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 adhesion friction angle
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
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
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 F = Gecko weight / active µspines l active µspines = Coeff% x density x foot_contact_surface d = function (F, l, Ø, E material ) 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 30 o. 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)
Foot testing Climbing strategy is fundamental (dead fly) 10A not testable: adhesive ==> 35A Performance –benchmark configuration (tripod with fixed weight, inclined spine being tested) –Static friction (inclined table) Tear –# of successful steps (renovating toe) W
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 123time
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]?