Thesis title: Analysis and rendering of contact vibrational stimuli for tactile perception
Among the 5 senses, which connect us with the surrounding world, the sense of touch is between the most articulated and the least understood. While we are able to master the signals that underlie sight and hearing, rendering them using loudspeakers and visual interfaces, the mechanisms underlying the sense of touch are still largely unknown. Touch, originating by the contact between the skin and the explored surface and involving several types of stimuli, requires a strongly multidisciplinary approach and involves a wide range of disciplines, such as Medicine, Neurosciences, Psychology, Dynamics, Tribology, Materials Sciences, and beyond. Tribology and Dynamics are involved in the study of all those complex phenomena that occur at the contact and that generate mechanical stimuli such as Friction-Induced Vibrations and contact forces, at the origin of the stimulation of skin’s mechanoreceptors. This Ph.D. thesis is collocated into a research line closely dedicated to the investigation of the role of different features (amplitude, frequency distribution) of Friction-Induced Vibrations (FIV) in mediating between the characteristics of surface textures and the way in which textures are perceived and discriminated. Analyses of vibrational stimuli originating from the exploration of rigid periodic and isotropic textures have been carried out in the present work, revealing different key features in the discrimination of such textures: respectively the amplitude for isotropic textures, and the frequency distribution for periodic ones. At the same time, a tactile rendering device, named PIEZOTACT, has been developed to reproduce/mimic the FIV previously measured during the exploration of real surfaces. The developed vibrotactile device has been used to simulate periodic and isotropic textures and then to conduct campaigns on groups of volunteers, in order to evaluate their ability to discriminate real and simulated textures starting from the sole vibrational tactile stimuli. The good performances achieved by the volunteers in the discrimination campaigns made it possible to validate the device for rendering the tested textures and to correlate the different characteristics of amplitude and frequency distribution of FIV with perception of textures. Because the rendering methodology underlying the device is based on the characterization of the Transfer Function of the overall electromechanical system (consisting of the device and the user's finger), parametric analyses have been carried out on the Transfer Function of the human finger and the device as the participant and the contact conditions vary. Since the analysis revealed a slight deviation between the Transfer Function for different volunteers under the same contact conditions, the possibility of using an averaged Transfer Function to simulate textures using the PIEZOTACT device has been then validated by discrimination campaigns. Finally, as part of a multi-disciplinary collaboration, a joint investigation, with laboratories form Neurosciences and Psychology, has been performed to evaluate the brain's response to the mechanical stimuli generated by the exploration of real and simulated surfaces, dealing as well with the role of signal attenuation during the touch motion.