around truss beams and moving brachiatingly through the base-truss.<br>
More on that below ...<br>

=== Subsystem transporter ===

See dedicated page: [[RepRec subsystem transporter]]

The idea here is basically about making the [[RepRec crawler]] <br>
completely detachable and independent from the [[RepRec poser]]. <br>
Idea is not to have it restricted to the just 1D motion but rather <br>
more advanced so it can reach all positions in the [[RepRec base truss]]. <br>
Though restricted capability may work too.

A [[RepRec subsystem transporter]] could act as a [[RepRec motor backpack]] too. <br>
Or carry [[RepRec motor backpack]]s around as yet another separate subsystem.

== Brachiating ==

Issue: Locomotion by brachiating through the base-truss gets more impractical with bigger more sophisticated crawlers,<br>
as likely for crawlers in nanoscale systems with [[Mechanical through joint threading]]<br>
Also at the macroscale gravitative mass becomes a problem leading to:<br>
– no longer ignoreable downward deflections (aka sagging) needing nontrivial software compensation<br>
– need for high forces due to high mechanical (dis)advantage => stronger heavier motors, a vicious cycle<br>
– potentially damaging load on the truss<br>
=> thus this approach will not be taken here (for macroscale prototypes) <br>
There are examples of existing prototype systems taking this approach (The [[Center For Bits And Atoms]] is working on this)<br>
Note that the complex crawlers there are fully pre-produced. They do not assemble other crawlers from more fundamental base-parts.<br>
Maybe (likely?) suitable for space-tech context, but not suitable for nanoscale-tech.<br>

== Toothed racks –vs– brachiating ==

Racks are a much faster mode of motion than brachiating. A [[RepRec base truss]]es<br>
3D printed racks are probably less accurate than ''properly designed'' 3D printed rotative bearings (??).<br>
But given that the accuracy requirements here are not at the level of a 3D printer it should be ok.<br>

Motion by racks either locks [[RepRec crawler]]s into a specific motion axis (e.g. a specific octet truss channel)<br>
or require complex means to switch between them (does not seem worth the effort ??)<br>
''(In an octet truss there are three channels in a truss layer and six channels in full 3D space).''<br>

'''More complex math:'''<br>
A brachiating mode of locomotion changes assembly coordinates in quite complicated ways<br>
if assembly is meant to be allowed not just in a few special positions but in all positions .<br>
And the double usage of the locomotion DOF as assembly DOF is desirable.<br>

For continuous racks the nodes of the [[RepRec base truss]] are in the way.<br>
In an octet truss rack crossings are probably worse than in a square grid like the one in [[Moses2014]].<br>

== Alternative names ==

– Crawler … troublesome and misleading bio-assozialtin (crawling proliferating out of control bug infestation)<br>
– IncompRec … making it more clear that this is not a complete RepRec without its [RepRec base truss]]<br>
– MobileUnit … bland & inconvenient & too unspecific<br>
'''Note:''' [[RepRec poser]] and [[repRec positioner]] are<br>
not applicable as names because these are<br>
sub and sub-sub systems respectively.<br>

= Ideas =

Macroscale: If rotations of big/heavy motors are involved:<br>
Maybe its possible to rotate motors almost around their center of gravity ??<br>

'''Regarding driving a crawler (all scales):'''<br>
[[RepRec mechanical demultiplexing]] could be done to use the same motors<br>
for the motion of the [[RepRec crawler]] crawler as are used for the [[RepRec poser]].<br>
Not advisable though if the motion of the [[RR/Crawler]] serves as one of the axes of the [[RR/Poser]] as in [[Moses2013]].<br>

'''Regarding energy supply for a crawler (macroscale specific)'''<br>
The energy supply for the [[RepRec crawler]] could be a strapped on battery and data can be transmitted wirelessly.<br>
This way there is no requirement for …<br>
★ either threading mechanical motion across all of the [[BaseTruss]], See: [[Mechanical through joint threading]]<br>
★ or cheating with …<br>
– either free floating flexing electrical coil-spring cables or<br>
– or electrical sliding contacts<br>

'''Regarding energy supply for the driving of a crawler (nanoscale specific)'''<br>
★ If motors are kept static in the base-truss then [[Mechanical through joint threading]]<br>
becomes even more of a headache. Requiring base-truss to crawler threading and long range transmission along<br>
some changing length of base-truss struts/tracks.<br>
★ If motors get integrated into crawlers (may make sense for big crawlers!)<br>
– Wired electric drive: Graphene sliding or rolling contacts.<br>
– Elastomechanical battery drive: Recharging internal springs (how long can that last when operating efficiently?)<br>
– Chemomechanical battery drive: Gathering and dumping bond-zipper energy storage capsules  (complex & very big?)<br>

= Pre APM-wiki page history =

Created 2020-01-03 Friday, review 2022-11-24
[[RepRec assembly crawler]] (shall redirect here)<br>

= Related =

* [[RepRec base truss]]
* [[Mechanical through joint threading]]