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Artificial Sweat for Humanoid Finger

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Abstract

To achieve favorable Frictional Tactile Sensation (FTS) for robot and humanoid fingers, this report investigated the effects of human finger sweat on Friction Coefficient (FC) and verified the effectiveness of artificial sweat on FTS for a humanoid finger. The results show that the model sweat (salt and urea water faked real sweat) increases the FC of the real finger sliding on the high hygroscopic and rough surface (paper), whereas on the low hygroscopic and smooth surface (PMMA), the sweat forms a fluid film and decreases FC, restricting severe finger adhesion. Further, the film formation and capillary adhesion force of sweat were discussed. The experimental results with the artificial sweats (ethanol and water) and humanoid finger (silicone rubber skin with tactile sensors) verifies the effectiveness. The artificial sweat restricts severe adhesion (stick-slip vibration), and enhances cognitive capability of FTS.

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Abbreviations

a:

radius of a droplet on a solid plane surface

D c :

distance between solid surfaces (sweat film thickness)

F:

raw data of finger friction force

F̄:

average of finger friction forces at the consta finger load force

f c :

Capillary Adhesion Force (CAF)

f p :

capillary pressure force

f s :

surface tension force

g:

acceleration of gravity

h:

film thickness (= a tan θd)

hmin:

minimum film thickness (sweat)

i, k:

integer variable for sigma

L:

finger load force (normal force)

m:

total count number of sampled-data at the constant finger load force

n:

total count number of friction tests under individual conditions

Ra:

calculated average roughness (JIS B 0601, 2001, in Japanese)

r K :

radius of meniscus curvature (equal to the Kelvin’s radius)

r w :

radius in water-solid contact

S f :

contact area of a finger

S w :

water contact area of a finger

γ L :

surface tension of water

θ:

equilibrium contact angle that the liquid makes with both solid surfaces

θ d :

droplet contact angle on a solid plane surface

σ:

equivalent surface roughness

λ:

the film parameter (= h / σ)

μ:

Friction Coefficient (FC)

̄μ:

average of friction coefficients at the constant finger load force

ρ:

density of a droplet

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  1. NSSL Ltd., 87-2, Ohkuri, Nishigamo, Kita, Kyoto, 603-8828, Japan

    Makoto Tomimoto

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  1. Makoto Tomimoto
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Correspondence to Makoto Tomimoto.

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Tomimoto, M. Artificial Sweat for Humanoid Finger. J Bionic Eng 11, 98–108 (2014). https://doi.org/10.1016/S1672-6529(14)60024-X

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