Haptic Interface Technology Lab
Pressing the fingertips into surfaces causes skin deformations that enable humans to grip objects and sense their physical properties. This process involves intricate finger geometry, non-uniform tissue properties, and moisture, complicating the underlying contact mechanics. Here, we explore the initial contact evolution of dry and hydrated fingers to isolate the roles of governing physical factors. Two participants gradually pressed an index finger on a glass surface under three moisture conditions: dry, water-hydrated, and glycerin-hydrated. Gross and real contact areas were optically measured over time, revealing that glycerin hydration produced strikingly higher real contact area, while gross contact area was similar for all conditions. To elucidate the causes for this phenomenon, we investigated the combined effects of tissue elasticity, skin-surface friction, and fingerprint ridges on contact area using simulation. Our analyses show the dominant influence of elastic modulus over friction and an unusual contact phenomenon, which we call friction-induced hinging.
Certain ungrounded asymmetric vibrations create a unidirectional force that makes the user feel as though their fingers are being pulled in a particular direction. However, although researchers have discovered this haptic feedback technique and showcased its success in a variety of applications, there is still little understanding about how different attributes of the asymmetric vibration signal affect the perceived pulling sensation. This ongoing work aims to use dynamic modeling and measurement to bridge this gap between the design of the control signals and human perception.
- Principal investigator's earlier works related to current research topics at HITLab -
Physical Variables Underlying Tactile Stickiness During Fingerpad Detachment
One may notice a relatively wide range of tactile sensations even when touching the same hard, flat surface in similar ways. Little is known about the reasons for this variability, so here we investigated how the perceptual intensity of light stickiness relates to the physical interaction between the skin and the surface. Our results show that stickiness perception mainly depends on the pre-detachment pressing duration, the time taken for the finger to detach, and the impulse in the normal direction after the normal force changes sign; finger-surface adhesion seems to build with pressing time, causing a larger normal impulse during detachment and thus a more intense stickiness sensation.
Saekwang Nam, Yasemin Vardar, David Guerguiev, and Katherine J. Kuchenbecker
Effective utilization of electrovibration can only be accomplished by simultaneously investigating both the physical and perceptual aspects of the finger-touchscreen interaction. Towards this goal, present work blends the available knowledge on electromechanical properties of the human finger and human tactile perception with the results of new psychophysical experiments and physical measurements. By following such an approach that combines both theoretical and experimental information, the study proposes new methods and insights on generating realistic haptic effects, such as textures and edges on these displays.
Both vision and touch contribute to the perception of real surfaces. Although there have been many studies on the individual contributions of each sense, it is still unclear how each modality’s information is processed and integrated. Here, we investigated the similarity of visual and haptic perceptual spaces, as well as how well they each correlate with fingertip interaction metrics. We found that real surfaces are similarly organized within the three-dimensional perceptual spaces of both modalities. These dimensions for each modality can be represented by roughness/smoothness, hardness/softness, and friction, and that these dimensions can be estimated by surface vibration power, tap spectral centroid, and kinetic friction coefficient, respectively.
Yasemin Vardar, Christian Wallraven, and Katherine J. Kuchenbecker
Realistic display of tactile textures on touch screens is a big step forward for haptic technology to reach a wide range of consumers utilizing electronic devices on a daily basis. Since the texture topography cannot be rendered explicitly by electrovibration on touch screens, it is important to understand how we perceive the virtual textures displayed by friction modulation via electrovibration. Here, we investigated the roughness perception of real gratings made of plexiglass and virtual gratings displayed by electrovibration through a touch screen for comparison. The results showed that the roughness perception of real and virtual gratings are different. We argue that this difference can be explained by the amount of fingerpad penetration into the gratings. For real gratings, penetration increased tangential forces acting on the finger, whereas for virtual ones where skin penetration is absent, tangential forces decreased with spatial period.
Yasemin Vardar, Aykut Isleyen, and Cagatay Basdogan
A Novel Texture Rendering Approach for Electrostatic Displays
Generating realistic texture feelings on tactile displays using data-driven methods has attracted a lot of interest in the last decade. However, the need for large data storage and transmission rates complicates the use of these methods for future commercial displays. Here, we propose a new texture rendering approach that can compress the texture data significantly for electrostatic displays. Using three sample surfaces, we first explain how to record, analyze and compress the texture data, and render them on a touchscreen. Then, through psychophysical experiments conducted with nineteen participants, we show that the textures can be reproduced by a significantly fewer number of frequency components than the ones in the original signal without inducing perceptual degradation.
Tamara Fiedler, Matti Strese, Eckehard Steinbach, Cagatay Basdogan, and Yasemin Vardar
Effect of Remote Masking on Detection of Electrovibration
Here, we investigated whether it is possible to change detection threshold of electrovibration at fingertip of index finger via remote masking, i.e. by applying a (mechanical) vibrotactile stimulus on the proximal phalanx of the same finger. We found that vibrotactile masking stimuli generated sub-threshold vibrations around fingertip, and hence did not mechanically interfere with the electrovibration stimulus. However, there was a clear psychophysical masking effect due to central neural processes. Electrovibration absolute threshold increased approximately 0.19 dB for each dB increase in the masking level.
Milad Jamalzadeh, Burak Guclu, Yasemin Vardar, and Cagatay Basdogan
Tactile Masking by Electrovibration
Future touch screen applications will include multiple tactile stimuli displayed simultaneously or consecutively to a single finger or multiple fingers. These applications should be designed by considering human tactile masking mechanism since it is known that presenting one stimulus may interfere with the perception of the other. Here, we investigate the effect of masking on tactile perception of electrovibration displayed on touch screens. Moreover, in order to investigate the effect of tactile masking on our haptic perception of edge sharpness, we compared the perceived sharpness of edges separating two textured regions displayed with and without various masking stimuli. Our results suggest that sharpness perception depends on the local contrast between background and foreground stimuli, which varies as a function of masking amplitude and activation levels of frequency-dependent psychophysical channels.
Yasemin Vardar, Burak Guclu, Cagatay Basdogan
Effect of Waveform on Tactile Perception by Electrovibration Displayed on Touchscreens
Here, we investigated the effect of input voltage waveform on our haptic perception of electrovibration on touch screens. We found that the subjects were more sensitive to stimuli generated by square wave voltage than sinusoidal one for frequencies lower than 60 Hz. Using Matlab simulations, we showed that the sensation difference of waveforms in low fundamental frequencies occurred due to the frequency-dependent electrical properties of human skin and human tactile sensitivity. To validate our simulations, we actuated the touch screen at the threshold voltages and then measured the contact force and acceleration acting on the index fingers of the subjects moving on the screen with a constant speed. We analyzed the collected data in the frequency domain using the human vibrotactile sensitivity curve. The results suggested that Pacinian channel was the primary psychophysical channel in the detection of the electrovibration stimuli caused by all the square-wave inputs tested in this study.
Yasemin Vardar, Burak Guclu, Cagatay Basdogan