Design of Variable-friction devices for shoe-floor contact

In rehabilitation training, high-fidelity simulation environments are needed for reproducing the effects of slippery surfaces, in which potential balance failure conditions can be reproduced on demand. Motivated by these requirements, this article considers the design of variable-friction devices for use in the context of human walking on surfaces in which the coefficient of friction can be controlled dynamically. Various designs are described, aiming at rendering low-friction shoe-floor contact, associated with slippery surfaces such as ice, as well as higher-friction values more typical of surfaces such as pebbles, sand, or snow. These designs include an array of omnidirectional rolling elements, a combination of low- and high-friction coverings whose contact pressure distribution is controlled, and modulation of low-frequency vibration normal to the surface. Our experimentation investigated the static coefficient of friction attainable with each of these designs. Rolling elements were found to be the most slippery, providing a coefficient of friction as low as 0.03, but with significant drawbacks from the perspective of our design objectives. A controlled pressure distribution of low- and high-friction coverings allowed for a minimum coefficient of friction of 0.06. The effects of vibration amplitude and frequency on sliding velocity were also explored. Increases in amplitude resulted in higher velocities, but vibration frequencies greater than 25 Hz reduced sliding velocities. To meet our design objectives, a novel approach involving a friction-variation mechanism, embedded in a shoe sole, is proposed.