Fig. 4.

The nine reference textures used in this study represented by 6-mm long samples of the space-domain TPaD texture maps. TPaD amplitude refers to the friction reduction amplitude achieved at any point through ultrasonic vibration between the TPaD surface and the test subject’s finger, scaled by the maximum amplitude of the device. Inset: Fast Fourier Transform magnitude traces for each of the textures, normalized for visibility. Red: commanded friction reduction spectral magnitude; purple: lateral fingertip velocity spectral magnitude acquired via laser Doppler vibrometer. (Note that dB units are calculated with a reference value of 1 cycle/mm).