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Stress Fluctuations in Sliding of Textured Objects and the Sense of Touch Tribology Gordon Conference 2010 Georges Debrégeas - Alexis Prevost R. Candelier,

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Presentation on theme: "Stress Fluctuations in Sliding of Textured Objects and the Sense of Touch Tribology Gordon Conference 2010 Georges Debrégeas - Alexis Prevost R. Candelier,"— Presentation transcript:

1 Stress Fluctuations in Sliding of Textured Objects and the Sense of Touch Tribology Gordon Conference 2010 Georges Debrégeas - Alexis Prevost R. Candelier, J. Scheibert, S. Leurent Laboratoire de Physique Statistique – ENS Paris Patrice Rey (CEA-LETI) Joël Frelat (LMM, Paris 6) 1

2 Information transduction in tactile perception Object + motion SKIN DEFORMATIONS & VIBRATIONS NERVOUS SIGNALS REPRESENTATION Tribology Gordon Conference 2010 2

3 Glabrous skin mechanoreceptors Merkel's cell complex Meissner's corpuscule Pacinian corpuscule Ruffini ending 3

4 Tribology Gordon Conference 2010 Glabrous skin mechanoreceptors Stimulus t Slow Adaptation Fast Adaptation Merkel's cell complex Meissner's corpuscule Pacinian corpuscule Ruffini ending Stimulus t Bolanowski et al., 1988 4

5 Tribology Gordon Conference 2010 Roughness perception: the duplex theory Coding of coarse roughness > ~200µm SA I (Merkel) channel Resolution limited by the small receptive field (few hundred µm) Spatial coding (static) Fairly independent of finger's motion Coding of fine roughness < ~200µm Mediated by Pacinian corpuscules exclusively Requires active tactile exploration Intensity coding 5 Hollins and Bensmaia, 2008

6 Tribology Gordon Conference 2010 Questions : 1 – How can one relate the physical properties of the object and exploratory conditions to the mechanical signals experienced by mechanoreceptive nerve endings. 2 – What are the consequences of this filtering process on the transduction and neural encoding of tactile information. Outline 6 1 – Biomimetic tactile sensing – design and calibration. 2 – Dynamic impulse response. 3 – Response to randomly rough substrates. 4 – A possible role for fingerprints. 5 – Conclusions and perspectives.

7 7 Tribology Gordon Conference 2010 The biomimetic approach Real finger Artificial finger Sensitive area Sensor deth Contact diameter Skin elastic modulus Human fingertip 0.5 -10 mm2-3 mm ~13mm (P~0.5N) 1-4 MPa Artificial fingertip 2 mm2.5 mm~6mm (P~1.5N) 2.2±0.1Mpa MEMS sensor

8 8 Tribology Gordon Conference 2010 Response to a localized force Indentation protocol: Apply a ponctual force at on the surface with a rod. Receptive fields measured by our MEMS sensors Predicted receptive field for a ponctual sensor in a perfectly elastic material

9 9 Tribology Gordon Conference 2010 Response to a localized force Indentation protocol: Apply a ponctual force at on the surface with a rod.

10 10 Tribology Gordon Conference 2010 Response to a localized force Indentation protocol: Apply a ponctual force at on the surface with a rod. Without exploration: roughly the same response for the 10 sensors

11 11 Tribology Gordon Conference 2010 A linear model for tactile transduction + Coulomb law: Green function for a ponctual force at the surface: Hertz contact + The stress felt by the sensor is given by:

12 12 Tribology Gordon Conference 2010 A linear model for tactile transduction + Coulomb law: Green function for a ponctual force at the surface: Hertz contact + The stress at the sensor location reads:

13 Tribology Gordon Conference 2010 Dynamic impulse response. 13

14 Tribology Gordon Conference 2010 14 Dynamic impulse response.

15 Tribology Gordon Conference 2010 15 Dynamic impulse response.

16 16 Tribology Gordon Conference 2010 Dynamic impulse response Perturbation in force signal associated with a small, isolated defect : The modification in stress profile at the interface reads Perturbation in force signal for a sensor at : The response highly depends on the sensor's position within the contact zone u

17 17 Tribology Gordon Conference 2010 Dynamic impulse response – normal stress Experiment Model MiddleRightLeft

18 18 Tribology Gordon Conference 2010 Dynamic impulse response – tangential stress Experiment Model MiddleRightLeft

19 Midline profiles 19 Tribology Gordon Conference 2010 Experiment Model

20 20 Tribology Gordon Conference 2010 Receptive field variability in cortical neurons DiCarlo et. al., 1998 The journal of Neuroscience « The shape, area and strength of exitatory and inhibitory receptive fields regions ranged widely. »

21 21 Tribology Gordon Conference 2010 Response to randomly rough substrates Scanning over a binary patterned substrate

22 22 Tribology Gordon Conference 2010 The Volterra kernels give a mapping from to. The Volterra decomposition The Volterra series is the analog of the Taylor series, but for functionals: NB: it is hard to extract the Volterra kernels...

23 23 Tribology Gordon Conference 2010 For Gaussian white noise inputs, the Wiener kernels are orthogonal. The Wiener decomposition They can be computed through correlations:...

24 Extracting the linear kernel 24 Tribology Gordon Conference 2010 measured Predicted

25 25 Tribology Gordon Conference 2010 Consequences of skin patterning (e.g. fingerprints) Artificial fingerprints Square-wave gratings (period 220  m) on the skin's surface Smooth skin Fingerprinted skin

26 Linear model of mechanical transduction Tribology Gordon Conference 2010 Square wave gratings: 26 Interfacial stress profile: Force signal :

27 Stimulus- signal response function Tribology Gordon Conference 2010 27

28 Tribology Gordon Conference 2010 Stimulus- signal response function 28

29 Numerical illustration of the filtering process Tribology Gordon Conference 2010 29

30 Tribology Gordon Conference 2010 Numerical illustration of the filtering process 30

31 Tribology Gordon Conference 2010 Numerical illustration of the filtering process 31

32 Tribology Gordon Conference 2010 Numerical illustration of the filtering process 32

33 Tribology Gordon Conference 2010 Numerical illustration of the filtering process 33

34 2 mm Typical inter-ridge distance  ~ 500 µm « Natural » exploratory finger/substrate velocity V ~ 10 cm/s Frequency f = V / ~ 200 Hz Order of the best frequency of Pacinian fibers Pacinian fibers = mediate the coding of fine texture Scenario  Fingerprints select one spatial frequency  Velocity chosen to match Pacinian best response Consequence of fingerprints for fine texture perception Tribology Gordon Conference 2010 34

35 Conclusions Tribology Gordon Conference 2010 Biomimetic approach allowed to characterize the linear mechanical transduction of texture information, and clarify the roles played by intrinsic sensor’s response, interfacial contact stress field and skin topography. But : - Limited to binary topography. - Non-linear effects should be important (stress coupling within the contact zone, normal stress dependence of the friction coefficient, etc.) Reverse correlation should allow to probe that. Important open question : How does the tactile system deal with such context dependent variability of individual sensors’ response. What encoding strategies may yield a stable representation of the probed surface. 35

36 Comparing biomimetic and human touch 36 Tribology Gordon Conference 2010 JP Roll - LNH – Marseille Can one relate the subcutaneous stress field measured with the biomimetic sensor with actual neurographic data ?

37 Comparing whisker and digital touch Tribology Gordon Conference 2010 Dan Shulz – Yves Boubenec UNIC - Gif-sur-Yvette 37

38 Tribology Gordon Conference 2010 Daniel Shulz (Gif-CNRS) Rodents whisker touch Wolfe & Feldman, ‘08 38

39 Tribology Gordon Conference 2010 39

40 Tribology Gordon Conference 2010 Johnson & Phillips, 1981 F 40

41 Tribology Gordon Conference 2010 41

42 Tribology Gordon Conference 2010 The cochlea Georg von Békésy, ‘47 inner hair cells outer hair cells Nobili, Mammano and Ashmore, ‘98 42

43 …Back to the actual finger Can we see this effect on a real finger ? Communicative & Integrative Biology, 2009 Tribology Gordon Conference 2010 43


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