Fundamental Acoustical Finger Force Calculation for Out-of-Plane Ultrasonic Vibration and its Correlation with Friction Reduction

Ultrasonic surface-haptic touchscreens produce compelling tactile sensations directly on the users' fingertips. The tactile sensations stem from the modulation of friction produced by acoustic radiation pressure, which reduces the contact between the skin and the glass plate. During this process, some of the vibrations are partly absorbed by the tissues, resulting in a conspicuous change in the vibration amplitude of the plate upon contact with the finger, which manifests as a net change in the system mechanical impedance. In this study, we leverage the observable change of impedance to estimate the acoustic levitation and the frictional force. The self-sensing method utilizes a model of the first principles governing the physical interaction between the plate and the skin, which relies on multi-scale contact theory. The model accurately describes the experimental influence of the amplitude on the observed impedance (i.e., the amount of energy absorbed and reflected) and can be used to estimate the friction coefficient (R 2 = 0.93). These results provide additional evidence of the partial levitation mechanism at play in ultrasonic friction-modulation. This finding can be useful for designing energy-efficient devices and provide design suggestions for using ultrasonic impedance for self-sensing friction forces.

Section: Squeeze-film Levitation and Friction Reductionsupporting

confidence: 85%

Section: Harmonic Behaviormentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Estimating Friction Modulation From the Ultrasonic Mechanical Impedance

Huloux

Bernard

Wiertlewski

2021

“…10.b) when compared to the vibratory transfer function V . The variability between plate vibration and friction reduction efficiency reflects differences in the bio-mechanical properties of the finger skin, which were also observed in [45] and have been shown to significantly affect subjects' perception [46]. Therefore, stimuli generation on surface haptics should focus on controlling the frictional force and friction coefficient rather than the ultrasonic amplitudes, as proposed in [47], [48].…”

Section: Discussionmentioning

confidence: 99%

Detection of Friction-Modulated Textures is Limited by Vibrotactile Sensitivity

Bernard

Ystad

Monnoyer

et al. 2020

Modulation of the frictional force of a fingertip sliding over a surface-haptic device can produce compelling sensations of texture and relief. The virtual sensation is particularly apparent and feel as fixed in space if the stimulus is rigorously correlated with the displacement of the finger. While frictional textures tactually resemble their real counterparts, some exploratory conditions under which the sharpness of the texture declines exist. We postulate that this decline in sharpness is caused by the perceptual limitation of the attempt to interpret the variation in friction as an out-of-plane sinusoidal topography. To investigate these questions, we measured the detection thresholds of sinusoidal friction-modulated gratings for a wide range of spatial periods explored at two different speeds. We compared the results with the detection thresholds, reported in the literature, of real gratings and vibrotactile stimuli. We found that the detection of spatial friction-modulated textures does not follow the same trend as that of real textures but is more similar to the vibrotactile rendering, which is strongly influenced by the exploratory speed. This study provides a better understanding of the perception of friction-modulated textures and provides insight into how to design impactful stimuli on surface-haptic devices.

“…The vibration of the plate creates a levitation pressure, thereby reducing the number of asperities in contact. During this process, a portion of the acoustic energy is transferred to the fingertip [21], [22]. Upon a change in normal force or actuation force, we observe that both the contact and the impedance are affected, see Fig.…”

Section: Introductionmentioning

confidence: 99%

How to Measure the Area of Real Contact of Skin on Glass

Huloux

Willemet

Wiertlewski

2021

The contact between the fingertip and an object is formed by a collection of micro-scale junctions, which collectively constitute the real contact area. This real area of contact is only a fraction of the apparent area of contact and is directly linked to the frictional strength of the contact (i.e., the lateral force at which the finger starts sliding). As a consequence, a measure of this area of real contact can help probe into the mechanism behind the friction of skin on glass. In this article, we present two methods to measure the variations of contact area; one that improves upon a tried-and-true fingertip imaging technique to provide ground truth, and the other that relies on the absorption and reflection of acoustic energy. To achieve precise measurements, the ultrasonic method exploits a recently developed model of the interaction that incorporates the non-linearity of squeeze film levitation. The two methods are in good agreement (ρ = 0.94) over a large range of normal forces and vibration amplitudes. Since the real area of contact fundamentally underlies fingertip friction, the methods described in the article have importance for studying human grasping, understanding friction perception, and controlling surface-haptic devices.