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

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.

Figures (45)

• Figure 1: Schematic representation of electroadhesive devices. Dielectric 1 is in contact with dielectric 2 and there is an air gap between them due to their surface roughness. The electrodes embedded in dielectric 1 are connected to the power source, and hence induced electrical charges are accumulated at the interface of dielectric 2. The opposite charges at the interfaces of the dielectrics generate electrostatic forces and attract the dielectrics to each other.
• Figure 2: The electroadhesion technology has applications in tactile displays, spacecraft docking, climbing and crawling robots, robotic grippers, electrostatic chucks, etc. The images are reproduced with permission: Copyright 2023, Wiley shintake2016versatile, Copyright 2023, Wiley xiao2015advances, and Copyright 2023, Elsevier koh2016hybrid.
• Figure 3: A cross-sectional representation of the capacitive touchscreen in contact with a human finger. The touchscreen is composed of a conductive (ITO) layer beneath an insulator layer of SiO$_2$. Only the outermost layer of the human finger (SC) is displayed in the figure, which has a finite conductivity.
• Figure 4: The experimental set-up used in our study to measure the electrostatic forces acting on human finger.
• Figure 5: Experimental results; (a) Coefficient of friction (CoF), normal force, and the velocity profile of the horizontal stage as a function of displacement (b) steady-state values of CoF (solid) and electrostatic attraction force (dashed) as a function of stimulation frequency.