An investigation of skin tribology phenomena involved in tactile communication through braille and its associated psychophysical response during task-based discrimination

Darden, Matthew
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Mechanical Engineering
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Most individuals utilize all five senses, especially their sense of sight, to create a unique sensory experience depicting the surrounding environment. Unfortunately, individuals in the blind and visually impaired (BVI) community lack the sense of sight and rely primarily on tactile means to acquire valuable or potentially vital information, leading to the advent of tactile communication methods like braille. A key challenge in controlling the haptic experience of a surface is the lack of fundamental understanding of how various surface attributes, such as friction and texture, affect the tactile response. Oftentimes, braille users experience tactile confusion when scanning complex tactual codes such as tactile graphics or advanced mathematics commonly seen in the STEM fields, but coding standards and limitations in perceptive resolution reduce the opportunity for innovating or redesigning the language to aid the reader.

This dissertation aims to address confusion in tactile information transfer by identifying, characterizing, and developing an understanding of the skin-surface contact interactions experienced during braille reading in order to promote innovations in surface engineering and material design that can improve existing tactile communication methods. The authors first propose a method to directly observe an individual’s cognitive response to tactile experiences through an “oddball paradigm” discrimination task using event-related potential (ERP) via electroencephalography (EEG), a technique that is common in visual and auditory psychological sensory studies. Results indicate that varying levels of friction and roughness from textured samples (i.e. sandpaper) elicit different magnitudes of cognitive activity, suggesting that this technique may prove to be a valuable tool in identifying and understanding the root causes of tactile confusion.

The second aspect of the research seeks to characterize the fundamental frictional forces that occur during braille reading by investigating the loading interactions as the fingerpad slides over a single braille dot and then progressively increasing the complexity of the topographies (i.e. dot spacing, orientation, count). Derived from Greenwood and Tabor, the authors develop and propose a multi-term friction model that predicts the adhesion and deformation frictional effects of a single feature during skin-on-dot sliding, identifying deformation as the dominant friction mechanism when a soft body slides over a spherical geometry. Incorporating both computational modeling and large-scale tribological tests under displacement-controlled sliding further decomposes the frictional loading mechanisms showing that surface tension and compression are driven by the elastic material’s Poisson effect dependent on the bulk’s position with respect to the dot feature. Here, loads in the vertical direction are governed by bulk material deformation due to contact pressure and loads in the lateral direction are governed by bulk material deformation due to both contact pressure and frictional shear.

Contact, Friction, Perception, Psychophysiology, Skin, Tribology