A Review of Surface Haptics: Enabling Tactile Effects on Touch Surfaces

Non-monotonic frictional behavior in the lubricated sliding of soft patterned surfaces

We study the lubricated contact of sliding soft surfaces that are locally patterned but globally cylindrical, held together under an external normal force. We consider gently engineered sinusoidal patterns with small slopes. Three dimensionless parameters govern the system: a speed, and the amplitude and wavelength of the pattern. Using numerical solutions of the Reynolds lubrication equation, we investigate the effects of these dimensionless parameters on key variables such as contact pressure and the coefficient of friction of the lubricated system. For small pattern amplitudes, the coefficient of friction increases with the amplitude. However, our findings reveal that increasing pattern amplitude beyond a critical value can decrease the friction coefficient, a result that contradicts conventional intuition and classical studies on the lubrication of rigid surfaces. For very large amplitudes, we show that the coefficient of friction drops even below the corresponding smooth case. We support these observations with a combination of perturbation theory and physical arguments, identifying scaling laws for large and small speeds, and for large and small pattern amplitudes. This study provides a quantitative understanding of friction in the contact of soft, wet objects and lays theoretical foundations for incorporating the friction coefficient into haptic feedback systems in soft robotics and haptic engineering.

Haptic Perception and Its Relation to Action

Haptic perception uses signals from touch receptors to detect, locate, and mentally represent objects and surfaces. Research from behavioral science, neuroscience, and computational modeling advances understanding of these essential functions. Haptic perception is grounded in neural circuitry that transmits external contact to the brain via increasingly abstracted representations. Computational models of mechanical interactions at the skin predict peripheral neural firing rates that initiate the processing chain. Behavioral phenomena and associated neural processes illustrate the reciprocal relationship by which perception supports action and action gates experience. The interaction of sensation and action is evident in how features of surfaces and objects such as softness and curvature are encoded. By incorporating touch sensations in conjunction with motor control, biologically embedded prosthetics enhance user capabilities and may elicit feelings of ownership. Efforts to create virtual haptic experience with advanced technologies underscore the complexity of this fundamental perceptual channel and its relation to action.

How to determine hands' vibration perception thresholds - a systematic review

The vibration perception threshold (VPT) is the minimum amplitude required for conscious vibration perception. VPT assessments are essential in medical diagnostics, safety, and human-machine interaction technologies. However, factors like age, health conditions, and external variables affect VPTs. Various methodologies and distinct procedures have been used to access VPTs, leading to challenges in establishing standardized protocols. Following the PRISMA methodology, this systematic review was conducted to answer the research question: "How are vibration perception thresholds assessed on the glabrous skin of the hands and fingers of healthy humans?" Searches were conducted across five databases to locate recent studies reporting data from VPT assessments, published in English, between 2012 and 2023. Thirty-nine studies met the inclusion criteria. Data on study goals and various methodological aspects was categorized and analyzed. Information gaps were identified, and this review offers recommendations for future studies to enhance standardization and facilitate data comparison. This review also suggests directions for future research, aiming to improve our understanding of how humans perceive haptic information.

High sound pressure piezoelectric micromachined ultrasonic transducers using sputtered potassium sodium niobate

This work presents air-coupled piezoelectric micromachined ultrasonic transducers (pMUTs) with high sound pressure level (SPL) under low-driving voltages by utilizing sputtered potassium sodium niobate K0.34Na0.66NbO3 (KNN) films. A prototype single KNN pMUT has been tested to show a resonant frequency at 106.3 kHz under 4 Vp-p with outstanding characteristics: (1) a large vibration amplitude of 3.74 μm/V, and (2) a high acoustic root mean square (RMS) sound pressure level of 105.5 dB/V at 10 cm, which is 5-10 times higher than those of AlN-based pMUTs at a similar frequency. There are various potential sensing and actuating applications, such as fingerprint sensing, touch point, and gesture recognition. In this work, we present demonstrations in three fields: haptics, loudspeakers, and rangefinders. For haptics, an array of 15 × 15 KNN pMUTs is used as a non-contact actuator to provide a focal pressure of around 160.3 dB RMS SPL at a distance of 15 mm. This represents the highest output pressure achieved by an airborne pMUT for haptic sensation on human palms. When used as a loudspeaker, a single pMUT element with a resonant frequency close to the audible range at 22.8 kHz is characterized. It is shown to be able to generate a uniform acoustic output with an amplitude modulation scheme. In the rangefinder application, pulse-echo measurements using a single pMUT element demonstrate good transceiving results, capable of detecting objects up to 2.82 m away. As such, this new class of high-SPL and low-driving-voltage pMUTs could be further extended to other applications requiring high acoustic pressure and a small form factor.

A neurocognitive pathway for engineering artificial touch

Artificial haptics has the potential to revolutionize the way we integrate physical and virtual technologies in our daily lives, with implications for teleoperation, motor skill acquisition, rehabilitation, gaming, interpersonal communication, and beyond. Here, we delve into the intricate interplay between the somatosensory system and engineered haptic inputs for perception and action. We critically examine the sensory feedback's fidelity and the cognitive demands of interfacing with these systems. We examine how artificial touch interfaces could be redesigned to better align with human sensory, motor, and cognitive systems, emphasizing the dynamic and context-dependent nature of sensory integration. We consider the various learning processes involved in adapting to artificial haptics, highlighting the need for interfaces that support both explicit and implicit learning mechanisms. We emphasize the need for technologies that are not only physiologically biomimetic but also behaviorally and cognitively congruent with the user, affording a range of alternative solutions to users' needs.

HaptoFloater: Visuo-Haptic Augmented Reality by Embedding Imperceptible Color Vibration Signals for Tactile Display Control in a Mid-Air Image

We propose HaptoFloater, a low-latency mid-air visuo-haptic augmented reality (VHAR) system that utilizes imperceptible color vibrations. When adding tactile stimuli to the visual information of a mid-air image, the user should not perceive the latency between the tactile and visual information. However, conventional tactile presentation methods for mid-air images, based on camera-detected fingertip positioning, introduce latency due to image processing and communication. To mitigate this latency, we use a color vibration technique; humans cannot perceive the vibration when the display alternates between two different color stimuli at a frequency of 25 Hz or higher. In our system, we embed this imperceptible color vibration into the mid-air image formed by a micromirror array plate, and a photodiode on the fingertip device directly detects this color vibration to provide tactile stimulation. Thus, our system allows for the tactile perception of multiple patterns on a mid-air image in 59.5 ms. In addition, we evaluate the visual-haptic delay tolerance on a mid-air display using our VHAR system and a tactile actuator with a single pattern and faster response time. The results of our user study indicate a visual-haptic delay tolerance of 110.6 ms, which is considerably larger than the latency associated with systems using multiple tactile patterns.

Effect of Finger Moisture on Tactile Perception of Electroadhesion

We investigate the effect of finger moisture on the tactile perception of electroadhesion with 10 participants. Participants with moist fingers exhibited markedly higher threshold levels. Our electrical impedance measurements show a substantial reduction in impedance magnitude when sweat is present at the finger-touchscreen interface, indicating increased conductivity. Supporting this, our mechanical friction measurements show that the relative increase in electrostatic force due to electroadhesion is lower for a moist finger.

Perceptual Constancy in the Speed Dependence of Friction During Active Tactile Exploration

Fingertip friction is a key component of tactile perception. In active tactile exploration, friction forces depend on the applied normal force and on the sliding speed chosen. We have investigated whether humans perceive the speed dependence of friction for textured surfaces of materials, which show either increase or decrease of the friction coefficient with speed. Participants perceived the decrease or increase when the relative difference in friction coefficient between fast and slow sliding speed was more than 20%. The fraction of comparison judgments which were in agreement with the measured difference in friction coefficient did not depend on variations in the applied normal force. The results indicate a perceptual constancy for fingertip friction with respect to self-generated variations of sliding speed and applied normal force.

Crystallization of piezoceramic films on glass via flash lamp annealing

Integration of thin-film oxide piezoelectrics on glass is imperative for the next generation of transparent electronics to attain sensing and actuating functions. However, their crystallization temperature (above 650 °C) is incompatible with most glasses. We developed a flash lamp process for the growth of piezoelectric lead zirconate titanate films. The process enables crystallization on various types of glasses in a few seconds only. The functional properties of these films are comparable to the films processed with standard rapid thermal annealing at 700 °C. A surface haptic device was fabricated with a 1 μm-thick film (piezoelectric e33,f of -5 C m-2). Its ultrasonic surface deflection reached 1.5 μm at 60 V, sufficient for its use in surface rendering applications. This flash lamp annealing process is compatible with large glass sheets and roll-to-roll processing and has the potential to significantly expand the applications of piezoelectric devices on glass.

The Single-Pitch Texel: A flexible and practical texture-rendering algorithm

As the number of applications for tactile feedback technology rapidly increases, so too does the need for efficient, flexible, and extensible representations of virtual textures. The previously introduced Single-Pitch Texel rendering algorithm offers designers the ability to produce textures with perceptually wide-band spectral characteristics while requiring very few input parameters. This paper expands on the capabilities of the rendering algorithm. Diverse families of fine textures, with widely varied spectral characteristics, were shown to be rendered reliably using the Texel algorithm. Furthermore, by leveraging an assistive algorithm, subjects were shown to consistently navigate the Texel parameter space in a matching task. Finally, a psychophysical study was conducted to demonstrate the rendering algorithm's resilience to spectral quantization, further reducing the data required to represent a virtual texture.

Understanding the Effects of Tactile Grating Patterns on Perceived Roughness Over Ultrasonic Friction Modulation Surfaces

OBJECTIVE

Our study aims to investigate the effects of grating patterns of perceived roughness on surfaces with ultrasonic friction modulation, and also to examine user performance of identifying different numbers of grating patterns.

BACKGROUND

In designing grating-based tactile textures, the widths of low- and high-friction zones are a crucial factor for generating grating patterns that convey roughness sensation. However, few studies have explored the design space of efficient grating patterns that users can easily distinguish and identify via roughness perception.

METHOD

Two experiments were carried out. In the first experiment, we conducted a magnitude estimation of perceived roughness for both low- and high-friction zones, each with widths of 0.13, 0.25, 0.38, 0.5, 1.0, 1.5, 2.0, 3.5, and 5.5 mm. In the second experiment, we required participants to identify 5 pattern groups with 2-6 patterns respectively.

RESULTS

Perceived roughness fitted a linear trend for low- or high-friction zones with widths of 0.38 mm or lower. Perceived roughness followed an inverted U-shaped curve for low- or high-friction zones with widths greater than 0.5 mm but less than 2.0 mm. The peak points occurred at the widths of 0.38 mm for both low- and high-friction zones. The statistical analysis indicates that both low- and high-friction zones had similar effects on human perception of surface roughness. In addition, participants could memorize and identify up to four tactile patterns with identification accuracy rates higher than 90% and average reaction time less than 2.2 s.

CONCLUSIONS

The relation between perceived roughness and varying widths of grating patterns follows linear or inverted U-shape trends. Participants could efficiently identify 4 or fewer patterns with high accuracy (>90%) and short reaction time (<2.2 s).

APPLICATION

Our findings can contribute to tactile interface design such as tactile alphabets and target-approaching indicators.

Temporal Detection Threshold of Audio-Tactile Delays With Virtual Button

Synchronization of audio-tactile stimuli represents a key feature of multisensory interactions. However, information on stimuli synchronization remains scarce, especially with virtual buttons. This work used a click sensation produced with traveling waves and auditory stimulus (a bip-like sound) related to a virtual click for a psychological experiment. Participants accomplish a click gesture and judge if the two stimuli were synchronous or asynchronous. Delay injection was performed on the audio (haptic first) or the click (audio first). In both sessions, one stimulus follows the other with a delay ranging from 0-700 ms. We use weighted and transformed 3-up/1-down staircase procedures to estimate people's sensitivity. We found a threshold of 179 ms and 451 ms for the auditory first and haptic first conditions, respectively. Statistical analysis revealed a significant effect between the two stimuli' order for threshold. Participants' acceptable asynchrony decreased when the delay was on the haptic rather than on the audio. This effect could be due to the natural experience in which the stimuli tend to be first tactile and then sonorous rather than the other way around. Our findings will help designers to create multimodal virtual buttons by managing audio-tactile temporal synchronization.

Friction and neuroimaging of active and passive tactile touch

Two types of exploratory touch including active sliding and passive sliding are usually encountered in the daily life. The friction behavior of the human finger against the surface of objects is important in tactile perception. The neural mechanisms correlating to tribological behavior are not fully understood. This study investigated the tactile response of active and passive finger friction characterized with functional near-infrared spectroscopy (fNIRS). The friction test and fNIRS test were performed simultaneously using the tactile stimulus of polytetrafluoroethylene (PTFE) specimens. Results showed that the sliding modes did not obviously influence the friction property of skin. While three cortex regions were activated in the prefrontal cortex (PFC), showing a higher activation level of passive sliding. This revealed that the tribological performance was not a simple parameter to affect tactile perception, and the difference in cortical hemodynamic activity of active and passive touch was also recognised. The movement-related blood flow changes revealed the role of PFC in integrating tactile sensation although there was no estimation task on roughness perception.

Haptic Feedback Device Using 3D-Printed Flexible, Multilayered Piezoelectric Coating for In-Car Touchscreen Interface

This study focuses on the development of a piezoelectric device capable of generating feedback vibrations to the user who manipulates it. The objective here is to explore the possibility of developing a haptic system that can replace physical buttons on the tactile screen of in-car systems. The interaction between the user and the developed device allows completing the feedback loop, where the user's action generates an input signal that is translated and outputted by the device, and then detected and interpreted by the user's haptic sensors and brain. An FEM (finite element model) via ANSYS multiphysics software was implemented to optimize the haptic performance of the wafer structure consisting of a BaTiO3 multilayered piezocomposite coated on a PET transparent flexible substrate. Several parameters relating to the geometric and mechanical properties of the wafer, together with those of the electrodes, are demonstrated to have significant impact on the actuation ability of the haptic device. To achieve the desired vibration effect on the human skin, the haptic system must be able to drive displacement beyond the detection threshold (~2 µm) at a frequency range of 100-700 Hz. The most optimized actuation ability is obtained when the ratio of the dimension (radius and thickness) between the piezoelectric coating and the substrate layer is equal to ~0.6. Regarding the simulation results, it is revealed that the presence of the conductive electrodes provokes a decrease in the displacement by approximately 25-30%, as the wafer structure becomes stiffer. To ensure the minimum displacement generated by the haptic device above 2 µm, the piezoelectric coating is screen-printed by two stacked layers, electrically connected in parallel. This architecture is expected to boost the displacement amplitude under the same electric field (denoted E) subjected to the single-layered coating. Accordingly, multilayered design seems to be a good alternative to enhance the haptic performance while keeping moderate values of E so as to prevent any undesired electrical breakdown of the coating. Practical characterizations confirmed that E=20 V/μm is sufficient to generate feedback vibrations (under a maximum input load of 5 N) perceived by the fingertip. This result confirms the reliability of the proposed haptic device, despite discrepancies between the predicted theory and the real measurements. Lastly, a demonstrator comprising piezoelectric buttons together with electronic command and conditioning circuits are successfully developed, offering an efficient way to create multiple sensations for the user. On the basis of empirical data acquired from several trials conducted on 20 subjects, statistical analyses together with relevant numerical indicators were implemented to better assess the performance of the developed haptic device.

The transition from Elasto-Hydrodynamic to Mixed Regimes in Lubricated Friction of Soft Solid Surfaces

Lubricated contacts in soft materials are common in various engineering and natural settings, such as tires, haptic applications, contact lenses, and the fabrication of soft electronic devices. Two major regimes are elasto-hydrodynamic lubrication (EHL), in which solid surfaces are fully separated by a fluid film, and mixed lubrication (ML), in which there is partial solid-to-solid contact. The transition between these regimes governs the minimum sliding friction achievable and is thus very important. Generally, the transition from EHL to ML regimes is believed to occur when the thickness of the lubricant layer is comparable with the amplitude of surface roughness. Here, it is reported that in lubricated sliding experiments on smooth, soft, poly(dimethylsiloxane) substrates, the transition can occur when the thickness of the liquid layer is much larger than the height of the asperities. Direct visualization of the "contact" region shows that the transition corresponds to the formation of wave-like surface wrinkles at the leading contact edge and associated instabilities at the trailing contact edge, which are believed to trigger the transition to the mixed regime. These results change the understanding of what governs the important EHL-ML transition in the lubricated sliding of soft solids.

HaptoMapping: Visuo-Haptic Augmented Reality by Embedding User-Imperceptible Tactile Display Control Signals in a Projected Image

This article proposes HaptoMapping, a projection-based visuo-haptic augmented reality (VHAR) system, that can render visual and haptic content independently and present consistent visuo-haptic sensations on physical surfaces. HaptoMapping controls wearable haptic displays by embedded control signals that are imperceptible to the user in projected images using a pixel-level visible light communication technique. The prototype system is comprised of a high-speed projector and three types of haptic devices-finger worn, stylus, and arm mounted. The finger-worn and stylus devices present vibrotactile sensations to a user's fingertips. The arm-mounted device presents stroking sensations on a user's forearm using arrayed actuators with a synchronized hand projection mapping. We identified that the developed system's maximum latency of haptic from visual sensations was 93.4 ms. We conducted user studies on the latency perception of our VHAR system. The results revealed that the developed haptic devices can present haptic sensations without user-perceivable latencies, and the visual-haptic latency tolerance of our VHAR system was 100, 159, 500 ms for the finger-worn, stylus, and arm-mounted devices, respectively. Another user study with the arm-mounted device discovered that the visuo-haptic stroking system maintained both continuity and pleasantness when the spacing between each substrate was relatively sparse, such as 20 mm, and significantly improved both the continuity and pleasantness at 80 and 150 mm/s when compared to the haptic only stroking system. Lastly, we introduced four potential applications in daily scenes. Our system methodology allows for a wide range of VHAR application design without concern for latency and misalignment effects.

Closed-Loop Control of Electroadhesion Using Current Regulation

Electroadhesion displays provide controllable friction between the fingertip and screen. However, the change of contact condition causes variability in the produced friction. In this paper, we demonstrate a novel method for closed-loop control using current regulation to improve the precision of the electroadhesion force regardless of contact conditions. The current sensor obtains static current (when the finger is stationary) and dynamic current (when the finger is sliding). The static current is used to estimate the apparent contact area. The estimated contact area modulates the driving voltage along with the dynamic current. To verify the proposed method, we measured electroadhesion forces under open-loop control and closed-loop control. The benefit of using this closed-loop control is shown by comparing the relative static error of open-loop control and closed-loop control. The relative error reductions achieved over 34 % (max 112 %) for four changing contact conditions.

Usability Tests for Texture Comparison in an Electroadhesion-Based Haptic Device

Haptic displays have been gaining more relevance over the recent years, in part because of the multiple advantages they present compared with standard displays, especially for improved user experience and their many different fields of application. Among the various haptic technologies, electroadhesion is seen as capable of better interaction with a user, through a display. TanvasTouch is an economically competitive haptic device using electroadhesion, providing an API and respective haptic engine, which makes the development of applications much easier and more systematic than in the past, back when the creation of these haptic solutions required a greater amount of work and resulted in ad-hoc solutions. Despite these advantages, it is important to access its ability to describe textures in a way understandable by the user’s touch. The current paper presents a set of experiments using TanvasTouch electroadhesion-based haptic technology to access how a texture created on a TanvasTouch device can be perceived as a representation of a real-world object.

Comparative Experimental Research on Haptic Display Methods of Virtual Surface Shape Based on Touch Screen

With the wide application of touch screen technology in daily life, more and more studies have tried to use touch screen as a carrier to convey the surface shape features of virtual objects to people. Current studies have proposed methods such as direct height mapping and haptic illusion based on lateral force to display the geometric shape of virtual surfaces on the touch screen. However, whether there are differences in the performance of these methods in displaying the shape features, and whether the lateral force-based haptic illusion can be used to display the shape features of complex surfaces, has not been fully explored. Aiming at this situation, in this article, we carry out user experiments on two regular shapes and eight natural images under displacement field and two types of lateral force fields with the help of three haptic devices previously designed. We also evaluate the effect of image smoothing on users' perception of natural image shape features. Experimental results show that the current shape display devices and methods that can simulate the lateral force field proposed by (Minsky et al., 1990) cannot achieve the display effect of the displacement field on complex shapes, and the shape display effect is the worst when only resistive lateral force feedback is employed. Nevertheless, since methods based on lateral force field, such as electrovibration, allow people to directly perceive the shape features of the virtual surface on the touch screen with their bare fingers, continuous research on this shape expression method still has a very attractive prospect.

Design of a "Cobot Tactile Display" for Accessing Virtual Diagrams by Blind and Visually Impaired Users

Access to graphical information plays a very significant role in today's world. Access to this information can be particularly limiting for individuals who are blind or visually impaired (BVIs). In this work, we present the design of a low-cost, mobile tactile display that also provides robotic assistance/guidance using haptic virtual fixtures in a shared control paradigm to aid in tactile diagram exploration. This work is part of a larger project intended to improve the ability of BVI users to explore tactile graphics on refreshable displays (particularly exploration time and cognitive load) through the use of robotic assistance/guidance. The particular focus of this paper is to share information related to the design and development of an affordable and compact device that may serve as a solution towards this overall goal. The proposed system uses a small omni-wheeled robot base to allow for smooth and unlimited movements in the 2D plane. Sufficient position and orientation accuracy is obtained by using a low-cost dead reckoning approach that combines data from an optical mouse sensor and inertial measurement unit. A low-cost force-sensing system and an admittance control model are used to allow shared control between the Cobot and the user, with the addition of guidance/virtual fixtures to aid in diagram exploration. Preliminary semi-structured interviews, with four blind or visually impaired participants who were allowed to use the Cobot, found that the system was easy to use and potentially useful for exploring virtual diagrams tactually.

Where's My Button? Evaluating the User Experience of Surface Haptics in Featureless Automotive User Interfaces

Advancements in user interface technologies and demands of design engineering led to increasing implementation of large and mostly flat interactive surfaces in automotive. Recent discussions in the context of in-vehicle usage of touchscreens advocate for the use of haptic feedback to restore the explore- and feel-qualities typically experienced in traditional physical button interfaces that contribute to intuitive, eyes-free, and tactually rich interactions. Haptic technologies that include a friction modulation approach seem especially promising to convey a high-quality feeling. This research reports an experience-oriented evaluation of an electrostatic friction haptic display in an in-vehicle direct touch interaction context. The evaluation was based on an automotive multitask setting (primary driving-task and secondary target-selection-task) with a 2 × 2 feedback modality design (factors haptic/audio with levels absent/present). The objective variables (response time, errors, and performance on the primary task) did not differ between feedback modalities. Any additional feedback to a visual baseline enhanced the user experience, with the multimodal feedback being preferred by most participants. Surface haptics was perceived as a novel yet unexpected type of haptic feedback. We discuss the implications for the haptic design of programmable friction displays and provide an initial set of guidelines for this innovative technology.

The Haptic Fidelity Framework: A Qualitative Overview and Categorization of Cutaneous-Based Haptic Technologies Through Fidelity

After decades of research and development, haptic feedback is increasingly appearing in consumer products. While the prevalence of haptic feedback is increasing, the integration rarely offers increased fidelity to previous generations. We argue this is because of the tremendous complexity of successful haptic design engineering, but critically, also because of information saturation. With novel cutaneous feedback technologies and companies emerging almost daily, the multi-disciplinary nature of haptics and the marketing-driven terminology used to stand out in a crowded market makes it challenging to select and integrate actuators correctly. To manage this complexity and facilitate the interdisciplinary exchange of user requirements and material affordances, we introduce a novel classification criterion for haptic actuators focused on the bandwidth and fidelity of potential effects. We introduce vocabulary for describing the precise experience the actuators and corresponding systems should deliver. Lastly, we summarize currently commercially available cutaneous-based haptic technology. In the nearby future, the same criterion and language can also prove valuable for steering technology development of new and improved actuators and enabling novice and experienced practitioners to understand and integrate cutaneous feedback in their products.

Feeling Illusory Textures Through a Hole: Rotating Frame At Skin-Object Interface Modifies Perceived Tactile Texture

Modulating tactile texture perception for the surface of real objects is a promising way to artificially present various tactile textures. Here, we propose a simple method of modulating tactile textures for various materials, which is named the rotating-frame method. In the method, one touches an arbitrary material's surface through a hole in a cardboard frame. When the frame is rotated between the hand and material, the tactile texture of the material is perceived as if it has turned into another material. We investigated the qualitative and quantitative characteristics of the illusory modulation created by the method in a series of psychophysical experiments. We found that the method altered the tactile textures of the surfaces of touched materials such as glass and carpet to seem softer, smoother, slipperier, and warmer than they actually are. The illusory texture change occurred robustly when the method was applied with different categories of materials. Our method paves the way for the development of simple techniques for texture augmentation that can be applied to a wide range of materials and do not disrupt stable direct contact between the hand and the materials.

Frequency-Dependent Behavior of Electrostatic Forces Between Human Finger and Touch Screen Under Electroadhesion

The existing lumped parameter circuit models do not capture the true (experimentally observed) behavior of electrostatic forces between human finger and a touch screen under electroadhesion, changing as a function of stimulation frequency. In order to address this problem, we first conducted an experiment to measure the voltage-induced frictional forces acting on the finger of a user sliding on a touch screen under constant normal force for stimulation frequencies ranging from 1 to 10 ⁶ Hz. The steady-state values of coefficient of sliding friction for those frequencies and the value for voltage-free sliding (no electroadhesion) were utilized to estimate the magnitude of electrostatic force as a function of frequency. The experimental data shows that electrostatic force follows an inverted parabolic curve with a peak value around 250 Hz. Following the experimental characterization of electrostatic forces, an electro-mechanical model based on the fundamental laws of electric fields and Persson's multi-scale contact mechanics theory was developed. Compared to the existing ones in the literature, the proposed model takes into account the charge accumulation and transfer at the interfaces of finger and touch screen. The model is in good agreement with the experimental data and shows that the change in magnitude of electrostatic force is mainly due to the leakage of charge from the Stratum Corneum (SC) to the touch screen at frequencies lower than 250 Hz and electrical properties of the SC at frequencies higher than 250 Hz.

Surface haptic rendering of virtual shapes through change in surface temperature

Compared to relatively mature audio and video human-machine interfaces, providing accurate and immersive touch sensation remains a challenge owing to the substantial mechanical and neurophysical complexity of touch. Touch sensations during relative lateral motion between a skin-screen interface are largely dictated by interfacial friction, so controlling interfacial friction has the potential for realistic mimicry of surface texture, shape, and material composition. In this work, we show a large modulation of finger friction by locally changing surface temperature. Experiments showed that finger friction can be increased by ~50% with a surface temperature increase from 23° to 42°C, which was attributed to the temperature dependence of the viscoelasticity and the moisture level of human skin. Rendering virtual features, including zoning and bump(s), without thermal perception was further demonstrated with surface temperature modulation. This method of modulating finger friction has potential applications in gaming, virtual and augmented reality, and touchscreen human-machine interaction.

Tactile Texture Rendering for Electrostatic Friction Displays: Incorporation of Low-Frequency Friction Model and High-Frequency Textural Model

Tactile texture presentation on touch panels enhances their usability and realizes immersive user interfaces. This study develops a tactile texture rendering method for electrostatic friction displays. The method combines two rendering models for material textures compared with previous studies which focused on either of these two models. One of these models is a physical model that simulates low-frequency frictional signals depending on the exploratory finger velocities and contact loads. The other is an autoregression-based data-driven model for high-frequency textural friction. For user studies, we compared combining the two models with using only the physical model for the four types of materials. Although the effectiveness varied across the materials, the subjectively judged realism and identification of the materials were improved for the combined condition. The new method combining high-frequency textural information and low-frequency physical model-based friction is expected to provide realistic tactile textures for electrostatic surface tactile displays.

Elastohydrodynamic friction of robotic and human fingers on soft micropatterned substrates

Frictional sliding between patterned surfaces is of fundamental and practical importance in the haptic engineering of soft materials. In emerging applications such as remote surgery and soft robotics, thin fluid films between solid surfaces lead to a multiphysics coupling between solid deformation and fluid dissipation. Here, we report a scaling law that governs the peak friction values of elastohydrodynamic lubrication on patterned surfaces. These peaks, absent in smooth tribopairs, arise due to a separation of length scales in the lubricant flow. The framework is generated by varying the geometry, elasticity and fluid properties of soft tribopairs and measuring the lubricated friction with a triborheometer. The model correctly predicts the elastohydrodynamic lubrication friction of a bioinspired robotic fingertip and human fingers. Its broad applicability can inform the future design of robotic hands or grippers in realistic conditions, and open up new ways of encoding friction into haptic signals.

Nanotexture Shape and Surface Energy Impact on Electroadhesive Human-Machine Interface Performance

With the ubiquity of touch screens and the commercialization of electroadhesion-based surface haptic devices, modeling tools that capture the multiphysical phenomena within the finger-device interface and their interaction are critical to design devices that achieve higher performance and reliability at lower cost. While electroadhesion has successfully demonstrated the capability to change tactile perception through friction modulation, the mechanism of electroadhesion in the finger-device interface is still unclear, partly due to the complex interfacial physics including contact deformation, capillary formation, electric field, and their complicated coupling effects that have not been addressed comprehensively. A multiphysics model is presented here to predict the friction force for finger-surface tactile interactions at the nanoscale. The nanoscopic multiphysical phenomena are coupled to study the impacts of nanotexture and surface energy in the touch interface. With macroscopic friction force measurements as verification, the model is further used to propose textures that have maximum electroadhesion effect and minimum sensitivity to relative humidity and user perspiration rate. This model can guide the performance improvement of future electroadhesion-based surface haptic devices and other touch-based human-machine interfaces.

A Novel Method for Surface Exploration by 6-DOF Encountered-Type Haptic Display Towards Virtual Palpation

Surface exploration in virtual reality has a large potential to enrich the user's experience. It could for example be used to train and simulate medical palpation. During palpation, users tap, indent, and rub the surface of a sample to estimate the underlying properties. However, up to now there is no good approach to render such intricate interaction realistically. This paper introduces 6 degrees of freedom (DoF) encountered-type haptic display technology for simulating surface exploration tasks. Among the different phases of exploration, this article focuses on the 'in-contact sliding' phase. Two novel control approaches to render sliding over a virtual surface are elaborated. A first rendering method generates lateral frictional forces as the finger slides over the surface. A second method adjusts the inclination of the end-effector to render tissue properties. With both methods a stiff nodule embedded in a soft tissue was prepared. User experiments were carried out to find proper parameter and intensity ranges and to confirm the feasibility of the new rendering schemes. Participants indicated that both rendering schemes felt realistic. Compared to earlier work, where only the vertical stiffness was altered, lower thresholds to detect and localise embedded virtual nodules were found. Users also made fewer errors in detecting nodule edges. Furthermore, the method that used end-effector inclination allowed faster discovery of the nodule's edges. It is expected that approaches that combine both rendering methods could provide an even more realistic feel.

Predicting human touch sensitivity to single atom substitutions in surface monolayers for molecular control in tactile interfaces

The mechanical stimuli generated as a finger interrogates the physical and chemical features of an object form the basis of fine touch. Haptic devices, which are used to control touch, primarily focus on recreating physical features, but the chemical aspects of fine touch may be harnessed to create richer tactile interfaces and reveal fundamental aspects of tactile perception. To connect tactile perception with molecular structure, we systematically varied silane-derived monolayers deposited onto surfaces smoother than the limits of human perception. Through mechanical friction testing and cross-correlation analysis, we made predictions of which pairs of silanes might be distinguishable by humans. We predicted, and demonstrated, that humans can distinguish between two isosteric silanes which differ only by a single nitrogen-for-carbon substitution. The mechanism of tactile contrast originates from a difference in monolayer ordering, as quantified by the Hurst exponent, which was replicated in two alkylsilanes with a three-carbon difference in length. This approach may be generalizable to other materials and lead to new tactile sensations derived from materials chemistry.

Remote Friction Reduction on Resonant Film Surface by Airborne Ultrasound

We propose a film device that can be attached to flat surfaces, including touch panels, to remotely reduce surface friction by irradiating airborne ultrasound. In this article, we present a film-air resonance structure that produces large-amplitude surface vibrations excited by airborne ultrasound. We confirmed via simulation that the surface amplitude increases to a level sufficient to reduce friction at the designed frequency. It was also observed in an experiment using a prototype that the friction between a finger and film surface is sharply reduced, and the surface vibrates with sufficient amplitude when touched with a finger.

Finger motion and contact by a second finger influence the tactile perception of electrovibration

Electrovibration holds great potential for creating vivid and realistic haptic sensations on touchscreens. Ideally, a designer should be able to control what users feel independent of the number of fingers they use, the movements they make, and how hard they press. We sought to understand the perception and physics of such interactions by determining the smallest 125 Hz electrovibration voltage that 15 participants could reliably feel when performing four different touch interactions at two normal forces. The results proved for the first time that both finger motion and contact by a second finger significantly affect what the user feels. At a given voltage, a single moving finger experiences much larger fluctuating electrovibration forces than a single stationary finger, making electrovibration much easier to feel during interactions involving finger movement. Indeed, only about 30% of participants could detect the stimulus without motion. Part of this difference comes from the fact that relative motion greatly increases the electrical impedance between a finger and the screen, as shown via detailed measurements from one individual. By contrast, threshold-level electrovibration did not significantly affect the coefficient of kinetic friction in any conditions. These findings help lay the groundwork for delivering consistent haptic feedback via electrovibration.

Development of a Human-Display Interface with Vibrotactile Feedback for Real-World Assistive Applications

It is important to operate devices with control panels and touch screens assisted by haptic feedback in mobile environments such as driving automobiles and electric power wheelchairs. A lot of consideration is needed to give accurate haptic feedback, especially, presenting clear touch feedback to the elderly and people with reduced sensation is a very critical issue from healthcare and safety perspectives. In this study, we aimed to identify the perceptual characteristics for the frequency and direction of haptic vibration on the touch screen with vehicle-driving vibration and to propose an efficient haptic system based on these characteristics. As a result, we demonstrated that the detection threshold shift decreased at frequencies above 210 Hz due to the contact pressure during active touch, but the detection threshold shift increased at below 210 Hz. We found that the detection thresholds were 0.30–0.45 gpeak with similar sensitivity in the 80–270 Hz range. The haptic system implemented by reflecting the experimental results achieved characteristics suitable for use scenarios in automobiles. Ultimately, it could provide practical guidelines for the development of touch screens to give accurate touch feedback in the real-world environment.

Effect of Remote Masking on Tactile Perception of Electrovibration

Masking has been used to study human perception of tactile stimuli, including those created by electrovibration on touch screens. Earlier studies have investigated the effect of on-site masking on tactile perception of electrovibration. In this article, we investigated whether it is possible to change the absolute detection threshold and intensity difference threshold of electrovibration at the fingertip of index finger via remote masking, i.e., by applying a (mechanical) vibrotactile stimulus on the proximal phalanx of the same finger. The masking stimuli were generated by a voice coil (the Haptuator). For 16 participants, we first measured the detection thresholds for electrovibration at the fingertip and for vibrotactile stimuli at the proximal phalanx. Then, the vibrations on the skin were measured at four different locations on the index finger of subjects to investigate how the mechanical masking stimulus propagated as the masking level was varied. Later, masked absolute thresholds of eight participants were measured. Finally, for another group of eight participants, intensity difference thresholds were measured in the presence/absence of vibrotactile masking stimuli. Our results show that vibrotactile masking stimuli generated sub-threshold vibrations around the fingertip, and hence, probably did not mechanically interfere with the electrovibration stimulus. However, there was a clear psychophysical masking effect due to central neural processes. We measured the effect of masking stimuli, up to 40 dB SL, on the difference threshold at four different intensity standards of electrovibration. We proposed two models based on hypothetical neural signals for prediction of the masking effect on intensity difference thresholds for electrovibration: amplitude and energy models. The energy model was able to predict the effect of masking more accurately, especially at high intensity masking levels.