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Understanding and Exploiting Electroadhesion of Human Fingertips for High Performance Surface Haptic Applications

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The aim of this dissertation is to make sense of nearly a century and a half's worth of observations concerning skin based electroadhesion phenomena. While this is a noble goal in and of itself, further motivation of this work is drawn from fact that the electroadhesive effect is increasingly being utilized in modern day practical applications, and appears poised for integration into the now ubiquitous touchscreen interface. It was with each of these contexts in mind that I set about resolving decades of disparate observational reports and various measurements in the literature with one another. In the course of this reckoning I was able to construct a simple, yet flexible, working model which seemingly brought everything together, and which led to predictions of future capabilities of the effect. It is my aim that the model and experiments given here will essentially show how aspects of electroadhesion may not just be better understood, but may be, in the end, actively \textit{exploited} for practical finger based applications. I begin by demonstrating the previously overlooked DC capabilities of electroadhesion, based on work performed in the 1920s by Johnsen and Rahbek, which is capable of producing forces on the finger an order of magnitude greater than those previously reported in the haptics literature. To model the ability of electroadhesion to generate such high force (especially at DC), I propose a unified force model, based on lumped electrical impedance parameters and an interfacial air gap, and resolve this model with those in previous reports. In this process I briefly discuss the background and specifics of the Johnsen-Rahbek effect, and include friction measurements made with my own electroadhesive surface and experimental apparatus. Expanding from this initial DC understanding of the effect, I then set about characterizing two different variable friction electroadhesive displays using careful electrical and electrochemical impedance measurements across a broad range of frequencies. I qualitatively and quantitatively examine the properties of the skin, body, surface coating, and various electrode interface impedances in isolation using different contact interface conditions, measurement types, and custom electrical hardware. My lumped series impedance model is filled out and used to explain how all impedances are related during normal usage. The linearity of this model is shown to be valid under certain assumptions, such as high applied frequencies or small applied currents, and speculation as to the physical mechanisms underlying each impedance element is given. This analysis unambiguously verifies and expands upon the existence of the hypothesized key electrical system parameter: the air gap impedance (or sliding interfacial impedance). This parameter represents a large increase (100- 1000%) in overall impedance observed when a finger is sliding versus when it is stationary which cannot be explained by other electrical impedance measures and which vanishes again should the finger come to rest. Finally, I report on an extremely high bandwidth electroadhesive approach to controlling friction forces on sliding fingertips which is capable of producing vibrations across an exceedingly broad range of tactile, audible, and ultrasonic frequencies. Vibrations on the skin can be felt directly, and vibrations in the air can be heard emanating from the finger. Additionally, I present evidence of how the interfacial air gap voltage is primarily responsible for the induced electrostatic attraction force underlying the electroadhesion effect. I develop an experimental apparatus capable of recording friction forces up to a frequency of 6 kHz, and use it to characterize my two electroadhesive systems, both of which exhibit flat current-to-force magnitude responses throughout the measurement range. These systems use custom electrical hardware to modulate a high frequency current and apply surprisingly low distortion, broadband forces to the skin. Recordings of skin vibrations with a laser Doppler vibrometer demonstrate the tactile capabilities of the system, while recordings of vibrations in the air with a MEMS microphone quantify the audible response and reveal the existence of ultrasonic forces applied to the skin via electronic friction modulation.

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  • 02/08/2019
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