Eric Vezzoli

Electrovibration Modeling Analysis

Electrostatic attraction may be used to modulate the apparent friction coefficient between two surfaces. Applied to the human finger and a polarized interface, the principle can modify the user perception of the interface surface. In this paper, the different steps towards the modeling of the electrovibration phenomenon are developed. An investigation on the current modeling will be carried out, with a focus on the temporal evolution and frequency dependence of the stimulus. Thus, an improvement of the modeling will be proposed to take into account this major effect, and then, it will be checked with an experimental setup and compared with literature results. The last few years have seen an increasing interest for haptic stimulation and simulation. Different technologies are available to provide a tactile feedback to a user by modifying his perception of a surface. In particular, it is possible to control the friction between a surface and a finger thanks to squeeze film effect or to electrovibra-tion. This study is focused on the electrovibration effect: the modulation of the perceived friction coefficient due to the induced electrostatic force between a finger and a high voltage supplied plate [1]. The effect is known since the mid fifties [2], but the interest has raised only recently. Firstly, spatial division of electrode was developed to provide precise and complex stimulus pattern of conductive pads, but this solution suffered from its complexity and turned out difficult to apply [5]. Recently, electrovi-bration took advantage of technological improvements of fingertips position sensor based on optical or resistive solutions. The possibility to track precisely the position of the finger leads to fine gratings simulation thanks to spatial-stimulus relation. With spatio-temporal transformation, the stimulator itself becomes easier to manufacture and it becomes possible to produce tactile feedback on transparent surfaces [1] or merge it with another tactile stimulation technique [3]. If the efficiency of the process to provide successful tactile feedback is clear, the physical modeling of the phenomenon involving the finger is not yet satisfactory. The

Experimental evaluation of friction reduction in ultrasonic devices

Previously proposed models of the ultrasonic lubrication of a finger mediated by flat surfaces are not consistent with the experimental results for vibrational amplitudes greater than a few microns. This paper presents experimental data acquired through a dedicated tribometer and proposes an experimental model of ultrasonic lubrication at high vibrational amplitudes.

Physical and Perceptual Independence of Ultrasonic Vibration and Electrovibration for Friction Modulation

Two different principles are available to modulate the user perceived roughness of a surface: electrovibration and ultrasonic vibration of a plate. The former enhances the perceived friction coefficient and the latter reduces it. This paper will highlight the independence of the two effects on the physical and perceptual point of view to confirm the increased range of sensation and stimulation that can be supplied by the two coupled techniques to the users. Firstly, a tribometric analysis of the induced lateral force on the finger by the two coupled effects will be presented, then a study on the dynamic of the two effects will be reported. In the end, a psychophysical experiment on the perception of the two coupled techniques will be shown.

Texture Rendering Strategies with a High Fidelity - Capacitive Visual-Haptic Friction Control Device

Ultrasonic vibrations of a plate can modify the perception of the friction between a surface and a sliding finger. This principle, coupled with modern position sensing techniques, is able to reproduce textured materials. In this paper, an open loop control through model inversion of the friction force between the finger and the plate is presented. The device incorporating the control system is described, and two different reproduction strategies are formalized to address the reproduction of objects and textures. In the end, a psychophysical experiment evaluating the two control strategies is described.

Friction Reduction through Ultrasonic Vibration Part 1: Modelling Intermittent Contact

Ultrasonic vibration is employed to modify the friction of a finger pad in way that induces haptic sensations. A combination of intermittent contact and squeeze film levitation has been previously proposed as the most probable mechanism. In this paper, in order to understand the underlying principles that govern friction modulation by intermittent contact, numerical models based on finite element (FE) analysis and also a spring-Coulombic slider are developed. The physical input parameters for the FE model are optimized by measuring the contact phase shift between a finger pad and a vibrating plate. The spring-slider model assists in the interpretation of the FE model and leads to the identification of a dimensionless group that allows the calculated coefficient of friction to be approximately superimposed onto an exponential function of the dimensionless group. Thus, it is possible to rationalize the computed relative reduction in friction being (i) dependent on the vibrational amplitude, frequency, and the intrinsic coefficient of friction of the device, and the reciprocal of the exploration velocity, and (ii) independent of the applied normal force, and the shear and extensional elastic moduli of the finger skin provided that intermittent contact is sufficiently well developed. Experimental validation of the modelling using real and artificial fingertips will be reported in part 2 of this work, which supports the current modelling.

Role of fingerprint mechanics and non-Coulombic friction in ultrasonic devices

Ultrasonic vibration of a plate can be used to modulate the friction of a finger pad sliding on a surface. This modulation can modify the user perception of the touched object and induce the perception of textured materials. In the current paper, an elastic model of finger print ridges is developed. A friction reduction phenomenon based on non-Coulombic friction is evaluated based on this model. Then, a comparison with experimental data is carried out to assess the validity of the proposed model and analysis.

Friction Reduction through Ultrasonic Vibration Part 2: Experimental Evaluation of Intermittent Contact and Squeeze Film Levitation

In part 1 of the current study of haptic displays, a finite element (FE) model of a finger exploring a plate vibrating out-of-plane at ultrasonic frequencies was developed as well as a spring-frictional slider model. It was concluded that the reduction in friction induced by the vibrations could be ascribed to ratchet mechanism as a result of intermittent contact. The relative reduction in friction calculated using the FE model could be superimposed onto an exponential function of a dimensionless group defined from relevant parameters. The current paper presents measurements of the reduction in friction, involving real and artificial fingertips, as a function of the vibrational amplitude and frequency, the applied normal force and the exploration velocity. The results are reasonably similar to the calculated FE values and also could be superimposed using the exponential function provided that the intermittent contact was sufficiently well developed, which for the frequencies examined correspond to a minimum vibrational amplitude of

Localized Haptic Texture: A Rendering Technique based on Taxels for High Density Tactile Feedback

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Feeling multiple edges: The tactile perception of short ultrasonic square reductions of the finger-surface friction

µm P-P. It was observed that the reduction in friction depends on the exploration velocity and is independent of the applied normal force and ambient air pressure, which is not consistent with the squeeze film mechanism. However, the modelling did not incorporate the influence of air and the effect of ambient pressure was measured under a limited range of conditions, Thus squeeze film levitation may be synergistic with the mechanical interaction.

A novel method using EEG to characterize the cortical processes involved in active and passive touch

We investigate the relevance of surface haptic rendering techniques for tactile devices. We focus on the two major existing techniques and show that they have complementary benefits. The first one, called textsc{S}urface textsc{H}aptic textsc{O}bject (textsc{SHO}), which is based on finger position, is shown to be more suitable to render sparse textures; while the second one, called textsc{S}urface textsc{H}aptic textsc{T}exture (textsc{SHT}), which is based on finger velocity, is shown to be more suitable for dense textures and fast finger movements. We hence propose a new rendering technique, called textsc{L}ocalized textsc{H}aptic textsc{T}exture (textsc{LHT}), which is based on the concept of textit{taxel} considered as an elementary tactile information that is rendered on the screen. By using a grid of taxels to encode a texture, textsc{LHT} is shown to provide a consistent tactile rendering across different velocities for high density textures, and is found to reduce user textit{error rate} by up to 77.68% compared to textsc{SHO}.