Onthe Electrical Characterization of Electroadhesive Displays and the Prominent Interfacial Gap Impedance Associated with Sliding Fingertips (2018)

Observations of electroadhesion, or the increased attraction between two contacting surfaces caused by an electric field applied across their interface, have been recorded for over 140 years. Throughout the late 19th and 20th centuries the effect intrigued scientists and engineers, causing tactile vibrations and sounds to be emitted via their fingertips, almost as if by magic. Modern day application of this effect, however, has so far been limited. It has been used for semiconductor wafer chucking, for industrial material handling and robotic gripping, as a tactile display technology for the visually impaired, and, most recently, as a visuo-tactile display technology for general tablet and smartphone type interactions.

It is this last application that has garnered renewed interest in electroadhesion, specifically as it pertains to the development of high performance variable friction surface haptic devices. With this interest comes the need for effective modeling techniques that are tailored to the specific application of fingertip adhesion, and metrics for creating optimized skin based electroadhesive systems. To date, a variety of groups have used perceptual and/or force measurements to characterize displays, as this is usually the output that is ultimately desired from the interaction. Typically, a voltage is input to the electroadhesive system, and the resulting force or perception output is measured, with an approximate theoretical model linking the two. While essential for evaluating the capabilities of existing hardware, this black box type approach is limited in terms of designing new electroadhesive systems. It lacks empirical measurements of electrical parameters for the systems in question, and usually relies heavily on values pulled from the literature to roughly map applied voltage input to recorded force and perceptual output.

In this work, we propose an alternate approach, and carry out a set of electrical characterization techniques to examine two electroadhesive surfaces (one common to the literature, and one relatively new) and experimentally look inside the black box. We asked these two questions: what factors affect how an applied voltage (or current) results in an effective interfacial voltage in the system, and how does this interfacial voltage translate into an additional lateral force on the finger? The latter is addressed in a separate work, while former is investigated here.

To answer this first question, we developed a model inspired by the semiconductor chuck literature and previous surface haptic research, which uses parameters taken almost entirely from empirical measurement. We then validate this model with I versus V Lissajous curve observations and use small signal electrical and electrochemical impedance measurements to capture model parameters. Finally, we discuss how isolated measurements and those taken as a whole converge to unambiguously confirm the existence of a prominent interfacial gap impedance. Finally, we show how this gap impedance compares to other elements in the system.

Award: Best Paper Award

Citation: C. D. Shultz, M. A. Peshkin and J. E. Colgate, "On the electrical characterization of electroadhesive displays and the prominent interfacial gap impedance associated with sliding fingertips," 2018 IEEE Haptics Symposium (HAPTICS), 2018, pp. 151-157, doi: 10.1109/HAPTICS.2018.8357168.