Localized Tactile Feedback on a Transparent Surface through Time-Reversal Wave Focusing

This article addresses the problem of producing independent tactile stimuli to multiple fingers exploring a transparent solid surface without the need to track their positions. To this end, wave time-reversal was applied to re-focus displacement impulses in time and in space at one or several locations in a thin glass plate. This result was achieved using ultrasonic bending waves produced by a set of lamellar piezoelectric actuators bonded at the periphery of the plate. Starting from first principles, the relations linking implementation parameters to the performance of the display are developed. The mechanical design of the display, signal processing, and driving electronics are described. A set of engineering tradeoffs are made explicit and used for the design of a mock up device comprising a glass plate 148 x 210 x 0.5 mm³. Tests indicate that a peak amplitude of 7 μm confined to a 20 mm² region could be obtained for an average power consumption of 45 mW. Simultaneous focusing at several locations was successfully achieved. We showed that a lumped-mass model for the fingertip can effectively describe the effect of an actual fingertip load at the focus point. Lastly, we elucidated a likely stimulation mechanism that involves the transient decoupling of the finger skin from the plate surface. This phenomenon explains the observed tactile effect.