  One well-known class of surface haptic devices that we have called TPaDs
(Tactile Pattern Displays) uses ultrasonic transverse vibrations of a touch
surface to modulate fingertip friction. This paper addresses the power
consumption of glass TPaDs, which is an important consideration in the context
of mobile touchscreens. In particular, based on existing ultrasonic friction
reduction models, we consider how the mechanical properties (density and
Young's modulus) and thickness of commonly-used glass formulations affect TPaD
performance, namely the relation between its friction reduction ability and its
real power consumption. Experiments performed with eight types of TPaDs and an
electromechanical model for the fingertip-TPaD system indicate: 1) TPaD
performance decreases as glass thickness increases; 2) TPaD performance
increases as the Young's modulus and density of glass decrease; 3) real power
consumption of a TPaD decreases as the contact force increases. Proper
applications of these results can lead to significant increases in TPaD
performance.
