Collective Construction

8/14/2019 Collective Construction

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Josefa Z. Hernndez, Nik Swoboda{phernan,nswoboda}@fi.upm.es

Master Universitario en Inteligencia Artificial


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! Social insects collectively build nests whosearchitectural complexities exceed theperceptual and cognitive capabilities of singleindividuals

! Scientific question: How these insects collectively build thestructures they do?

! Engineering question: How one could design and program asystem in which an artificial swarm collectively builds complexstructures?


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! Deal with traditional construction problems: low efficiency,high accident rates, low quality, and skills shortages

! Facilitate the production of low-cost housing! Introduce automation in settings difficult or dangerous for

humansto work in, e.g. other planets, underwater.

! Maintain and repair structures during their lifetimes! Modify structures according to changing circumstances


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! Models of positive and negative feedback have been used toexplain such engineering feats

! Stigmergic communication: the perception of the result ofprevious work triggers specific construction behaviors

" quantitative(continuous) stigmergy: the different stimuli thattrigger behavior are quantitatively different (e.g. termite-moundbuilding)

" qualitative(discrete) stigmergy: stimuli can be classified intodifferent classes that differ qualitatively (e.g. wasp nest building)


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(Brooks, 86): Proposed for robots and extendedinto a new view of AI

! It is a hierarchy of task-accomplishing behaviors (situation#action rules)

! Multiple behaviors may fire at the same timeo Layered hierarchyo Lower layersbehaviors inhibit higher levelones

! The resulting systems are, in terms of the amount of computation they do,extremely simple

! Some of the robots do tasks that would be impressive if they wereaccomplished by symbolic AI systems

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Steels Mars explorer system (Steels, 90), using the subsumption

architecture achieves near-optimal cooperative performance insimulated rock gathering on Mars domain:

The objective is to explore a distant planet, and in particular, tocollect samples of a precious rock. The location of the samples isnot known in advance, but it is known that they tend to be

clustered. A number of autonomous vehicles are available thatcan drive around the planet collecting samples and later re-entera mother ship spacecraft to go back to Earth. There is no detailedmapof the planet, although it is known that the terrain is full ofobstacles hills, valleys, etc.- which prevent the vehicles fromexchanging any communication

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The system uses two mechanisms:

! Gradient field: determine direction to mother-ship viaradio-signal gradient

! Indirect communication: by dropping beacons(crumbs

). Inspired by ant-populations communicationthrough pheromones.

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!Individual robots behavior:(1) IF detect an obstacle THEN change direction

(2) IF carrying samples AND at the base THEN drop samples

(3) IF carrying samples and NOT at the base THEN travel up

gradient

(4) IF detect a sample THEN pick sample up

(5) IF true THEN move randomly

! Subsumption hierarchy: (1) < (2) < (3) < (4) < (5)! The solution is cheap(minimal computing power) androbust

(loss of a single robot will not affect overall system too much)

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:If sample is found, drop crumb trailwhile

returning to ship (special rocks appear in clusters!). Other robots willweaken trail on way to samples. If sample cluster is empty $no trailreinforcement$trail dies.

(1) IF detect an obstacle THEN change direction

(2) IF carrying samples AND at the base THEN drop samples(3) IF carrying samples and NOT at the base THEN drop 2 crumbs

AND travel up gradient

(4) IF detect a sample THEN pick sample up

(6) IF sense crumbs THEN pick up 1 crumb AND travel downgradient

(5) IF true THEN move randomly

! Subsumption hierarchy: (1) < (2) < (3)< (4) < (6)< (5)

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v1


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! Nest building by wasps:o attach the future nest to a

substrate with a stalk-like pedicel

o build two cells on either side of thepedicel

o subsequent cells are added to theouter circumference of the combsbetween two previouslyconstructed cells

o as more cells are added, parallelrows of cells are formed

o a row tends to be finished beforeinitiating a new row

o rows are initiated by theconstruction of a centrally locatedfirst cell

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! (Theraulaz, Bonabeau, 95): Formal model of distributed buildinginspired by social wasps

! Lattice swarms modelo stateless agents move randomly on a 3D

cubic lattice

o an agent deposits a brick every time itfinds a stimulating configuration

o a lookup rule table specifies all suchconfigurations and defines an algorithm


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nest-like structures

What makes the difference?


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! Biological-like architectures require coordinated algorithmso the desired architecture is

decomposed into a finitenumber of building steps

o the local configurations that arecreated by a given state andwhich trigger building actions,differ from those created by aprevious or forthcoming buildingstep (disjoint steps)

o all individuals cooperate in thecurrent building state at any time

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z+1 z z1

(1.1)

2 2 2

2 2 2

2 2 2

0 0 0

0 0

0 0 0

0 0 0

0 0 0

0 0 0(2.1.)

0 0 0

0 1 0

0 0 00 0 0

0 0

0 0 00 0 0

0 0 0

0 0 0(2.2)

0 1 00 0 0

0 0 0

0 2 00 0

0 0 0

0 0 00 0 0

0 0 0(2.3)

0 1 0

0 0 0

0 0 02 2 2

0 0

0 0 00 0 0

0 0 0

0 0 0

(2.4)1 0 0

0 0 0

0 0 02 2 0

2 0

0 0 00 0 0

0 0 0

0 0 0(2.5)

0 0 0

0 0 0

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2 2 2

0 0

0 0 0

0 0 0

0 0 0

0 0 0(2.6)

0 0 0

0 0 0

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2 0

2 0 00 0 0

0 0 0

0 0 0(2.7)

0 0 00 0 0

0 0 0

2 2 02 0

0 0 0

0 0 00 0 0

0 0 0(2.8)

0 0 0

0 0 0

0 0 02 2 2

2 2

0 0 00 0 0

0 0 0

0 0 0

Set of local stimulating configurations used to produce these architectures:

the wasp in the centralposition of slice z, will putdowno a brick of type 1 in the

case of configuration

1.x ando a brick of type 2 in the

cases 2.x.


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Take an unspecified number of robots and a supply of buildingmaterial, give the system a blueprint or other set of specificationsfor any arbitrary structure desired, and have a guarantee thatthe system will produce that structure without further intervention


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Building two-dimensional user-specified structures (Werfel et al, 06)

o robots and blocks are deployed at random into an obstacle-free workspaceo blocks are square and can be attached to each other on all four sideso a markerindicates the location for the start of constructiono robots can not communicate with one anothero robots fetch blocks from elsewhere in the workspace, bring them to the

growing structure, and travel along its perimeter for some distance beforeattaching their block


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D#..)8/"5) 8#%&/(-8/"#%How and where robots decide to attach blocks?

!specifying the desired shapeo shape map: lattice-based representation with a coordinate system whose

origin is the marker

o robots can use the structure as a reference to determine their location!

ordering attachment(A): A block can be attached to 1 or 2, not to 3 or 4

(B): Two separated blocks (sites 1, 5) may not be attached in the same row if allsites between them are meant to be occupied

(C): Building up a structure by layers, always starting new rows from the end andextending them clockwise


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Establishing location of robots with inert blocks:

! identical blockso the marker serves as a landmarko robots follow the perimeter of the

structure until reaching the landmark

! distinct blockso all blocks are potential landmarkso robots maintain a dynamic label map, storing the labels and locations of all

blocks in the structure

!writable blockso robots can change the state

of block labels

o blocks store their coordinatesexplicitly


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Establishing location of blocks with communicating blocks:! determining whether or not a site is available for block attachment is

shifted from the robots to the structure itself

! blocks connect to each other when connected to the structure! each block in the structure stores:

o the shape mapo its location in the shared coordinated systemo a state for each of its faces


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Block algorithm:! when a robot is given permission to attach a block, the structure block

sends a message along that row in both directions

! all the blocks in the row update the state of their faces! the newly attached block obtains from its neighbors the shape map

and its coordinates, and sets the state of its faces

Example of

block algorithm

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Types of constraints on block placement:! geometric: ensure that the structure and its perimeter is of the desired

shape (independent of block type)

! functional: encompass whatever restrictions based on block typesmay be dictated by the particular application

o patterned structures: use absolute coordinateso adaptive structures: use relationships between block types

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Types of functional constraints:

! Absolute: no reference to neighboring blocks (e.g., type A may beattached; B is forbidden).

! Proximity: the types of neighboring blocks are important but theirlocations are not (e.g., every A must have at least one B somewhere

within a neighborhood of radius 3; no C can be adjacent to a D).

! Relational: involving both types and locations of neighboring blocks(e.g., every A must have a B bordering its west edge and a Cbordering its north edge).

square structures built with two blocktypes and the constraint that no two

yellow blocks be adjacent

patterned structurewith a complexshape


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Sets of constraints can be organized in regionsand multiple levels

o Region 1, border of upper-left area: only blueblocks allowed.

o Region 2, interior of upper-left area: blue andwhite blocks both allowed; no white blockmay be in the eight neighborhood ofanother white block.

o Region 3, elsewhere: blocks must be eitherred or white depending on their y-coordinate

Regions definedby reference togeometricshapes and lines


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v2

Experiments on the construction of square structures of varying side length:


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A: The robot initially knows onlythat the marker must bepresent.

B: Using the line on the floor as areference, it picks up a blockfrom the cache.

C: It can use its RFID reader todetermine its position, and its

camera to follow the perimeter.Existing blocks are added tothe robots map as it observesthem.

D: The robot reaches an emptysite where a block is desired

E: It maneuvers to attach its blockat that site, dropping it intoplace...

F: ...and writes the block's newcoordinates to its tag.

v3


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! 3D structures with communicating blocks (Werfel, Nagpal, 08)! Extends the 2D assumptions on blocks and robots to 3D

structures: weightless environment, robots move freely in anydirection, blocks can communicate with physically attached

neighbors, ..! Problem decomposed in two parts:

o where blocks need to go?o how they get there?


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Where to put blocks?

(A) A block could be attached at site A but not at site B

(B) A 3D version of the separation rule violated

(C) Even though this partial structure can occur without violating theseparation rule, attaching further blocks at any of the twelve sitesmarked * would violate that restriction


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GH 8#%&/(-8/"#%Blocks rules - Where to put blocks?

Attachment allowed iff the answer to these three questions is YES:

1. Does the structure design specify this site is to be occupied?2. For each of the three axes passing through this site, is the separation

rule satisfied?

3. For each of the three planes passing through this site, is a block herebeing attached contiguously to the locally contiguous group?

Site A violates the first checkSite B passes checks 1 and 3 but fails 2Site C passes checks 1 and 2 but fails 3

Desired structure


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Robots rules - How to find allowed attachment sites?

! Random movement: Movement in any available direction along thestructure surface.

! Systematic search: Robots circle the structure at a fixed height; ifthey return to a previously visited site they move up or down onelevel and continue with the next layer.

! Gradient-following: Robots receive explicit directions from theblocks of the structure. Information includes the number of stepsaway from the closest allowed attachment site, as well as the localdirection to travel to reach it.


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successivesnapshots duringthe process ofconstruction by

ten robots, usinggradient-basedmovement


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S: Steps taken along surfaceM1: Messages passed from blocks to robots

M2: Messages passed between blocks v


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!TERMES (Werfel et al., 11), a multi-robot construction systeminspired by the building activities of termites.

! The system takes as input a high-level representation of adesired structure, and provides rules for robots to build thatstructure.


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