Designing spines

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Designing µspines

• brainstorming

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Increasing friction

• Brakes dynamic friction
• Static friction sport shoes
• penetrable surfaces (grass) football shoes with
  needles
• athletics rubber tracks rubber bumps (lamellae?)
• when running, half of one feet pushing very high
  normal force

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Why spines ?

• Motivation
• Improve friction
• Most insects have spined legs various Gorbs
  papers
• Adhesion possible?
• We want to take advantage of asperities

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At microscopic level

• For the interaction of spine and surface we can
  have two cases
• Hooking on asperity
• Pure friction

friction angle
adhesion
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Interaction characterization

• Wall is flat
• Spine
• shape (tip size and roughness)
• Interaction
• relative hardness (penetrable/ impenetrable)
• relative approach angle
• relative force
• at microscopic level
• roughness and texture

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Artificial vision

• If looking for asperities, why not just hook
  using vision remote image transmission and
  analysis, or a simple algorithm on board

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Geometric considerations
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Hooking on asperities

• Definition
• any stable, almost flat and horizontal part of
  the wall surface
• can be a protrusion or a hole (more stable)
• What is the chance of hooking on asperities?
• n. of vertical asperities whose size is greater
  than the tip, facing the climbing direction
  (half), with no obstruction to spine insertion,
  per unit of surface (can be on a linear
  dimension). Increases considering the tolerance
  on the spine number and transverse compliance and
  foot movement/climbing strategy

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What spine angle?

• If we are looking for asperities, the spine angle
  with respect to the surface should be very steep
  for easier (non obstructed) insertion. We do not
  want a high normal force (for friction), just
  shear. We will reduce the spine length for higher
  load
• A spine that is also inclined horizontally
  (insect leg spines) is more stable on protrusions

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Why a lower angle works fine?

• A lower angle is good for surfaces that are
  softer or brittle because the plastic deformation
  or the fragile break by shear is lower the
  compression strength (a normal force)
• With lower angle we have chances of getting the
  friction effect too

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How many spines?

• Few
• Easier design up to three spines on a rigid
  plate are intrinsically compliant

• Many
• Lower load ? lower deflection
• Higher chance of finding an asperity
• Can be a combination of spine triplets

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Axial compliance

• Benefits
• Many spines can adapt to protrusions or holes of
  different depth
• Notes
• Keep force to a minimum (lubrication)
• Low excursion almost constant force

• Drawbacks
• Design and fabrication complexity
• Non uniform force distribution
• in holes (good hooks) spines have lower axial
  force (if proportional to displacement)

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Tear

• Due to high load and small surface, spines will
  only allow a short time use because they tear
  quickly and become blunt
• Possible reason for so few commercially available
• Ways of reducing tear
• Harder material (diamond tips)
• Renovating material (very thin metal wire in a
  stiff resin support)
• Additives in resin (sphere glass or fine talc)

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Quick-cast with chopped fibers in wax. Various
shapes
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Gecko vs. Roach (observations from movies)

• 4 vs. 6 legs
• Large vs. no toes (claws)
• Adhesion vs. crawling
• Long sure steps vs many short quick attempts
  (trials errors)

• Normal force for both?
• How many steps per second (on same surface type)?

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Foot specialization

• Front legs
• few reliable hooks (standing)
• Intermediate legs?
• Back legs
• many less reliable hooks (propulsion)

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Desired µspine features

• Thinner spine
• higher specific load ? bending and instability ?
  increase density
• better behavior on low roughness surfaces
• Higher roughness surfaces
• compliant toe
• Will water mattress
• Running strategy many quick steps

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Types of spine shapes

• Cylinders (legs)
• Demos with magnets
• Flat - compliant
• various 10A Urethane samples
• Any shape
• Quick cast sample

• Resin coated metal wire magnetically attracting
  metal micro fibers
• Magnetically aligned pins in cast resin with
  fluid cushion
• Mould of resin skin with protrusions

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Axial compliance solutions tested

• Individual std. pins lubricated with Vaseline in
  copper tube with tension or compression spring.
  Can be put in casts for embedding in feet
• Lower scale. 100 and 200 um pins in elastic
  medium (10A urethane) in a thermally shrinked
  tube. Should be aligned (with magnet) or put in
  metal tubes and filled with liquid resin
• The water cushion does not allow for high axial
  compliance. 10A very sticky, covered with Teflon
• Spines embedded in soft resin have higher
  deflection than axial compliance

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Axial compliance by a viscous medium. Protrusion
to minimize deflection. Rigid outside to support
shear force.
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Is transverse spine compliance desirable?

• High compliance may impact the friction angle and
  loose friction force
• Affects the load distribution
• The most stable configuration is of minimum
  energy and less force, so with higher compliance
  the spines will tend to loose good contact
  configuration
• Small compliance increases the chance of finding
  good contact points
• Optimal value should be investigated further

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Axial compliance (by hand)
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The importance of load application (low moment)
flat foot, close to the wall
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Materials

• Legs
• stiffness DECT paper, rope?
• ferromagnetic fibers free samples from Bekaert
• sacrificial material for cushion
• paraffin wax Kevin
• Any shape
• 35A urethane in vacuum with Tap Plastics
  additives to improve hardness

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Constraint on load µspine dimension
F Gecko weight / active µspines
l
d function (F, l, Ø, Ematerial)
active µspines Coeff x density x
foot_contact_surface
density µspines / surface
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Pinned wheels and shells





http//www.ramsco-inc.com
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Fabrication technique

• Legs
• density of spines defined by their
• uniform distribution achieved by vibration
  Sangbae
• inclination gravity and centrifugal force
• Tested .8 wire coated with 10A and 100 um spines
  at 30o. Good interaction with carpet and paper.
  Good for propulsion?

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Scale

• Is insect spine effect scalable (larger) and
  still work with most roughness or do we want to
  keep them small and many?
• The size of spines is probably defined by what is
  available (Kevins pins and Bekaert fibers)

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Multiscale (tree) spines
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Foot testing

• Climbing strategy is fundamental (dead fly)
• 10A not testable adhesive gt 35A
• Performance
• benchmark configuration (tripod with fixed
  weight, inclined spine being tested)
• Static friction (inclined table)
• Tear
• of successful steps (renovating toe)

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Test with two plates and Quick cast

• Parameters
• resin layer thickness
• time before raising
• elevation
• lateral displacement
• time before separation

• Variables
• resin
• top material, surface finish

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time
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To do next

• Legs
• dimensions/scale?
• feasibility of magnetic assembly
• Flat
• obtaining the thinnest skin with cushion with 35A
  and pins
• Any shape
• What (cone) shape and density Will paper?

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Triplets

• Intrinsic compliance

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Three rigidly connected spines
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Claws or spines?

• claws Sangbae intuition on twiki

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Squeezing
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