Role of arm reaching movement kinematics in friction perception at initial contact with smooth surfaces.

When manipulating objects, humans begin adjusting their grip force to friction within 100 ms of contact. During motor adaptation, subjects become aware of the slipperiness of touched surfaces. Previously, we have demonstrated that humans cannot perceive frictional differences when surfaces are brought in contact with an immobilised finger, but can do so when there is submillimeter lateral displacement or subjects actively make the contact movement. Similarly, in, we investigated how humans perceive friction in the absence of intentional exploratory sliding or rubbing movements, to mimic object manipulation interactions. We used a two-alternative forced-choice paradigm in which subjects had to reach and touch one surface followed by another, and then indicate which felt more slippery. Subjects correctly identified the more slippery surface in 87 ± 8% of cases (mean ± SD; n = 12). Biomechanical analysis of finger pad skin displacement patterns revealed the presence of tiny (<1 mm) localised slips, known to be sufficient to perceive frictional differences. We tested whether these skin movements arise as a result of natural hand reaching kinematics. The task was repeated with the introduction of a hand support, eliminating the hand reaching movement and minimising fingertip movement deviations from a straight path. As a result, our subjects' performance significantly declined (66 ± 12% correct, mean ± SD; n = 12), suggesting that unrestricted reaching movement kinematics and factors such as physiological tremor, play a crucial role in enhancing or enabling friction perception upon initial contact. KEY POINTS: More slippery objects require a stronger grip to prevent them from slipping out of hands. Grip force adjustments to friction driven by tactile sensory signals are largely automatic and do not necessitate cognitive involvement; nevertheless, some associated awareness of grip surface slipperiness under such sensory conditions is present and helps to select a safe and appropriate movement plan. When gripping an object, tactile receptors provide frictional information without intentional rubbing or sliding fingers over the surface. However, we have discovered that submillimeter range lateral displacement might be required to enhance or enable friction sensing. The present study provides evidence that such small lateral movements causing localised partial slips arise and are an inherent part of natural reaching movement kinematics.

Initial contact shapes the perception of friction.

Humans efficiently estimate the grip force necessary to lift a variety of objects, including slippery ones. The regulation of grip force starts with the initial contact and takes into account the surface properties, such as friction. This estimation of the frictional strength has been shown to depend critically on cutaneous information. However, the physical and perceptual mechanism that provides such early tactile information remains elusive. In this study, we developed a friction-modulation apparatus to elucidate the effects of the frictional properties of objects during initial contact. We found a correlation between participants' conscious perception of friction and radial strain patterns of skin deformation. The results provide insights into the tactile cues made available by contact mechanics to the sensorimotor regulation of grip, as well as to the conscious perception of the frictional properties of an object.

Friction sensing mechanisms for perception and motor control: passive touch without sliding may not provide perceivable frictional information.

Perception of the frictional properties of a surface contributes to the multidimensional experience of exploring various materials; we slide our fingers over a surface to feel it. In contrast, during object manipulation, we grip objects without such intended exploratory movements. Given that we are aware of the slipperiness of objects or tools that are held in the hand, we investigated whether the initial contact between the fingertip skin and the surface of the object is sufficient to provide this consciously perceived frictional information. Using a two-alternative forced-choice protocol, we examined human capacity to detect frictional differences using touch, when two otherwise structurally identical surfaces were brought in contact with the immobilized finger perpendicularly or under an angle (20° or 30°) to the skin surface (passive touch). An ultrasonic friction reduction device was used to generate three different frictions over each of three flat surfaces with different surface structure: 1) smooth glass, 2) textured surface with dome-shaped features, and 3) surface with sharp asperities (sandpaper). Participants (n = 12) could not reliably indicate which of the two surfaces was more slippery under any of these conditions. In contrast, when slip was induced by moving the surface laterally by a total of 5 mm (passive slip), participants could clearly perceive frictional differences. Thus making contact with the surface, even with moderate tangential forces, was not enough to perceive frictional differences, instead conscious perception required a sufficient size slip.NEW & NOTEWORTHY This study contributes to understanding how frictional information is obtained and used by the brain. When the skin is contacting surfaces of identical topography but varying frictional properties, the deformation pattern is different; however, available sensory cues did not get translated into perception of frictional properties unless a sufficiently large lateral movement was present. These neurophysiological findings may inform how to design and operate haptic devices relying on friction modulation principles.

Partial squeeze film levitation modulates fingertip friction.

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.

Classification and Evaluation of Octopus-Inspired Suction Cups for Soft Continuum Robots.

The emergence of the field of soft robotics has led to an interest in suction cups as auxiliary structures on soft continuum arms to support the execution of manipulation tasks. This application poses demanding requirements on suction cups with respect to sensorization, adhesion under non-ideal contact conditions, and integration into fully soft systems. The octopus can serve as an important source of inspiration for addressing these challenges. This review aims to accelerate research in octopus-inspired suction cups by providing a detailed analysis of the octopus sucker, determining meaningful performance metrics for suction cups on the basis of this analysis, and evaluating the state-of-the-art in suction cups according to these performance metrics. In total, 47 records describing suction cups are found, classified according to the deployed actuation method, and evaluated on performance metrics reflecting the level of sensorization, adhesion, and integration. Despite significant advances in recent years, the octopus sucker outperforms all suction cups on all performance metrics. The realization of high resolution tactile sensing in suction cups and the integration of such sensorized suction cups in soft continuum structures are identified as two major hurdles toward the realization of octopus-inspired manipulation strategies in soft continuum robot arms.

Ultraloop: Active Lateral Force Feedback Using Resonant Traveling Waves.

The sensation of touching virtual texture and shape can be provided to a touchscreen user by varying the friction force. Despite the saliency of the sensation, this modulated frictional force is purely passive and strictly opposes finger movement. Therefore, it is only possible to create forces along the direction of movement and this technology cannot stimulate a static fingertip or provide forces that are orthogonal to the direction of movement. The lack of orthogonal force limits the guidance to a target in an arbitrary direction and there is a need for active lateral forces to give directional cues to the fingertip. Here, we introduce a surface haptic interface that uses ultrasonic traveling waves to create an active lateral force on bare fingertips. The device is built around a ring shape cavity where two degenerate resonant modes around 40 kHz are excited with 90 ° phase shift. The interface provides active forces up to 0.3 N to a static bare finger uniformly over a 140×30 mm 2 surface. We report the model and design of the acoustic cavity, force measurements, and an application to create a key-click sensation. This work demonstrates a promising method for uniformly producing large lateral forces on a touch surface.

Rapid change of friction causes the illusion of touching a receding surface.

Shortly after touching an object, humans can tactually gauge the frictional resistance of a surface. The knowledge of surface friction is paramount to tactile perception and the motor control of grasp. While potent correlations between friction and participants' perceptual response have been found, the causal link between the friction of the surface, its evolution and its perceptual experience has yet to be demonstrated. Here, we leverage new experimental apparatus able to modify friction in real time, to show that participants can perceive sudden changes in friction when they are pressing on a surface. Surprisingly, only a reduction of the friction coefficient leads to a robust perception. High-speed imaging data indicate that the sensation is caused by a release of a latent elastic strain over a 20 ms timeframe after the activation of the friction-reduction device. This rapid change of frictional properties during initial contact is interpreted as a normal displacement of the surface, which paves the way for haptic surfaces that can produce illusions of interacting with mechanical buttons.

Simple and Fast Locomotion of Vibrating Asymmetric Soft Robots.

To be fully integrated into the activities of our daily lives, robots need to be capable of traversing unstructured environments and interacting safely with their surroundings. Soft robots are perfect candidates since they can adapt to their surroundings through passive material compliance, rather than relying on complex control. However, the same compliance hinders the generation of propelling forces, and current approaches face a trade-off between traveling speed, action range, and control complexity. We overcome this trade-off by developing a locomotion mechanism based on the synergistic interaction between symmetric vibrations, elasticity, and asymmetric morphology. We then realize a rapid soft locomotor using inexpensive off-the-shelf components and requiring only elementary actuation and control. A single robotic unit can travel at speeds up to 100 mm/s when tethered and 35 mm/s when untethered. We derive a model that predicts the speed of the robot as a function of several design parameters and physical properties, highlighting the role of geometric asymmetries in the resulting anisotropic motion. Moreover, these elementary units can be added together to create more complex behaviors. By adding 2 units in parallel, the assembly can locomote and be steered following nonholonomic constraints. Our approach opens the door to a new class of low-cost soft robots that can travel fast and far with elementary fabrication and control, and which can be combined to achieve complex functions without compromising their essential simplicity.

Focused Vibrotactile Stimuli From a Wearable Sparse Array of Actuators.

Wearable vibrotactile actuators are non-intrusive and inexpensive means to provide haptic feedback directly to the user's skin. Complex spatiotemporal stimuli can be achieved by combining multiple of these actuators, using the funneling illusion. This illusion can funnel the sensation to a particular position between the actuators, thereby creating virtual actuators. However, using the funneling illusion to create virtual actuation points is not robust and leads to sensations that are difficult to locate. We postulate that poor localization can be improved by considering the dispersion and attenuation of the wave propagation on the skin. We used the inverse filter technique to compute the delays and amplification of each frequency to correct the distortion and create sharp sensations that are easier to detect. We developed a wearable device stimulating the volar surface of the forearm composed of four independently controlled actuators. A psychophysical study involving twenty participants showed that the focused sensation improves confidence in the localization by 20% compared to the non-corrected funneling illusion. We anticipate our results to improve the control of wearable vibrotactile devices used for emotional touch or tactile communication.

Efficient tactile encoding of object slippage.

When grasping objects, we rely on our sense of touch to adjust our grip and react against external perturbations. Less than 200 ms after an unexpected event, the sensorimotor system is able to process tactile information to deduce the frictional strength of the contact and to react accordingly. Given that roughly 1,300 afferents innervate the fingertips, it is unclear how the nervous system can process such a large influx of data in a sufficiently short time span. In this study, we measured the deformation of the skin during the initial stages of incipient sliding for a wide range of frictional conditions. We show that the dominant patterns of deformation are sufficient to estimate the distance between the frictional force and the frictional strength of the contact. From these stereotypical patterns, a classifier can predict if an object is about to slide during the initial stages of incipient slip. The prediction is robust to the actual value of the interfacial friction, showing sensory invariance. These results suggest the existence of a possible compact set of bases that we call Eigenstrains. These Eigenstrains are a potential mechanism to rapidly decode the margin from full slip from the tactile information contained in the deformation of the skin. Our findings suggest that only 6 of these Eigenstrains are necessary to classify whether the object is firmly stuck to the fingers or is close to slipping away. These findings give clues about the tactile regulation of grasp and the insights are directly applicable to the design of robotic grippers and prosthetics that rapidly react to external perturbations.

Rhythm perception is shared between audio and haptics.

A surface texture is perceived through both the sound and vibrations produced while being explored by our fingers. Because of their common origin, both modalities have a strong influence on each other, particularly at above 60 Hz for which vibrotactile perception and pitch perception share common neural processes. However, whether the sensation of rhythm is shared between audio and haptic perception is still an open question. In this study, we show striking similarities between the audio and haptic perception of rhythmic changes, and demonstrate the interaction of both modalities below 60 Hz. Using a new surface-haptic device to synthesize arbitrary audio-haptic textures, psychophysical experiments demonstrate that the perception threshold curves of audio and haptic rhythmic gradients are the same. Moreover, multimodal integration occurs when audio and haptic rhythmic gradients are congruent. We propose a multimodal model of rhythm perception to explain these observations. These findings suggest that audio and haptic signals are likely to be processed by common neural mechanisms also for the perception of rhythm. They provide a framework for audio-haptic stimulus generation that is beneficial for nonverbal communication or modern human-machine interfaces.

Submillimeter Lateral Displacement Enables Friction Sensing and Awareness of Surface Slipperiness.

Human tactile perception and motor control rely on the frictional estimates that stem from the deformation of the skin and slip events. However, it is not clear how exactly these mechanical events relate to the perception of friction. This study aims to quantify how minor lateral displacement and speed enables subjects to feel frictional differences. In a 2-alternative forced-choice protocol, an ultrasonic friction-reduction device was brought in contact perpendicular to the skin surface of an immobilized index finger; after reaching 1N normal force, the plate was moved laterally. A combination of four displacement magnitudes (0.2, 0.5, 1.2 and 2 mm), two levels of friction (high, low) and three displacement speeds (1, 5 and 10 mm/s) were tested. We found that the perception of frictional difference was enabled by submillimeter range lateral displacement. Friction discrimination thresholds were reached with lateral displacements ranging from 0.2 to 0.5 mm and surprisingly speed had only a marginal effect. These results demonstrate that partial slips are sufficient to cause awareness of surface slipperiness. These quantitative data are crucial for designing haptic devices that render slipperiness. The results also show the importance of subtle lateral finger movements present during dexterous manipulation tasks.

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

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 ($\rho =0.94$) 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.

Estimating Friction Modulation From the Ultrasonic Mechanical Impedance.

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 article, 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.

Detection of Friction-Modulated Textures is Limited by Vibrotactile Sensitivity.

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 article provides a better understanding of the perception of friction-modulated textures and provides insight into how to design impactful stimuli on surface-haptic devices.

Contact geometry and mechanics predict friction forces during tactile surface exploration.

When we touch an object, complex frictional forces are produced, aiding us in perceiving surface features that help to identify the object at hand, and also facilitating grasping and manipulation. However, even during controlled tactile exploration, sliding friction forces fluctuate greatly, and it is unclear how they relate to the surface topography or mechanics of contact with the finger. We investigated the sliding contact between the finger and different relief surfaces, using high-speed video and force measurements. Informed by these experiments, we developed a friction force model that accounts for surface shape and contact mechanical effects, and is able to predict sliding friction forces for different surfaces and exploration speeds. We also observed that local regions of disconnection between the finger and surface develop near high relief features, due to the stiffness of the finger tissues. Every tested surface had regions that were never contacted by the finger; we refer to these as "tactile blind spots". The results elucidate friction force production during tactile exploration, may aid efforts to connect sensory and motor function of the hand to properties of touched objects, and provide crucial knowledge to inform the rendering of realistic experiences of touch contact in virtual reality.

Perception of Ultrasonic Switches Involves Large Discontinuity of the Mechanical Impedance.

The distinct tactile feedback provided by mechanical keyboards notifies users that their actions have been successfully recorded. The presence of these subtle yet informative tactile cues is one of the reasons why mechanical keyboards are still preferred to their virtual counterparts. An artificial sensation of pressing a mechanical switch can be produced by varying the coefficient of friction as the user is pressing down on a glass surface using ultrasonic vibration. We examined the factors involved in producing a vivid sensation of a stimulus by measuring the mechanical impedance, the frictional behavior of the fingertip and the perceptual thresholds. Subjects who experienced weaker sensations also showed a weaker sensitivity to friction modulation, which may in turn be attributable to the presence of a larger or a smaller than average impedance. In the second experiment, the user's finger impedance was measured during the click, and it was observed that the successful detection of the stimulus was correlated with the presence of considerable discontinuity in the mechanical impedance added to the plate by the finger. This discontinuity in the evolution of the impedance supports the idea that the skin is being reconfigured towards a new equilibrium state after the change in friction.

Anticipatory Vibrotactile Cueing Facilitates Grip Force Adjustment during Perturbative Loading.

Grip force applied to an object held between the thumb and index finger is automatically and unconsciously adjusted upon perception of an external disturbance to the object. Typically, this adjustment occurs within approximately 100 ms. Here, we investigated the effect of anticipatory vibrotactile cues prior to a perturbative force, which the central nervous system may use for rapid grip re-stabilization. We asked participants to grip and hold an instrumented, actuated handle between the thumb and index finger. Under computer control, the handle could suddenly be pulled away from a static grip and could independently provide vibration to the gripping fingers. The mean latency of corrective motor action was 139 ms. When vibrotactile stimulation was applied 50 ms before application of tractive force, the latency was reduced to 117 ms, whereas the mean latency of the conscious response to vibrotactile stimuli alone was 229 ms. This suggests that vibrotactile stimulation can influence reflex-like actions. We also examined the effects of anticipatory cues using a set of perturbative loads with different rising rates. As expected, facilitation of grip force adjustment was observed for moderate loads. In contrast, anticipatory cues had an insignificant effect on rapid loads that evoked an adjustment within 60-80 ms, which approaches the minimum latency of human grip adjustment. Understanding the facilitative effects of anticipatory cues on human reactive grip can aid the development of human-machine interfaces to enhance human behavior.

Power optimization of ultrasonic friction-modulation tactile interfaces.

Ultrasonic friction-modulation devices provide rich tactile sensation on flat surfaces and have the potential to restore tangibility to touchscreens. To date, their adoption into consumer electronics has been in part limited by relatively high power consumption, incompatible with the requirements of battery-powered devices. This paper introduces a method that optimizes the energy efficiency and performance of this class of devices. It considers optimal energy transfer to the impedance provided by the finger interacting with the surface. Constitutive equations are determined from the mode shape of the interface and the piezoelectric coupling of the actuator. The optimization procedure employs a lumped parameter model to simplify the treatment of the problem. Examples and an experimental study show the evolution of the optimal design as a function of the impedance of the finger.