Friction measurements on a Large Area TPaD

We describe a variable friction haptic display on a large area of glass, and characterize its performance. The Large Area Tactile Pattern Display (LATPaD) uses modulation of surface friction created by ultrasonic vibration, as in previous smaller devices [8]. Unlike those, the LATPaD does not operate in the lowest resonant mode. As a consequence, the vibration amplitude varies across the surface of the LATPaD (being zero at nodal lines) and therefore the friction reduction effect varies spatially. In practice, the variation in friction reduction is less than might be expected, due to the finite extent of the finger pad. We measure friction over the surface for several mode shapes.