Role of indentation depth and contact area on human perception of softness for haptic interfaces

Abstract

In engineering, the “softness” of an object, as measured by an indenter, manifests as two measurable parameters: (i) indentation depth and (ii) contact area. For humans, softness is not well defined, although it is believed that perception depends on the same two parameters. Decoupling their relative contributions, however, has not been straightforward because most bulk—“off-the-shelf”—materials exhibit the same ratio between the indentation depth and contact area. Here, we decoupled indentation depth and contact area by fabricating elastomeric slabs with precise thicknesses and microstructured surfaces. Human subject experiments using two-alternative forced-choice and magnitude estimation tests showed that the indentation depth and contact area contributed independently to perceived softness. We found an explicit relationship between the perceived softness of an object and its geometric properties. Using this approach, it is possible to design objects for human interaction with a desired level of perceived softness.

Electropneumotactile Stimulation: Multimodal Haptic Actuators Enabled by a Stretchable Conductive Polymer on Inflatable Pockets

“…Such a capability could be useful for applications needing to recapitulate more than one property of a surface at the same time. This demonstration is part of an effort to use the tools of organic materials chemistry in haptics research—“organic haptics.” [ 2–4 , 5 ]…”

An elasticity-curvature illusion decouples cutaneous and proprioceptive cues in active exploration of soft objects

“…This percept is informed by some combination of cutaneous inputs from mechanosensitive afferents signaling skin deformation and proprioceptive inputs signaling body movements. Efforts to define the precise cues within skin deformation and body movements have focused on contact area at the finger pad (14)(15)(16)(17)(18), spatiotemporal deformation of the skin's surface (19)(20)(21), and kinesthetic inputs of displacement, force, and joint angle (22)(23)(24)(25). Such an array of sensory contact inputs, mediated by independent cortical mechanisms, are recruited and integrated in the primary somatosensory cortex, and form the perceptual basis from which compliances are recognized and discriminated (26).…”

Organic Haptics: Intersection of Materials Chemistry and Tactile Perception

“…In the second area, we attempted to isolate the effects of indentation depth, contact area, and elastic modulus on the perception of softness using silicone slabs with microstructured relief features. Our (IRB-approved) experiments, described below, [41,78] used either a two-or three-alternative forced-choice test (i.e., "odd-man-out") to measure the tactile discriminability of surfaces that differed in surface energy (e.g., hydrophobicity vs. hydrophilicity) or softness. This study measured sensitivity: the goal was not to characterize the threshold difference at which surfaces could be distinguished, but rather to determine whether a difference could be detected at all.…”

“…Reproduced with permission. [78] Copyright 2019, American Association for the Advancement of Science.…”

“…Where δ is the indentation depth, a is the contact radius, πa 2 is the contact area, and m is meters. [78] For example, for a thick sample, the equation predicts that an approximately three-fold reduction in the elastic modulus will result in a sample that is perceived as twice as soft as the original. Among other insights, this study also suggests that "softness" is mathematically transitive (by the lack of blue boxes above, or red boxes below the diagonal line in Fig.…”