The Application of Tactile, Audible, and Ultrasonic Forces to Human Fingertips Using Broadband Electroadhesion (2018)

Surface haptic displays aim to apply programmable forces to bare fingertips on flat surfaces. Geared towards enriching the user experience of touchscreen devices, these displays show promise of being seamlessly incorporated into everyday human computer interactions. One class of surface haptic device that has received great interest is the variable friction display. When operated at steady state, variable friction displays are able to arrest the motion of a sliding finger, or reduce friction to a near negligible level. These effects are achieved by applying additional electrostatic force, which increases friction, or by altering the skin/surface interface via low amplitude, ultrasonic oscillations, which decreases friction. These effects can also be combined for greater dynamic range.

While the quasi-static behavior of these displays has been the primary subject of research for some time by myself and others, more recent applications utilize increasingly rapid modulation of friction. This transition towards dynamic actuation stems from the fact that all known variable friction displays (including those presented here) actuate the entire fingerpad in spatial synchrony, i.e. the entire fingerpad is altered at once. This synchrony offers impoverished information to slowly adapting type I tactile afferents (Merkel’s discs), which are sensitive to quasi-static (< 10 Hz) spatial distributions of strain energy across the fingerpad. In contrast, dynamic modulation of friction (> 10 Hz) is thought to offer rich information for both fast adapting type I (Messiner’s Corpuscles) and type II (Pacinian Corpuscles) tactile afferents, which are most sensitive to transient and broadband vibrations in the range of approximately 10-1,000 Hz, and which can have less spatial acuity. The fast adapting afferents, therefore, appear to be prime candidates for spatially synchronous variable friction actuation. In fact, properly actuating the fingertip at these dynamic frequencies (10-1,000 Hz) may be critical to new surface haptic applications such as virtual texture display.

A principal goal of this research, therefore, is to develop an approach to variable friction surface haptics that is sufficiently broadband to offer rich excitation of fast adapting type I and II afferents. Additionally, this approach should have an ideally flat dynamic force response in the range of 10-1,000 Hz. In this paper we demonstrate that not only is this broadband tactile excitation possible, but the method and hardware developed may be easily extended to produce programmable audio emanating from a fingertip, adding a complementary sensory modality to the interaction experience. As a side-effect of the modulation method, it is also shown that ultrasonic friction forces (up to at least 50-60 kHz) can also be applied to the finger. These results indicate that the only major limitation to the bandwidth of the electroadhesion effect appears to be the speed at which the electric field may be applied to the system.

Award: Best Application Paper of the Year

Citation: C. Shultz, M. Peshkin and J. E. Colgate, "The Application of Tactile, Audible, and Ultrasonic Forces to Human Fingertips Using Broadband Electroadhesion," in IEEE Transactions on Haptics, vol. 11, no. 2, pp. 279-290, 1 April-June 2018, doi: 10.1109/TOH.2018.2793867.