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.

We lift glasses of water, regardless of whether they are empty or full and whether they are dry or wet. The sensorimotor mechanisms responsible for this astonishing performance are far from being understood. The grip forces required to lift an object are known to be unconsciously regulated to a value typically 20% above what would cause slippage (1). Remarkably, this regulation starts from the moment our fingers touch the surface. It has been shown that just a hundred milliseconds of contact with a surface are enough to start adjusting fingertip forces to friction. Humans provide larger grasping forces if the surface is made of slippery silk, but smaller if it is made of sandpaper since it provides better grasp (2, 3). It has been further demonstrated that it is friction, and not texture, that determines these adjustments (3). Since 1 mm of indentation of the fingertip is sufficient to reach 80% of the final gross contact area, and fingers often move faster than 10 mm/s toward an object, within this time frame, the sensorimotor system already should be able to extract some estimates of the frictional properties from the initial deformation of the finger pad, before any net load forces start developing.On a physical level, the overall so-called “frictional strength” of the contact is given by the number of asperities in intimate contact and their individual shear strength (4–6). It is the measure of the maximum lateral force on the contact that will lead to slippage. This frictional strength is the main determinant in regulating grip force applied to lift an object of a given weight (3). Failure to properly assess the frictional strength of the surface at initial contact—due to the presence of gloves or anesthesia, for instance—is followed by larger-than-usual grip forces, consequently increasing the real area of contact (7–9).Despite its crucial importance, the mechanical deformation that underpins the encoding of the frictional strength on initial contact remains unclear. It is well known that the timing of the impulses of tactile afferents encodes the information related to force direction (10), local curvature (11), edges (12), and shapes (13) and also contains information about the frictional strength (14, 15). One hypothesis suggests that, at the mechanical level, microslip events at the finger–object interface induce vibrations of the skin (16, 17). Another hypothesis postulates that the sensation of friction is mediated by a radial pattern of skin strain within the contact area. The magnitude of the strain induces internal stresses, which are 21% smaller on a slippery surface than on a high-friction surface (18).Interestingly, roboticists have leveraged these findings to estimate friction on initial contact from the gradient of the lateral traction field. This metric is used to control the force applied by robotic grippers to soft and fragile objects (19–21). In haptic rendering, it is possible to produce tactile sensations by releasing the accumulated stress using ultrasonic friction modulation (22). However, the perception of the frictional strength with a single normal motion is not as salient. Khamis et al. (23) recently showed that participants were unable to differentiate a 73% reduction in friction of a glass plate when it was pressed against their fingertips by a robotic manipulator.Friction is consciously perceived in a passive condition only when the plate starts sliding (24–26). The change in the frictional state from stuck to sliding is perceived after a global lateral displacement of 2.3 mm (27). This transition induces large deformations of the skin, along with a particular strain pattern (25, 28–30). These results suggest that large or rapid deformations can elicit a tactile sensation, but the quasistatic radial strain pattern is too subtle to induce a reliable percept.We hypothesize that the frictional strength can be perceived when actively touching the surface. Active exploration is known to promote acute sensitivity (31–33). We present evidence that during the first instant of contact between the finger and an object, a radial strain pattern exists. Its magnitude is affected by the interfacial friction and correlates with the perception of friction. Combined with the results of the motor-control literature, a picture emerges explaining the mechanical basis upon which friction is encoded.     

Performance metrics for guidance active constraints in surgical robotics

Active constraint (AC)/virtual fixture (VF) is among the most popular approaches towards the shared execution of subtasks by the surgeon and robotic systems. As more possibilities appear for the implementation of ACs in surgical scenarios, the need to introduce methods that guarantee a safe and intuitive user‐interaction increases. The presence of the human in the loop adds a layer of interactivity and adaptability that renders the assessment of such methods non‐trivial. In most works, guidance ACs have been evaluated mainly in terms of enhancement of accuracy and completion time with little regard for other aspects such as human factors, even though the continuous engagement of these methods can considerably degrade the user experience. This paper proposes a set of performance metrics and considerations that can help evaluate guidance ACs with reference to accuracy enhancement, force characteristics and subjective aspects. The use of these metrics is demonstrated through two sets of experiments on 12 surgeons and 6 inexperienced users.     

Valveless microliter combustion for densely packed arrays of powerful soft actuators

Existing tactile stimulation technologies powered by small actuators offer low-resolution stimuli compared to the enormous mechanoreceptor density of human skin. Arrays of soft pneumatic actuators initially show promise as small-resolution (1- to 3-mm diameter), highly conformable tactile display strategies yet ultimately fail because of their need for valves bulkier than the actuators themselves. In this paper, we demonstrate an array of individually addressable, soft fluidic actuators that operate without electromechanical valves. We achieve this by using microscale combustion and localized thermal flame quenching. Precisely, liquid metal electrodes produce sparks to ignite fuel lean methane–oxygen mixtures in a 5-mm diameter, 2-mm tall silicone cylinder. The exothermic reaction quickly pressurizes the cylinder, displacing a silicone membrane up to 6 mm in under 1 ms. This device has an estimated free-inflation instantaneous stroke power of 3 W. The maximum reported operational frequency of these cylinders is 1.2 kHz with average displacements of ∼100 µm. We demonstrate that, at these small scales, the wall-quenching flame behavior also allows operation of a 3 × 3 array of 3-mm diameter cylinders with 4-mm pitch. Though we primarily present our device as a tactile display technology, it is a platform microactuator technology with application beyond this one.

Through the senses, human beings constantly gain rich information about the external world; ultimately, everything one knows comes from what one first learns through one’s sense powers (1). Though sight is generally considered to be our strongest sensorial asset, touch (pressure, pain, vibration, temperature, etc.) intimately connects us with our nearby environment and our own bodies. Touch is, perhaps, more necessary for survival than any other sense (2–8). It is unsurprising, then, that our skin is our bodies’ largest organ (9, 10), comprising in part a diverse array of mechanoreceptive organelles, allowing people to feel skin deformations of different types, durations, and intensities (11). For example, human fingertips have over 200 mechanoreceptive units per square centimeter (10), perceiving static deformations of down to 0.1 mm (12) and vibrations of up to 400 Hz (10, 13).Despite the importance of touch, our visual and auditory senses dominate the experience of digital information. The most proliferate form of haptic actuation is vibrotactile, but this technique does not allow the type of displacement and persistence of touch required to provide a natural experience. Vibrations alone cannot meaningfully recreate the pressure felt from a bag on the shoulders or the impact of a ball caught in the hands. Because of this lack of haptic experience, at least two societal needs remain unfulfilled: artificial touch recreation in immersive virtual reality (VR) and braille displays that compete with analogous visual media. For braille, specifically, there are no full-page, affordable, portable, refreshable displays on the market (14).The dearth of available tactile display options is not from lack of trying; manufacturing arrays of actuators at the size and density suitable for reading computer information from a tactile screen requires reducing actuator volume, weight, power draw, and cost, all together. The diverse set of designs conceived to achieve this haptic challenge have employed an equally numerous suite of physical principles, and each actuation method has presented its own failure mode (14). For example, thermal actuators usually take seconds (without thermal management accessories) (15) to finish a work loop because of heat transport limitations (16). Pulsed electromagnetic systems suffer from low actuation forces and interference between individual actuators (crosstalk) when made close to the size of a braille dot (17). Piezoelectric devices have large production costs at scale (HyperBraille systems cost ∼$50,000), also needing long cantilevered geometries that impede their ability to be densely arranged (14, 18).Fluidic elastomer actuators (FEAs) displace rubber forms with liquids and gases, showing promise as dense actuator arrays because of their manufacturing simplicity and favorable mechanical characteristics (19). An elastomeric membrane ∼1 mm in diameter can displace more than 0.5 mm as was previously shown by ref. 20 in which a 1.5-mm diameter viscoelastic membrane displaced 0.56 mm in about 1 s. Beyond simplicity, these soft haptic devices also have the convenient ability to conform to complex body shapes (21). For example, HaptX has developed a commercial, tethered VR glove technology that integrates 130 individually addressable fluidic actuators into each glove (22). This paper’s lead author has experienced this technology and testifies to its natural feel (23). As designed, the glove’s microfluidic channels are tethered to a large box housing a pump and many valves, limiting the user’s range of motion. One major deficiency of FEAs is how the system scales with actuator number density: there is generally a linear relationship between the number of valves and actuators. As electromechanical valves are themselves actuators, the size, weight, power, and cost (SWaP-C) requirements of FEA arrays soon become untenable for portable tactile display systems. For example, the most popular valve choice for FEAs is the Parker X-Valve, with dimensions of 7.87 × 23.37 × 12.30 mm³ at a unit cost of ∼$40 (24); if a single, six-dot braille cell (∼6 × 10 × 10 mm³) was controlled by six valves, that is, one valve per actuator, the array of valves would take up 18.4 times the cell area and cost $240 (14). Though there are multiplexing solutions to the valving challenge (25), we are currently unaware of any high-resolution tactile interaction being enabled by these methods.Counterintuitively, microscale combustion could provide an alternative actuation motif for haptic arrays, given its own engineering tradeoffs. Combining high-energy density fuels (26, 27) with small-volume mechanical elements results in a potentially safe and enduring actuation mechanism. Previously, microscale combustion research primarily focused on replacing batteries with high–power density micro-electromechanical systems (MEMS) thermoelectric generators (28). These systems may have failed to become practical because of unwanted flame extinction, thermal degradation, and frictional wear (29). More recently, combustion has been used in FEAs for macroscale soft robots and pumps (30–34). A spark ignites a combustible gas mixture that rapidly heats the product gas and expands the soft FEA cavity to move a robot or separate fluid. This research direction, however, has not been previously expanded into the realm of small gas-powered FEAs (35).In this paper, we make two contributions: 1) the use of combustion in microliter-scale FEAs for powerful, high-stroke, millimeter-scale actuations and 2) the exploitation of rapid thermal quenching at these scales to individually actuate fluidically coupled arrays without valving. As we no longer need valves, we can space the actuators more closely because their flow and electrical control components occupy less area than the actuator footprint. Primarily composed of molded silicone and microfluidic liquid metal (LM) traces, our design is an inexpensive, thin rubber sheet that provides more favorable SWaP-C scaling than prior FEA systems. We elementally characterize our device’s mechanical performance as a general microactuation strategy. As tactile display systems represent one of the oldest, broadest, and most contemporary microactuator research initiatives, we focus our discussion and demonstration on this system’s potential to serve a similar purpose.     

A Novel Approach to Robotic Cardiac Surgery Using Haptics and Vision

Cardiovascular disease is one of the leading causes of death in the United States and also a major disease nationwide. Over 700,000 coronary artery bypass graft (CABG) procedures are performed annually all around the world, of which 350,000 are performed in the United States. The use of mechanical stabilizers to isolate and immobilize the surface region of the heart is not without its limitations such as hemodynamic deterioration, and arrythmia induction requiring inotropic support. Consequently, the use of mechanical stabilizers leads to a poor immobilization of the surgical field in spite of significant forces of traction and retraction used with these devices. The primary goal of this research is to develop effective haptic (sense of touch) and visual servoing methods with the long-term goal of eliminating the need for mechanical stabilizers and extracorporeal support for CABG procedures. We present in this paper the results from our initial work in the area of tracking a deformable membrane using vision and providing haptic feedback to the user, based on the visual information through the vision hardware and the material properties of the membrane. In our first experiment, we track the deformation of a rubber membrane in real-time through stereovision while providing haptic feedback to the user interacting with the reconstructed membrane through the PHANToM haptic device. In the second experiment, we verify the ability of our vision system to track a point on a surface undergoing a complex 3D motion.     

A Sensaptic ADAS Device Using Shape Memory Alloy Wires: Design and Control

This paper presents an active accelerator pedal system based on an integrated sensor and actuator using shape memory alloy (SMA) for speed control and to create haptics in the accelerator pedal. A device named sensaptics is developed with a pair of bi-functional SMA wires instrumented in a synergistic configuration function as an active sensor for positioning the accelerator pedal (pedal position sensing) to control the vehicle speed through electronic throttle and as a variable impedance actuator to generate active force (haptic) feedback to the driver. The reaction force emanated from the pedal alerts the driver and takes appropriate control action by slowing down the vehicle, in harmony with the road’s condition. The design is developed as a proof-of-concept device and is tested and evaluated in a real-time common rail diesel system for rail pressure regulation and over speeding tests, and the responses and performances are found to be promising.