Electro-Mechanical Contact Interactions Between Human Finger and Touchscreen Under Electroadhesion

TL;DR

This study develops and validates an electro-mechanical model of electroadhesion between human fingers and touchscreens, revealing how electrical properties and coatings influence tactile perception and electrostatic forces.

Contribution

It introduces a novel impedance-based method to estimate electrostatic forces and air gaps, and explores the effects of coatings and moisture on tactile perception under electroadhesion.

Findings

Electrostatic force varies with charge leakage and frequency.

Electrical impedance measurements can estimate finger-to-screen air gaps.

Coatings significantly influence tactile perception and electroadhesion effects.

Abstract

Electroadhesion (EA) has potential in robotics, automation, space missions, textiles, and tactile displays, but its physics remains underexplored due to limited models and experimental data. This thesis develops an electro-mechanical model to estimate electrostatic forces between human finger and touchscreen under EA and compares it to experimentally measured friction forces. The model aligns well with the data, showing that the electrostatic force changes mainly due to charge leakage from the Stratum Corneum at frequencies below 250 Hz and its electrical properties above 250 Hz. Additionally, a novel approach using electrical impedance measurements estimates electrostatic forces by subtracting skin and touchscreen impedances from the total impedance. This method is the first to experimentally estimate the average air gap between finger and voltage-induced capacitive touchscreen. The effect of electrode polarization impedance, particularly at low frequencies, was also studied, revealing its role in the charge leakage phenomenon. Tactile perception via EA was investigated using DC and AC voltage signals on a touchscreen with 10 participants of varying finger moisture levels. Results showed that AC voltage detection thresholds were significantly lower than for DC, explained by charge leakage at lower frequencies. Participants with moist fingers exhibited higher threshold levels, supported by impedance measurements. The thesis also investigated how touchscreen top coatings influence tactile perception, focusing on EA-free interactions. Psychophysical experiments and physical measurements demonstrated that coating materials significantly affect tactile perception, likely due to molecular interactions. These findings offer insights into finger-touchscreen interactions under EA and have potential applications in designing robotic systems and haptic interfaces using this technology.