Augmentation Of Stiffness Perception With a 1-Degree-of-Freedom Skin Stretch Device

During tool-mediated interactions with objects, we experience force and fingerpad skin stretch resulting from shear forces caused by friction between the fingerpad skin and the stylus. When probing an object, for the same penetration distance, a stiffer object causes a larger load force and, thus, greater fingerpad skin stretch. We hypothesized that rendering additional artificial skin stretch together with force will increase perceived stiffness. We created a Skin Stretch Stylus that renders skin stretch through tactor displacement, attached it to a force-feedback device, and performed a study to characterize the effect of tactor displacement-induced skin stretch on stiffness perception. Results showed that adding artificial skin stretch causes additive augmentation of perceived stiffness across a range of surface stiffness, and the addition is a linear function of tactor displacement gain. However, intersubject variability in the estimated slope coefficient was large. We propose a model that explains the additive effect and suggests potential sources for the intersubject variability. We conclude that augmenting force feedback with skin stretch can increase users' perception of stiffness, but the effect is user-specific. Such augmentation may be useful in virtual environment and teleoperation scenarios when force feedback gains must be kept low to prevent feedback-induced instabilities, or when force feedback is limited due to actuator force limits.