Michael Wiertlewski

The Spatial Spectrum of Tangential Skin Displacement Can Encode Tactual Texture

The tactual scanning of five naturalistic textures was recorded with an apparatus that is 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 with no distributed sensing, which is a principle that could be applied to robots.

Mechanical behavior of the fingertip in the range of frequencies and displacements relevant to touch.

It was previously suggested that the mechanical properties of the fingertip could be characterized by elasticity from dc to about 100Hz and by viscosity above this frequency. Using a specifically designed high mobility probe, we accurately measured the impedance of the fingertips of seven participants under a variety of conditions relevant to purposeful touch. Direct measurements vindicated previous indirect observations. We also characterized the dependency of the fingertip impedance upon normal load, orientation, and time.

On the 1/f noise and non-integer harmonic decay of the interaction of a finger sliding on flat and sinusoidal surfaces

Fluctuations of the frictional force arising from the stroke of a finger against flat and sinusoidal surfaces are studied. A custom-made high-resolution friction force sensor, able to resolve milli-newton forces, was used to record those fluctuations as well as the net, low-frequency components of the interaction force. Measurements show that the fluctuations of the sliding force are highly non-stationary. Despite their randomness, force spectra averages reveal regularities. With a smooth, flat, but not mirror-finish, surface the background noise follows a 1/f trend. Recordings made with pure-tone sinusoidal gratings reveal complexities in the interaction between a finger and a surface. The fundamental frequency is driven by the periodicity of the gratings and harmonics follow a non-integer power-law decay that suggests strong nonlinearities in the fingertip interaction. The results are consistent with the existence of a multiplicity of simultaneous and rapid stick-slip relaxation oscillations. Results have implications for high fidelity haptic rendering and biotribology.

Physical Factors Influencing Pleasant Touch during Tactile Exploration

Background When scanning surfaces, humans perceive some of their physical attributes. These percepts are frequently accompanied by a sensation of (un)pleasantness. We therefore hypothesized that aspects of the mechanical activity induced by scanning surfaces with fingertips could be objectively associated with a pleasantness sensation. Previously, we developed a unidimensional measure of pleasantness, the Pleasant Touch Scale, quantifying the pleasantness level of 37 different materials. Findings of this study suggested that the sensation of pleasantness was influenced by the average magnitude of the frictional forces brought about by sliding the finger on the surface, and by the surface topography. In the present study, we correlated (i) characteristics of the fluctuations of frictional forces resulting from the interaction between the finger and the surface asperities as well as (ii) the average friction with the sensation of pleasantness. Results Eight blindfolded participants tactually explored twelve materials of the Pleasant Touch Scale through lateral sliding movements of their index fingertip. During exploration, the normal and tangential interaction force components, fN and fT, as well as the fingertip trajectory were measured. The effect of the frictional force on pleasantness sensation was investigated through the analysis of the ratio fT to fN, i.e. the net coefficient of kinetic friction, μ. The influence of the surface topographies was investigated through analysis of rapid fT fluctuations in the spatial frequency domain. Results showed that high values of μ were anticorrelated with pleasantness. Furthermore, surfaces associated with fluctuations of fT having higher amplitudes in the low frequency range than in the high one were judged to be less pleasant than the surfaces yielding evenly distributed amplitudes throughout the whole spatial frequency domain. Conclusion Characteristics of the frictional force fluctuations and of the net friction taking place during scanning can reliably be correlated with the pleasantness sensation of surfaces.

Dynamics of ultrasonic and electrostatic friction modulation for rendering texture on haptic surfaces

Surface haptic devices modulate the friction between the surface and the fingertip, and can thus be used to create a tactile perception of surface features or textures. We present modeling and experimental results on both ultrasonic and electrostatic surface haptic devices, characterizing their dynamics and their bandwidth for rendering haptic effects.

A compact tactile display for the blind with shape memory alloys

This paper presents a novel tactile display device that exhibits some advantageous features for the blind: low-cost, lightweight, compactness and high portability. The prototype consists of an 8 times 8 arrangement of tactile pins actuated by shape memory alloys (SMAs) with 2.6 mm spatial resolution and 1 mm vertical excursion. Unlike existing devices, the full display is only 200 g weight and its compact dimensions make it easily carried by the user. A technical overview of the system is presented as well as preliminary results on tactile perception to evaluate its performance on information transmission

A High-Fidelity Surface-Haptic Device for Texture Rendering on Bare Finger

We present the design and evaluation of a high fidelity surface-haptic device. The user slides a finger along a glass plate while friction is controlled via the amplitude modulation of ultrasonic vibrations of the plate. A non-contact finger position sensor and low latency rendering scheme allow for the reproduction of fine textures directly on the bare finger. The device can reproduce features as small as 25 \(\upmu \)m while maintaining an update rate of 5 kHz. Signal attenuation, inherent to resonant devices, is compensated with a feedforward filter, enabling an artifact-free rendering of virtual textures on a glass plate.

Partial squeeze film levitation modulates fingertip friction

Significance Touchscreens have redefined human–computer interfaces. Although flexibility in the design of interfaces has dramatically increased, users are still confronted with a flat, featureless glass plate that cannot provide any tactile cues. Exciting this plate with ultrasonic waves reduces the friction experienced by a user’s finger, enabling tactile feedback directly on the surface. For vibration amplitudes of ±3 μm, we measured a reduction of up to 95% in the friction force experienced by a sliding finger. Using special illumination techniques and microsecond imaging, we show that this reduction of friction is because of the skin bouncing on a layer of air trapped between the plate and the surface of the finger. When touched, a glass plate excited with ultrasonic transverse waves feels notably more slippery than it does at rest. To study this phenomenon, we use frustrated total internal reflection to image the asperities of the skin that are in intimate contact with a glass plate. We observed that the load at the interface is shared between the elastic compression of the asperities of the skin and a squeeze film of air. Stroboscopic investigation reveals that the time evolution of the interfacial gap is partially out of phase with the plate vibration. Taken together, these results suggest that the skin bounces against the vibrating plate but that the bounces are cushioned by a squeeze film of air that does not have time to escape the interfacial separation. This behavior results in dynamic levitation, in which the average number of asperities in intimate contact is reduced, thereby reducing friction. This improved understanding of the physics of friction reduction provides key guidelines for designing interfaces that can dynamically modulate friction with soft materials and biological tissues, such as human fingertips.

Causality inversion in the reproduction of roughness

When a finger scans a non-smooth surface, a sensation of roughness is experienced. A similar sensation is felt when a finger is in contact with a mobile surface vibrating in the tangential direction. Since an actual finger-surface interaction results in a varying friction force, how can a measured friction force can be converted into skin relative displacement. With a bidirectional apparatus that can measure this force and transform it into displacement with unambiguous causality, such mapping could be experimentally established. A pilot study showed that a subjectively equivalent sensation of roughness can be achieved betweem a fixed real surface and a vibrated mobile surface.

Bioinspired artificial fingertips that exhibit friction reduction when subjected to transverse ultrasonic vibrations

This paper presents the design of a bioinspired artificial fingertip that resembles the mechanical behavior of a human fingertip under conditions of both static deformation and high frequency excitation. The artificial fingertip is constructed around a deformable spherical membrane filled with a cellulose sponge, itself connected to a rigid structure that acts as a bone. Force-deformation characteristics and response to a transient mechanical perturbation are both shown to be in good qualitative agreement with those of a real finger. More importantly, the fingertip exhibits friction reduction when interacting with TPads (variable friction tactile displays based on transverse ultrasonic vibrations). Comparison with artificial fingertips that do not exhibit friction reduction suggests that mechanical damping characteristics play a key role in the amount of friction reduction achieved.