Substituting friction by weak vibration on a tactile pin array

During scanning a tactile relief with the finger tips, only a small fraction of the whole surface is touched and perceived simultaneously. In contrast to the saccadic eye movements, the positions of the fingers are changed slowly and continually most of the time. Consequently, even if a large relief is to be presented, the size of the display surface can be limited to the size of one or a few fingertips. Instead of presenting a complete real relief, small fractions of a mostly virtual relief will be presented. In order to involve the same actions and haptic feedback signals as with exploring real reliefs, the user should select the information displayed under his or her fingertips by moving the fingers on the resting virtual relief. Obviously, the displays under the fingertips have to be moved along with the fingers. Thus, there is no relative movement between the skin of the fingertips and the surfaces of the displays. The implications of this lack of friction are not obvious because an exactly equal situation does never occur when dealing with natural tactile objects. It is known that rating the roughness of a grooved surface is independent of decreasing the friction by the application of grease to the fingertips. However in these experiments which deal with roughness, determining the location of the stimuli was not involved which is crucial in the reading of Braille and tactile graphics. In order to learn about this problem, we designed and built a tactile display which at present is limited to the presentation of Braille characters because the Braille reading performance can be measured more reliably than the exploration of graphics.