Surface waves and spatial localization in vibrotactile displays

The locus of vibrotactile stimulation is often used as an encoding cue in tactile displays developed for spatial orientation and navigation. However, the ability to localize the site of stimulation varies as a function of the number and configuration of the vibrating motors (tactors) in the display. As the inter-tactor distance decreases it has been found that the ability to localize a point of stimulation diminishes. One factor that may limit tactile localization is the surface wave elicited by vibration that propagates across the skin at a velocity that depends on the frequency of vibration and the viscoelastic properties of the skin. A material that simulates the stress-strain characteristics of human skin was used to measure the characteristics of surface waves during vibrotactile stimulation. Accelerometers glued to the simulated skin at fixed distances from the activated tactors were used to measure the amplitude of the acceleration as a function of distance as well as the propagation velocity. It was determined that at a distance of 60 mm from the site of activation, the surface wave was on average attenuated to less than 1 m/s
²
. This suggests that for this type of tactor an inter-tactor distance of at least 60 mm would be optimal for a display in which the locus of stimulation is used as an encoding variable. It seems that much of the difficulty encountered in identifying the locus of a vibrotactile stimulus in multi-tactor displays may result from small intertactor distances.