Contact geometry and mechanics predict friction forces during tactile surface exploration

International audienceWhen we touch an object, complex frictional forces are produced, aiding us in perceiving surface features that help to identify the object at hand, and also facilitating grasping and manipulation. However, even during controlled tactile exploration, sliding friction forces fluctuate greatly, and it is unclear how they relate to the surface topography or mechanics of contact with the finger. We investigated the sliding contact between the finger and different relief surfaces, using high-speed video and force measurements. Informed by these experiments, we developed a friction force model that accounts for surface shape and contact mechanical effects, and is able to predict sliding friction forces for different surfaces and exploration speeds. We also observed that local regions of disconnection between the finger and surface develop near high relief features, due to the stiffness of the finger tissues. Every tested surface had regions that were never contacted by the finger; we refer to these as " tactile blind spots ". The results elucidate friction force production during tactile exploration, may aid efforts to connect sensory and motor function of the hand to properties of touched objects, and provide crucial knowledge to inform the rendering of realistic experiences of touch contact in virtual reality

A Real-time Synthesizer of Naturalistic Congruent Audio-Haptic Textures

International audienceThis demo paper presents a multi-modal device able to generate real-time audio-haptic signal as response to the users' motion and produce naturalistic sensation. The device consists in a touch screen with haptic feedback based on ultrasonic friction modulation and a sound synthesizer. The device will help investigate audio-haptic interaction. In particular the system is built to allow for an exploration of di↵erent strategy of mapping audio and haptic signal to explore the limits of congruence. Such interactions could be the key to more informative and user-friendly touchscreens for Human-Machine-Interfaces

Reproduction des Textures Tactiles: Transducteurs, Mécanique, et Représentation du Signal

The textures of surfaces are tactually perceived mostly from the vibrations generated when sliding our fingertips on them. Despite its prevalence in everyday behavior, the study of the interaction of a finger with a textured surface, for virtual reality purposes, has not been much studied.This thesis explores some factors that contribute to the mechanics of interaction between a bare finger and a surface with a view to their artificial reproduction. The recording and reproduction of tactual textures are first discussed, along with a specifically designed apparatus able to precisely measure the interaction force arising from the friction of a sliding finger. The same piezoelectric-based apparatus was employed to rapidly deform the fingertip during exploratory movement, in order to replicate the presence of a texture, resulting in a new approach to simulate the roughness and texture of virtual surfaces. The problem of recording-reproducing textured surfaces motivated the question of the determination of the mechanical behavior of the fingertip. Investigations revealed that fingertips behave like elastic springs at low frequencies, and that after a corner frequency of about 100 Hz, the response is dominated by viscous damping, something that was never directly shown before. Next, the features of the vibratory signals created by the friction of a finger on various textures were analyzed. Expressing the fluctuations of the frictional force as function of space, rather than of time, indicated a number of possible signal characteristics that could play a key role in the tactual perception textures. The thesis highlights the importance of the mechanics and biomechanics during the haptic exploration of surfaces and their possible contribution to perception. Collectively, the findings reported in this thesis are pertinent to the design of effective virtual reality systems and other applications.La texture des surfaces est tactilement perçue principalement par les vibrations générées lors du glissement de nos doigts sur celles-ci. Malgré sa prévalence quotidienne, l'étude de l'interaction du doigt avec une texture, dans le cadre de la réalité virtuelle, n'a pas été approfondie en détail. Cette thèse explore une partie des facteurs qui contribuent à la mécanique de l'interaction entre un doigt et une surface avec pour objectif sa reproduction artificielle. L'enregistrement et la reproduction des textures tactiles sont premièrement discutés, accompagnés de la conception d'un appareil capable de précisément mesurer la force d'interaction émanant de la friction d'un doigt qui glisse. Le même dispositif piézoélectrique fut employé pour déformer à haute vitesse le bout du doigt, dans le but de reproduire la présence d'une texture. Ce travail est une nouvelle approche pour simuler la rugosité et la texture d'une surface virtuelle. La question de l'enregistrement et de la reproduction des surfaces texturées a motivé l'étude du comportement mécanique du bout du doigt. Cette étude révèle que le bout des doigts se comporte comme un ressort élastique dans les basses fréquences, et qu'après une fréquence de coupure d'environ 100 Hz, la réponse est dominée par l'amortissement visqueux, un fait qui n'a jamais été directement observé auparavant. Ensuite, les spécificités des signaux vibratoires créés par le frottement d'un doigt sur plusieurs textures furent analysées. L'expression des fluctuations de la force de friction en fonction de l'espace plutôt que du temps, indique que plusieurs caractéristiques du signal peuvent jouer un rôle majeur dans la perception tactile des textures. Cette thèse met en lumière l'importance de la mécanique et de la biomécanique pendant l'exploration haptique des surfaces et leur potentielle contribution à la perception. Collectivement, les résultats présentés dans cette thèse sont utiles pour la conception de meilleurs systèmes de réalité virtuelle et à d'autres applications

The contribution of air to ultrasonic friction reduction

International audience— The origin of friction reduction on an ultrason-ically vibrating plate has been the subject of debate. Recent work suggests that friction may be reduced due to intermittent contact caused by bouncing upon the vibrating surface [8], leaving the question of whether other phenomena such as levitation on a squeeze film of air also play a role. To probe the contribution of squeeze film levitation, we investigated the dependence of the friction reduction effect upon air pressure. An artificial finger was placed inside a vacuum chamber, touching an ultrasonic friction reduction device composed of a glass plate vibrated by piezo-actuators. Friction between the finger and the glass was measured by rotating the finger with a motor, and measuring the motor's torque load. Decreased friction is signaled by decreased motor current. Compared to atmospheric pressure, a 98% vacuum inside the chamber was observed to markedly diminish the friction reduction effect, suggesting that squeeze film levitation does indeed play a substantial role in ultrasonic friction reduction

Sensing the Frictional State of a Robotic Skin via Subtractive Color Mixing

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Anticipatory vibrotactile cueing facilitates grip force adjustment

Human grip forces are automatically adjusted upon occurrence of an external disturbance experienced by an object that is held by a thumb and index finger. We investigated some of the cues that may be used by the brain to perform rapid grip restabilization. To this end we ask subjects to grip and hold an instrumented and actuated parallelepiped-shaped handle between the index finger and the thumb. Under computer control, the handle could be jerked from the still grip and could independently provided vibration of 250 or 100 Hz to the gripping fingers. We found that the latency of the motor corrective action was 139 ms on average, but when a vibrotactile stimulation was applied 50 ms before the application of the pulling force, the latency was reduced on average to 117 ms. The average latency of the conscious response to the vibrotactile stimuli was 230 ms, suggesting that vibrotactile stimulation was capable of influencing the reflex action. A) Overall view grip handle B) Differential capstan drive encoder

The Spatial Spectrum of Tangential Skin Displacement Can Encode Tactual Texture

International audienceThe tactual scanning of five naturalistic textures was recorded with an apparatus capable of measuring the tangential interaction force with a high degree of temporal and spatial resolution. The resulting signal showed that the transformation from the geometry of a surface to the force of traction, and hence to the skin deformation experienced by a finger is a highly nonlinear process. Participants were asked to identify simulated textures reproduced by stimulating their fingers with rapid, imposed lateral skin displacements as a function of net position. They performed the identification task with a high degree of success, yet not perfectly. The fact that the experimental conditions eliminated many aspects of the interaction, including low-frequency finger deformation, distributed information, as well as normal skin movements, shows that the nervous system is able to rely on only two cues: amplitude and spectral information. The examination of the " spatial spectrograms " of the imposed lateral skin displacement revealed that texture could be represented spatially despite being sensed through time and that these spectrograms were distinctively organized into what could be called " spatial formants ". This finding led us to speculate that the mechanical properties of the finger enables spatial information to be used for perceptual purposes in humans without any distributed sensing, a principle that could be applied to robots

Estimating Friction Modulation From the Ultrasonic Mechanical Impedance

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The Role of Damping in Ultrasonic Friction Reduction

24th IEEE Haptics Symposium, Philadelphia, PA, APR 08-11, 2016International audienceWe observed the dynamic interaction between a fingertip and an ultrasonically vibrating plate using Laser Doppler Vibrometry in order to investigate the causes of ultrasonic friction reduction. Observations were made both for a human finger and for artificial fingertips constructed to exhibit different amounts of damping. The data suggest that fingertip dynamics play an important role in friction reduction. In particular, the fingertips were all found to exhibit forced oscillations in response to the plate motion, but with different relative phases. Fingertips with lower damping oscillated more in-phase with the plate, while fingertips with higher damping oscillated more out-of-phase with the plate, and also exhibited greater friction reduction. These results are reflected in a model