Active Lateral Force Feedback on Bare Fingertips and Its Applications


Active lateral force feedback is essential for haptic applications in which forces on the fingertip is perpendicular to or in the same direction of finger movement, such as virtual shape rendering and button click rendering. In this thesis, I first present a novel device, the UltraShiver, that can provide large active lateral forces (400 mN peak) while operating in the ultrasonic regime. UltraShiver consists of a sheet of anodized aluminum excited in a compression-extension mode via piezoelectric actuators. By combining in-plane ultrasonic oscillation and out-of-plane electrostatic attraction ("electroadhesion"), both operating at about 30 kHz, lateral forces are generated. The lateral force is a function of pressing force, lateral vibration velocity, and electroadhesive voltage, as well as the relative phase between the velocity and voltage. I perform experiments to investigate characteristics of the UltraShiver and their influence on lateral force. Also, a lumped-parameter model is developed to understand the physical underpinnings of the influences. The UltraShiver, however, relies on a single longitudinal resonance to produce oscillations, resulting in an inconsistent force profile. To overcome this limitation, I propose another device, the SwitchPaD, that also synchronizes in-plane ultrasonic oscillation and out-of-plane electroadhesion, but switches between the first and the second longitudinal mode based on the finger position, resulting in a much more consistent force profile across the touch surface. Experiments are used to compare the performance of two different modal switching strategies. Results indicate that the SwitchPaD can generate 250 mN peak active lateral force over a large area, and that, with the proper switching strategy, the switch itself is imperceptible. As controllable lateral force sources, the UltraShiver and the SwitchPaD are used to render haptic features on the surface. Via integration with finger position sensing, these two devices can be used to render 2.5D shape, which can help users identify objects and guide their exploration on the surface. Additionally, once integrated with a pressing force sensor, these two devices can generate a button click sensation on a flat surface without macroscopic motion of the surface in the lateral or normal direction. This effect can be localized because electroadhesion between a finger and a surface can be localized. Further, both the perception experimental results and the subjects' comments confirm all subjects could clearly perceive the click sensations among all the stimuli. Subjects also reported that some rendered click sensations felt similar to those in commercial trackpads. Overall, the ability to produce localized, highly controllable button click rendering, suggests that the technique described here is a promising candidate for touch typing on a flat surface.

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