Enhancement of vibrotactile sensitivity: Effects of stationary boundary contacts

Tactile sensations experienced by humans, such as roughness and softness, require not only high-frequency (200 Hz) but also lower-frequency vibrations (< 50 Hz). However, such low frequencies are difficult to achieve with small actuators that can be integrated into mobile devices. Therefore, it is necessary to develop methods for enhancing human vibrotactile sensitivity. We focused on a phenomenon whereby simultaneous contact with vibratory and stationary surfaces enhances human vibrotactile sensitivity, which we call stationary-boundary-contact (SBC) enhancement. SBC produces a line sensation along the gap between the vibratory and stationary surfaces. In this study, we determined the detection thresholds for SBC-enhanced sensitivity under several conditions. Psychophysical experiments showed that the detection thresholds of SBC were reduced by more than three times at low frequencies as compared to those under normal conditions. We then investigated the mechanism behind SBC enhancement by using a finite element model for the skin. Static and dynamic deformation analyses indicated that the dynamic impact of skin against the edge of a stationary surface contributes to an increase in the vibration frequency of the skin. This hypothesis was also supported by the psychophysical experiment, which showed that an edge-rounded stationary surface had less effect on sensitivity enhancement. Finally, we investigated possible SBC arrangements for practical applications on the basis of line sensation.