Measuring tactile cues at the fingerpad for object compliances harder and softer than the skin

Abstract

Distinguishing an object’s compliance, into percepts of “softness” and “hardness,” is crucial to our ability to grasp and manipulate it. Biomechanical cues at the skin’s surface such as contact area and force rate have been thought to help encode compliance. However, no one has directly measured contact area with compliant materials, and few studies have considered compliances softer than the fingerpad. Herein, we developed a novel method to precisely measure the area in contact between compliant stimuli and the fingerpad, at given levels of force and displacement. To determine the method’s robustness, we conducted psychophysical and biomechanical experiments with human subjects. The results indicate that cues including contact area at stimulus peak force of 3 Newtons, force rate over stimulus movement and at peak force, displacement and/or time to reach peak force may help in discriminating compliances while the directional spread of contact area is less important. Between softer and harder compliances, some cues were slightly more evident, though not yet definitively. Based upon the method’s utility, the next step is to conduct broader experiments to distill the mixture of cues that encode compliance. The importance of such work lies in building haptic displays, for example, to render virtual tissues.

A Standard Methodology to Characterize the Intrinsic Material Properties of Compliant Test Stimuli

“…They had been poured with target moduli of about 18, 22, 120 and 150 kPa. Those initial estimates had been based upon different means of characterization, using spherical indentation [25]–[28]. These values span compliances both harder and softer than fingertip skin, which range from 42-130 kPa [19], [20].…”

“…The first mimics the 10 mm diameter by 10 mm tall substrate and 24 mm diameter compression platen, and the second mimics a 38 mm diameter by 10 mm tall substrate and 6 mm diameter compression platen. Note that the geometry of the former follows the standard characterization methodology and the second was built to match a set of cylinders that had been used in prior study [12], [28].…”

Imaging the 3-D deformation of the finger pad when interacting with compliant materials

“…In this experiment, we sought to validate the measurement of the gross contact area, at the peak indentation magnitude, comparing the 3-D reconstruction method to the prior “ground truth” ink-based method [8]. Hard and soft stimuli were indented into the finger pad (inclined at an angle of 30º) at 5 mm/s to depths of 0–5 mm in independent trials.…”

“…Within each trial, the stimulus was pressed continuously into the finger pad up to the terminal depth, at which force was recorded and a stereo image was taken of the finger pad. Immediately afterward, the stimulus was withdrawn and an ink print was taken from the stimulus to be measured via custom software [8]. Each trial was repeated three times.…”

“…As an intermediate step between measures of bulk compressive characteristics and imaging techniques constrained by interactions with rigid bodies, our group previously developed a way to measure the flattened geometric shape and size of the contact region [8]. Ink marked on the stimulus surface was deposited on the skin after an indentation and imaged to generate stimulus- and indentation depth- specific contact areas.…”