SpinesPhotos.ppt
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Transcript SpinesPhotos.ppt
Designing µspines
brainstorming
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
Why spines ?
• Motivation
– Improve friction
• Most insects have spined legs [various Gorb’s
papers]
– Adhesion possible?
– We want to take advantage of asperities
At microscopic level
• For the interaction of spine and surface we
can have two cases:
– Hooking on asperity
– Pure friction
friction angle
adhesion
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
Artificial vision
• If looking for asperities, why not just hook
using vision: remote image transmission
and analysis, or a simple algorithm on board
Geometric considerations
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
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
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
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
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)
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)
Quick-cast with chopped fibers in
wax. Various shapes
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)?
Foot specialization
• Front legs
– few reliable hooks (standing)
• Intermediate legs?
• Back legs
– many less reliable hooks (propulsion)
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
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
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
Axial compliance by a viscous
medium. Protrusion to minimize
deflection. Rigid outside to
support shear force.
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
Axial compliance (by hand)
The importance of load
application (low moment): flat
foot, close to the wall
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
Constraint on load: µspine
dimension
l
F = Gecko weight /
active µspines
d = function (F, l, Ø, Ematerial)
active µspines = Coeff% x density x foot_contact_surface
density = µspines / surface
Pinned wheels and shells
http://www.ramsco-inc.com
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?
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 (Kevin’s pins and Bekaert
fibers)
Multiscale (tree) spines
Foot testing
• Climbing strategy is fundamental (dead fly)
• 10A not testable: adhesive ==> 35A
• Performance
– benchmark configuration (tripod with fixed
weight, inclined spine being tested) W
– Static friction (inclined table)
• Tear
– # of successful steps (renovating toe)
Test with two plates and Quick
cast
• Parameters
–
–
–
–
–
1
• Variables
resin layer thickness
time before raising
elevation
lateral displacement
time before separation
2
– resin
– top material, surface
finish
3
time
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]?
Triplets
Intrinsic compliance
Three rigidly connected spines
Claws or spines?
claws [Sangbae intuition on twiki]
Squeezing